Clay modification for the production of polystyrene nanocomposites

Transcription

Clay Modification for the Production of
Polystyrene Nanocomposites by Melt Processing
by
Jorge Alonso Uribe Calderón
A thesis submitted to the Faculty of Graduate Studies and Research in
partial fulfillment of the requirement of the degree of Doctor of Philosophy
Department of Chemical Engineering
McGill University
Montreal, Quebec, Canada
September 2007
Copyright © Jorge Alonso Uribe Calderón
Abstract
Natural Montmorillonite was modified with thermally stable phosphonium
surfactants to produce new organoclays for the production of polymer
nanocomposites. The organoclays were characterized to determine thermal
stability, basal spacing, and the surface energy at room temperature and at the
processing temperature. Polystyrene (PS)/organoclay nanocomposites were
prepared by melt compounding, using three different PS resins. Experimental data
were obtained to elucidate the influences of temperature and molecular weight
and structure of the surfactant on the surface energy of the organoclays. The
phosphonium-based
organoclays
exhibited
better
thermal
stability
than
commercially available ammonium-based organoclays. The basal spacing was
similar to that found in commercially available organoclays. Transmission
electron microscopy (TEM) showed that the degree of dispersion of the various
organoclays in nanocomposites was related to the Hamaker constant of the
organoclay at the processing temperature. Significant improvement in the degree
of dispersion was realized, when blends of polystyrene with a styrene- maleic
anhydride (SMA) copolymer were used. It appeared that delamination in the
SMA systems was achieved directly without undergoing an intermediate
intercalated structure.
The influence of organoclay concentration on flexural
modulus of PS- organoclay nanocomposites was determined, using the HalpinTsai and Hui-Shai models. The predictions were in good agreement with
experimental results. The modulus of PS nanocomposites correlated well with the
work adhesion at room temperature, in agreement with the equation of Shang.
Barrier properties showed reasonable agreement with the predictions of models
reported in literature. However, the values of aspect ratios predicted by the
models were quite different from those observed experimentally.
The
permeability of nanocomposites to oxygen correlated with both the Hamaker
constant A131 at the processing temperature and the initial basal spacing of the
organoclay. In both cases, permeability decreased with the corresponding
parameter.
i
Résumé
De la montmorillonite naturelle a été modifiée avec des surfactants de
phosphonium qui sont thermiquement stables pour objectif de produire de
nouvelles organoargiles pour la production de nanocomposites polymériques. Les
organoargiles ont été caractérisées pour déterminer la stabilité thermique,
l’espacement basal et l’énergie superficielle à température ambiante et à la
température de préparation. Des nanocomposites de polystyrène
(PS) et
organoargile ont été préparés en fondant le PS, avec trois différentes résines de
PS. On a évalué l’influence de la température, de la masse molaire et de la
structure des surfactants sur l’énergie superficielle des organoargiles. Les
organoargiles préparées avec des surfactants de phosphonium ont démontré une
meilleure stabilité thermique que les organoargiles commerciales préparées avec
des surfactants d’ammonium. L’espacement basal a été similaire à celui des
organoargiles commerciales.
La microscopie électronique en transmission a
démontré que le degré de dispersion des organoargiles dans les nanocomposites
est en relation avec la constante de Hamaker de l’organoargile à la température de
préparation. Le degré de dispersion de l’argile a été amélioré quand un mélange
de polystyrène avec un copolymère de styrène et d’anhydride maléique (SAM) a
été employé. Apparemment, la delamination dans les systèmes de SAM a été
réussie directement sans avoir une structure intercalée intermédiaire. On a
déterminé l’influence de la concentration d’argile sur les propriétés mécaniques
des nanocomposites de PS en utilisant les modèles de Halpin-Tsai et Hui-Shai.
Les prédictions ont été en accord avec les résultats expérimentaux. Les modules
des nanocomposites de PS sont bien corrélés avec le travail d’adhésion à la
température ambiante, selon l’équation de Shang. Les propriétés de barrière ont
été mesurées et sont en accord avec les prédictions des modèles publiés. Mais les
valeurs de facteurs de forme des particules calculées par les modèles sont
différentes de celles observées expérimentalement. La perméabilité à l’oxygène
des nanocomposites de PS a été corrélée avec la constante de Hamaker à la
ii
température de préparation et avec l’espacement initial de l’organoargile. La
perméabilité à l’oxygène a diminué avec les deux paramètres.
iii
Acknowledgements
The help and support of the following individuals and institutions are highly
appreciated as without their support this work would not been possible.
Firstly, I would like to express my deep gratitude to my supervisor,
Professor Musa R. Kamal, for his financial support, guidance, patience and
invaluable suggestions, and for being a constant source of encouragement.
My co-supervisor R. Bruce Lennox of the Department of Chemistry,
McGill University for his participation and cooperation to this project. His
comments were highly appreciated.
The National Council for Science and Technology (CONACytT, Mexico),
Department of Chemical Engineering, Natural Science and Engineering Council
Research of Canada (NSERC), and Centre de Recherche en Plasturgie et
Composites (CREPEC) for the financial support.
I would like to thankthe following persons:
Dr. Nitin Borse for introduce me to the use of twin screw extruder, and
compression molding, and for his interesting comments and suggestions.
Dr. Weiliang Chan and Dr. Lijun Feng, for introducing me to the pendant
and sessile drop techniques and to the use of atomic force microscopy
Dr. Mohammad Al-Wohoush, for introducing me to the use of the
software for drop contour analysis, and for all his suggestions and interest.
Dr. Kelly Sear, for introducing me to the use of transmission electron
microscopy.
Mr. Slavek Poplawski and Dr. Xue-Dong Lui, for carrying out the x ray
experiments and some transmission electron microscopy.
Mr. Changzheng Xue and Gonzalo Guerrica-Echevarria, for helping to
obtain the some nanocomposites and some of the sessile drop data.
M. Jinsong Chu and M. Kam-Wa Lee for their interest, comments, help
and above all, their friendship. And, Jean-Michael Lavoie for correcting the
résumé.
The administrative staff of the Department of Chemical Engineering,
McGill University.
Nova Chemicals and Cytec Industries for donating the materials used in
this research.
Last, but not the least, my mother, father, Julio, brothers, sisters and all my
family for their love and support.
iv
Table of Content
Abstract ........................................................................................................................... i
Résumé................................................................................................................ ii
Acknowledgements............................................................................................ iv
Table of Content ................................................................................................. 1
List of Figures ..................................................................................................... 5
List of Tables ...................................................................................................... 9
Chapter 1 Introduction .......................................................................................... 11
1.1
Introduction........................................................................................... 12
1.2
Motivation............................................................................................. 15
1.3
Approach............................................................................................... 16
1.4
Thesis Overview ................................................................................... 17
Chapter 2
2.1
Background ....................................................................................... 18
Polymer Nanocomposites ..................................................................... 19
2.1.1
General considerations...................................................................... 19
2.1.2
Preparations methods ........................................................................ 20
2.1.3
Nanocomposite structures ................................................................. 21
2.1.4
Mechanical, thermal, and barrier properties ..................................... 24
2.2
Modified Montmorillonite (Organoclay).............................................. 25
2.2.1
Montmorillonite ................................................................................ 25
2.2.2
Structure of organoclays ................................................................... 27
2.2.3
Surfactants......................................................................................... 28
2.3
Theoretical Considerations ................................................................... 33
2.3.1
Thermodynamic aspects.................................................................... 33
2.3.2
Dynamic aspects ............................................................................... 37
2.4
PS Nanocomposites .............................................................................. 38
2.4.1
Melt intercalation .............................................................................. 38
2.4.2
In-situ polymerization ....................................................................... 45
2.4.3
Solution blending .............................................................................. 47
Chapter 3
Objectives ......................................................................................... 48
1
3.1
Main Objective...................................................................................... 49
3.2
Specific Objectives ............................................................................... 49
Chapter 4
4.1
Experimental: Materials & Procedures ............................................. 50
Materials ............................................................................................... 51
4.1.1
Polymers ........................................................................................... 51
4.1.2
Layered silicates................................................................................ 51
4.1.3
Phosphonium surfactants .................................................................. 52
4.2
Experimental Procedures ...................................................................... 52
4.2.1
Preparation of organoclay and PS nanocomposites .......................... 52
4.2.2
Characterization procedures.............................................................. 56
Chapter 5
Thermally Stable Phosphonium-Montmorillonite Organoclays ....... 64
5.1
Abstract ................................................................................................. 65
5.2
Introduction........................................................................................... 65
5.3
Results and Discussion ......................................................................... 67
5.3.1
Thermogravimetric analysis.............................................................. 67
5.3.2
X-ray analysis ................................................................................... 82
5.4
Chapter 6
Summary ............................................................................................... 86
Surface Energy of Modified Montmorillonite .................................. 88
6.1
Abstract. ................................................................................................ 89
6.2
Introduction........................................................................................... 89
6.3
Surface Energy of the Organoclays and Polymers at Room Temperature
91
6.3.1
Surface roughness ............................................................................. 91
6.3.2
Surface energies at room temperature............................................... 93
6.4
Surface Energy of Organoclay at the Processing Temperature ............ 99
6.4.1
Thermal stability of the materials ..................................................... 99
6.4.2
Surface tension of the resins ........................................................... 103
6.4.3
Contact angles and surface energies at high temperature ............... 105
6.5
Thermodynamic Work of Adhesion and Interfacial Surface Tension 109
6.6
Hamaker Constant............................................................................... 114
6.7
Correlation of Surface Parameters with Organoclay Performance..... 117
2
6.8
Chapter 7
Summary ............................................................................................. 122
Polystyrene/Phosphonium Organoclay Nanocomposites by Melt
Compounding...................................................................................................... 124
7.1
Abstract ............................................................................................... 125
7.2
Introduction......................................................................................... 125
7.3
Nanocomposite Characterization ........................................................ 127
7.3.1
Wide angle x-ray diffraction (WAXD)........................................... 127
7.3.2
Transmission electron microscopy (TEM) ..................................... 129
7.4.3
Thermal stability ............................................................................. 133
7.4.4
Oxygen permeability....................................................................... 136
7.4.5
Mechanical properties ..................................................................... 140
7.4.6
Mechanical and oxygen permeability and work adhesion.............. 147
7.5
Chapter 8
Summary ............................................................................................. 150
PS-SMA-Phosphonium Organoclay Nanocomposites ................... 151
8.1
Abstract ............................................................................................... 152
8.2
Introduction......................................................................................... 152
8.3
Surface Energy of SMA...................................................................... 153
8.4
Clay Dispersion................................................................................... 155
8.5
Thermal Stability of Nanocomposites ................................................ 163
8.6
Oxygen Permeability .......................................................................... 164
8.7
Mechanical Properties......................................................................... 165
8.8
Summary ............................................................................................. 173
Chapter 9
Conclusions and Recommendations ............................................... 174
Conclusions..................................................................................................... 175
Original Contributions to Knowledge............................................................. 177
Recommendations........................................................................................... 178
References........................................................................................................... 179
Appendix A..................................................................................................... 202
Phosphoinium surfactants ........................................................................... 202
Polymer resins............................................................................................. 204
Clays............................................................................................................ 208
3
Appendix B ..................................................................................................... 211
Appendix C ..................................................................................................... 212
4
List of Figures
Figure 2.1. Schematic representation of nanoplatelet based polymer
nanocomposites (taken from Hussain et al, 2006). ............................................... 22
Figure 2.2. Schematic representation of Montmorillonite crystal (taken from Ray
et al, 2006). ........................................................................................................... 26
Figure 2.3. Schematic representation of surfactant molecules arrangements (taken
from Murray, 2000). ............................................................................................. 28
Figure 2.4. (a) Contributions of polymer and surfactant chains to the total ideal
combinatorial entropy change per surface area as a function of surface separation
(Vaia and Giannelis, 1997a). (b) Changes of total free energy per area as a
function of surface separation, subscripts refer to different interaction parameters
polymer-clay surface values, negative values indicate a favourable interaction
polymer-clay, ε=0(a), -2(b), -4(c) and -12(d) mJ/m2, respectively....................... 34
Figure 2.5. Free energy change as a function of surface separation. The degree of
cation exchange is expressed as packing density (molecule/area): a) 0.04 and b)
0.12. (Balazs et al, 1998). ..................................................................................... 35
Figure 2.6.- Free energy change as a function surface separation. N, χand φare
polymerization degree, interaction parameter and volume fraction of
functionalized polymer, respectively. (Balazs et al 1998b, 2000). ....................... 36
Figure 2.7. The influence of polymer branching on free energy change as a
function of surface separation (Singh et al, 2000). ............................................... 36
Figure 2.8. Schematic representation of clay platelets in the polymer flow. Greek
letters represent the different angles describing the platelets orientation, S is the
overlapped clay surface, d is the basal spacing and h the clay platelet thickness. 38
Figure 2.9. Mechanical properties of nanocomposites prepared with Cloisite 10-A
and PS resins with different molecular weights (Tanoue et al, 2005). ................. 45
Figure 4.1. Graphic representation of a static mixer (Borse, 2006)...................... 55
Figure 4.2. Schematic representation of data manipulation for drop profile
analysis. a) drop profile in pixel units, and b) the sorted drop profile in mm....... 59
Figure 5.1. (a) TGA and (b) derivative TGA for phosphonium surfactants. ........ 68
5
Figure 5.2. (a) TGA and (b) derivative TGA for Ph1 organoclays with different
amounts of added surfactant. ................................................................................ 71
Figure 5.3. (a)TGA and (b) derivative TGA of different organoclays. ................ 75
Figure 5.4. Example of Hoffman elimination reaction. ........................................ 76
Figure 5.5. TGA derivative curves for surfactants and the corresponding
organoclays. Sharp peaks at low temperatures correspond to the surfactant salts. (a)
Ph1, (b) Ph2, (c) Ph3 and (d) Ph4. ........................................................................ 78
Figure 5.6. TGA curves for different organoclays at 220 °C................................ 81
Figure 5.7. X-ray diffractograms of Montmorillonite treated with Ph1 at several
CEC ratios............................................................................................................. 83
Figure 5.8. Basal spacing and organic content as a function of Ph1 surfactant
addition. ................................................................................................................ 84
Figure 5.9. X-ray diffractograms for different organoclays.................................. 86
Figure 6.1. AFM images of organoclay surfaces. ................................................. 92
Figure 6.2. Plots of γl(1+cosθ)/2 (γld)1/2 against γlp)1/2/(γld)1/2 for solid-liquids
system, dispersive and polar components of diiodomethane: 50.4 and 0.4 mJ/m2 ,
respectively. .......................................................................................................... 97
Figure 6.3. TGA isothermal curves of polymer resins at 220 °C, negative numbers
represent the percentage of mass loss. ................................................................ 101
Figure 6.4. Thermal stability of organoclay at isothermal conditions. ............... 102
Figure 6.5. Drop profile evolution with time of PS1220 at 220 °C. ................... 104
Figure 6.6. Surface tension values of PS1220 at 220 °C with time. ................... 104
Figure 6.7. ST of PS resins with temperature. .................................................... 105
Figure 6.8. Typical sessile drop picture, PS1220 on Ph1 surface....................... 106
Figure 6.9. Thermodynamic work of adhesion of clay-Styrenic systems........... 111
Figure 6.10. Interfacial tension clay-polymer at room temperature. .................. 113
Figure 6.11 a) Tem pictures of PS nanocomposites prepared with different
organoclays (the clay content is 2 % in all cases), b) effect of molecular weight of
surfactant on the basal spacing and A131 (clay-polymer-clay)............................ 118
6
Figure 6.12. Influence of the thermodynamic work of adhesion on the modulus of
PS nanocomposites prepared with different organoclay and clay content. PS resins
have different flow rate: a) PS 1510 and b) PS 1220.......................................... 120
Figure 6.13. Influence of the A131 at 220 °C (a) and the initial basal spacing of
organoclay (b) on permeability to oxygen in PS 1220 nanocomposites prepared
with different organoclays and clay concentrations............................................ 121
Figure 7.1. TEM pictures of PS1220 nanocomposites prepared with different
organoclays. ........................................................................................................ 131
Figure 7.2. TGA curves of PS1220 nanocomposites containing different
organoclays (5 % clay content)........................................................................... 134
Figure 7.3. Permeability coefficient ratios of PS nanocomposites prepared with
phosphonium organoclays: (a) PS1510 and (b) PS1220. Symbols represent the
experimental observations and lines represent the fit generated with Cussler
Model where φ represents the volume fraction of MMT and A the aspect ratio of
particles. .............................................................................................................. 139
Figure 7.4. Flexural modulus of PS nanocomposites as a function of type and
concentration of organoclay: a) PS1510 and b) PS1220. ................................... 141
Figure 7.5. Maximum strength of PS nanocomposites as a function of type and
concentration of organoclay: a) PS1510 and b) PS1220. ................................... 143
Figure 7.6. Maximum deformation of PS nanocomposites as a function of type
and concentration of organoclay: a) PS1510 and b) PS1220.............................. 144
Figure 7.7. Comparison of some experimental moduli with calculated values from
models: a) PS 1510 and b) PS 1220.................................................................... 146
Figure 7.8. Effect of thermodynamic work of adhesion on oxygen permeability
and mechanical properties of nanocomposites. a) PS1510, and b) PS1220. Solid
and open symbols correspond to oxygen permeability and modulus, respectively.
............................................................................................................................. 148
Figure 7.9. Nanocomposite moduli as a function of thermodynamic work of
adhesion at two organoclay concentration: a) 2% and b) 5%. ............................ 149
Figure 8.1. X-ray patterns of PS1220/Dylark nanocomposites having different
copolymer proportions with phosphonium organoclays (MMT content 2 %).... 157
7
Figure 8.2. X-ray patterns of PS1510/ Dylark 10 % with phosphonium
organoclays (MMT content 2 %). ....................................................................... 158
Figure 8.3. TEM pictures of Ph1/Dylark nanocomposites.................................. 159
Figure 8.4. TEM pictures of PS1510/ Dylark 10 % with phosphonium organoclays
(clay content 2 %). .............................................................................................. 160
(clay content 2 %). .............................................................................................. 162
Figure 8.6. TGA traces of PS1220/Ph1 nanocomposites with different Dylark
contents (2 % clay content)................................................................................. 163
Figure 8.7. Effect of Dylark content on oxygen permeability for samples of
PS1220 having several phosphonium organoclay. MMT content 2 % in all cases.
Graph show the half of error bars for a better data appreciation. ....................... 165
Figure 8.8. Flexural modulus of PS1220-Dylark nanocomposites as a function of
type of organoclay and Dylark concentration. .................................................... 166
Figure 8.9. Flexural strength of PS1220 nanocomposites as a function of type of
organoclay and Dylark concentration. ................................................................ 167
Figure 8.10. Maximum deformation of PS1220 nanocomposites as a function of
type of organoclay and Dylark concentration. .................................................... 168
Figure 8.11. Variation of modulus with MMT content for PS-Dylark
nanocomposites prepared with different phosphonium organoclays. a) PS1510Dylark, and b) PS1220-Dylark. .......................................................................... 169
Figure 8.12. Variation of strength with MMT content for PS-Dylark
nanocomposites prepared with different phosphonium organoclays. a) PS1510Dylark, and b) PS1220-Dylark. .......................................................................... 171
Figure 8.13. Variation of maximum deformation with MMT content for PSDylark nanocomposites prepared with different phosphonium organoclays. a)
PS1510-Dylark, and b) PS1220-Dylark.............................................................. 172
8
List of Tables
Table 2.1. Thermal de gradation onset, maximum degradation rate and basal
spacing for some organically modified montmorillonites. Cloisite
montmorillonite contain ammonium surfactants, where HT is Hydrogenated
Tallow (~65% C18; ~30% C16; ~5% C14). The length of alkyl chains included
in the structure is indicated by the number of carbon atoms (C#). ....................... 32
Table 2.2. Mechanical properties of PS nanocomposites prepared with an
oligomerically (Triclay II)-modified organoclay (Zang et al, 2005). ................... 43
Table 2.3 Mechanical properties of PS nanocomposites containing a polymeric
ammonium organoclay (COPS) (Sepehr et al, 2005). .......................................... 44
Table 5.1. TGA of surfactants............................................................................... 70
Table 5.2. Residual mass of Ph1 organoclay at different temperatures................ 73
Table 5.3. Residual mass of organoclays at different temperatures...................... 77
Table 6.1. Dispersive and polar component of surface tension of liquids used in
the sessile drop experiments in mJ/m2 (Shimizu and Demarquette, 2000)........... 94
Table 6.2. Contact angles (degree) and the resulting surface free energies of
organoclays, mJ/m2. .............................................................................................. 96
Table 6.3. Contact angles (degree) and surface energy of polymers and
organoclays (mJ/m2) at 25 C. r2 the correlation coeficient ................................. 108
Table 6.4. Interfacial Tension: Polymer–Organoclay (mJ/m2) at 220 °C. .......... 114
Table 6.5. Hamaker constant for different organoclays and PS-clay systems.... 117
Table 7.1. Summary of basal spacing (nm). ....................................................... 128
Table 7.2. Summary of aspect ratio of clay agglomerates. Standard deviation in
parenthesis........................................................................................................... 132
Table 7.3. Surface properties and Hamaker constant of organoclays and polymers.
............................................................................................................................. 133
Table 7.4. Summary of TGA results for PS nanocomposites. ............................ 135
Table 7.5. Oxygen permeability coefficient for PS nanocomposites (cc-mm/m2day-atm), values in brackets represent the standard deviation............................ 137
Table 8.1 Contact angles of sessile drops on SMA. ........................................... 154
9
Table 8.2. Interfacial tension and thermodynamic work of adhesion................. 155
10
Chapter 1 Introduction
11
1.1 Introduction
The aim of this chapter is to give a general background about polymer
nanocomposites and to explore the main factors influencing the preparation and
behavior of polymer/clay nanocomposites.
Polymers are widely used versatile materials owing to their advantageous
attributes, such as ease of production, light weight, and flexibility of chemical and
physical design and manipulation. However, in many instances, it is necessary to
modify polymers, in order to satisfy some performance requirements. A common
approach for polymer modification involves mixing polymers with solid fillers
(for example, fibers, whiskers, platelets, or particles), thus producing polymeric
composite systems (Maiti and Singh, 1986, Angles and Dufresne, 2000, Salaniwal
et al, 2002, Ragista et al, 2005, Chen, 2004, Jordan et al, 2005).
A nanocomposite incorporates a filler with dimensions in the nanometer
scale (nanofiller). The small particle size produces a large specific surface area.
The large interfacial area of the nanofiller could produce both cost and
performance benefits for a wide spectrum of applications in the aerospace, food
packaging, biomedical, automotive and other fields. The potential property
improvements could affect mechanical, thermal, barrier, and flammability
behavior, among other properties (Utracki, 2004). Such benefits are not only of
scientific and academic interest, but they could be of great industrial and
commercial importance.
Toyota Research Laboratories pioneered in the development of polymerclay nanocomposites. Their researchers demonstrated that the addition of small
12
amounts of montmorillonite clay to Nylon-6 enhanced mechanical properties,
raised heat distortion temperature (HDT), and reduced the rate of water absorption
(Jordan et al, 2005). At the present, nanocomposites are produced using a wide
range of polymer matrices, i.e., thermoplastics, thermosets, and elastomers. A
wide variety of both synthetic and natural reinforcements, such as clays, silica,
carbon nanotubes, and metal oxides, have been used. Clays (layered silicates) are
commonly used in the preparation of polymer nanocomposites (Jordan et al, 2005,
Alexandre and Dubois, 2000).
The size and morphology of clay particles, the polymer-filler surface
interaction, and the quality of dispersion are critical for achieving the desired
property improvements. Clay is an inorganic material (predominantly
hydrophilic),
whereas
polymers
are
organic
substances
(predominantly
hydrophobic). It is evident that the resulting blend of these materials is
thermodynamically unfavorable. The resulting composite would be an immiscible
blend with poor clay dispersion and unsatisfactory properties, unless
modifications are made. To overcome this difficulty, the layered silicates are
usually treated with surfactants (usually organic modifiers), not only to match the
surface tension of the modified silicate platelets with the surface tension of the
polymer, but also to expand the galleries between the silicate layers. The
organically modified clay is referred to as organoclay. Since the organoclay
surface tension is lowered (in comparison with neat clay) and the interlayer
distance is increased, polymer diffusion into the organoclay galleries becomes
more likely. This would contribute to increasing the polymer/clay interfacial area
13
and enhancing clay dispersion within the polymer (Gilman et al, 2002a, LeBaron
et al, 1999, Porter et al 2000). The appropriate choice of both the chemical
structure and the concentration of the surfactant could lead to improved
compatibility between the organoclay and the polymer, thus leading to the
formation of nanocomposites with enhanced properties (Murray, 2000).
Thermal stability of the organoclay is an important issue, especially if the
nanocomposite is prepared via melt compounding, because the polymer is
exposed to high temperatures in melt processing. Surfactant molecules can
decompose thermally or exude from the organoclay galleries, under melt
processing conditions. Byproducts obtained during thermal decomposition can
catalyze polymer decomposition and thus cause deterioration of polymer
properties (Xie et al, 2001, 2002). Furthermore, thermal decomposition/exudation
of surfactant molecules could reduce the interlayer spacing and cause an increase
in the hydrophilicity of the organoclay. The overall result is the reduction of the
probability of polymer diffusion into the clay galleries. Moreover, free surfactant
molecules (usually with low molecular weight) could cause undesirable
plasticization effects, the production of smoke during processing or odors in the
products (Vaia and Giannelis, 1997a, 1997b).
A combination of entropic and enthalpic factors determines the
thermodynamic probability of intercalation or exfoliation of the organoclay in the
polymer. Organoclay dispersion (i.e. intercalation or exfoliation) requires
sufficiently favorable enthalpic contributions to overcome any entropic penalties.
Favorable enthalpy of mixing for a polymer/organoclay system is realized when
14
the polymer/clay interactions are more favorable than the surfactant/clay
interactions (Chen, 2004, Vaia and Giannelis, 1997a).
The chemical structure of the polymer is another important factor in
nanocomposite preparation. Polymers containing hydrophilic chemical groups
(polyimide, polyesters or thermoset resins) are more likely to yield welldispersed, modified layered silicates. On the other hand, polymers containing only
non-polar structural units are less likely to promote clay dispersion (polyethylene,
polypropylene or polystyrene) (Vaia and Giannelis, 1997a, 1997b). However,
modified polyethylene or polypropylene, grafted with maleic anhydride, could
produce nanocomposites with acceptable quality of clay dispersion and enhanced
properties (Jordan et al, 2005).
The molecular weight of polymers and the processing conditions are
important factors that influence organoclay dispersion during melt compounding.
It has been shown that long residence time and low polymer molecular weight
favor enhanced dispersion (Dennis et al, 2001). However, it was found that the
degree of intercalation in clay-polystyrene systems is independent of the residence
time (mixing time under certain processing conditions). This suggested that other
factors, such as the surface properties of the filler, determine the quality of the
intercalation or delamination processes (Nassar, 2003, Uribe, 2003).
1.2 Motivation
Polystyrene nanocomposites are of industrial interest. World consumption
of PS was around 15 million tonnes in 2005, with a sustained growth rate of 3 %
15
annually (Gobi International, 2007). The largest end use of polystyrene is for
packaging, but it is also used in a variety of commercial and consumer
applications.
The motivation of this project rests on the fact that many important issues
in the preparation of polymer/clay nanocomposites remain unresolved.
In
particular, polystyrene (PS) nanocomposite preparation by melt processing has
represented a challenge, due to various chemical, thermodynamic, and processing
factors. Some of the critical issues contributing to this challenge are related to the
interfacial interactions between the polymer and the candidate organoclays and to
the thermal stability of the surfactants and organoclays (Dharaiya and Jana, 2005).
Thus, the evaluation of clay-surfactant-polymer interactions for a group of
thermally stable surfactants and corresponding clays could provide a valuable
basis for the design and selection of suitable organoclays.
1.3 Approach
The approach employed in this work is to study the factors influencing the
melt compounding and properties of PS/clay nanocomposites. The main
nanocomposites of interest are based on some thermally stable phosphoniummodified montmorillonites. A substantial part of the work considers the quality of
clay dispersion in relation to the surface properties of the PS resin, a
compatibilized polymer blend, and the phosphonium organoclays.
16
1.4 Thesis Overview
Chapter 2 includes an extensive background and literature review of
various theoretical and experimental issues relating to the preparation by melt
processing and the behavior of polymeric nanocomposites. Emphasis is placed on
the use and modification of montmorillonite,
Chapter 3 outlines the general and specific objectives of this work. The
experimental part is described in Chapter 4, which contains a complete
description of materials used to prepare samples. Emphasis is placed on the
modification of montmorillonite with phosphonium compounds.
The experimental results are presented and discussed in three chapters, in
the form of three manuscripts submitted for publications. Chapter 5 deals with the
organic modification of montmorillonite and its influence on the thermal stability
and structure of the organoclay. Chapter 6 considers the surface properties and
interfacial interactions associated with the modified montmorillonite.
The
structure and properties of the nanocomposites, especially those based on
phosphonium organoclays, are given and discussed in Chapter 7.
The use of a copolymer in the preparation of nanocomposites is discussed
in the Chapter 8. Chapter 9 outlines the overall conclusions, recommendations
based on this project and its contributions to knowledge.
17
Chapter 2
Background
18
The present chapter reviews relevant theoretical considerations concerning
the synthesis and behavior of polymer nanocomposites. After a general discussion
of polymer nanocomposites, a description of nanofillers used in the preparation of
nanocomposites is presented. This is followed by a literature review regarding the
chemical structure of surfactants used to modify nanofillers. A brief description of
thermodynamics predictions work is included. Finally, an extensive literature
review concerning PS nanocomposite is presented.
2.1 Polymer Nanocomposites
2.1.1 General considerations
Polymer nanocomposites may be defined as a mixture of a polymer matrix
with other materials, one of which has at least one dimension in the nanometer
scale. In general, polymer nanocomposites refer to a polymer matrix incorporating
a nanofiller, but polymer blends with polymer nano-domains can be also
considered as a nanocomposites (Utracki, 2004).
Nanofiller can be divided into three groups, depending on the size of the
characteristic dimensions of the filler: (i) nano-spheres, in which the three
dimensions of the filler are in the nanoscale regime, (ii) nano-tubes or nano-fibers,
having two dimensions in the nanometer range, (iii) nano-platelets, in which only
the thickness of the filler is in the manometer scale (Alexandre and Dubois, 2000).
Nano-platelets, such as silicate minerals, are the most common nanofillers used in
19
the production of polymer nanocomposites, but oxides or sulphides of some
elements can be included in this category (Sukpirom and Lerner, 2003).
This work deals with the production of polymer nanocomposites
containing silicate nano-platelets, in particular montmorillonite. The importance
of polymer nanocomposites rests on the possibility to achieve a significant
improvement in properties by incorporating small amount of nanofiller (1 to 5%
in volume fraction) (Lebaron et al, 1999, Porter et al, 2000). The improvements
of properties depend on the quality of filler–polymer adhesion. It is also important
to benefit from the very large surface area of fully separated (exfoliated)
nanofiller particles. For example, fully exfoliated particles of montmorillonite
have a surface area of 657 m2/g (Helmy et al, 1999, Giese et al, 1998). The fillerpolymer adhesion and the surface area of nanofiller available after processing
depend on several chemical, structural, interfacial and processing factors.
2.1.2 Preparations methods
Polymer nanocomposites are prepared by in-situ polymerization, solution
mixing and melt compounding. In In-situ polymerization, the nanocomposite is
obtained via polymerization of a homogeneous mixture of nanofiller-monomer (or
prepolymer).
The above procedure has different variants, depending on the
polymerization medium (solution, bulk or emulsion polymerization) or initiation
process (free radical or irradiation, for example) (Fan et al, 2003, Wang et al,
2002, Zang et al, 2003a). In solution mixing, the polymer and nanofiller are
separately dissolved/dispersed in an appropriate solvent. Subsequently, the two
20
solutions are combined and mixed together (by stirring or ultrasonic mixing). The
nanocomposite is obtained after solvent evaporation (Giese et al, 1998). In melt
intercalation or melt processing or compounding, the nanofiller is dispersed
within heat-softened or molten polymer. This preparation technique may or may
not involve the use of shear. Processing conditions and polymer-nanofiller
interactions play an important role in the dispersion of the nanofiller (Nassar et al,
2005, Tanoue et al, 2006, Tokihisa et al, 2006). In the so-called annealing process,
the polymer and nanofiller, in powder form, are mixed at room temperature and
usually pressed. The mixture is heated to above the polymer glass transition
temperature (Tg), and the polymer intercalation into the nanofiller structure is
expected to take place without shear (Vaia et al, 1993). When shear is employed,
batch or continuous mixing techniques, including static, batch mixers and
extruders, are used. The above are the most common methods, but combinations
of them have been employed (Yilmazer and Ozden, 2006).
2.1.3 Nanocomposite structures
The silicate nano-layers (or silicate layers) in the filler-polymer mixtures
normally appear in one or more of the following three structures: exfoliated,
intercalated or immiscible.
The exfoliated structure is characterized by the
homogeneous dispersion of platelets within the polymer matrix, and by the
complete delamination of the filler agglomerates to individual silicate layer. A
more strict definition considers that the minimal distance among individual
silicate layer should be at least 8 nm (Alexandre and Dubois, 2000). Significant
21
property enhancement is achieved, when the agglomerates are exfoliated. The
intercalated structure is obtained, when the polymer chains penetrate the spaces
between the individual silicate layers of the nanofiller, without causing complete
exfoliation. In such a case, the enhancement in properties is moderate. Immiscible
or phase separated structures are produced, when large agglomerates persist in the
mixture, with only small degrees of intercalation or exfoliation. The large
agglomerates of nanofiller observed in this structure are comparable in size to
particles found in conventional microcomposites or composites. In such a case,
the material properties are improved only slightly or, possibly, diminished
(Seperh et al, 2005). Figure 2.1 shows a schematic representation of the above
structures.
Figure 2.1. Schematic representation of nanoplatelet based polymer
nanocomposites (taken from Hussain et al, 2006).
Two complementary techniques are used to characterize layered silicate
nanocomposite structures: x-ray diffraction (XRD) and transmission electron
22
microscopy (TEM). According to x-ray diffraction theory, a diffracted beam is
produced when an x-ray beam impinges on a crystal plane surface, which is
oriented at a given angle in relation to the beam. The angles of incidence and
reflection must be equal, and the incoming and outgoing beams and the normal to
the reflecting planes must themselves all lie in one plane. Three integers are used
to identify the plane with respect to the three unit cell edges, hkl. Usually, h, k,
and l are associated with x, y, and z axes, respectively. For rays reflected by two
adjacent parallel planes, the distance between planes can be calculated using
Bragg’s Law:
2d hkl sin θ = nλ
Equation 2.1.
where λ corresponds to the wave length of the x-ray radiation (usually 0.15418
nm), d is the spacing between diffractional lattice planes, and θ is the measured
diffraction angle or glancing angle (Clegg, 2001). In practice, the value of n can
be set to 1.0 for the primary reflection. In layered silicate nanocomposites, the
interlayer distance between platelets is determined using the above relationship
and taking into account only the distance in the z-axis direction (d001). An increase
of the interlayer distance, possibly due to intercalation, leads to a shift of the
diffraction peak toward lower angles. Exfoliated structures do not show
diffraction peaks in the XRD pattern, either because of the large spacing between
the layers (i.e. exceeding 8 nm) or because the nanocomposite does not exhibit
ordering.
Transmission electron microscopy (TEM) is used to complement XRD for
the characterization of nanocomposite morphology. Each technique has
23
advantages and disadvantages. X-ray analysis provides a fast characterization and
gives a general view of nanocomposite structure, but it is very sensitive to the
nanofiller concentration. On the other hand, TEM, is a more elaborate and
cumbersome technique, which reveals details of nanocomposite structure.
However, as the TEM specimen is usually very small, the resulting observations
are exclusive to the sampling area.
2.1.4 Mechanical, thermal, and barrier properties
The large interfacial area provided by the nanofiller produces a dramatic
improvement in properties.
For example, tensile mechanical properties of
polymers are generally improved with nanofiller concentration. Substantial
improvements are observed in Young’s modulus in nanocomposites exhibiting
exfoliated structures, even at low filler content (Dennis et al, 2001). However,
stress at break and elongation at break may increase or decrease, depending on the
polymer and the interaction between the matrix and the filler. Nanocomposites
based on amorphous polymer usually exhibit a decrease in elongation at the break.
The influence of the nanofiller is also observed in the changes in flexural and
impact properties (Vaia et al, 1993, Chen, 2004, Gilman et al, 2002). Nonuniform dispersion of nanofiller may limit the enhancement in properties (Murray,
2000).
Thermal stability of nanocomposites is generally enhanced with filler
concentration, because the nanofiller hinders the diffusion of the volatiles during
thermal decomposition. This behavior is more significant in exfoliated
24
nanocomposites (Vaia et al, 1993, Lui et al, 2004, Gilman, 1999, Giannelis,
1998). Delaminated/intercalated structures of nanocomposite collapse during
combustion, forming a well ordered insulating skin, which prevents the diffusion
of volatile materials. Consequently, the heat generated during the combustion is
reduced, and the flame retardancy of the composites is enhanced (Gilman, 1999,
Giannelis, 1998).
Nanofillers enhance barrier properties, chemical resistance and solvent
uptake of the materials. The characteristic high aspect ratio and the impermeable
structure of silicate platelets generate a tortuous pathway for a permeating
substance through nanocomposites. The best gas barrier properties are obtained in
fully exfoliated nanocomposite (Yano et al, 1997).
2.2 Modified Montmorillonite (Organoclay)
2.2.1 Montmorillonite
Montmorillonite (MMT) is commonly used as a nanofiller in the
preparation of polymer nanocomposites. It is a 2:1 phyllosilicate mineral, that
belongs to the smectite family. The chemical formula of MMT is
(Na,Ca)x(Al,Mg)2(Si4O10)(OH)2 nH2O, where x and n vary depending on the type
of clay and degree of hydration. Potassium, iron, and other cations are common
substitutes. The exact ratio of cations varies with clay source. MMT is a lowtemperature product of the weathering of igneous minerals, that have become
thermodynamically unstable in the presence of water at a temperature below 300
25
°C (Giese et al, 1998). Figure 2.2 shows a schematic representation of the
crystalline structure of MMT.
Figure 2.2. Schematic representation of Montmorillonite crystal (taken from
Ray et al, 2006).
It is common to find a net negative electrostatic charge on the MMT
layers, as a result of unbalanced substitution of ions of lower charge for ions of
higher charge. The net charge is naturally balanced by Na+ or Ca 2+ cations in the
layers (LeBaron, 1999, Porter et al, 2000). The negative surface charge (called
cation exchange capacity CEC, expressed in meq/100g) is an important
characteristic of MMT. The exchangeable cations are located in the interlayer
positions. The thickness of an individual MMT platelet is 0.96 nm. MMT layers
are arranged in stacks with a regular gap (interlayer distance) between the
individual layers. The gap between layers (1.1-1.33 nm) depends on the drying
conditions (Tseng et al, 2001, Yao et al, 2002, Zang et al, 2004), and it is
26
sufficiently large to permit penetration or intercalation by small molecules
(Lagaly, 1986, Supiron and Lerner, 2003). The average particle size is 500 nm
(Fisher et al, 1999)
By exchanging of sodium or calcium cations for organic cations
(commonly called surfactants, modifiers, or intercalants), the surface energy of
MMT decreases and the interlayer spacing expands. The resulting material is
called organoclay. Surface energy, interlayer distance and thermal stability of
organoclays depend strongly on the chemical structure, grafting density and the
type of cation head included in the surfactant (Beyer et al, 2002, Lagaly, 1986,
Vaia et al, 1996, Xie et al, 2003).
2.2.2 Structure of organoclays
Surfactants for clay modification usually include long aliphatic chains in
their molecular structure. Aliphatic chains arrange themselves depending on the
size and concentration of surfactant molecules into monolayer, bilayer, pseudotrimolecular layer, or an inclined paraffin structure. The molecular arrangement
determines the final interlayer distance (Lagaly, 1986). A schematic
representation of organoclay structure is shown in Figure 2.3.
27
Figure 2.3. Schematic representation of surfactant molecules arrangements
(taken from Murray, 2000).
2.2.3 Surfactants
MMT has been treated with ammonium, sulfonium, phosphonium, and
imidazolium surfactants, in order to modify surface characteristics and to improve
compatibility with polymers. The first generation of MMT-based organoclays
employed ammonium surfactants for the organic modification of the clay.
Ammonium surfactants used in commercially available organoclays usually
incorporate short aliphatic chains and hydroxyl and benzyl groups. They also
contain at least one long aliphatic chain (C12-C18) to cause expansion of the
spacing between the layers (Carastran and Demarquette, 2006, Chen et al 2001a,
2001b, Chigwada and Wilkie, 2003, Ding et al, 2005, Dolgovskij et al, 2004,
Dong et al, 2004, Essawy et al, 2004, Fan et al, 2002, Fu and Qutubuddin, 2005,
Gilman et al, 2000, Han et al, 2003, Hwu et al, 2004, Kim et al, 2003, Lee and
Kin, 2004, Lee et al, 2006, Lim and Park, 2000, Liu et al, 2005, 2006, Morgan et
al, 2002, Okamoto et al, 2001, Sepehr et al, 2005, Shen et al, 2005, Sohn et al,
2003, Tanoue et al, 2004, 2005, Wang et al, 2003, 2004, 2005a, Wu et al, 2004,
Xie et al, 2003, Yurekli et al, 2004, Zeng and Lee, Zha et al, 2005, Zhang et al,
28
2003a, 2003b, 2004, 2001, Zheng and Wilie, 2003b). Other MMT modifiers
include pyridium (Tseng et al, 2002a, Yei et al, 2004, 2005), alkyl amines (Li and
Ishida, 2003, 2005, Vaia et al, 1994, Zhao and Samulski, 2006) or alkyl
carbazoles (Chigwada et al, 2005). Other ammonium surfactants incorporate
siloxane (Zhao and Samulski, 2006), reactive groups (Bourbigota et al, 2003, Fu
and Qutubuddin, 2000, 2005, Ren et al, 2000, Su and Wilkie, 2003, Zhang et al,
2005, Zhu et al, 2001a), oxyethyl groups (Zhang et al, 2003, Zhogh et al, 2005) or
initiators for polymerization reactions (Fan et al, 2003ª, Jeong et al, 2006,
Vyazovkin et al, 2004, Uthirakumar et al, 2005, Zhao et al, 2004,). Several
oligomers of styrene (Beyer et al, 2002, Chigwada et al, 2005a, Fan et al, 2003b,
Hoffman et al, 2000, Kurian et al, 2004, Sepehr et al, 2005, Zheng and Wilkie,
2003a, Zheng et al, 2006), butadiene (Su et al, 2004a), propylene (Burmistr et al,
2005, Gilman et al, 2000, Okamoto et al, 2000), or siloxane groups (Maiti, 2003)
have been used in the modification of clay. Complex surfactants have been
employed such as silsesquioxane (Yei et al, 2004a), crown ethers and cryptands
(Yao et al, 2002), and cyclodextrin (Yei et al, 2004b), phenylacetophenone
(Chigwada et al, 2006b) or zwitterions complex (Li et al, 2005).
The low thermal stability of ammonium surfactants presents a problem for
melt compounding and processing of polymer nanocomposites, where high
processing temperatures exceeding 200 °C are commonly encountered. Thermal
degradation during processing could initiate/catalyze polymer degradation, in
addition to causing a variety of undesirable effects during processing and in the
final product (Nassar et al, 2005).
29
Efforts have been made to synthesize thermally stable organoclays based
on stibonium (Wang and Wilkie, 2003) or imidazolium surfactants (Gilman et al,
2002a, Morgan and Harris, 2004, Wang et al, 2003, Zhao et al, 2005, Zhu et al,
2001). Phosphonium surfactants have been used in the preparation of organoclays
(Arrollo et al, 2006, Bourbigot et al, 2003, Chu et al, 2004, Hartwing et al, 2003,
Hrobarikova et al, 2004, Kim et al, 2004a, Maiti et al, 2002, Morgan et al, 2005,
Ray et al, 2003, Uribe, 2003, Wang et al, 2003, Xie et al, 2002). Phosphonium
surfactants incorporate mainly short alkyl chains, phenyl and usually a long alkyl
chain.
The interlayer spacing of the resulting organoclays depends on the
chemical structure of the surfactant, the CEC ratio of cation exchange and silicate
layer thickness (Maiti et al, 2002).
The thermal stability of phosphonium
organoclays is superior to that ammonium organoclays (Stoeffler et al, 2006,
Uribe, 2002, Xie et al, 2002). Additionally, phosphorus compounds induce flame
retardancy and heat stabilization.
Table 2.1 shows the thermal stability and the basal spacing of some
examples of organoclays. Cloisite montmorillonites are commercially available
organoclays containing quaternary ammonium surfactants with at least one
hydrogenated tallow chain. It is notorious the low thermal stability for those
organoclays (onset).
The thermal stability of ammonium organoclay can be
improved by introducing a complex molecule to the original surfactant, as it is the
case of the combination of cetylpyridinium chloride/ cyclodextrin. Quaternary
phosphonium surfactants produce organoclays with higher thermal stability in
comparison with ammonium organoclays depending on the chemical structure of
30
surfactant. Imidazolium and stibonium surfactants produce as well organoclay
with high thermal stability. The basal spacing depends on the degree of cation
exchange and the surfactant molecule size, in the case of ammonium organoclays
the basal spacing was increased up to 3.15 nm.
The combination of
cetylpyridinium chloride/ cyclodextrin exhibited higher spacing. Phosphonium
organoclays showed basal spacing comparable to the ammonium organoclays.
Contrarily, imidazolium organoclays exhibited the lowest basal spacing. From
above experimental observations, phosphonium organoclays present at the same
time good thermal stability and considerably large basal spacing.
Several authors have documented the collapsing of basal spacing for
ammonium organoclay during the production of polymer nanocomposites by melt
compounding associated to the low thermal stability of ammonium organoclays
(Carastan and Demarquette, 2006; Li and Ishida, 2003, Nassar et al, 2005; Tanoue
et al, 2004; Tanoue et al, 2005; Tanoue et al, 2006).
31
32
Table 2.1. Thermal de gradation onset, maximum degradation rate and basal spacing for some organically modified
montmorillonites. Cloisite montmorillonite contain ammonium surfactants, where HT is Hydrogenated Tallow (~65% C18;
~30% C16; ~5% C14). The length of alkyl chains included in the structure is indicated by the number of carbon atoms (C#).
Surfactant/organoclay
Initial Thermal
Max. Decomposition d001(nm)
Reference
Decomposition ( C) Rate ( C)
Cloisite 10A (1-HT)
160
245, 310, 395
1.92Cervantes-Uc et al, 2007
Cloisite 15A (2-HT)
192
331, 447
3.15
Cloisite 20A (2-HT)
198
336, 451
2.42
Cloisite 25A (1-HT)
192
330, 390
1.86
Cloisite 30B (1-HT)
174
298, 427
1.85
Cloisite 93A (1-HT)
212
347
2.63
Alkyl ammonium (C18)
2.31
Okamoto et al, 2003
Yei et al, 2005
Cetylpyridinium chloride/
284
4.22
cyclodextrin
Alkyl phosphonium (C12-C18) 193-309
301-407
1.82-2.20 Xie et al, 2002
Alkyl ammonium (C8-C18)
162-170
212-266
2.21
Alkyl phosphonium (C16)
220
2.40
Kim et al, 2003
2.87
Zuh et al, 2001
2.32
Trans-2-butene-1,4270
1.70
Takana et al, 2006
bis(triphenylphosphonium)
270
1.95
Hartwig et al, 2003
200
1.84
Alkyl imidazolium (C12-C18)
354-423
474-564
1.61-1.88 Bottino et al, 2003
Alkyl imidazolium (C3-C16)
320-343
406-448
1.2-1.7
Gilman et al, 2002a
styryltropylium
300
1.6
Zang and Wilkie, 2003
Triphenylhexadecylstibonium
286
2.00
Wang and Wilkie, 2002
Crown ethers
1.5-1.8
Yao et al, 2002
2.3 Theoretical Considerations
2.3.1 Thermodynamic aspects
Vaia and Giannelis (1997a) proposed a mean-field, lattice-based model to
estimate the total free energy change associated with layer separation and polymer
intercalation for organoclays. The change in internal energy (ΔE) associated with
the establishment of new intermolecular interactions and the ideal combinatorial
entropy change (ΔS) associated with configurational changes of the various
constituents contribute to the total free energy associated with the process. Free
energy change (ΔF) can be calculated as:
ΔF = F(h) - F(ho) = ΔE - TΔS
Equation 3.2
where h and ho are the final and initial interlayer distances between clay platelets
and T is the temperature. Layer separation (polymer intercalation) is favorable
when ΔF < 0.
Figure 2.4-a shows the polymer and chain (surfactant) contributions to the
total entropy, while Figure 2.4-b indicates the change in free energy as a function
of surface separation. Complete layer separation depends on the establishment of
favorable polymer-clay surface interactions to overcome the penalty of polymer
confinement (lower values of ε, pairwise interaction energy). Polar polymers
containing groups capable of associative-type interaction, such as Lewis-acid/base
interaction or hydrogen bonding, lead to intercalation or exfoliation.
33
a
b
Figure 2.4. (a) Contributions of polymer and surfactant chains to the total ideal
combinatorial entropy change per surface area as a function of surface separation
(Vaia and Giannelis, 1997a). (b) Changes of total free energy per area as a function
of surface separation, subscripts refer to different interaction parameters polymerclay surface values, negative values indicate a favourable interaction polymer-clay,
ε=0(a), -2(b), -4(c) and -12(d) mJ/m2, respectively.
Vaia et al (1997b) also reported that there is an optimum interlayer spacing
for platelets to favour polymer intercalation, which corresponds to an intermediate
between a monolayer and a solid-like paraffinic arrangement of alkyl chains of
modifier (1.32 – 2.27 nm) (Kurian et al, 2004). Their calculations suggest that the
intercalation and/or exfoliation depend on the polymer-clay interactions. The
formation of nanocomposites with intercalated or exfoliated structure would
require lowering the surface tension of the clay or increasing the surface tension
of the polymer and finding the optimum concentration of intercalant.
Additionally, the use of surfactant with several long chains may increase the
system entropy, making it more likely to intercalate the organoclay.
Balazs and coworkers (Balazs et al, 1998a, 1998b, 2000, Ginzburg et al,
2000, Singh and Balazs, 2000, Zhulina et al, 1999) systematically modeled the
34
intercalation process for polymers and platelet-like fillers. The model indicates
how the composition of the mixture affects the thermodynamic stability of the
product, but it does not describe the kinetics of the process. Parameters such as
surfactant length and packing density together with the initial interlayer spacing
are considered, in addition to the molecular weight and chemical composition of
the polymer. Figures 2.5-a and 2.5-b show the change in free energy as a function
of surface separation for different levels of grafted density of surfactant. The
packing density is critical for promoting intercalation or exfoliation of clay
(similar to Vaia’s model). In both cases, the polymer-filler interaction parameter
(χ) plays an important role.
a
b
Figure 2.5. Free energy change as a function of surface separation. The
degree of cation exchange is expressed as packing density (molecule/area): a)
0.04 and b) 0.12. (Balazs et al, 1998).
Figure 2.6-shows the relationship between the change in free energy with surface
separation as a function of the degree of polymerization and χ. Figure 2.6-b
shows the influence of volume fraction of modified polymer (i.e. polar groups in
the polymer structure). Calculations suggest that modified polymers with low
35
molecular weight are more likely to intercalate clay. Polymer architecture has a
strong influence on the thermodynamic stability of the composites (Figure 2.7).
The enhanced miscibility between the organically modified clay and the polymers
with higher number of branches is primarily due to the compactness of the
macromolecules. The radius of gyration of the polymers decreases as the number
of branches increases, and the polymer can more easily interact with and
interpenetrate grafted layer (organoclay).
a
b
Figure 2.6.- Free energy change as a function surface separation. N, χ and
φ are polymerization degree, interaction parameter and volume fraction of
functionalized polymer, respectively. (Balazs et al 1998b, 2000).
Figure 2.7. The influence of polymer branching on free energy change as a function
of surface separation (Singh et al, 2000).
36
The calculations indicate that the formation of stable (exfoliated)
composites is promoted by increasing the attraction between the polymer and
surfactant. Functionalized polymer chains containing polar groups may promote
intercalation. A polymer with a high degree of polymerization promotes phase
separation. Surfactant concentration is an important factor in the intercalation
process. Long chain surfactants may contribute to clay intercalation. The use of a
small volume fraction of diblock copolymers (having polar monomers) could
promote intercalation.
Kim et al (2004b) extended the Balazs treatment by adding the clay-clay,
polymer-polymer and clay-polymer interactions and by extending the modeling to
two dimensions. They found that the use of surfactant with short chain length at
low grafting density could lead to intercalation/exfoliation.
2.3.2
Dynamic aspects
Cho and Kamal (2004) proposed a theoretical model based on
hydrodynamics to describe platelet separation of clays in polymer melt flows.
According to the model, platelet separation occurs if the hydrodynamic dispersive
force is greater than the attractive force between the clay platelets. These forces
depend on shear rate, viscosity of the matrix, basal spacing, value of the Hamaker
constant of clay, and geometrical variables. The effects of the above variables on
the separation of two clay platelets were estimated by calculating the stress ratios:
the stretching stress due to the polymer flow divided by the van der Waals’ stress
due to the attraction forces between clay platelets. Figure 2.8 shows a schematic
representation of clay platelets in the polymer flow.
37
Figure 2.8. Schematic representation of clay platelets in the polymer flow.
Greek letters represent the different angles describing the platelets orientation, S is
the overlapped clay surface, d is the basal spacing and h the clay platelet thickness.
Calculations indicate that clays having large aspect ratio and high
Hamaker constant are more difficult to exfoliate. Similarly, the stress ratios
increase with a decrease of the Hamaker constant. In summary, clays with larger
interlayer spacing and lower Hamaker constant are easier to exfoliate. High
polymer molecular weight and high shear rate promote clay exfoliation.
2.4
PS Nanocomposites
2.4.1 Melt intercalation
Melt compounding can be roughly divided into two groups: static
(annealing) and shear melt intercalation. The matrix in melt compounded styrenicclay nanocomposites can be PS homopolymer, styrene-containing copolymer, or a
38
polymer blend containing polystyrene.
Styrenic nanocomposites containing
nanoparticles or nanofiber other than clay are included in this review.
The earliest papers (Vaia et al, 1993, 1995, 1996, Krishnamoorti et al,
1996) report on PS nanocomposite preparation by annealing, using several
ammonium-based organoclays and PS or poly(3-bromostyrene) with different
molecular weights. The intercalation process was reversible and depended on
polymer molecular weight and processing conditions, such as annealing time and
temperature. In addition, the initial interlayer distance and surface tension of clay
were important. Polar polymers promoted polymer intercalation (Vaia et al, 1995,
1996).
Kurian et al (2004) modified MMT with amine terminated PS having
different molecular weights. Thus, attractive and repulsive enthalpic interactions
between the surfactant and polymer were eliminated. The results suggested that
high levels of surfactant coverage (packing density) of the layered silicate clay
mineral inhibited polymer intercalation. In addition, surfactant length was an
important factor in determining nanocomposites morphology (Kurian et al, 2006).
The results from static melt intercalation of styrenic copolymer containing
polar groups indicated that the intercalation process occurred after very short
annealing time and that the polymer-silicate surface interactions determine the
resulting nanocomposite structure (Hasegawa et al, 1999, Lee et al, 2002, Yoon et
al, 2000). Maleated PS produced intercalated nanocomposites (Park et al, 2001).
The common processing techniques for preparing nanocomposites under
shear include the use of twin-screw extruders, microcompounders and internal
39
mixers under different processing conditions. In general, intercalated structures
are obtained with melt processing of neat PS with ammonium treated MMT.
Mechanical, flame, and thermal properties are improved with clay content in
comparison with the neat resin (Carastan and Demarquette, 2006, Ding et al,
2005, Dolgovskij et al, 2004, Essawy et al, 2004, Han et al, 2003, Morgan and
Harris, 2002, Sepher et al, 2005, Tanoue et al, 2004, 2005, Wang et al, 2003,
2004). X-ray diffraction results indicate a reduction or collapse of interlayer
spacing for ammonium organoclay based nanocomposites. The latter is attributed
to thermal degradation of the intercalating surfactant and/or its diffusion out of the
galleries into the matrix (Carastan and Demarquette, Ding et al, 2005, Essawy et
al, 2004, 2006, Nassar et al, 2005, Tanoue et al, 2005, Wang et al, 2003, Zheng
and Wilkie, 2003, 2004).
Phosphonium organoclays were mixed with PS to produce intercalated
structures (Bourbigot et al, 2003), depending on the chemical structure of the
surfactant and processing conditions. Properties were enhanced. On the other
hand, imidazolium, and carbazole organoclays yielded nanocomposites with a low
degree of polymer intercalation (Chigwada et al 2005, Gilman et al, 1999), while
surfactants containing vinylpyridine units produced PS nanocomposites with a
mixture of intercalated/exfoliated structures (Zheng et al, 2006).
Melt processing of clay modified with styrene-oligomer or maleated
organoclays yielded intercalated/exfoliated PS nanocomposites, depending on the
chemical composition of the polymer matrix (Kurian et al, 2004, Hoffmann et al,
2000, Zhang et al, 2005, Zheng et al, 2006). Similarly, the use of swelling agents
40
(epoxy, polydimehylsiloxane or poly-caprolactone), during melt processing,
enhanced polymer intercalation in PS or SAN nanocomposites (Ishida et al, 2000,
Kim et al, 2001, Sikka et al, 1996).
Styrene-maleic anhydride copolymers (Schleidt et al, 2006), high impact
polystyrene
or
polystyrene-block-polybutadiene
block-polystyrene
triblock
copolymers (SBS) (Dazhu et al 2005, Lim and Park, 2001, Zhang et al, 2006),
with modified MMT, produced intercalated structures and, as a result, the
mechanical properties of the corresponding nanocomposites were improved.
Sulfonated polystyrene ionomer (SPS) was used as compatibilizer to produce
intercalated and even exfoliated structures in unmodified MMT (Zhang, 2005).
Processing conditions and compositional variables have a strong effect on
clay intercalation/exfoliation and, consequently, on property enhancement. Nassar
et al (2005) studied the effect of stress field on clay intercalation. According to
their results, a combined shear/elongational or mainly elongational stress field is
suitable to intercalate ammonium organoclay with PS. Tanoue et al (2005)
discussed the effect of processing variables on the properties of nanocomposites
prepared by melt intercalation in a TSE. The nanocomposites incorporated
ammonium organoclay. Polymer intercalation is lowered with residence time,
while the interlayer distance collapsed. Processing temperatures and polymer
molecular weight did not have a strong influence on the mechanical properties. In
another study, Tanoue et al (2006) reported the effect of processing conditions on
properties of ammonium organoclay-PS nanocomposite having a polar copolymer
as compatibilizer.
41
Jang and Wilkie (2005) studied the relationship between polymer
solubility parameters and the dispersion of clay via melt processing. They found
that larger clay spacing is achieved with polymers having higher solubility
parameter. Clay modification appears to be less important than polymer polarity.
However, a combination of enlarged organoclay spacing and high solubility
parameter of the polymer promote polymer intercalation.
Hectorite, saponite, synthetic MMT, zinc oxide, titanium dioxide or
graphite were been used as nanofillers (Bhiwankar et al, 2005, Fischer et al, 1999,
Ma et al, 2005, Ryul et al, 2001, Uhl and Wilkie, 2002, Wang et al, 2006,
Yamaguchi and Yamada, 2006, Yang and Nelson., 2006). Polymer intercalation
and enhancement in properties were achieved.
Other nanoparticles, such as
alumina, magnetite, carbon nanotubes, graphite, double-layered ZnAl, zinc oxide,
carbon nano-fibers or silica, have been used to obtain nanocomposites by melt
blending (Caprari et al, 2006, Chae and Kim, 2005, Hadjiev et al, 2005, He et al,
2006a, Jiang and Kim, 2006, Litina et al, 2006, Saito et al, 2005, Xu et al, 2005b).
Tables 2.2 to 2.4 show some experimental results reported in the literature
concerning mechanical properties of mainly PS nanocomposites prepared by melt
compounding. For example, the inclusion of oligomerically modified organoclay
to PS resin contributed to increase the tensile strength and modulus (Table 2.2).
Maximum increments were observed at 8% organoclay (strength was increased by
15 % and modulus by 50%). Higher concentration of organoclay reduced the
mechanical properties. Elongation was decreased with organoclay content at all
proportions (Zang et al, 2005). Sepehr et al (2005) reported the production of PS
42
nanocomposites containing a styrenic polymer ammonium organoclay (COPS).
Samples PS1-PS2 in Table 2.3 were prepared using a twin screw extruder (TSE)
while the samples PS3-PS5 were prepared with the TSE in addition to a
extensional flow mixer. The addition of COPS was detrimental to mechanical
properties due to the immiscibility and plasticating effect of organoclay. The use
of additional elongational flow mixer yielded into a slightly property
improvement.
Burmistr et al (2005) reported the preparation of organically
modified bentonite with a polymeric quaternary ammonium salts (PQAS) and the
corresponding polyamide, polystyrene and polypropylene nanocomposites (Table
2.4). Unmodified bentonite tended to increased slightly the tensile strength and
sharply impact except for polypropylene.
Modified bentonite increased
mechanical properties at low bentonite content, higher concentrations promoted a
decrement in all mechanical properties.
Table 2.2. Mechanical properties of PS nanocomposites prepared with an
oligomerically (Triclay II)-modified organoclay (Zang et al, 2005).
43
Table 2.3 Mechanical properties of PS nanocomposites containing a
polymeric ammonium organoclay (COPS) (Sepehr et al, 2005).
Table 2.4. Mechanical properties of polymer nanocomposites prepared with
organically modified bentonite (Burmistr et al, 2005)
Tanoue et al (2005) reported the reparation and characterization of PS
nanocomposites prepared with a commercially available ammonium organoclay
(Cloisite 10-A) and resins with different molecular weight by melt compounding
in a TSE. The modulus of resulting nanocomposites increased with organoclay
concentration by 20 % regardless the molecular weight of polymeric resins. Other
mechanical properties decreased monotonically with the clay content. Strength
was reduced by 12% in average, and impact strength was diminished by 30%.
Elongation at the break was drastically reduced with clay content (by 70% for the
44
high molecular weight nanocomposites). Increments on mechanical properties
have been correlated to the extent of intercalation (Nassar et al, 2005).
Figure 2.9. Mechanical properties of nanocomposites prepared with Cloisite
10-A and PS resins with different molecular weights (Tanoue et al, 2005).
2.4.2 In-situ polymerization
Polymerization of PS nanocomposites can be carried out in suspension,
solution, bulk, emulsion, or microemulsion. There is an extensive amount of
reported work on the synthesis and characterization of PS/ammonium organoclay
nanocomposites prepared by in-situ polymerization. Generally, the resulting
nanocomposites exhibited either intercalated or exfoliated structures, which
produced improvements in properties (mechanical, barrier, thermal or fire
properties). The relationship between the degree of intercalation and the
enhancement of properties has been attributed to the structural affinity between
45
styrene and the intercalant (Chen and Qi, 2000, Chen et al, 2000, 2001a, Doh and
Cho, 1998, Gilman et al, 2002, Gu et al, 2005, Jang et al, 2005, Jang and Wilkie,
2005, Li and Ishida, 2005, Liu et al, 2005a, 2005b, Moet and Akelah, 1993,
Okamoto et al, 2003, Tseng et al, 2002, Wang et al, 2002, 2005, Zhu and Wilkie,
2000).
Thermally stable organoclays, such as phosphonium organoclays (Gu et al,
2005, Jang and Wilkie, 2005, Fu and Qutubudin, 2000) or imidazolium
organoclays (Gilman et al, 2002) were used to synthesize PS nanocomposites. The
resulting materials exhibited a mixture of intercalated/exfoliated structures with
improvements mainly in flame resistance. The use of reactive and silane modified
organoclays produced exfoliated nanocomposites, with enhanced mechanical,
flame resistance and thermal properties (Akelah et al, 1996, Fu and Qutubuddin,
2000, 2002, Li et al, 2005, Qutubuddin et al 2002, 2005).
The copolymerization of styrene with acrylonitrile or butyl acrylate and
methyl methacrylate yielded a variety of structures depending on the organoclayconomomer interactions. Usually, exfoliated structures were obtained, and the
resulting nanocomposites exhibited improved properties (Aphiwantrakul et
al2005, Jang et al, 2001, Li and Ishida, 2003, Noh and Lee, 1999, Noh et al, 1999,
Qi et al, 2005, Su and Wilkie, 2004, Wang et al, 2005, Zeng and Lee, 2001, Zhao
and Samulski et al, 2006, Zhu et al, 2001).
Unmodified MMT was used in the synthesis of PS nanocomposites. The
resulting nanocomposites exhibited a range of immiscible to exfoliated structures,
depending on the preparation conditions (Bruzaud et al, 2005, Kim et al, 2002,
46
Kong et al, 2005, Noh and Dong, 1999, Shen et al, 2006, Uribe, 2003, Yan et al,
2005). Other nanofillers, such as silicates, titanium dioxide, silica, and nanotubes
have been used for preparing PS nanocomposites by in-situ polymerization
(Bartholomea et al, 2005, Ding and Qu, 2000, Kotyoky et al, 2006, Leroux et al,
2005, Loos et al, 2005, Rong et al, 2005, Qiu and Qu, 2006, Tong and Deng,
2006).
2.4.3 Solution blending
Ammonium modified MMT was used to produce nanocomposites by
solution blending.
The nanocomposites exhibited intercalated or exfoliated
structures, depending on the solution medium and processing conditions (Ji et al,
2006, Limpanarta et al, 2005, Ren et al, 2000, 2001, Yurekli et al, 2004).
Imidazolium and phosphonium organoclays produced intercalated structures upon
solution blending with PS or syndiotactic polystyrene (sPS) (Torre et al, 2006,
Tseng et al, 2001a, 2001b, Wu et al, 2001, 2002, Zheng and Wilkie, 2003a). sPS
ionomers produced intercalated/exfoliated structures, depending on the levels of
ionomer content (Govindaiah et al, 2006).
SEBS (Lee et al, 2006), polystyrene-polyisoprene diblock copolymer (Zha
et al, 2005, Ha et al, 2005), and SBS (Limpanarta et al, 2005) were incorporated
in various nanocomposite formulations.
47
Chapter 3 Objectives
48
3.1 Main Objective
The main objective of this study is to evaluate the feasibility and
advantages of the melt processing and properties of polystyrene-based
nanocomposites
incorporating
phosphonium-modified
montmorillonite
organoclay with due consideration to issues relating to thermal stability and
interfacial interactions.
3.2 Specific Objectives
In order to achieve the above objective, it is necessary to carry out the
following tasks.
(1)
To modify montmorillonite with thermally stable phosphonium surfactants
and to evaluate the thermal stability of the surfactants and the
corresponding organoclays;
(2)
To measure or estimate the surface tension and interfacial interactions for
the various components of the relevant systems and to evaluate the effect
of surface tension and initial interlayer distance or basal spacing on the
intercalation/exfoliation process;
(3)
To evaluate the effects of polymer and surfactant composition, structure,
and molecular weight, and polymer functionalization on the intercalation
process;
(4)
To characterize the thermal, mechanical, and barrier properties of the
various nanocomposites and to evaluate some of the available structureproperty models.
49
Chapter 4 Experimental: Materials & Procedures
50
4.1
Materials
4.1.1 Polymers
Two polystyrene (PS) resins with different molecular weights were used:
PS 1510, and PS 1220 (Melt Flow Indices: 6.5 and 1.9 g/10 min, Mw 230 and 310
kg/mol, and polydispersity (PD) 4.56 and 3.27, respectively) and styrene-maleic
anhydride (SMA) copolymer, 14 % maleic content (Dylark 332, Mw 181 kg/mol,
PD 2.10). Styrene homopolymers contain zinc stearate in different proportions
(1000 and 850 ppm for PS 1510 and PS 1220, respectively).
High density
polyethylene (HDPE) Sclair 2714 was used in order to validate some of the
measurements in this work. More detailed technical information may be found in
Appendix A.
4.1.2 Layered silicates
Natural sodium montmorillonite (MMT), with the commercial name
Cloisite Na+, (cation exchange capacity (CEC) 92.6 meq/100g clay) from
Southern Clay Products (USA), was used to prepare thermally stable organoclays.
In addition, the following two organoclays were obtained from Southern Clay
Products (USA): (i) Cloisite 10A, which refers to MMT treated with dimethyl,
benzyl, hydrogenated tallow ammonium, 125 meq/100 g clay; and (ii) Cloisite
15A, which refers to MMT treated with dimethyl, di-hydrogenated tallow
ammonium, 125 meq/100 g clay. These organoclays were used for comparison
purposes. The basal spacing (d001) for these materials are 1.92 and 3.15 nm for
51
Cloisite 10-A and 15-A, respectively. More detailed technical information may be
found in Appendix A.
4.1.3 Phosphonium surfactants
Sodium MMT was organically modified with four different phosphonium
surfactants to yield the four organoclays (Ph1- Ph4). Chemical composition,
molecular weight and melting point of surfactants follow:
Ph1: Cyphos IL 167, Tributyl-tetradecyl-P+Cl-
MW: 434 g/mol; MP 45 C
Ph2: Cyphos IL 101, Trihexyl-tetradecyl-P+Cl-
MW: 487 g/mol; MP -50 C
Ph3: Cyphos IL 166, Tetra n-octyl P+Br-
Ph4: Cyphos IL 164, Tetra n-butyl-P+Cl-
All of the phosphonium surfactants were supplied by Cytec Inc, Canada.
Additional technical information in Appendix A.
4.2 Experimental Procedures
4.2.1 Preparation of organoclay and PS nanocomposites
4.2.1.1 Clay modification
Two procedures for modifying sodium MMT were used, depending on the
phosphonium salt solubility. The following procedure was used for one-phaseaqueous suspension systems (Cyphos IL 167 and Cyphos IL 164) (2). Fifty grams
of sodium MMT were dispersed into 5 l of distilled water for 24 h at room
temperature, using a mechanical mixer (Caframo, Canada) with a dispersion
blade. An aqueous solution (1000 ml) of Cyphos salt with the desired amount of
salt was added slowly. The amount of salt used was the amount required on the
52
basis of the CEC of clay.
The cation exchange reaction occurred rapidly,
producing a whitish precipitate. The resulting organoclay suspension was mixed
further for 12 h. The suspended organoclay was filtered under vacuum, using
coarse Watmman filter paper. The resulting organoclay paste was dispersed into 5
l of fresh distilled water and mixed for 24 h. This procedure was repeated twice.
No chloride traces were detected by addition of silver nitrate, after the third
washing. Subsequently, the organoclay paste was mixed manually with 300 ml of
petroleum ether, using a spatula. After evaporation of the free petroleum ether, the
organoclay was dried at 80 °C for 24 h under vacuum. Finally, the resulting
material was ground, using a concentric grinder (SiebTecnhik Type T100,
Germany) for 30 s, in order to obtain a fine powder. The organoclay product was
stored in a desiccator.
The procedure to produce organoclays with water insoluble phosphonium
salts (Cyphos IL 101and Cyphos IL 166) was as follows. Twenty five grams of
sodium MMT were dispersed into 2.5 l of distilled water at room temperature, in a
4 l glass beaker equipped with a stirring bar. After 24 h, mixing was stopped and
1000 ml of diethyl ether solution of Cyphos salt, containing the stochiometric
amount of salt corresponding to the CEC of pristine MMT, was slowly poured
into the clay dispersion. The resulting system contained a clear upper organic
phase and a turbid bottom mineral phase. After 12 h of moderate mixing (no
vortex), the mineral phase became transparent and the organic phases became
turbid. Special care was taken to avoid diethyl ether evaporation. At this point,
the system was warmed up to evaporate the diethyl ether (60 °C). After solvent
53
evaporation, the organic phase became a sticky solid precipitate. The precipitated
organoclay was filtered and dispersed in hot water (80 °C) for 4 h. Washing was
repeated three times, until no chloride traces were detected with silver nitrate after
the third washing. The resulting organoclay paste was manually mixed with 200
ml of petroleum ether using a spatula. After free petroleum ether evaporation, the
organoclay was dried at 80 °C for 24 h under vacuum. Then it was ground, using
a concentric grinder.
4.2.1.2 PS nanocomposite preparation
PS nanocomposites were compounded in a twin-screw extruder (TSE) ZE
25, supplied by Berstorff GmbH (Hannover, Germany).
The extruder was
operated in the co-rotating, intermeshing mode, with a high-shear screw
configuration (Appendices B and C). A static mixer and a specially designed slit
die were used, in order to increase the mean residence time and to provide a
significant level of chaotic mixing (Borse, 2006). The static mixer is known as
the ISG (interfacial surface generator) motionless mixer having 10 elements of
25.4 mm in diameter. The overall nested length of the assembly is 323.85 mm. A
graphic representation of IGS mixer is presented in Figure 4.1.
54
Figure 4.1. Graphic representation of a static mixer (Borse, 2006).
The polymer and clay were fed to the TSE hopper using individual
volumetric feeders (Model T20 from K-Tron Corporation. Pitman, USA). Two
levels of nominal mineral content were employed: 2 and 5 %. The maximum
processing temperature was fixed at 220 °C in the slit die. Screw speed was 200
rpm and the feeding rate was 2.3 kg/h. The extruded ribbons (45 mm in width
and 0.5 mm in thickness) were cooled using air fans.
When maleated polystyrene (Dylark) was used, neat PS and Dylark were
initially blended in the standard unmodified twin screw extruder, using a circular
die. The resulting cylindrical blend extrudates were cut with a pelletizer (Berlyn
Focus1, Berlyn Corp. Worcester, USA). The pellets were then melt compounded
with the organoclay, according to the procedure described above.
4.2.1.3
Compression moulding
The extruded ribbons were compression molded at 200 °C under 98 kJ
clamping force for 5 min to obtain samples for further characterization.
Subsequently, the samples were water cooled under pressure to room temperature.
55
Genesis series compression molding press model G-30 manufactured by Wabash
Metal Products Inc., Indiana, USA was used for compression molding.
Compression molding yielded square plates (140x140x1.5 mm).
4.2.2 Characterization procedures
4.2.2.1 Thermal Stability and Mineral Content
Thermal stability of the surfactant salts and the corresponding organoclays
and the organic content of the organoclays were determined using
thermogravimetric analysis (TGA). The experiments were carried out in a TGA 7
Perkin-Elmer apparatus (Norwalk, CT. USA), controlled by Pyris 1 software
(version 4.0). The microbalance was calibrated with a reference weight of 100
mg, and the furnace was calibrated using the Curie point of Alumel, Nickel and
Perkalloy (calibration procedures are reported in the TGA user manual).
Solid samples from 15 to 20 mg were placed in an open platinum crucible
and heated from 50 to 650 °C at 20 °C/min under a nitrogen atmosphere (40
ml/min). Isothermal experiments were performed on organoclays and polymers;
samples were exposed to 220 °C for a pre-determined period under the argon
atmosphere.
Derivative
mass/temperature
curves
and
the
maximum
decomposition temperature were obtained by using the Pyris software. The results
presented are the average of at least five runs.
56
4.2.2.2 Determination of basal spacing
The x-ray diffraction patterns were obtained using a Rotaflex x-ray
diffractomer supplied by Rigaku (Tokyo, Japan) with CuKα radiation (λ=0.1458
nm). The available scanning angle range (2Θ) ranges from 0.5° to 135°, and the
scanning angle rate ranges from 0.05 to 1.0 °/min.
A sample weighing 3 g of neat MMT or phosphonium organoclay was
compressed into tablets in a stainless steel sample holder for several minutes
under 5 ton load. The experiments were run at room temperature with an angle
range (2θ) from 1° to 30°, at 0.1°/min (a single run was carried out). The machine
was operated at 50 kV and 150 mAmpere. A typical sample of PS
nanocomposites for x-ray experiments consisted of a 10x10x2 mm piece, obtained
by compression molding. Sample surface must be very smooth to avoid undesired
x-ray reflections. The experiments were conducted as mentioned above.
4.2.2.3 Clay dispersion and structure: TEM
Samples were ultramicrotomed with a diamond knife, using a Leica
Ultracut S microtome (Austria) at room temperature. Samples were trimmed to
obtain a truncated pyramid section, in order to reduce the cutting pressure,
avoiding damage to the diamond knife edge. The sample is first trimmed
manually with a razor blade to obtain a proper tip, and then the sample was
trimmed in the microtome with a glass knife.
The resulting micro-thin sections (50 nm nominal thickness) were received
in a water bath and transferred from the water bath to 200-mesh cooper grids.
57
The materials were sampled by taking images at various magnifications, over 3 to
4 sections per grid, to ensure that the image was representative of the sample
(Mollet and Kamal, 2006). Transmission electron microscopy (TEM) micrographs
were obtained with a Jeol JEM 2011 transmission electron microscope (200 kV)
and recorded with a digital camera Gatan Bioscan Model 792. Also, a TEM
Phillips CM 200 (200 kV) was used. Images received from this device were
recorded on photographic plates and digitalized.
4.2.2.4 Determination of polymer surface tension
A specially designed apparatus was used for pendant and sessile drop
measurements to determine the surface tension for melt polymers and contact
angles.
Details of the design and operation of the apparatus may be found
elsewhere (Demarquette, 1993, Demarquette and Kamal, 1994, Kamal et al,
1994).
Polymer resins were extruded into filaments (approximately 1.5 mm in
diameter), using a capillary rheometer (Instron model TT CM, Instron
Corporation, UK) at 180 °C and ram velocity of 1 cm/min. A 15 mm long
filament of cold polymer was introduced into the preheated syringe of the surface
tension pendant drop apparatus, according to the procedure proposed by Alam
(1998). Subsequently, the evolution of the drop was monitored continuously with
a CCD camera (Pulnix TCM-50).
Images were processed using Sampera
software (Coreco Inc, Canada). Images of the drop evolution with time were
captured every 30 min for 8 hrs, in a weak argon atmosphere.
58
Drop profiles were obtained from the images using DropProfile.exe
software (McGill University). The program detects the edge of the pendant drop
from a tif or jpg file. The obtained drop profile is converted from pixel units to
mm units (using a proper reference pixels/mm). The drop profile is sorted into
two parts (right and left), to be used in the calculation of surface tension. Figure
0
2.5
50
2.0
Height, mm
Pixels
4.2 shows the steps followed in the profile analysis.
100
150
200
1.5
1.0
0.5
0.0
250
0
50
100
150
200
Pixels
(a)
250
300
-0.5
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
Diameter, mm
(b)
Figure 4.2. Schematic representation of data manipulation for drop profile
analysis. a) drop profile in pixel units, and b) the sorted drop profile in mm.
Differential equations describing the drop equilibrium were solved with a
computer program (surfacetension.exe, McGill). Gas phase density was neglected
in the calculations, and polymer melt density was calculated using empirical
expressions:
ρPS= 1.1000-0.0006800T
ρPE=0.8683-0.0005664T
where T is temperature in °C and ρ is the density in g/cm3 (these relationships are
valid in the range 180 °C -290 °C) (Garmabi et al, 1998, Fox and Flory, 1950).
59
4.2.2.5
Surface roughness of organoclay
Roughness parameters were determined for the surface of organoclay
tablets.
Average and root-mean-square roughness Ra, RMS, respectively, in
addition to average height and the maximum range, were obtained using atomic
force microscopy (AFM) images. Ra refers to the average roughness, which is the
arithmetic mean of the deviations in height Z i from the image mean value Z :
Ra =
1
N
∑
N
i =1
Zi − Z
Equation 4.1
RMS is the root-mean-square roughness, defined as the square root of the mean
value of the squares of the distance of the points from the image mean value.
RMS =
1
N
∑
N
Zi − Z
i =1
2
Equation 4.2
Average Height is the arithmetic mean defined as the sum of all height values
divided by the number of data points N.
Z=
1
N
∑
N
i =1
Zi
Equation 4.3
Max range is the maximum peak-valley range in the area.
The organoclay was dried for 24 hr at 80 °C under vacuum. A 500 mg
sample of the dried organoclay was carefully poured into a KBr die and pressed
under vacuum at room temperature and 8 tons for 5 min, using a pneumatic press.
Images of the surface of the resulting tablet were obtained with an Aurora nearfield scanning optical microscope (NSOM) from Topometrix (Santa Barbara,
USA) in the non-contact mode. The microscope is equipped with SPMLab
software version 4.0 for scanning and feedback control. The scanning area
60
covered was 35x35 μm at a scanning rate of 17.5 μm/s. The maximum signal of
organoclay spectrum occurred from 45 to 56 kHz, depending on the sample.
Lower voltages are recommended. The experiments were run at room
temperature. NSOM has been proved an effective technique for studying the
optical properties of polymer films (Nguyen et al, 2001, Wang et al, 2000). In
NSOM, a tapered optical fiber with a sub-wavelength-sized aperture is scanned
over the sample with the tip held close to the surface in the near-field regime. The
feedback system used to hold the sample-tip distance constant is suitable for
simultaneous measurement of the surface topography and the optical properties of
a sample.
4.2.2.6 Contact angle measurements
For sessile drop experiments at room temperature, a rectangular piece of
organoclay tablet (approximately 8x8 mm) was placed inside a clean glass
cuvette, and the assembly was introduced into the test chamber (experimental
setup of surface tension measurements). Special care was taken to avoid the
presence of undesirable sharp edges during preparation of the organoclay tablet.
A drop of liquid was gently placed on the even clay surface using a Pasteur
pipette. Subsequently, the evolution of the drop was monitored continuously with
a CCD camera (Pulnix TCM-50).
Images were processed using Sampera
software (Coreco Inc, Canada). Pictures of drop evolution were taken every 5
min, until the drop shape reached equilibrium (no changes larger than 1 degree
were recorded in two consecutive measurements of sessile drops). Sessile drop
experiments were also carried out using 3 mm thick polymer platelets. The
61
polymer platelets were obtained by compression molding for 5 min at 200 °C
under 8 ton load. After water cooling, a clean scalpel was used to cut samples into
8 mm x8 mm squares.
The contact angle was determined using specially developed software
(DropProfile.exe, 2005, McGill). The program detects the edge of the sessile
drop (from a tif or jpg file) and fits a polynomial equation (polynomial degree 6,
for instance) to the sessile drop contour. Contact angles at the drop edges were
obtained by evaluating the derivative of the fitted equation at the drop edges.
4.2.2.7 Determination of Mechanical Properties
Flexural mechanical properties were determined using a Universal Testing
Machine Instron Model 1123 R controlled by Series IX software, with appropriate
features for flexural test, following the ASTM D 790-98 standard test method. At
least six 50x12x1.5 mm specimens were tested in the flexural mode at 5 mm/mim
at room temperature for each formulation. The modulus, strength and maximum
deformation were obtained.
4.2.2.8 Oxygen Permeability
The oxygen permeability coefficient was used as an index of barrier
resistance. The coefficient was determined following the ASTM D 3985-95
standard test method on thin films of PS nanocomposites. The permeation cell
consisted of a Mocon coulumetric oxygen detector, connected to an oxygen gas
transmission apparatus Oxtran Model 100. The carrier gas (nitrogen/hydrogen
mixture, hydrogen 2 %) was combined with oxygen permeating though the thin
62
polymer film. Hydrogen oxidation in the sensor generates a current proportional
to the permeating oxygen flux.
Thin films of the composites were prepared by compression molding,
using a stainless steel frame (0.2 mm) at 200 °C with 98 kJ clamping force.
Special care was paid to avoid any bubble generation in the films. Sample
thickness was measured by using a digital micrometer, Mitutoyo model 293 705
(Japan) with an accuracy of 0.001 mm. Plastic thin films were cut into circular
specimen and were masked between two aluminum foils, which yielded an
exposed area of 18.86 cm2. The above assembly was placed in the diffusion cell.
63
Chapter 5 Thermally Stable PhosphoniumMontmorillonite Organoclays
64
5.1
Abstract
Sodium montmorillonite (MMT) was modified with several organic
phosphonium salts. Organoclays with water soluble surfactants were prepared by
the traditional cation exchange reaction. An alternative procedure was used to
prepare organoclays with water insoluble salts.
The effect of chemical
composition and molecular weight of the salts on the thermal stability and basal
spacing were evaluated.
The phosphonium montmorillonites exhibit higher
thermal stability than conventional ammonium organoclays. The basal spacing is
generally larger for the phosphonium montmorillonites. These properties provide
a good potential for the use of phosphonium organoclays for the synthesis of
polymer/clay nanocomposites by melt processing.
5.2 Introduction
Montmorillonite (MMT) is commonly used as a nano-filler in the
preparation of polymer nanocomposites. By exchanging of sodium or calcium
cations for organic cations, the surface energy of MMT decreases and the basal
spacing expands (Favre and Lagaly, 1991). Surface energy, basal spacing and
thermal stability of these organoclays depend strongly on the chemical structure,
packing density and the type of cation head included in the surfactant.
The first generation of MMT-based organoclays employed ammonium
surfactants. Ammonium surfactants used in commercially available organoclays
usually incorporate short aliphatic chains and benzyl and sometimes hydroxyl
groups. They also contain at least one long aliphatic chain (C12-C18) to cause
65
expansion of the distance between the layers (Liu et al, 2005, Zha et al, 2005,
Tanoue et al, 2004, Carastan and Demarquette, 2006, Kim et al, 2003, Lee and
Kim, 2004, Zhang et al, 2004). Other MMT modifiers include alkyl amines (Li
and Ishida, 2003), alkyl carbazol (Chigwada et al, 2005), poly(dimethylsiloxane)
(Li and Ishida, 2005), and quinolinium or pyridinium (Chigwada et al, 2006).
Other ammonium surfactants are more complex molecules (Yao et al, 2002, Yei
et al, 2005, Chigwada et al 2006b), oligomers (Chen and Vyazovkin, 2006,
Lagaly and Ziesmer, 2005, Sepehr et al, 2005, Zheng and Wilkie, 2003, Gilman et
al, 2000) and reactive groups (Ding et al, 2005, Zhang and Wilkie, 2004,
Bourbigot et al, 2003).
The low thermal stability of ammonium surfactants presents a problem for
melt compounding and processing of polymer nanocomposites, where high
processing temperatures exceeding 200 °C are commonly encountered. Thermal
degradation during processing could initiate/catalyze polymer degradation, in
addition to causing a variety of undesirable effects during processing and in the
final product.
Efforts have been made to synthesize thermally stable organoclays based
on stibonium (Wang and Wilkie, 2003) or imidazolium surfactants (Bourbigot et
al, 2003). Phosphonium surfactants have been used in the preparation of
organoclays (Maiti et al, 2002, Zhu et al, 2001, Hartwig et al, 2003, Hrobarikova
et al, 2004, Kim et al, 2004, Xie et al, 2002). These phosphonium surfactants
incorporate mainly short alkyl chains, benzene and usually a long alkyl chain.
Arroyo et al (2006) reported the synthesis of a promising organoclay based on
66
triphenyl vinylbenzyl phosphonium chloride.
The organoclay exhibited
substantially higher thermal stability than ammonium surfactant modified
organoclays. The basal spacing of the resulting organoclays depends on the
chemical structure of the surfactant, the degree of cation exchange, and silicate
layer thickness (Maiti et al, 2002). Xie et al (2002) found that the thermal
stability of phosphonium organoclays is superior to that of ammonium
organoclays. Additionally phosphonium compounds enhance flame retardancy.
The present study describes the preparation and characterization of four
phosphonium-based organoclays. The resulting organoclays are intended for use
in the production of polymer/MMT nanocomposites by melt compounding. Thus,
the study evaluates the effect of packing density of surfactants on the thermal
stability and on basal spacing in the resulting organoclays. Emphasis is placed on
systems incorporating styrene based polymers.
5.3 Results and Discussion
5.3.1 Thermogravimetric analysis
TGA shows that the thermal decomposition of surfactants occurred in one
step, and that the maximum decomposition rate was similar for the four
substances (Fig. 5.1, Table 5.1). In contrast, the derivative TGA indicated that
the thermal decomposition of the phosphonium surfactants occurred in two steps.
The first mass loss was observed at 100 °C – 270 °C and accounted for up to 1.4
% of the original mass, depending on surfactant type. Water and impurities may
67
also be evaporated. The second mass loss occurred above 270 °C, due to the main
thermal decomposition.
100
Ph4
Mass, %
80
60
a
100
Ph3
Ph1
40
Ph1
Ph2
Ph3
Ph4
20
0
Ph2
95
200
100
250
300
200
350
300
400
500
Temperature, °C
Figure 5.1. (a) TGA and (b) derivative TGA for phosphonium surfactants.
68
d(% Mass)/dT(°C)
0.0
b
-0.05
-1.0
Ph1
Ph2
Ph3
Ph4
-1.5
-2.0
100
200
300
400
500
Temperature, °C
Figure 5.1. (a) TGA and (b) derivative TGA for phosphonium surfactants.
According to TGA (inset in Fig. 5.1-a), thermal decomposition of Ph4
(temperature at 5 % mass loss in Table 5.1) started at 324 °C.
The other
surfactants reached the same level of mass loss between 344 °C (Ph2) and 349 °C
(Ph1). The decomposition rate was maximum at 406 °C for Ph2 and 395 °C for
Ph3.
Thus, the phosphonium surfactants exhibited good thermal stability,
considering normal polymer processing temperatures in the range 200 °C - 300
°C.
69
Table 5.1. TGA of surfactants.
Surfactant % Mass Loss at
200 °C
Temp. at 5 % Mass Loss
Temp. at Max.
Mass Loss Rate
Ph1
0.93
349
409.7
Ph2
1.19
344
406.2
Ph3
0.18
346
395.0
Ph4
1.4
324
388.1
TGA curves of Ph1-MMT with different amounts of phosphonium ions
are shown in Fig. 5.2. Two parallel measurements were carried out for each
system to verify the reproducibility of experiments. Since the experimental results
were similar, the figure presents only the results of one experimental run. MMT
lost 4.8 % of the original mass due to water evaporation in the early stages of the
experiment. MMT exhibited excellent thermal stability between 150 °C and 400
°C. Higher temperatures promoted dehydroxylation of the structure (Cheng et al,
2001a). Water desorption was recorded at temperatures above 500 °C, accounting
for an average of 4.4 % at 700 °C.
The mass loss in the early stages in samples with surfactant amounts
added of 0.25 and 0.5 CEC was due to evaporation of absorbed water. As the
modifier content increased above 0.75 CEC, the absorbed water percentage
became negligible. TGA curves corresponding to surfactant additions of 1.25 and
1.75 CEC were similar to samples prepared at 1.5 CEC. Derivative TGA for 0.25
and 0.5-CEC organoclays showed one decomposition peak at 500 °C, due to mass
loss of surfactant adsorbed. As the surfactant addition was further increased, a
double peak was observed, at 490 °C and 550 °C, suggesting a different
70
decomposition mechanism.
At 1 CEC to 2 CEC surfactant addition, the
derivative TGA curves exhibited three peaks. The peak occurring around 420 °C
was probably due to the thermal decomposition of the free surfactant molecules
that were not adsorbed (maximum decomposition of surfactant occurred at 409
°C).
The peak temperature was higher than the maximum decomposition
temperature of pure phosphonium salt due to the protecting effect of the MMT
layers
100
0
% Mass
90
0.25
a
80
70
0.5
0.75
1
1.5
2
100
200
300
400
500
600
700
Temperature, ºC
Figure 5.2. (a) TGA and (b) derivative TGA for Ph1 organoclays with
different amounts of added surfactant.
71
d(% Mass)/ d T (ºC)
0.00
0
0.5
0.75
-0.04
-0.08
0.25
1
-0.12
b
1.5
-0.16
2
-0.20
100
200
300
400
500
600
700
Temperature, ºC
Figure 5.2. (a) TGA and (b) derivative TGA for Ph1 organoclays with
different amounts of added surfactant.
Table 5.2 shows the residual mass at different temperatures for the Ph1
MMTs. The actual content of organic material lost during heating was always
lower than the amount added. Thus, the real surfactant content in the organoclay
was derived from lost mass at 700 °C. The surfactant content of the sample 2.00
CEC corresponds to 32.51% of the total mass of the organoclay, which is
equivalent to 1.14 CEC. For samples with surfactant addition >1 CEC, some
surfactant ions (together with their counterions) will also be adsorbed in the
interlayer spaces. Consequently, organoclay contains a combination of surfactant
cations, bound by ion exchange; with higher decomposition temperature and free
surfactant molecules with decomposition/evaporation temperature similar to that
of the pure phosphonium salt.
72
Table 5.2. Residual mass of Ph1 organoclays at different temperatures.
Relative
Conc.
% Mass
% Mass
% Lost Mass
% Lost mass
(200 °C)
(700 °C)
(200 °C)
(700 °C)
Theoretical
Organic
Content
(%w/w)
CEC
95.20
90.77
4.80
4.43
0
0.25 CEC
95.38
82.10
4.12
9.12
9.13
0.50 CEC
97.50
77.60
2.51
15.23
16.74
0.75 CEC
99.06
72.99
0.94
21.41
23.18
1.00 CEC
99.28
68.09
0.72
26.53
28.69
1.25 CEC
99.33
64.61
0.67
30.06
33.46
1.50 CEC
99.51
64.86
0.49
29.99
37.63
1.75 CEC
99.68
66.55
0.32
29.01
41.31
2.00 CEC
99.01
66.50
0.99
32.51
44.58
0
Xi et al (2005) distinguished three different molecular environments for
surfactants in montmorillonite-ammonium organoclays: (1) surfactant cations
intercalated into the interlayer spaces through cation exchange and bound to
surface sites via electrostatic interaction; (2) surfactant (cations and/or molecules)
physically adsorbed on the external surface of the particles; and (3) surfactant
molecules located within the interlayer spaces.
The authors found that the
organoclays prepared at low surfactant concentrations exhibited better thermal
stability than those prepared at high surfactant concentrations. Consequently, the
thermal stability of organoclays was influenced significantly by the surfactant
adsorbed on the external surfaces.
Our results are in agreement with these
observations.
Xi et al (2007) indicated that the molecules of surfactant exceeding the
CEC adhere to the clay mineral surface by van der Waals forces, and their
73
properties are very similar to those of the pure surfactant.
The surfactant,
physically adsorbed on the external surface, can be removed after washing,
resulting in an increase in thermal stability and a decrease in surface energy of the
resultant organoclays (He et al, 2006).
Fig. 5.3 shows TGA and TGA derivative curves for the other
phosphonium MMTs, together with ammonium organoclays Cloisite 10-A and
Cloisite 15-A for comparison. Table 5.3 summarizes the main results. Using 5 %
of mass loss as an indicator of thermal stability, the ammonium organoclays,
Cloiste10-a and 15-A, exhibited significant thermal degradation at 233 °C and
287 °C, followed by Ph4 (379 °C), Ph1 (304 °C), Ph2 (344 °C) and Ph3 (405 °C).
Mineral content was taken as an indirect measure of organic content, which varied
depending on surfactant molecular weight.
According to the supplier, the
surfactant content for both Cloisite 10-A and 15-A was 1.25 CEC. Thermal
decomposition started first for Cloisite 10-A (1 % mass loss occurred at 203 °C),
then for Cloisite 15-A (241 °C).
Derivative TGA curves indicated that the
thermal decomposition of ammonium surfactants, particularly Cloisite 10-A,
occurred in three steps. Thermal decomposition of ammonium salts generally
follows either a Hoffmann elimination reaction or an SN2 nucleophilic
substitution (Fig. 5.4). Hoffmann elimination occurs in the presence of basic
anions, such as hydroxyl groups, which extract hydrogen from the alkyl chain of
the quaternary ammonium, yielding an olefinic and tertiary amino group (Xie et
al, 2001, 2002). Nassar et al (2005) showed that, in the case of polystyrene resins,
the olefinic group from the Hoffmann reaction may react with oxygen to generate
74
free radicals.
Such radicals could attack the polymer, causing polymer
degradation and possible deterioration of properties. Free radicals could promote
degradation, as in the case of polypropylene (Aulagner et al, 2000), or
crosslinking, as in the case of polyethylene (Smedberg et al, 2003).
a
Mass, %
100
MMT
Ph3
90
Ph1
Clo-10A
Ph2
Ph4
80
70
Clo-15A
60
100
200
300
400
500
600
Temperature, °C
Figure 5.3. (a)TGA and (b) derivative TGA of different organoclays.
75
MMT
d(%Mass)/dT(°C)
0.00
Ph4
-0.05
-0.10
-0.15
Clo-10A
Ph1
-0.20
Ph3
Ph2
-0.25
Clo-15A
-0.30
100
200
300
400
500
600
Temperature, °C
Figure 5.3. (a)TGA and (b) derivative TGA of different organoclays.
Figure 5.4. Example of Hoffman elimination reaction.
Phosphonium-modified MMT decomposed at higher temperatures than
ammonium organoclays, although phosphonium surfactants are susceptible to
similar reactions. Surfactants with higher molecular weight (i.e. Ph2 and Ph3)
yielded organoclays with higher thermal stability. Ph2, with Cl, and Ph3, with Br,
have the same molecular weight (after exchange of the Cl and Br groups).
Nevertheless, the corresponding organoclays showed different thermal stability.
This suggested that the molecule of Ph3 increases the steric resistance around the
76
phosphorus atom, thus inhibiting the decomposition reactions (Xie et al, 2002).
The importance of molecule isometry can be seen more evidently in the case of
the isometric surfactant Ph4, which has the lowest molecular weight in the group,
yet it exhibited thermal stability comparable to the higher molecular weight
organoclays.
Table 5.3. Residual mass of organoclays at different temperatures.
Organoclays
Temp. at 1 % Temp. at 5 % Temp. at
Organic
Mass Loss, Mass Loss, Max. Decomp. Content
(°C)
(°C)
Rate, (°C)
(%w/w)
Cyphos 167 Ph1
291
304
387, 449, 487
28.2
Cyphos 101 Ph2
281
344
517
29.86
Cyphos 166 Ph3
329
405
519
32.42
Cyphos 164 Ph4
304
379
474
20.27
Cloisite 10-A
203
233
244, 304, 370
29.13
Cloisite 15-A
241
287
332
38.76
Fig. 5.5 shows derivative TGA curves of the phosphonium surfactants and
the corresponding organoclays.
In general, the sharp peak observed in the
surfactant derivative curves was replaced in the organoclay by a shoulder or a
smaller peak, followed by other decomposition peaks at higher temperatures.
Surprisingly, organoclays began to lose mass at temperatures lower than those for
the corresponding pure surfactants. Organoclays prepared with low molecular
weight surfactants (i.e. Ph1 and Ph4, Fig. 5.5 a and d, respectively) started to
decompose earlier than the organoclays incorporating higher molecular weight
surfactants (Fig. 5.5 b and c, respectively).
Ph1 and Ph4 could have a
77
combination of attached surfactant cations and physically adsorbed surfactant
molecules, as mentioned before. The early thermal decomposition of organoclay
has been attributed to the fact that Lewis/Bronsted acid sites in the aluminosilicate
could catalyze the initial stages of thermal decomposition of the organoclay (Xie
et al, 2001 and 2002). In addition, the nano-scale dimensions of the interlayer
spaces significantly influence reaction kinetics, product transfer, and volatilization
of surfactant molecules. Consequently, the organoclay appeared to decompose
earlier, compared to the pure surfactant (Xie et al, 2001). Hedley et al (2007)
correlated the thermal degradation of phosphonium organoclays with the degree
d(% Mass)/dT(°C)
0.0
-0.5
Surfactant
-1.0
0.00
-1.5
-0.05
-2.0
-0.10
100
200
250
200
300
350
300
0.00
a -0.02
-0.04
-0.06
-0.08
-0.10
-0.12
-0.14
Organoclay -0.16
400
-0.18
400
500
600
d(% Mass)/dT(°C)
of arrangement of surfactant molecules within the organoclay galleries.
Temperature, °C
organoclays. Sharp peaks at low temperatures correspond to the surfactant
salts. (a) Ph1, (b) Ph2, (c) Ph3 and (d) Ph4.
78
0.00
b
-0.5
-0.05
Surfactant
-0.10
-1.0
0.00
-0.15
-1.5
-0.05
-0.20
-0.10
-0.25
-2.0
200
100
250
300
200
350
400
300
Organoclay
400
d(% Mass)/dT(°C)
d(% Mass)/dT(°C)
0.0
-0.30
600
500
Temperature, °C
0.00
c
-0.05
-0.5
Surfactant
-1.0
-0.10
-0.15
0.00
-1.5
-0.05
-2.0
-0.10
100
-0.20
200
250
200
300
350
300
400
Organoclay
400
500
-0.25
d(% Mass)/dT(°C)
d(% Mass)/dT(°C)
0.0
-0.30
600
Temperature, °C
79
d
-0.5
-1.0
-1.5
-2.0
Surfactant
0.00
-0.02
-0.04
-0.06
0.00
-0.08
-0.10
-0.05
-0.10
200
100
250
200
300
350
300
400
Organoclay
400
500
-0.12
d(% Mass)/dT(°C)
d(% Mass)/dT(°C)
0.0
-0.14
600
Temperature, °C
Isothermal TGA experiments were performed at 220 °C under nitrogen
atmosphere.
Fig. 5.6 shows the mass loss with time for the phosphonium
organoclays, Cloisite 10-A, and Cloisite 15-A.
Cloisite 10 A lost weight
continuously, upon exposure to 220 °C. The total mass loss accounted for 9.7 %
after 10 min exposure. At that point, the organoclay would have lost not only the
excess surfactant, but also 25% of the chemically bound surfactant (this amount
accounts for 5.8% of the total mass of the organoclay). Cloisite 15-A lost 1.4% in
the same period, reflecting better thermal stability.
80
Ph1
Mass, %
100
Ph2 Ph3
200
Ph4
95
160
Cloisite 15 A
90
120
Cloisite 10 A
85
80
Sample
Temperature
0
10
20
Temperature, °C
240
40
30
40
50
60
Time, min
Figure 5.6. TGA curves for different organoclays at 220 °C.
Dharaiya and Jana (2005) exposed samples of ammonium organoclay
(Cloisite 30 B) to high temperature for several periods. The resulting surface
properties of the organoclay were determined. Surface energy values were better
indicators of decomposition of ammonium organoclays than XRD.
They
suggested that changes in surface polarity of the clay mineral during
nanocomposite preparation had a direct influence on the affinity between the
organoclay and polymer. Such an influence would, in turn, have a significant
impact on the dispersion of the clay mineral particles in the polymer matrix.
The average mass loss for the phosphonium organoclays ranged from 0.1
% to 0.22 % after 50 min of exposure at 220 °C, compared to 1.4% for Cloisite
15A and 9.7% for Cloisite 10A, after only 10 minutes, as indicated above. Ph4,
which has the lowest molecular weight, loses most mass. Phosphonium salts
81
behaved differently from ammonium salts, due to the higher steric tolerance of the
phosphorus atom and the participation of its low-lying d-orbitals in the processes
of making and breaking chemical bonds (Xie et al, 2002).
5.3.2 X-ray analysis
The enhancement of properties in polymer nanocomposites is directly
related to the extent and quality of dispersion and to the degree of clay mineral
exfoliation and/or polymer intercalation in the nanocomposite. Another important
factor is the quality of adhesion at the polymer-clay interface. The initial basal
spacing in the organoclay is an important parameter for the determination of the
potential for polymer intercalation and clay mineral delamination. Organoclays
with smaller interlayer distances have reduced probabilities for polymer
intercalation. Nevertheless, intercalated structures can also be prepared from clay
mineral with smaller basal spacing via in situ polymerization (Gilman et al, 2002,
Wang and Chen, 2005). In general, it is desirable to start with organoclays that
exhibit large interlayer distances, in order to achieve desirable nanocomposites
properties. The long alkyl chains of surfactant molecules are thought to form
mono or bilayers, or even more paraffin-type bi-layers depending on surfactant
concentration (Lagaly, 1986).
Fig. 5.7 shows X-ray diffractograms of neat MMT and MMT treated with
Ph1. The basal spacing determined for pristine MMT was 1.17 nm. As the
content of Ph1 was raised, the basal spacing increased until it reached 2.38 nm,
82
corresponding to surfactant addition of 1.5 CEC (Fig. 5.7). Computer simulations
indicated that the interlayer distances generated by mono, bi and pseudo-trilayers
of alkyl chains would be 1.32, 1.8, and 2.27 nm, respectively (Murray, 2000).
However, Torok et al (1999) reported slightly different interlayer distances: 1.42
nm for monolayer and 1.77 nm for bilayer arrangements.
Following the
procedure employed by Torok, the distance for a pseudo-trilayer would be 2.34
nm. Thus, the surfactant cations were arranged in bilayers, at Ph1 addition of
0.75 CEC (1.92 nm). Alkyl chains appeared to exhibit a pseudo-trilayer structure
(2.32 – 2.38 nm) at higher surfactant addition.
Intensity, a. u.
1.5
1.25
1.0
0.75
MMT
2
4
6
8
10
2 Θ, °
Figure 5.7. X-ray diffractograms of Montmorillonite treated with Ph1 at
several CEC ratios.
83
Fig. 5.8 shows the dependence of basal spacing on the organic content of
the organoclay, as determined by TGA. The basal spacing and organic content
increased gradually with concentration of surfactant until 1 CEC. Subsequently,
both properties reached a plateau at higher surfactant addition. The above results
suggested that at least a surfactant concentration equivalent to 1 CEC was needed
to ensure the optimum interlayer distance. Janek and Lagaly (2003) reported that
the content of organic cation required for the transition from fine into voluminous
flocs of organically modified clay correspond to the cation exchange capacity.
Therefore, MMT was treated with the other phosphonium surfactants at
35
2.8
30
2.4
25
20
2.0
15
1.6
Organic Content
Interlayer distance
1.2
10
5
0
Organic Content, %
Interlayer distance, nm
concentrations equivalent to 1 CEC.
0.8
0.0
0.5
1.0
1.5
2.0
Surfactant Concentration, CEC
Figure 5.8. Basal spacing and organic content as a function of Ph1 surfactant
addition.
84
The X-ray diffraction patterns of the resulting phosphonium organoclays
are plotted in Fig. 5.9. Data for neat MMT and Cloisite 10-A are also included.
The ammonium surfactant raised the basal spacing of MMT to 2.05 nm (Cloisite
10-A). This distance corresponds roughly to bilayer assembly of hydrogenated
tallow chains, methyl and phenyl groups. However, the supplier reports a larger
spacing for Cloisite 15-A (3.15 nm). Phosphonium organoclays exhibited a range
of basal spacings from 1.84 to 2.52 nm. The low molecular weight surfactant
(Ph4 with the shortest alkyl chains) led to the smallest basal spacing (1.84 nm),
corresponding to a bilayer arrangement of alkyl chains. High molecular weight
surfactants (Ph2 and Ph3) produced organoclays with the higher basal spacing
(2.52 nm). The behavior of Ph1 surfactant, with medium molecular weight, was
discussed above (2.32 nm at 1 CEC).
85
Ph2
2.52 nm Ph1
2.32 nm
Closite 10-A
2.05 nm
Ph4
MMT
1.84 nm
1.17 nm
Intensity, a.u.
Ph3
2.52 nm
2
4
6
8
10
2Θ, °
Figure 5.9. X-ray diffractograms for different organoclays.
5.4
Summary
Montmorillonite was modified with four commercially available
phosphonium surfactants, using a standard technique with water soluble
surfactants and a two-phase reaction with surfactants exhibiting low water
solubility. The phosphonium organoclays exhibited higher thermal stability than
ammonium organoclays.
Therefore, the former should be useful for melt
compounding and processing of nanocomposites at high temperature. The extent
of improvement depended on the molecular weight of the surfactant, with higher
86
molecular weight materials exhibiting higher thermal stability. However, steric
effects appear to be important, especially for Ph3.
The basal spacing in the organoclays correlated with molecular weight. It
was generally larger for higher molecular weight surfactants. Basal spacing also
increased with the amount of surfactant used, up to the CEC equivalent of the
clay.
87
Chapter 6 Surface Energy of Modified Montmorillonite
88
6.1 Abstract.
Surface
properties
of
thermally
stable
phosphonium-modified
montmorillonite were investigated at both room temperature and 220 °C. These
properties were compared with those of pristine and ammonium-modified
montmorillonite. Surface properties at room temperature were calculated from
contact angles in sessile drop measurements. Several liquids with known polar
and dispersive components of surface tension were used. Surface energy of
nanofillers at 220 °C was calculated from contact angles, using sessile drops of
polymer melts. Two commercial polystyrene (PS) resins, with different melt flow
characteristics, and high density polyethylene (HDPE) were used. Isothermal
TGA experiments were used to determine the thermal stability of the resins and
nanofillers.
The dispersion behavior and mechanical properties of the
nanocomposites may be correlated with the values of the Hamaker constant and
thermodynamic work of adhesion for these systems.
6.2 Introduction
The surface energy of nanofillers, especially at elevated
temperatures, has a great influence on the processability and the properties of
polymer nanocomposites. Surface properties of polymers, fibers, and fillers at
room temperature are available in the literature (Comyn et al, 1993, Lewin et al,
2005, Norris et al, 1992, Shimizu and Demarquette, 2000). Surface and adhesion
parameters have been used to explain the filler dispersion and mechanical
reinforcement in polymer nanocomposites. For example, Dai and Huand (1999)
89
correlated the values of work of adhesion with the degree of clay-rubber
interfacial tension. They also correlated the values of interfacial tension with clay
dispersion and the increase of filler-matrix contact area. Similarly, Kovacevic et
al (2005) studied the effect of adhesion on the mechanical properties of acrylate
and vinyl acetate nanocomposites prepared with kaolin. Borse and Kamal (2006)
estimated the thermodynamic work of adhesion at the clay-polyamide interface
and studied its influence on mechanical properties corresponding of
nanocomposites.
Additionally, Cho and Kamal (2004) demonstrated the
importance of the Hamaker constant, which indicates the attractive forces between
platelets, in evaluation of the stresses required for platelet delamination during
melt compounding. In the above studies, the work of adhesion and Hamaker
constant were estimated from surface energy values reported in the literature or
from calculations using the Group Contribution method at room temperature, due
the lack of reliable data at high temperatures, at which melt processing usually
occurs.
Phosphonium organoclays have been found to be more thermally stable than
ammonium based organoclays (Kamal and Uribe, 2006, Kim et al, 2002, Xie et al,
2002, Kim et al, 2004). Phosphonium organoclays have been employed in the
preparation of polymer nanocomposites, poly(trimethylene terephthalate) (Chang,
2006), polylactide (Maiti et al, 2002), polyethylene (Stoeffer et al, 2006) and
polystyrene (Akelah et al, 2007) are some examples of polymer matrices used in
the preparation of above nanocomposites.
In general, the incorporation of
phosphonium organoclay in polymer resins produced important improvements in
90
mechanical, thermal and barrier properties, due the achievement of polymer
intercalation within organoclay.
The present work reports the results of
measurements of the surface free energy of montmorillonite clay modified with
various ammonium and phosphonium surfactants. Surface energy measurements
were made using the sessile drop technique, at both room and high temperatures.
The measured surface energy values were employed to determine the relevant
values of the thermodynamic work of adhesion. The Hamaker constant and
interfacial tension for various PS-organoclay systems were estimated.
Correlations were sought between these parameters and both clay dispersion
behavior and mechanical properties.
6.3 Surface Energy of the Organoclays and Polymers at Room
Temperature
6.3.1 Surface roughness
Surface roughness has an important effect on the measurement of contact
angles.
Nakae et al (1998) found that the effect of surface roughness was
considerable for average height larger than 0.5 μm. However, modeling and
experimental work indicated that contact angles are independent of surface
roughness when the average roughness is lower than 0.15-0.1 μm (Xinping et al,
2004, Ponsonnet et al, 2003). Figure 6.1 shows some NSOM images of tablet
surfaces for montmorillonite and three organoclays. The average (Ra), root mean
square (RRMS), and maximum (RMAX ) roughness parameters, in nanometers, were
91
determined for the organoclay tablets used in the sessile drop experiments. The
values for (Ra, RRMS, RMAX) were as follows: montmorillonite (148, 203, 1624),
Cloisite 10-A (79, 99, 720), Ph1 (29, 37, 288), Ph2 (137, 167, 1056), Ph3 (120,
142, 667), and Ph4 (96, 128, 1392). Ph1, Cloisite 10-A and Ph4 exhibit low
values of Ra, whereas the highest values are observed for neat montmorillonite.
MMT
Ph1
Ph2
Ph3
Ph4
Cloisite 10A
Figure 6.1. AFM images of organoclay surfaces.
92
The roughness parameters of the organoclay tablet surfaces used in this
work were lower than reported in related work by Rogers and coworkers (Rogers
et al, 2005 ), they reported 10 and 7 μm as average roughness for disk prepared
with unmodified and ammonium modified Montmorillonite, respectively. Our
results suggest that smooth organoclay surfaces may be obtained by using a wellpolished die and anvil surfaces, in combination with high pressure.
6.3.2 Surface energies at room temperature
The surface free energy (usually expressed in milli Joules per square meter
or mJ/m2) consists mainly of dispersive and polar components. The latter includes
all the possible molecular interactions, such as acid-base interaction. The total
surface energy of solid polymers can be calculated, using the contact angle (θ)
from a sessile drop experiment, in combination with the value of the surface
tension of the liquid, according to Neumann’s equation (Li and Neumann, 1992,
Neumann and Li, 1990):
cos θ = 2 (γs/γl)1/2 exp (-β(γl-γs)2) - 1
Equation 6.1
where β is a parameter that is usually considered to be constant for different
liquid-solid systems (β= 0.0001247 (m2/mJ)2), γs and γl refer to the values of the
surface tension of the solid and liquid, respectively. The dispersive and polar
components of γs at room temperature can be calculated by means of a graphic
method employed by Comyn et al (1993). According to this method, linear
regression of plots of γl(1+cosθ)/2 (γld)1/2 against (γlp)1/2/( γld)1/2 yields (γsp)1/2as
slope and (γsd)1/2 as intercept, where γld, γlp, γsd, and γsp are the corresponding
93
dispersive and polar components of the surface tension of the liquid and solid,
respectively.
Table 6.1 shows the values of surface tension of liquids used in the
calculations, including the corresponding polar and dispersive components.
Diiodomethane is considered to be mostly a non-polar liquid having a small polar
component (0.4 mJ/m2). However, higher polar components (6.7 mJ/m2) have
been reported, consequently, results vary depending on the magnitude of polar
component (Shimizu and Demarquette, 2000).
Table 6.1. Dispersive and polar component of surface tension of liquids used
in the sessile drop experiments in mJ/m2 (Shimizu and Demarquette, 2000).
Liquid
Dispersive
Polar
Total
Ethylenglycol
29.0
1.0
48.0
Diiodomethane
50.4
0.4
50.8
Diiodomethane
44.1
6.7
50.8
Formamide
39.2
19.0
58.2
Glycerol
37.4
26.0
63.4
Water
21.8
51.0
72.8
Table 6.2 shows the contact angles obtained from the sessile drop
experiments at room temperature and Figure 6.2 shows examples of the
application of the graphic method to determine the components of surface energy
for some of the materials under consideration. The time required to reach
equilibrium depends on the rheology of the liquid and the extent of interaction
between the liquid and the solid.
For example, drops of water reached
equilibrium after few minutes in most cases, whereas glycerol required more time
(25 min) to reach equilibrium. Drops of ethylene glycol reached equilibrium after
94
one hour, in some cases. Contact angles of sessile drops of polar liquids on the
polystyrene surface are higher than those obtained with non-polar liquids.
Contact angles of a liquid drop on a flat plate can be measure by several
techniques. A common technique to determine contact angles the so-called
measure of advancing and receding angle (Kamusewitz and Possart, 2003, Spelt
and Vargha-Butler, 1996). However, in the present study, only the advancing
contact angles were recorded. The use of this method is widely reported in the
literature as a fast and easy way to obtain contact angles (Comyn et al, 1993,
Xinping et al, 2004, Ponsonnet et al, 2003, Chen et al, 2005).
Contact angles vary, depending on the composition of the PS resins used.
PS1220 resin exhibited higher contact angles for polar solvents than PS1510. The
differences in contact angles could be explained in terms of migration of the zinc
stearate lubricant to the sample surface, as discussed below (Akanni and Burrows,
1987, Foldes and Szigeti-Erde, 1997, Minnikanti and Archer, 2006, Owens, 1969,
1970). The γsd for zinc stearate was determined to be 22 mJ/m2 (Mitsuya et al,
1983).
95
73.7 + 1.6 66.5 + 0.6
63.0 + 0.3 57.6 + 1.6
Ph3
Ph4
* Norris et al, 1992
46.2*
78.0 + 0.8 67.7 + 0.1
Ph2
38.4*
28.6 + 0.3
62.0 + 0.7 51.4 + 0.9
Ph1
MMT
32.5 + 0.8
Clo. 10-A 61.7 + 1.4 50.0 + 0.5
37.4*
29.7 + 1.4
35.3 + 1.2
27.4 + 1.4
23.8 + 1.4
77.0 + 0.5 65.3 + 0.5
28.8 + 0.4
-
36.0 + 0.6
44.1 + 0.7
45.3 + 0.7
21.5 + 0.5
25.0 + 0.5
46.0 + 0.5
46.4 + 1.1
Diiodomethane Ethyleneglycol
PS1220
Glycerol
69.0 + 0.6 62.1 + 1.2
Water
PS1510
Material
Contact Angles Θ
49.38
43.16 + 2.5
38.77 + 2.7
38.61 + 4.6
45.52 + 0.6
45.16 + 1.6
39.27 + 5.0
39.66 + 4.7
γ (Neumann)
36.17
33.17
33.01
36.18
35.84
33.43
37.08
33.83
γd
24.42
11.83
6.84
4.36
11.21
11.85
3.75
7.73
γp
0.983
0.932
0.905
0.878
0.811
0.950
0.788
0.856
r2
Surface Tension
60.59
45.28 + 0.7
39.85 + 0.5
40.54 + 0.5
47.05 + 0.4
45.28 + 0.5
40.83 + 0.4
41.57 + 0.6
Total
96
Table 6.2. Contact angles (degrees) and the surface free energies of polymers and organoclays (mJ/m2) at 25°C. r2 is the correlation
coefficient.
d 1/2
γl(1+cosθ)/2 (γl )
20
Ph1
Ph2
MMT
PS1510
15
10
5
0
0.0
0.5
1.0
1.5
2.0
(γlp/ γld)1/2
Figure 6.2. Plots of γl(1+cosθ)/2 (γld)1/2 against (γlp)1/2/(γld)1/2 for solid-liquids system,
dispersive and polar components of diiodomethane: 50.4 and 0.4 mJ/m2 ,
respectively.
Values of surface energy for solids predicted by Neumann’s equation are
reasonably consistent, within experimental error (+13 %).
The averaged values of
surface energy for the PS resins reported in the literature are in the range between 33 to
40.7 mJ/m2 (Lewin et al, 2005, Moreira and Demarquette, 2001). Calculated values of
the dispersive and polar components of surface free energy of the PS resins are
comparable to those reported in the literature (Lewin et al, 2005, Shimu\izu and
Demarquette, 2000).
The total surface free energies for PS1510 and PS1220 were
similar, but the polar and dispersive components were different.
The dispersive
component calculated for PS1220 was higher than the corresponding dispersive
component for PS1510, but the polar component for PS1220 was lower in comparison
97
with PS1510. The migration of polymer additives could modify the surface tension of
resin. Polymer resin containing higher concentration of zinc stearate exhibited higher
polar contribution to the surface energy and vice versa.
The surface free energy of neat montmorillonite was calculated from data
reported in the literature using the Neumann’s equation (Norris et al, 1992), the results
were not consistent due to the limitations of the equation (Drelich and Miller, 1994). The
results from graphic method indicate that the dispersive component (36 mJ/m2) has an
important contribution to the surface free energy for neat montmorillonite. Norris et al
(1992) reported similar values for the dispersive component of montmorillonite.
However, the value for the polar component reported is lower (10.1 mJ/m2) than the value
estimated in this work (24.42 mJ/m2).
Surface energy of the organoclays ranges roughly from 38 – 46 mJ/m2, depending
on the organic modifier. For example, Cloisite 10-A and Ph1 exhibited similar values of
surface free energy (45 mJ/m2) followed by Ph4 (43.16 mJ/m2). Ph2 and Ph3 had similar
values of surface free energy (38 mJ/m2), which are close to the corresponding values of
pure PS resins. Ph2 had the highest values of the dispersive component, followed by
Ph1, Ph3, Cloisite 10-A, and Ph4. The dispersive contribution to the surface free energy
of the organoclay seems to correlate with the length of the main aliphatic chain of the
phosphonium surfactant.
Longer aliphatic chains produced a higher dispersive
contribution.
Norris et al (1992) studied the surface free energy of modified clay with several
ammonium surfactants. He found that the dispersive component of modified clays was
almost the same, regardless of the modifier used in the clay treatment. Moreover, he
98
found that the value of the dispersive component for modified montmorillonite was equal
to that for the untreated montmorillonite (40 mJ/m2). This behavior is especially observed
in montmorillonite treated with ammonium surfactants containing small aliphatic chains.
Dharaiya and Jana (2005) determined the surface tension of Cloisite 30 B at room
temperature (natural montmorillonite clay modified with N+(CH2CH2OH)2(CH3)T
quaternary ammonium ion derived from tallow amine, where T represents an alkyl group
with approximately 65% C18H37, 30% C16H33, and 5% C14H29). The authors estimated the
surface tension of the organoclay to be 35 mJ/m2. The reported dispersive component
was equal to 22.4 mJ/m2, using only water and diiodomethane (γd = 44.1 mJ/m2 and γp
=6.7 mJ/m2). The values estimated by Dharaiya and Jana (2005) for the dispersive
components were lower than the values reported in this work. The difference in values
can be explained in terms of surfactant chemical structure, methodology and the number
of liquids used in the measurements (Shimizu and Demarquette, 2000).
6.4 Surface Energy of Organoclay at the Processing Temperature
6.4.1 Thermal stability of the materials
The calculation of surface energy of organoclay at the processing temperature
(220 °C) involved several experiments, including the evaluation of the thermal stability
of resins and organoclays, the calculation of surface tension of the resins in the melt state,
the study of surface roughness of organoclay samples, and the measurement of contact
angles at high temperatures of sessile drops of the polymer melts on the corresponding
clay surfaces.
99
Thermal stability of the materials, after long time at the measurement
temperature, is important in both pendant and sessile drop experiments. This is due to the
fact that system equilibrium is usually reached after long periods of exposure at high
temperatures. Samples of the materials were exposed at 220 °C for several hours under
an argon atmosphere (40 cc/min). Thermal stability of the samples was correlated with
mass loss after the total exposure time. Obviously, this approach does not consider
degradation mechanisms that do not involve mass loss.
Figure 6.3 shows TGA curves obtained in the isothermal experiments at 220 °C.
Similar behavior was observed at others temperatures (i.e. 200 °C or 210 °C), but the
mass loss rate was reduced at lower temperatures. The low molecular weight PS resin
lost more mass than the other PS resins. After 12 hours of exposure, the total mass loss
was 1.4 % of initial mass for the low molecular weight PS, whereas high molecular
weight PS lost only 0.5 %. High density polyethylene behaved differently in similar
experiments. It lost mass in the first two hours of exposure at 220 °C, then HDPE mass
remained constant (total mass lost 0.14 %).
100
HDPE Sclair 2714 -0.14
100.00
Mass, %
99.75
PS 1220 -0.53
99.50
99.25
99.00
PS 1510 -1.44
98.75
98.50
0
1
2
3
4
5
6
7
8
9
10
11
12
Time, hr
Figure 6.3. TGA isothermal curves of polymer resins at 220 °C, negative numbers
represent the percentage of mass loss.
Figure 6.4 shows isothermal TGA curves of four phosphonium organoclay and
Cloisite 10-A. Cloisite 10-A lost weight continuously, when it was heated at 220 °C.
The total mass loss was 18 % after four hours of sample exposure at this temperature.
The ammonium organoclay lost approximately 60 % of its organic content during the 12
hour experiment. The sample lost 8% of the initial mass in the first 6 min of exposure.
This corresponds to the amount of excess surfactant (surfactant in excess corresponds to
5.8 % of total mass of organoclay) plus 10 % of surfactant bounded to the clay surface.
The thermal degradation of ammonium surfactants follows the Hoffman
elimination reaction. According to this reaction, an olefin and an amine are formed, and
a proton replaces the ammonium cation on the clay. The decomposition products may
start at temperatures above 185 °C, depending on the chemical composition of the
101
intercalants. They may include the corresponding alkenes, aldehydes, or ketones of the
intercalant components (Zhu and Wilkie, 2000).
Ph1
Mass, %
100
Ph2 Ph3
200
Ph4
95
160
Cloisite 15 A
90
120
Cloisite 10 A
85
80
Sample
Temperature
0
10
20
Temperature, °C
240
40
30
40
50
60
Time, min
Figure 6.4. Thermal stability of organoclay at isothermal conditions.
Low thermal stability of organoclay could affect the surface properties. Dharaiya
and Jana (2005) evaluated the surface properties Cloisite 30 B after to be exposed at high
temperature.
They concluded that changes in surface energy values were sensitive
indicators of decomposition of ammonium organoclay. They also noted that changes in
surface polarity of clay during nanocomposites preparation had a direct influence
(increasing or decreasing) on the affinity of the organoclay to the polymer, which could
either enhance or inhibit clay dispersion.
Phosphonium organoclays showed higher thermal stability in the isothermal
experiments. Mass loss occurred in the early stages of experiments (probably moisture
volatilization).
Then the mass remained almost constant.
Average mass loss was
102
between 1.1 % and 1.9 %. Ph4 organoclay lost more mass than the other phosphonium
organoclays. Thermal stability of phosphonium salts is higher than that of ammonium
salts due to the greater steric tolerance of the phosphorus atom and the participation of its
low-lying d-orbital in the processes of making and breaking chemical bonds (Xie et al,
2002).
6.4.2 Surface tension of the resins
Figure 6.5 shows a typical evolution of pendant drop profile with time for
PS1220 at 220°C.
The drop profile shows rapid changes in the first two hours.
Subsequently, the profile changes more slowly. Figure 6.6 shows the apparent surface
tension values calculated from the pendant drop profiles of three different experiments
for PS1220.
The surface tension of monodisperse resins decreases with temperature, but it
increases with molecular weight of the polymer. However, surface tension decreases
dramatically with degree of polydispersity, due to the migration to the surface of low
molecular weight chains (Demarquette and Kamal, 1994, Minnikanti and Archer, 2006).
In general, the experimental results follow the above tendency (Figure 6.7). The larger
experimental error associated with the measured surface tension of PS1220 may be due to
the higher sensitivity of surface tension of this polymer to temperature, due the degree of
polydispersity. The measured surface tension of HDPE was 25.63 mJ/m2 at 220 °C. The
measured values for both PS and HDPE were comparable to those reported in literature
(Lewin et al, 2005).
103
0 hr
1 hr
2 to 1 2 h r s
Figure 6.5. Drop profile evolution with time of PS1220 at 220 °C.
Apparent Surface tension, mJ/m2
50
45
40
35
30
25
20
15
0
2
4
6
8
Time, hr
Figure 6.6. Surface tension values of PS1220 at 220 °C with time.
104
28
Surface Tension, mJ/m2
PS1510
PS1220
26
24
22
20
200
210
220
Temperature, °C
Figure 6.7. ST of PS resins with temperature.
6.4.3 Contact angles and surface energies at high temperature
Sessile drop experiments at 220 °C employed PS and HDPE. Solid PS cylinders
placed on the clay surface rapidly become spherical at the beginning of the sessile drop
experiment, and the contact angle decreases continuously with time during the first 180
minutes. Figure 6.8 shows a typical sessile drop picture for PS1220 on a Ph1 tablet after
two hour exposure at 220 °C. The time required for the sessile drop to reach equilibrium
depends on the nature of surfactant used. In general, three hours were sufficient to
achieve the equilibrium for both unmodified montmorillonite and the phosphonium
organoclays. However, Cloisite 10 A, a less thermally stable organoclay, required longer
105
to reach a stable contact angle, due to the continuous thermal degradation of surfactant
molecules and/or polymer degradation.
Figure 6.8. Typical sessile drop picture, PS1220 on Ph1 surface.
Average values of contact angles for the various systems and corresponding
values of surface energy are shown in Table 6.3. The contact angles of systems using the
phosphonium surfactants decreased in the following descending order: Ph3, Ph2, and
Ph1, and Ph4. Montmorillonite systems showed the lowest contact angles, and they
followed by Cloisite 10A systems. The high-energy surface of neat montmorillonite
produced the lowest contact angles (Gennes, 1985). Contact angles of PS resins with
different molecular weights were similar, within experimental error. HDPE-clay systems
exhibited a similar tendency, but with higher values of contact angles.
The surface energy of the organoclays at 220 °C were measured, using contact
angle measurements in conjunction with resin melts. The surface tension was calculated
by numerical solution of Neumann’s equation, employing the corresponding values of
surface tension of the different resins at 220 °C. Calculated surface energies for the
organoclays were independent of the polymer used, within the experimental error. This
106
suggests that the procedure followed and the assumptions made are reasonable. Surface
properties of organoclay at 220 °C were substantially lower than at room temperature.
Surface tension values were, roughly one third of corresponding values at room
temperature.
Surfactants with high molecular weight (Ph2 and Ph3 cations having the same
molecular weight but different structure) exhibited the lowest values of surface energy.
The difference in chemical structure did not seem to affect considerably the resulting
surface energy. On the other hand, Ph1 and Ph4 had similar values of surface energy,
although the surfactant molecular weights were quite different (434 and. 294 g/mol,
respectively).
The highest value of surface tension was for neat montmorillonite,
followed by Cloisite 10 A. Thermal decomposition of Cloisite surfactant may contribute
to the high values of surface tension.
107
11.5 ± 0.1
67.0 + 0.5
72.8 + 1.1
45.0 + 0.5
17.5 + 1.3
Ph 2
Ph 3
Ph 4
MMT
21.9 ± 0.2
16.9 ± 0.1
10.1 ± 0.3
14.8 ± 0.2
53.9 + 0.9
Ph 1
18.96 ± 0.3
36.0 +1.3
γ
Clo. 10 A
θ
Nanofiller PS1510
18.0 + 1
46.0 + 1
67.5 + 4.5
71.0 + 2
55.0 + 2
37.0 + 1
θ
PS1220
22.3 ± 0.1
17.0 ± 0.2
11.6 ± 1.2
10.7 ± 0.5
14.8± 0.5
19.0 ± 0.2
γ
38 + 2
57 + 1.5
80.2 + 0.4
64.3 + 1.5
61.2 + 0.3
46.7 + 0.5
θ
HDPE
21.2 ± 0.3
15.6 ± 0.4
9.3 ± 0.1
13.6 ± 0.4
14.5 ± 0.1
18.4 ± 0.1
γ
21.8 + 0.5
16.5 + 0.7
10.3 + 1.1
11.9 + 1.3
14.7 + 03
18.8 + 0.5
γ
Table 6.3. Contact angles (degree) of polymer-clay sessile drops and surface energy of nanofillers (mJ/m2).
108
6.5 Thermodynamic Work of Adhesion and Interfacial Surface
Tension
The analysis of work of adhesion at the filler-polymer interface is useful in
understanding differences in mechanical properties of composites. Work of adhesion
(Wa) is usually considered in terms of the components of surface free energy. In general,
the greater is the work of adhesion, the stronger will be the adhesive bond. The work of
adhesion at the filler-matrix interface should be sufficiently high to allow stress transfer
from the ductile polymer medium to the rigid particles, in order to adequately reinforce
the polymeric material Other wise, the polymer-filler interface will be destroyed during
the application of load, and the system will behave as an unfilled material Composites
with a weak filler-polymer interface could exhibit mechanical properties similar to the
neat polymer matrix; or even worse they could exhibit lower strength if the filler particles
act as discontinuities that serve as stress concentrators.
The thermodynamic work of adhesion for PS-clay (Wpc) was calculated using the
dispersive and polar components of surface energy of the styrenic resins and the
organoclays according the following expressions for room temperature and for elevated
temperature, respectively (Comyn et al, 1993, Li and Neumann, 1992, Neumann and Li,
1990):
Wpc = 2((γpd γcd)1/2+ (γpp γcp)1/2)
Equation 6.2
Wpc = 2(γp γc)1/2
Equation 6.3
109
where γpd ,γcd, γpp, and γcp are the dispersive and polar component of surface energy of
polymer and clay, and γp and γc, are the surface tension and surface energy of polymer
and clay respectively.
Figure 6.9 shows the values of thermodynamic work of adhesion for the clay-PS
systems and the corresponding values of cohesive energy for the styrenic resins.
Thermodynamic work of adhesion for clay-PS systems can be divided into three
categories, depending on their magnitude: low work of adhesion systems (Ph2 and Ph3),
intermediate work of adhesion systems (Cloisite 10-A, Ph1 and Ph4), and high work of
adhesion systems (Na montmorillonite). The first group showed values of work of
adhesion similar to the work of cohesion of pure PS resins; the second group exhibited a
slight increase in work adhesion.
Finally, the calculations showed that sodium
montmorillonite produced an interface with the highest work of adhesion. Therefore, one
would expect that the mechanical strength of PS-neat montmorillonite should improve
with the incorporation of clay and increase with clay content (Uribe, 2003). However, it
should be important to recognize that such improvement is contingent on the exfoliation
or the separation of the platelets, in order to benefit from the large surface area of the
reinforcing particles.
Similarly, but to a smaller extent, PS nanocomposites having
Cloisite 10-A, Ph1 and Ph4 should yield similar improvements.
110
Figure 6.9. Thermodynamic work of adhesion of clay-Styrenic systems.
The interfacial tension of the polymer-clay systems at room temperature can be
estimated, using the values of the dispersive and polar components of surface tension of
the polymers and organoclays, employing the following expressions for room and higher
temperature, respectively (Biresaw and Carriere, 2004):
[
γ pc = (γ pd ) − (γ cd )
1/ 2
] + [(γ
1/ 2 2
)
p 1/ 2
p
( )
− γ cp
]
1/ 2 2
γc= γpc +γp cosθ
Equation 7.4
Equation 7.5
where θ is the contact angle from sessile drop experiments.
Figure 6.10 shows values of interfacial tension of clay-PS systems at room
temperature.
Montmorillonite produced higher values of interfacial tension than
111
organoclays. The poor affinity of montmorillonite-polymer was reflected in the large
value for interfacial tension. High MFI PS (PS1510) produced lower values of interfacial
tension than PS1220 for all organoclays analyzed, especially in the case of Ph3 (0.032
mJ/m2).
The results suggested that PS1510 should achieve better interaction with
organically modified clay. This observation is important, because it has been shown that
the molecular weight and flow characteristics of the surfactant play an important role in
achieving intercalation or exfoliation (Balazs and Lyatkaya, 1998, Balazs et al, 1998).
Calculations indicated lower values of interfacial tension for Ph2 and Ph3 with PS1220
systems (0.028 and 0.58 mJ/m2, respectively).
Table 6.4 shows the values of interfacial tension at 220 °C for polymer-
organoclay interface calculated from data shown in Table 6.3, calculated interfacial
tension at high temperature were lower than the corresponding values at room
temperature. Low values of interfacial tension with PS melt were obtained for neat
montmorillonite, Cloisite 10 A, Ph1 and Ph4, in that order.
possible interactions between the PS melt and these clays.
These data suggested
High molecular weight
surfactants (Ph2 and Ph3) lowered the montmorillonite surface energy so much, that the
resulting values of interfacial tension indicated a low degree of interaction between the
organoclays and PS polymers at high temperature.
112
Figure 6.10. Interfacial tension clay-polymer at room temperature.
Several factors affect the interfacial tension at polymer-polymer interfaces. For
example, temperature and the presence of small molecules (lubricants, for instance) tend
to decrease the interfacial tension (Minnikanti and Archer, 2006, Sakane et al, 2001).
However, temperature reduces the surface (energy) tension of polymer and mineral fillers
in different proportions. The thermal coefficient of surface tension (reduction in surface
tension with temperature) of most polymers ranges from 0.12-.065 mJ/m2-°C (Sauer and
Dee, 2002) from 25°C to 300°C.
Moreira et al (2001) reported values of thermal
coefficient for PS resins having different molecular weight.
They found that the
coefficient varies with polymer molecular weight and molecular weight polydispersity.
They reported the thermal coefficient for a PS resins similar to the used in this work as
0.0833 mJ/m2-°C. The thermal coefficient for calcined kaolin was reported to be 0.43
113
mJ/m2-°C (Ansaria and Price, 2004, Kubilay and Gurban, 2006). Surface treatment with
silane agents reduced the coefficient at 0.13 mJ/m2-°C (Price and Ansaria, 2003). The
surface energy of Sepiolite, another type of silicate, was reported to be 0.43 mJ/m2-°C
(Askin and Yazici, 2005), while the coefficient for untreated silica (0.42 mJ/m2-°C ) was
reduced, depending on the surface treatment with siloxane, up to 0.16 mJ/m2-°C (Matros
et al, 2001). The above coefficients were determined in the temperature range from 80°C
to 275 °C.
Consequently, the difference between surface tension/energy of
polymer/organoclay becomes larger with temperature assuming that the organoclays
behave as described above.
Table 6.4. Interfacial Tension: Polymer–Organoclay (mJ/m2) at 220 °C.
Nanofiller
PS1510
PS1220
HDPE
Cloisite 10 A
1.21
1.32
0.86
Ph 1
1.19
1.34
2.16
Ph 2
2.47
3.09
2.52
Ph 3
3.17
2.63
5.00
Ph 4
0.57
0.71
1.63
Montmorillonite 0.31
0.02
0.40
6.6 Hamaker Constant
The Hamaker constant (A11) represents the attractive forces between bodies (two
platelets). For bodies of the same material separated by vacuum, the Hamaker constant
can be calculated according to the following expression (Neumann et al, 1979):
114
A11= 24 π Do2 γLW
Equation 6.6
where Do is 1.65x 10-10m, and γLW is the dispersion forces contribution to surface tension.
The Hamaker constant can be calculated for systems having two platelets of one material
separated (1) by a different material (3):
A131 =
(
A11 − A33
)
2
Equation 6.7
where A11 and A33 are the corresponding Hamaker constants of the individual
components. Table 6.5 shows the calculated values of the Hamaker constant for
individual components and for PS-organoclay systems at different temperatures. The
Hamaker constant at room temperature was calculated using the corresponding dispersive
components of organoclay surface tension reported above.
On the other hand, the
Hamaker constant at high temperature was calculated using the total value of surface
energy, since it is difficult to estimate dispersive and polar components of surface energy
of solids at high temperature. In general, the values of the Hamaker constant are in the
range of 6.80 - 7.42 x 10-20 J. The calculated value of the Hamaker constant at room
temperature for neat montmorillonite was similar to values reported in the literature
(Médout-Marère, 2000).
The Hamaker constants for organoclays at high temperature indicate the degree of
attraction among the clay platelets during melt compounding. In this sense, it is possible
to predict which organoclays could be better dispersed within the polymer melt. Ph2 and
Ph3 have the lowest Hamaker constant values (the lowest attraction among platelets).
Thus, it is expected that they could be more readily dispersed in polymers than Ph1 or
Ph4, depending on the surface tension of polymer. Neat montmorillonite and Cloisite 10
115
A could present difficulties to be dispersed due to the high attraction existing among the
clay platelets. Organoclays having lower values of Hamaker constants could be more
easily dispersed into a polymer matrix that those having higher values, because the latter
platelets exhibit less resistance to be separated during processing.
The parameter A131 at room temperature indicates the attraction between the
organically modified clay platelets (1) when separated by the polymer medium (3). Thus,
the values of Hamaker constants for the systems should reflect the potential for
mechanical reinforcement, as in the case of thermodynamic work of adhesion. The
results showed that the Cloisite 10 A/low MFI PS (PS1220) exhibited the highest
Hamaker constant (attraction), compared to other clays. Systems incorporating high MFI
PS (PS1510) with Ph1, Ph2 and neat montmorillonite exhibited high attraction, but the
attraction was about three times lower than the corresponding systems with PS1220.
Ideally, the PS resins should have the same degree of attraction for a given organoclay, in
particular in this molecular weight range. However, the samples exhibited different
values of the dispersive component of surface tension, likely because of the role of zinc
stearate, as mentioned above
Hamaker constants A131 at high temperature for Phosphonium organoclay were in
the same order of magnitude, while neat Montmorillonite and Cloisite 10-A showed
lower values. Similar behavior was observed for both resins. The calculations suggested
that organoclays presented different polymer-organoclay interaction, depending on
organic modification of clay. Organoclay/PS systems containing high molecular weight
surfactants seemed to have stronger interactions at high temperature than systems
containing low molecular weight surfactants.
116
Table 6.5. Hamaker constant for different organoclays and PS-clay systems.
A11x 10 20J,
A131x10 23J, 25 °C
A131x10 21J, 220 °C
25 °C
220 °C
PS1220-Clay PS1510-Clay PS1220-Clay PS1510-Clay
Clo. 10–A
6.85
3.91
19.39
0.24
0.52
0.44
Ph1
7.35
3.40
2.16
5.94
2.10
1.92
Ph2
7.42
2.44
1.13
8.09
3.94
3.70
Ph3
6.77
2.11
24.26
1.03
5.40
5.13
Ph4
6.80
3.36
22.34
0.66
1.24
1.11
MMT
7.42
4.43
1.15
8.02
0.06
0.03
Hamaker Constant for PS1220 and PS1510 is 7.6 x10-20 and 6.94x10-20 J at 25 °C,
respectively.
6.7 Correlation of Surface Parameters with Organoclay Performance
As indicated above, clay dispersion and barrier and mechanical properties of
nanocomposites are influenced by the organic modification of clay and the interfacial
interactions in the polymer/organoclay system.
A detail discussion concerning clay
dispersion in PS nanocomposites and the evaluation of PS-clay nanocomposites
properties can be found in Chapter 7. Figure 6.11 (a) shows TEM pictures indicating
the influence of organic modification of montmorillonite on the clay dispersion in PS
nanocomposites. Figure 6.11 (b) shows the relationship between the Hamaker constant
(A131) of the different organoclays at high temperature and the basal spacing of the
organoclay. As expected, an increase in the molecular weight of surfactant produced a
lowering of the Hamaker constant and an increase in the basal spacing. The TEM
pictures of PS nanocomposites also indicate that the higher molecular weight surfactants
lead to better dispersion (Uribe et al, 2007).
117
Figure 6.11 a) Tem pictures of PS nanocomposites prepared with different
organoclays (the clay content is 2 % in all cases), b) effect of molecular weight of
surfactant on the basal spacing and A131 (clay-polymer-clay).
Figure 6.12 shows the effect of thermodynamic work of adhesion at room
temperature on the elastic modulus of nanocomposites prepared with PS resins at
118
different organoclay concentrations. Thermodynamic work of adhesion depends on the
surface treatment of clay (Uribe et al, 20079). In general, the organoclays that contained
higher molecular weight surfactant exhibited lower values of work of adhesion (Ph2 and
Ph3). Good adhesion is required to obtain improvements in mechanical properties.
Modulus was increased with clay content and the degree of adhesion at the polymer-filler
interface. In general, the organoclays that contained higher molecular weight surfactant
exhibited lower values of work of adhesion (Ph2 and Ph3). Good adhesion is required to
obtain improvements in mechanical properties. Modulus increased with clay content and
the degree of adhesion at the polymer-filler interface. This suggested that the modulus
was affected, not only by the quality of filler dispersion and/or polymer
intercalation/exfoliation, but also by the quality (strength) of adhesion at the claypolymer interface. Organoclays having surface tension close to that of the polymer, such
as Ph1, Ph4 or Cloisite 10A, exhibit low interfacial tension values.
Figure 6.13 shows the influences of the clay concentration, the Hamaker constant
A131 at the processing temperature, and the initial basal spacing of the clay on the
permeability of oxygen in the PS 1220 nanocomposites, prepared with the different
organoclays. The two unfilled PS resins showed different permeability levels, (149 + 7
and 110 + 4 cc-mm/m2-day-atm for PS 1510 and PS 1220, respectively). The data for PS
1220 suggested that the correlation between oxygen permeability and work of adhesion
was rather weak, especially for PS 1510, probably due to the influence of other factors on
permeability. However, the results in Figure 6.13 indicate that the permeability of PS
1220 nanocomposites to oxygen decreased with increases in clay concentration, the
Hamaker constant A131 at the processing temperature, and the initial basal spacing of the
119
clay. It should be pointed out that barrier properties depend on the quality of clay
dispersion and the aspect ratio of clay particles (Chang et al, 2001).
Figure 6.12. Influence of the thermodynamic work of adhesion on the modulus of PS
nanocomposites prepared with different organoclay and clay content. PS resins have
different flow rate: a) PS 1510 and b) PS 1220.
120
Figure 6.13. Influence of the A131 at 220 °C (a) and the initial basal spacing of
organoclay (b) on permeability to oxygen in PS 1220 nanocomposites prepared with
different organoclays and clay concentrations.
121
6.8 Summary
The results of this study quantify the relevant surface energy parameters and
illustrate the importance of surface energy and interfacial interactions in determination of
the quality of clay dispersion and the performance of nanocomposites.
The surface energy, including the dispersive and polar components, of modified
montmorillonites was determined using the sessile drop method, both at room
temperature and at 220 °C.
Surface tension of the polymer melts at 220 °C was
determined using the pendant drop technique. Organoclay surface energy at processing
temperatures was substantially lower than at room temperature. While the clays have
higher surface energies than the polymer at room temperature, the situation is reversed at
the processing temperature.
Surfactants with larger molecular weight exhibited lower surface energy at both
room and processing temperatures. The surface energy of the PS, especially the polar
component at room temperature, appeared to be significantly influenced by the presence
and migration of the lubricant (zinc stearate).
The quality of clay dispersion in PS resin was correlated with the initial basal
spacing of organoclay and the value of Hamaker constant for the polymer-organoclay
system at the processing temperature (A131). Organoclay with larger initial basal spacing
and higher Hamaker constant exhibited better clay dispersion. The flexural modulus of
nanocomposites correlated well with work of adhesion of the organoclay-polymer system
at room temperature. The modulus increased with the work of adhesion. On the other
hand, correlation between oxygen permeability and work of adhesion was rather weak,
especially for PS 1510, probably due to the influence of other factors on permeability.
122
However, the results indicated that the permeability of PS 1220 nanocomposites to
oxygen decreased with increases in clay concentration, the Hamaker constant A131 at the
processing temperature, and the initial basal spacing of the clay.
123
Chapter 7 Polystyrene/Phosphonium Organoclay
Nanocomposites by Melt Compounding
124
7.1 Abstract
Polystyrene-montmorillonite nanocomposites were prepared by melt
compounding, using several ammonium and phosphonium organoclays. Melt
processing was carried out in a twin screw extrusion system, specially modified to
produce improved dispersion and longer residence time. The effect of molecular
weight of polystyrene on clay dispersion and property enhancement was
evaluated.
Nanocomposite structure was characterized by wide angle x-ray
diffraction (WAXD) and transmission electron microscopy (TEM).
Thermal
stability, mechanical properties and permeability to oxygen were also determined.
The quality of dispersion of organically modified montmorillonite depended on
the molecular weight of the polystyrene resin. Barrier properties were measured
and compared to predictions of permeability models available in the literature.
Clay dispersion and property enhancement were explained in relation to the
surface characteristics of the organoclays, and the work of adhesion at the
polystyrene-clay interface was correlated with the tensile modulus of the
nanocomposites.
7.2 Introduction
There are many factors, including nanofiller and polymer characteristics
and processing conditions, which influence the final structure and properties of
nanocomposites obtained by melt processing. It is known that clay modification
and surface energy, thermal stability, and initial basal spacing play an important
125
role in the exfoliation-intercalation process (Chigwada et al, 2006, Dharaiya and
Jana, 2005, Le Baron et al, 1999, Vaia et al, 1996).
Similarly, processing
conditions such as shear stress, mixing field and mixing time are very important
(Dennis et al, 2001, Nassar et al, 2005, Tanoue et al, 2006).
Finally, the
composition of the polymer matrix has a great influence on the miscibility and
stability of nanostructures (Hasegawa et al, 1999).
Polystyrene (PS) polymers are suitable to study the influence of the
nanocomposite preparation process on nanocomposite properties, due to the
absence of crystallization, easy melt processing, molecular linearity and
availability, in addition to the industrial interest in developing styrenic
nanocomposites. However, the main drawbacks are the limited thermal stability
and the hydrophobicity of the resin.
The synthesis of PS nanocomposites,
especially by melt compounding, has represented a major challenge. Thus, it has
not been possible to realize the full benefits of property enhancements expected
from nanocomposites obtained by melt compounding PS with montmorillonite.
Such nanocomposites exhibit intercalated or immiscible structures. Thus, in most
cases, property enhancement has been limited (Bhiwarkar and Weiss, 2005,
Tanoue et al, 2004, Sepehr et al, 2005). The main objective of this study was to
evaluate the factors influencing the formation of nanocomposite structures during
melt processing of PS-montmorillonite systems, with the aim of producing
nanocomposites with improved performance. Special emphasis is placed on
phosphonium organoclay based nanocomposites. In order to improve the thermal
126
stability of the material during melt processing, compared to ammonium
organoclays.
The following considerations were taken into account in this work, in an
effort to enhance the conditions for producing improved PS nanocomposites: (i)
the use of thermally stable organoclays, and (ii) improved mixing conditions.
Natural montmorillonite was organically modified with several phosphonium
surfactants, producing organoclays with excellent thermal stability and enhanced
interlayer distances and surface properties (Kamal and Uribe, 2006). Polystyrene
nanocomposites were produced in a twin screw extrusion system, combining high
shear (screw configuration and screw velocity), long residence time, and chaotic
mixing.
Ammonium modified montmorillonite (Cloisite 10-A) was used for
comparison proposes.
7.3 Nanocomposite Characterization
7.3.1
Wide angle x-ray diffraction (WAXD)
Table 7.1 shows the interlayer distances of organoclays and the
corresponding basal spacing of organoclays in PS nanocomposites calculated
from x-ray diffraction patterns. Nominal MMT concentrations were 2 and 5%. In
the case of low molecular weight resin (PS1510) nanocomposite, Cloisite-10A
nanocomposites exhibited the typical reduction in interlayer distance from 2.04
nm (organoclay spacing) to 1.47 nm, due to surfactant loss during processing.
This behavior was caused by either thermal degradation or out-diffusion of the
surfactant molecules. The almost indistinguishable peak in the region of 2Θ
127
between 2° and 4° for Cloisite-10A suggests a low degree of polymer
intercalation. Diffraction patterns for PS composites incorporating phosphonium
organoclays did not show any increase in the basal spacing of these organoclays.
On the other hand, they did not exhibit any collapse of these galleries, in contrast
to the behavior of Cloisite-10A. Only a small decrease in interlayer spacing was
observed for Ph2 and Ph3 nanocomposites (-0.3 and -0.26 nm, respectively),
possibly due to rearrangement of the long alkyl chains of surfactants.
The
absence of gallery collapse may be attributed to the thermal stability of the
organoclays. Results obtained with high molecular weight polymer (PS 1220)
nanocomposites were similar to those described above. Cloisite 10-A gallery
showed greatly reduced gallery spacing, in this case. No indications of polymer
intercalation were observed, in agreement with Balazs’s prediction that
intercalation chances are reduced with increasing molecular weight (Balazs et al,
1998).
Table 7.1. Summary of basal spacing (nm).
Organoclay
Initial d001
PS1510 - d001
PS1220 - d001
Ph1
2.32
2.32
2.38 (+0.06)
Ph2
2.52
2.32 (-0.20)
2.32 (-0.20)
Ph3
2.52
2.26 (-0.26)
2.45 (-0.07)
Ph4
1.84
1.84
1.84
Cloisite
2.05
1.47 (-0.58)
1.47 (-.058)
Various reports were found in the literature on PS/Cloisite-10A
nanocomposites (Carascan and Demarquette, 2006, Nassar et al, 2005, Tanoue et
al, 2004, Tanoue et al, 2005, Tanoue et al, 2006). Others are available regarding
128
nanocomposites prepared in an internal mixer with phosphonium organoclays
(Uribe, 2003, Yilmazer and Ozden, 2006). The high pressure generated during
extrusion could limit polymer diffusion into the organoclay galleries, thus
hindering polymer intercalation. On the other hand, polymer intercalation in
phosphonium organoclay nanocomposites was reported in other melt compounded
nanocomposites, such as polylactides, ε-caprolactone, syndiotactic polystyrene,
styrene-acrylonitrile copolymers and poly(butylene succinate) (Chu et al, 2004,
Hrobarikova et al, 2004, Kim et al, 2004, Maiti et al, 2002, Okamoto et al, 2003).
7.3.2
Transmission electron microscopy (TEM)
Low magnification TEM pictures indicate that clay was dispersed within
the polymer matrices as elongated agglomerates, which haves different shapes and
dimensions, depending on matrix molecular weight and organoclay type.
Organoclay particles of Ph1, Ph2, and Ph3 in PS1510 nanocomposites appear as
thin agglomerates (tactoids). The thickness of these tactoids ranged from 10 to 20
nm (5 to 10 individual clay platelets). However, Ph4 particles appear as large
agglomerates. There are signs of polymer intercalation at the edges and in the
core of clay agglomerates.
Figure 7.1 shows the organoclay dispersion for
samples of PS1220/Ph1 and PS1220/Cloisite 10A at 2% organoclay content. In
PS1220, Ph1 was broken down into tactoids consisting of only a few intercalated
clay layers. Only Ph2 and Ph3 organoclay were dispersed within PS1220 resin as
round-like agglomerates. Clay platelets in Ph2 and Ph3 nanocomposites were
curled during processing showing the flexibility of clay platelets.
Ph4
129
agglomerates consisted of 8-10 clay platelets. On the hand, Cloisite 10-A was
dispersed as large agglomerates within both PS resins.
Table 7.2 summarizes data on aspect ratio of clay agglomerates
(determined with Scion software). Values of aspect ratio for clay agglomerates in
PS1220 nanocomposites are lower than those for PS1510 nanocomposites. High
shear stresses prevailing during the processing of PS1220 could enhance breakup
of the clay agglomerates.
130
PS1220/Ph1
PS1220/Cloisite 10A
Figure 7.1. TEM pictures of PS1220 nanocomposites prepared with different
organoclays.
131
Table 7.2. Summary of aspect ratio of clay agglomerates. Standard deviation
in parenthesis.
Clay PS1220
PS1510
Ph1
8.7 (3.5)
10.1 (6.2)
Ph2
2.4 (1.2)
15.4 (7.3)
Ph3
2.9 (1.3)
15.6 (6.9)
Ph4
6.3 (1.3)
17.6 (7.0)
The melt compounding process employed in this work was successful in
reducing the size of the clay agglomerates from several microns in thickness to
tactoids around ten nanometers thick. However, only weak intercalation by the
polymer, if any, was observed in most cases. The combination of shear and
chaotic mixing during melt compounding promotes the breaking of clay
agglomerates, until the major dimension (length) of clay clusters is reduced to
1μm with a thickness of few tens of nanometers.
The breakdown of clay
agglomerates is due to the lower interaction forces among platelets at high
temperature in comparison with neat MMT, as indicated by the value of the
Hamaker constant at the processing temperature (Table 7.3, Uribe et al., 2007).
Since the surface tension of modified clay is lower than the surface tension of the
polymer melt polymer diffusion into the galleries of modified clay is energetically
unfavorable, especially for the high molecular weight polymer. In summary, the
clay dispersion is strongly affected by the polymer molecular weight and surface
properties of the organoclay. Low molecular weight polymer produces better clay
dispersion and promotes intercalation.
132
Table 7.3. Surface properties and Hamaker constant of organoclays and
polymers.
7.4.3
A11x10 20J
Material
γ (mJ/m2)
PS1510
23.0+0.3
PS 1220
23.3+0.80
Ph 1
14.7 + 03
3.40
Ph 2
11.9 + 1.3
2.44
Ph 3
10.3 + 1.1
2.11
Ph 4
16.5 + 0.7
3.36
Cloisite 10 A
18.8 + 0.5
3.91
MMT
21.8 + 0.5
4.43
Thermal stability
In general, the presence of clay within the polymer matrix tends to
increase thermal stability of the polymer (Chigwada et al, 2006). The TGA
curves for unfilled PS1510 showed significant mass loss starting around 380 °C
and ending around 500 °C.
Mass loss for PS1510 occurred in one step
decomposition, with no carbonaceous residues. The higher molecular weight
resin, PS1220, started significant decomposition at 410 °C, also decomposing in
one step. The TGA curves of the nanocomposites were shifted towards higher
temperatures, indicating enhanced thermal stability, which depends on the type
and concentration of the organoclay.
However, one-step decomposition was
maintained. Higher clay concentration led to improvement in thermal stability.
Figure 7.2 shows TGA curves of PS1220 nanocomposites with 5% MMT
nominal content from various organoclays. Phosphonium organoclays increase
133
thermal stability of PS1220 almost equally.
However, Cloisite 10A
nanocomposites start thermal decomposition at relatively low temperatures,
possibly due to the lower thermal stability of the organoclay. Phosphonium
organoclays Ph3 and Ph4 provide the best thermal stability with PS1220. A
similar behavior was observed in nanocomposites prepared with PS1510. Table
7.4 indicates the temperatures corresponding to various levels of thermal
decomposition or maximum decomposition during the TGA scan. In the above
estimates, the actual MMT concentration was taken as the ash content at 550 °C.
100
Ph4
Ph2 Clo-10-A
80
Mass, %
PS1220
Ph3
60
Ph1
40
20
0
350
400
450
500
550
Temperature, °C
Figure 7.2. TGA curves of PS1220 nanocomposites containing different
organoclays (5 % clay content).
134
417
431
464
0
5%
10 %
50 %
Ash (w/w, %)
426
436
465
0
5%
10 %
50 %
Ash (w/w, %)
1.97
481
444
428
375
1.60
477
438
422
378
2%
2.67
491
451
435
375
2.87
486
444
427
366
5%
Cloisite 10 A
* Nominal concentration of MMT.
393
1%
PS1220
381
1%
Mass lost
MMT*
PS1510
479
445
433
394
482
450
439
417
3.01 3.99
478
445
435
415
Ph2
1.73
469
440
431
405
1.38
472
439
427
395
5% 2%
2.05 4.43
474
436
424
398
2%
Ph1
2.28
479
446
436
413
2.35
479
446
436
413
5%
1.81
471
446
436
406
1.35
472
389
426
394
2%
Ph3
Table 7.4. Summary of TGA results for PS nanocomposites.
2.68
485
450
437
416
3.23
479
444
433
407
5%
480
445
432
413
5%
479
448
437
406
1.85 4.13
469
444
434
402
0.71 3.09
474
444
433
404
2%
Ph4
135
7.4.4
Oxygen permeability
Under appropriate conditions, the presence of clay should contribute to reducing
the gas permeability in polymer nanocomposites (Khayankarn et al, 2003, Krook et al,
2002, Osman et al, 2004). The layered silicates form a tortuous path, which retards the
progress of gas molecules through the polymer matrix. Barrier performance depends on
two main parameters: the particle dimensions and the quality of dispersion (Maiti et al,
2002). Moreover, polymer crystallinity and morphology can have a significant influence
on the barrier properties (Osman et al, 2005). Ray et al (2003a) reported 40% reduction
in oxygen permeability, using phosphonium organoclay at 4 % concentration in
polylactide nanocomposites produced by melt processing. Chang et al (2001) found that
polyimide nanocomposites, containing two different organo-montmorillonites, showed a
dramatic improvement in barrier properties.
The two unfilled PS resins showed different permeability levels, depending on the
molecular weight of the polymer (149 + 7 and 110 + 4 cc-mm/m2-day-atm for low and
high molecular weight resins, respectively). Table 7.5 shows the values of oxygen
permeability coefficient for PS nanocomposites. Since dispersion of clay depends on the
molecular weight of the PS resins and on the surface properties of the organoclays, it is
expected that oxygen permeability should behave accordingly.
For nanocomposites
based on PS1510, the lowest values of oxygen permeability coefficient were observed for
Ph2 and Ph4 nanocomposites.
Other phosphonium organoclays and Cloisite 10-A
nanocomposites exhibited lowering of oxygen permeation, but to a smaller extent. The
clay dispersion of PS1510/Ph4 nanocomposite (at 5% wt.) reduced neat PS1510 oxygen
permeability by 33 %. Organoclays in PS1220 reduced the oxygen permeability to a
136
lesser extent than in low molecular weight PS nanocomposites. With this resin, the best
reduction in permeability was obtained with Ph3 nanocomposites (ca. 25% at 5% wt.).
Table 7.5. Oxygen permeability coefficient for PS nanocomposites (ccmm/m2-day-atm), values in brackets represent the standard deviation.
2%
5%
PS1510
Ph1
138.6 (8.2)
112.0 (6.5)
Ph2
116.4 (5.6)
103.5 (3.6)
Ph3
127.3 (8.0)
112.4 (6.5)
Ph4
101.1 (4.9)
95.4 (5.6)
Cloisite 10A
121.5 (4.6)
106.4 (1.3)
PS1220
Ph1
107.3 (3.0)
87.0 (5.0)
Ph2
96.9 (3.1)
91.0 (4.2)
Ph3
89.0 (4.3)
84.3 (3.2)
Ph4
98.6 (4.9)
96.5 (5.1)
Cloisite 10A
113.2 (6.9)
95.0 (4.2)
Various models have been proposed to estimate the effect of filler concentration
on permeability properties of composites. Most models consider clay particles to be well
dispersed and oriented parallel to the film surface, with constant particle aspect ratio (A).
The following models have been evaluated in the present work: Nielsen (Ray et al,
2003), Cussler (regular and random array) (Lape et al, 2004), Bharadwaj (Bharadwaj,
2001) and Gusev (Gusev and Lusti, 2001).
The above models were fitted to the
experimental data on permeability, in order to estimate the effective aspect ratio of the
particles (platelets, tactoids, or agglomerates) in each case. Figure 7.3 shows the oxygen
permeability ratio (P/P0) of PS nanocomposites, as a function of type and content of
137
phosphonium organoclay, where P and P0 are the oxygen permeability coefficients of the
nanocomposite and unfilled polymer, respectively. Lines in the graphs correspond to the
P/P0 values predicted by Cussler’s model (random array), which generated the closest
values for filler aspect ratio to the experimental observations.
Numbers on graphs
represent the aspect ratio used in the calculations. The model is represented by the
following expression:
P
1−φ
=
2
P0
⎞
⎛ 2
A
φ
1
+
⎟
⎜
⎠
⎝ 3
Equation 7.1
Cussler’s model generated the closest values for filler aspect ratio to the
experimental observations. Numbers on graphs represent the aspect ratio used in the
calculations. In the case of PS1510 nanocomposites, the values of fitted aspect ratio are
in the same order of magnitude as those based on TEM observations for Ph1, Ph2 and
Ph3 nanocomposites. However, differences among aspect ratios are higher for Ph4
nanocomposites. Fitted values for PS1220 are several times higher than those based on
TEM, especially for Ph2 and Ph3 composites. The discrepancy may be attributed to
differences between the average aspect ratio (estimated from permeability measurements)
and local estimation of aspect ratio of filler based on local measurement by TEM over a
micro-sized specimen.
An important factor could be attributed to the effect on
nanocomposite structure of post-processing of the material to prepare samples for
permeability measurement. Moreover, it should be noted that the important role of the
quality of adhesion is not considered in the permeability models.
138
Summarizing, organic modification of clay determines the quality of dispersion,
as a consequence the reduction in oxygen permeability in PS nanocomposites.
Nanocomposites having organoclays with lower surface energy exhibit larger reduction
in oxygen permeability. Cussler’s model produced the best fitting to experimental data.
Figure 7.3. Permeability coefficient ratios of PS nanocomposites prepared with
phosphonium organoclays: (a) PS1510 and (b) PS1220. Symbols represent the
experimental observations and lines represent the fit generated with Cussler Model
where φ represents the volume fraction of MMT and A the aspect ratio of particles.
139
7.4.5
Mechanical properties
Mechanical properties depend in great measure on clay dispersion (Lee et al,
2005b) and the development of adhesion between clay surface and the polymer (LópezQuintanilla et al, 2006). Figure 7.4 shows variation of flexural modulus as a function of
actual filler content for PS1220 and PS1510 nanocomposites containing different
organoclays.
The experimental results show that clay contributes to increasing the
modulus, as reported by many researchers (LeBaron and Pinnavia, 2001, Tanoue et al,
2005). Organoclays Ph1 and Ph4 make a significant contribution to modulus increase
with clay concentration, for both PS1220 and PS1510. The modulus of Ph1-PS1220
decreases at high clay concentration, possibly due to poor clay dispersion. Organoclays
Ph2 and Ph3 do not seem to have a significant influence on modulus (within experimental
error). Cloisite 10A nanocomposites showed improvement in modulus with clay content
for both polymer resins.
TEM observations suggested that Ph2 and Ph3 were better dispersed (small
agglomerate sizes) in the matrix, due to the lower values of the Hamaker constant at the
processing temperature. However, Ph1-PS and Ph4-PS systems yielded higher values of
the thermodynamic work of adhesion at the polymer-clay interface (Uribe et al., 2007).
Thus, while dispersion is an important factor in both permeability to oxygen and modulus
enhancement of nanocomposites, it appears that the work of adhesion plays an equally
important role in determination of both of these properties.
140
Figure 7.4. Flexural modulus of PS nanocomposites as a function of type and
concentration of organoclay: a) PS1510 and b) PS1220.
141
The strength and maximum deformation were slightly increased or remained
unchanged with clay content (Figures 7.5 and 7.6).
It is generally accepted that
maximum strength and deformation are usually lowered in nanocomposites, compared to
the polymer, especially in systems containing rigid brittle polymers, such as PS, SAN or
PMMA (Ma et al., 2005; Su et al., 2004; Tanoue et al., 2005; Tanoue et al., 2006; Fu and
Naguib, 2006). On the other hand, good polymer-filler adhesion might contribute to
higher elongations at break, in comparison with the neat resin (Ray et al., 2003a).
Ductile polymer systems could exhibit, in some cases, significant improvements in both
modulus and strength by incorporating clay and compatibilizer in the nanocomposite
formulation, as in the case of nylon, PE or PP (Burmistr et al., 2005; Gyoo et al., 2006;
Lee et al., 2005a).
142
Figure 7.5. Maximum strength of PS nanocomposites as a function of type and
143
Figure 7.6. Maximum deformation of PS nanocomposites as a function of type and
144
Flexural moduli of selected PS nanocomposites were compared with calculated
moduli from several models reported in the literature: Hui-Shia (Hui and Shia, 2001),
Modified Rule of the Mixture (MRM), Halpin-Tsai (Shia and Hui, 1998), Ji-Jiang (Ji et
al., 2002) and Brune-Bicerrano (Brune and Bicerrano, 2002). Mineral clay modulus was
taken from the literature (Wang et al., 2001). The above models assume uniform filler
dispersion. The Hui-Shia and Halpin-Tsai models predict the composite modulus (E)
dependence on aspect ratio of the filler (α), modulus of the polymer matrix (E0) and filler
(E1) and the ratio of the filler modulus to matrix modulus (Er). Filler aspect ratio is an
important parameter included in these models.
The Hui-Shia model leads to the
following equations:
E
E0
η=
=
1 + 2αηφ
1 − ηφ
Equation 7.2
Er − 1
Er + 2α
The Halpin-Tsai’s model is represented by the following equations:
E
E0
=
ξ =φ +
1
Equation 7.3
3 ⎤
φ ⎡1
1− ⎢ +
4 ⎣ ξ ξ + Λ ⎥⎦
⎡ (1 − g )α 2 − ( g / 2) ⎤
E1
+ 3(1 − φ ) ⎢
⎥
E0 − E1
α 2 −1
⎣
⎦
⎡ 3(α 2 + 0.25 )g − 2α 2 ⎤
Λ = ( 1 − φ)⎢
⎥
α 2 −1
⎣
⎦
g = π/ 2 α
Figure 7.7 shows the experimental flexural modulus results for Ph1 and Ph4
nanocomposites, prepared with different resins (solid symbols) and the calculated moduli
produced by Modified Hui-Shia and Halpin-Tsia’s models.
Moduli for PS1510
145
nanocomposites were estimated by fitting the Hui-Shia model, yielding aspect ratios
close to experimental observations (aspect ratio range from 11 to 12). Similarly, moduli
for PS1220 nanocomposites were compared with the calculation obtained by fitting the
Halpin-Tsia’s modulus model (calculated aspect ratio from 5 to 25).
Figure 7.7. Comparison of some experimental moduli with calculated values from
models: a) PS 1510 and b) PS 1220.
146
7.4.6
Mechanical and oxygen permeability and work adhesion
The thermodynamic work of adhesion (Wcp) for PS-clay, at room temperature can
be calculated using the following expression (Comyn et al, 1993; Li and Neumann, 1992;
Neumann and Li, 1990):
Wcp=2((γcd γpd)1/2+ (γcp γpp)1/2)
Equation 7.4
s
where γcd , γcp , γpd and γpp are the dispersive and polar component of surface energy of
clay (organoclay) and polymer, respectively. The values of the surface energies of the
various constituents have been reported elsewhere (Uribe et al. 2007).
Figures 7.8 shows the effect of thermodynamic work of adhesion on the elastic
modulus and permeability to oxygen of nanocomposites prepared with PS resins at
different levels of organoclays.
Thermodynamic work of adhesion depends on the
surface treatment of clay (Uribe et al. 2007). Basically, the organoclays that contain high
molecular weight surfactant exhibit the lowest values of work of adhesion (Ph2 and
Ph30). Good adhesion is required to obtain improvements in mechanical properties.
Modulus was increased with clay content and the degree of adhesion at the polymer-filler
interface. The overall performance of nanocomposites seems to be affected, not only by
the quality of filler dispersion and/or polymer intercalation/exfoliation but also by the
quality (strength) of adhesion at the clay-polymer interface. Organoclays having surface
tension close to the polymer, such as Ph1, Ph4 or Cloisite 10A, show improvements in
both properties at low filler content.
Shang et al (1994) proposed the following equation to calculate the
modulus of a material as a function of work adhesion (Wa):
EC= C exp [-KEc (1/Wa)]
Equation 7.5
147
where Ec is the modulus of the composite, and C and KEc are constants determined
experimentally. Figure 7.15 shows that the experimental data on moduli of PS1510 and
PS1220 nanocomposites follow the above relationship reasonably well. The parameters
in Equation 7.5 show some dependence on filler concentration. It is evident that the
modulus varies inversely with the work of adhesion, and thus, the degree of
reinforcement depends work of adhesion, as well as on the clay dispersion.
Figure 7.8. Effect of thermodynamic work of adhesion on oxygen permeability and
mechanical properties of nanocomposites. a) PS1510, and b) PS1220. Solid and open
symbols correspond to oxygen permeability and modulus, respectively.
148
Figure 7.9. Nanocomposite moduli as a function of thermodynamic work of
adhesion at two organoclay concentration: a) 2% and b) 5%.
149
7.5
Summary
PS nanocomposites were prepared with thermally stable phosphonium organoclays
in a twin screw extruder, employing long residence time and a combination of
elongational and shear flow in a chaotic mixing field.
Thermal stability of the
organoclays appeared to inhibit gallery collapse during compounding.
However,
modified clay was dispersed into aggregates without significant increase in basal spacing
in the PS nanocomposites.
Thermal stability, mechanical properties and barrier
properties were improved to varying degrees with organoclay content.
The use of
organoclays (Ph1 and Ph4) exhibiting values of surface energy, at the processing
temperature, close to the surface tension of the polymer produces PS nanocomposite with
enhanced properties. Property improvements were observed in organoclays having larger
basal; spacing.
Models describing permeability yielded reasonable fit of the data
regarding the effect of clay concentration on permeability. However, widely differing
values of clay aspect ratio were estimated using these models. Moreover, correlations of
oxygen permeability with the work of adhesion at room temperature were weak.
However, these models PS nanocomposite moduli yielded good correlation with
thermodynamic work of adhesion. Shang’s equation provides a good correlation between
modulus and the work of adhesion.
150
Chapter 8 PS-SMA-Phosphonium Organoclay
Nanocomposites
151
8.1 Abstract
The copolymer of styrene-maleic anhydride (SMA) was used as compatibilizer to
improve the clay dispersion during the preparation of PS nanocomposites through melt
compounding. The surface energy of SMA was calculated from the contact angle of
sessile drops of several liquids, the thermodynamic work of adhesion and interfacial
tension SMA-clay were calculated.
Nanocomposites containing SMA at different
proportions, in combination with four phosphonium organoclays, were prepared by
extrusion. XRD and TEM results indicate that the clay was dispersed in the PS-SMA
blends to different degrees, depending on the surface energy of phosphonium organoclay.
The clay was found as swollen agglomerates with partially exfoliated structures in PSSMA blends. XRD measurements indicated the absence of intercalation, suggesting that
partial exfoliation occurred without an intermediary intercalation step. Thermal stability
of nanocomposites was improved with SMA content, depending on the thermal stability
of phosphonium organoclays. In the nanocomposites, the elastic modulus increased with
clay content, but the strength and maximum deformation were generally decreased.
Barrier properties were improved in the presence the clay.
8.2 Introduction
There are many factors, including nanofiller type and composition, polymer
characteristics, and processing conditions, which influence the final structure and
properties of nanocomposites obtained by melt processing.
It is known that clay
modification, surface energy and interfacial interactions of the polymer and clay, thermal
stability, and initial basal spacing play important roles in the exfoliation-intercalation
152
process (Chigwada et al, 2006, Dharaiya and Jana, 2005, Le Baron et al, 1999, Vaia et al,
1996). Similarly, processing conditions are very important (Dennis et al, 2001, Nassar et
al, 2005, Tanoue et al, 2006). Finally, the composition of the polymer matrix has a great
influence on the miscibility and stability of nanostructures (Hasegawa et al, 1999).
Maleic anhydride copolymers are commonly used to compatibilize non-polar polymers
with the organoclay and to enhance clay dispersion and to promote MMT delamination
(Lopez-Quintanilla et al, 2006, Girish et al, 2001, Liang et al, 2004).
Natural
montmorillonite was organically modified with several phosphonium surfactants at the
CEC of MMT, producing organoclays (Ph1, Ph2, Ph3, Ph4) with excellent thermal
stability and enhanced interlayer distances and surface properties (Kamal and UribeCalderon, 2006). Nanocomposites based on polystyrene (PS)-copolymer of styrenemaleic anhydride (SMA)-clay nanocomposites were produced by melt compounding in a
twin screw extrusion system. The characteristics of these nanocomposites and their
constituents are discussed in this chapter.
8.3 Surface Energy of SMA
The SMA used in this study was a styrene-maleic anhydride (SMA) copolymer,
14 % maleic content (Dylark 332, Mw 181 kg/mol, PD 2.10). Surface energy of the SMA
was determined using the sessile drop technique. For this purpose, four different liquids,
with known polar and dispersive components of surface tension at room temperature,
were used. Contact angles of sessile drops on the flat surface of SMA plates were
measured, and the surface energy components were calculated. The results are shown in
Table 8.1.
Contact angles for water and glycerol were slightly lower than the
153
corresponding contact angles for PS1220 (homopolymer). The contact angles for polar
liquids on the styrene copolymer, containing a polar comonomer at low concentration,
tend to decrease due to the slight growth in the surface polarity promoted by the
concentration of polar comonomer on the surface, the contact angles tended to increase
with polar comonomer concentrations (Adao et al, 1999, Suchocka-Gałas and
Kowalonek, 2006).
Polar comonomers can interact among themselves and form
hydrogen bonds or dipole–dipole interactions. The polar species migrate to the sample
bulk, making the surface more nonpolar (Adao et al, 1999, Lee et al, 2004, SuchockaGałas and Kowalonek, 2006).
Table 8.1 Contact angles of sessile drops on SMA.
Liquid
Contact angle, °
Water
72.0 + 0.4
Glycerol
61.7 + 0.2
Diiodomethane
22.9 + 0.2
Ethyleneglycol
35.5 + 0.5
The procedure employed by Comyn et al (1993) was used to calculate the
components of surface energy. The dispersive and polar components of surface energy
for SMA were 39.06 mJ/m2 and 5.52 mJ/m2, and the total surface energy was 44.58 + 0.2
mJ/m2. The presence of the maleic anhydride component in Dylark copolymer generates
an increase in both the dispersive and polar components in comparison to PS
homopolyme resins used before. The surface energy of Dylark is slightly higher than that
of pure PS, due to the polar comonomer included in the copolymer structure.
The thermal decomposition of SMA did not allow the determination of surface
tension of the resin (blends) using sessile drop experiments at 220 °C on organoclay
154
surfaces. Maleated groups of SMA tend to react with the hydroxyl groups of clay or with
other carboxyl groups in the copolymer, thus forming bubbles leading to instabilities in
the measurement (Zeliazkow, 2001). Table 8.2 shows the calculated values of interfacial
tension (γ12) and thermodynamic work of adhesion (Wa) of interphase SMA-organoclay
and PS-organoclay for PS resins with different melt flow index. Values were calculated
using the equations 6.2 and 6.4.
As expected, the SMA systems exhibited lower
interfacial tension values with the organoclay than PS systems, depending on the surface
energy of the phosphonium organoclay. The SMA copolymer showed moderate affinity
to the organoclays, especially Ph2. Wa at the SMA-organoclay interface was higher than
that of the corresponding PS-organoclay.
Wa depended on the surface energy of
phosphonium organoclay. The highest values of work of adhesion were observed in
samples of Ph1 and the lowest values were observed for Ph2 and Ph3.
Table 8.2. Interfacial tension and thermodynamic work of adhesion.
SMA-Clay
Clay
γ12 (mJ/m2)
Wa (mJ/m2)
PS1510-Clay
PS1220-Clay
Wa (mJ/m2)
Wa (mJ/m2)
Ph1
1.0
90.5
88.2
85.8
Ph2
0.1
85.0
81.6
81.3
Ph3
0.3
84.1
81.3
80.1
Ph4
1.4
88.1
86.1
83.4
8.4 Clay Dispersion
Experimental evidence and modeling have shown that the incorporation in the
polymer of compatibilizing groups, such as maleated groups, enhances intercalation or
exfoliation in polymer-organoclay systems (Lee et al, 2002). This is in agreement with
the predictions of theoretical models (Balazs et al, 1998). The incorporation of maleic
155
anhydride could produce undesirable effects, such as odors and volatiles, during melt
compounding. Anhydride groups could react with each other or with hydroxyl groups at
the clay surface to form carboxylic acid (Chitanu et al, 1998, Steinert and Ratzsch, 1989).
The remaining carboxylic group of the anhydride could react with an oxygen atom at the
clay surface and produce CO2. The above reactions generate bubbles and a yellowish
color in the final product.
Furthermore, carboxylic acid could cause chemical
degradation of PS, resulting in reduction of molecular weight and deterioration of
properties (Jang and Wilkie, 2005, Nassar et al, 2005). Processing problems can be
avoided or minimized by using Dylark at low concentration (for example 5 – 10 % w/w).
Several authors (Bikiaris et al, 2004, Rosch et al, 1990) have reported that PS-SMA
blends are immiscible. Consequently, the clay is expected to be located in the SMA
phase rather than within the PS domains.
The effect of SMA concentration was studied, using PS1220 and several
phosphonium organoclays (at 2 % w/w MMT content). Nanocomposites were prepared
at different concentrations of Dylark (from 5 to 35 %, w/w, corresponding to 0.7 to 4.9 %
maleic anhydride). Figure 8.1 shows wide angle x-ray diffraction (WAXD) patterns for
various nanocomposites containing the four phosphonium organoclays at different
concentrations of Dylark.
The nanocomposite containing 5 % Dylark showed an
intercalation peak corresponding to 3.27 nm. The intercalation peak tended to disappear
at higher Dylark concentration (above 10 %). No evident diffraction peak was present
for formulations containing 35 % Dylark. However, a different effect was observed in
other phosphonium organoclays. The characteristic peaks of Ph2 and Ph3 were preserved
in samples containing Dylark, a slight reduction in basal spacing was observed. While
156
the intensity of the WAXD diffraction peak was reduced in the presence of Dylark, the
position of the peak did not change as a result of incorporation of the Dylark.
Figure 8.2 shows WAXD patterns for nanocomposites prepared with PS1510,
Dylark at 10 %, and several phosphonium organoclays (clay content 2% wt). The peak
observed for Ph1 organoclay disappeared completely, indicating good clay dispersion and
possible clay exfoliation. However, the characteristic organoclay peaks persisted, but
with considerably lower intensity, for composites with Ph2, Ph3 and Ph4.
Ph4
Intensity, A.U.
5%
10 %
35 %
Ph3
Ph2
Ph1
1
2
3
4
5
6
2Θ
Figure 8.1. X-ray patterns of PS1220/Dylark nanocomposites having different
copolymer proportions with phosphonium organoclays (MMT content 2 %).
157
Intensity, A.U.
Ph4
Ph3
Ph2
Ph1
1
2
3
4
5
6
2Θ
Figure 8.2. X-ray patterns of PS1510/ Dylark 10 % with phosphonium organoclays
(MMT content 2 %).
The effectiveness of copolymer in enhancing clay delamination depends on the
surface energy and basal spacing of the organoclay. Ph1 and Ph4 have the highest
surface energy in the group (14.6 and 16.4 mJ/m2 respectively) at 220 °C (Uribe et al,
2007), but Ph1 organoclay had a larger basal spacing (2.32 nm) than that of Ph4 (1.84
nm). Organoclays Ph2 and Ph3 have higher basal spacing (2.52 nm) but lower surface
energy at the processing temperature (11.9 and 10.3 mJ/m2), which tends to hinder clay
delamination.
The x-ray results for Dylark/Ph1 nanocomposites were supported by low
magnification TEM pictures for the Ph1 system (Figure 8.3). Clay agglomerate size was
around 0.3 μm. High magnification TEM pictures indicated that clay agglomerates
consisted of few individual clay platelets and many delaminated structures. Strong polar
groups in the copolymer interact with the hydroxyl groups at the clay surface, causing
158
some intercalation and partial exfoliation.
It has been shown that, in some cases,
polymer-clay interaction is stronger than polymer-surfactant interaction (Lee et al,
2005a).
Figure 8.3. TEM pictures of Ph1/Dylark nanocomposites.
TEM results supported WAXD observations for PS1510/Dylark 10 % blends
(Figure 8.4). Organoclay Ph1 was dispersed in domains that exhibited exfoliated/partial
exfoliated clay platelets.
The other organoclays appeared as small agglomerates or
tactoids with mainly unintercalated structures. However, some intercalation occurred at
the agglomerate edges. Organoclays Ph2 and Ph3 consisted of few tens of individual
clay platelets.
159
Ph1
Ph2
Ph3
Ph4
(clay content 2 %).
160
Composites based on PS1220/Ph1/Dylark 10% did not exhibit a diffraction peak
for the organoclay. While TEM images at low magnifications suggested that the clay
appeared as agglomerates (1 or 2 μm in length), high resolution images indicated the
existence of intercalated/exfoliated structures within the clay agglomerate (Figure 8.5).
Organoclays Ph2 and Ph3, which had the lowest values of surface tension at the
processing temperature, were dispersed into small compact clay aggregates. Organoclay
Ph4 appeared as a non-homogeneous mixture of micron-size domains, with partially or
fully exfoliated clay platelets.
Summarizing the results, SMA containing polar groups improved organoclay
dispersion/polymer intercalation.
The degree of intercalation depended on the
compatibilizer concentration. Low compatibilizer concentrations are preferred (5-.10
w/w) to prevent possible degradation reactions. The clay was dispersed into domains
which included delaminated or intercalated clay platelets. Organoclays with initially high
basal spacing and high surface tension (Ph1 or Ph4) were delaminated at low
concentration of Dylark.
161
Ph1
Ph2
Ph3
Ph4
(clay content 2 %).
162
8.5 Thermal Stability of Nanocomposites
The effect of maleic anhydride copolymer on the thermal stability of PS
nanocomposites was evaluated.
Thermal stability of Ph1/PS1220 (Figure 8.6) and
Ph4/PS1220 nanocomposites deteriorated with Dylark content, although clay was well
dispersed, according to TEM observations. Possible reactions involving the maleated
groups in Dylark, the organoclay and/or the polymer might contribute to the overall
thermal stability of the nanocomposites (Chitanu et al, 1998, Nassar et al, 2005). It
should be noted that the thermal stability of Ph2/PS 1220 and Ph3/PS 1220
nanocomposites was slightly improved by the addition of Dylark, possibly due to the
higher thermal stability of these organoclays (Kamal and Uribe, 2006).
100
Mass, %
80
65-35
85-15
90-10
60
40
95-05
PS1220
Dylark
20
PS1220+Ph1
0
350
400
450
500
550
Temperature, °C
Figure 8.6. TGA traces of PS1220/Ph1 nanocomposites with different Dylark
contents (2 % clay content).
163
8.6 Oxygen Permeability
The measured oxygen permeability of pure Dylark resin was 135 + 8 cc-mm/m2day-atm, which was higher than the corresponding value for PS1220 resin. The potential
effect of well dispersed clay on barrier properties could not be evaluated for Dylark
systems, due to the brittleness of Ph1/Dylark nanocomposite films. Brittle films could
include micro-cracks, which would compromise barrier behavior (Chaiko and Leyva,
2005). Oxygen permeability of neat PS1220/Dylark blends was not changed significantly
with compatibilizer content. In fact, the only significant changes in permeability were
observed at the highest maleic anhydride content. The oxygen permeability of Ph1
nanocomposites decreased, as compatibilizer concentration increased (Figure 7.9). The
highest reduction in oxygen permeability reached 24%. In other systems (e.g. Ph2 and
Ph3), either a plateau was reached after 5% Dylark content, or little change of
permeability was observed with compatibilizer content (e.g. Ph4).
The above results suggest that the presence of the compatibilizer promoted clay
dispersion and, as a result, reduced the oxygen permeability in the case of Ph1, which
was transformed into swollen clay domains having 3-4 μm in length and 2-3 μm in
thickness. These domains hosted intercalated or partially exfoliated organoclay. Ph2 and
Ph3 were dispersed in small aggregates (150-300 nm in length and 15-50 nm in
thickness), which had 6-25 individual clay platelets.
164
Oxygen Permeability Coefficient
cc*mm/m2*day*atm
120
110
100
90
80
Ph1
Ph2
Ph3
Ph4
70
60
0
5
10
Dylark Percentage
15
35
Figure 8.7. Effect of Dylark content on oxygen permeability for samples of PS1220
having several phosphonium organoclay. MMT content 2 % in all cases. Graph
show the half of error bars for a better data appreciation.
8.7 Mechanical Properties
Figure 8.8 shows a plot of flexural modulus of Ph1-PS1220 nanocomposites vs.
Dylark concentration (2 % clay content). The modulus of neat blends increased with
Dylark content, suggesting that maleic anhydride augmented the stiffness of PS1220
resin.
The modulus of PS1220 nanocomposites behaved differently with Dylark
concentration, depending on the type of organoclay used. The moduli of Ph1, Ph2 and
Ph4 nanocomposites reached a maximum at 15% Dylark, whereas the modulus of Ph3
nanocomposites showed erratic behavior.
The results suggested that there was an
optimum maleic anhydride content to improve the mechanical properties at low filler
165
concentration, which ranged from 1.4 to 2.1 % w/w (10-15 % Dylark), depending on the
surface characteristics of organoclay.
4000
Modulus, MPa
3500
Ph1
Ph2
Ph3
Ph4
PS/Dylark
3000
2500
2000
0
5
10
15
35
Dylark Concentration, %
Figure 8.8. Flexural modulus of PS1220-Dylark nanocomposites as a function of
type of organoclay and Dylark concentration.
Flexural strength and maximum deformation showed monotonic decrease with
Dylark content, as the clay was better dispersed with the polymer. Figures 8.9 and 8.10
show the behavior of strength and maximum deformation of PS1220 nanocomposites
with Dylark concentration. Neat PS1220-Dylark blends showed a continuous
deterioration in strength as copolymer content increased. Similar behavior was observed
for nanocomposite strength, which showed monotonic decrease in maximum strength
with increasing Dylark content. SMA immiscibility has a significant effect on the
166
mechanical properties (Utracki, 1989). Moreover, nanocomposite samples exhibited a
monotonic decrement in maximum deformation with increasing Dylark content. The
extent of reduction in maximum strength and deformation depended on the type of
organoclay.
The use of a compatibilizer (in this case maleic anhydride-styrene block
copolymer) can yield better clay dispersion and improvement of polymer intercalation or
even some clay exfoliation (Carastan and Demarquette, 2006, Deenadayalan et al, 2006,
Girish et al, 2001, Jacquelot et al, 2005, Mingqian and Uttandaraman, 2006, Nikhil and
Weiss, 2006).
Maximun Strength, MPa
100
80
Ph1
Ph2
Ph3
Ph4
PS/Dylark
60
40
20
0
0
5
10
15
35
Figure 8.9. Flexural strength of PS1220 nanocomposites as a function of type of
organoclay and Dylark concentration.
167
Maximun Deformation, %
4
3
Ph1
Ph2
Ph3
Ph4
PS/Dylark
2
1
0
0
5
10
15
35
Figure 8.10. Maximum deformation of PS1220 nanocomposites as a function of type
of organoclay and Dylark concentration.
PS1220-Dylark and PS1510-Dylark blends (90/10 w/w in each case) were melt
compounded with the four phosphonium at two nominal concentrations of MMT, using
twin-screw extrusion. Figure 8.11 shows the behavior of nanocomposite modulus with
MMT content for the different phosphonium organoclays (the actual MMT content was
determined by TGA, as the final residual weight at 550 °C). The modulus of PS1510SMA increased slightly upon incorporation of a small amount of MMT, for all the
phosphonium organoclays used. Additional MMT content did not cause further increase
in modulus.
Different behavior was observed in the case of PS1220-SMA
nanocomposites. Modulus increased monotonically with MMT concentration, up to an
average increase of around 13 %.
168
3400
Modulus, MPa
3300
Ph1
Ph2
Ph3
Ph4
3200
3100
3000
2900
0
1
2
3
4
5
3
4
5
MMT, % w/w
a
3600
Modulus, MPa
3400
Ph1
Ph2
Ph3
Ph4
3200
3000
2800
2600
0
1
2
MMT, % w/w
b
Figure 8.11. Variation of modulus with MMT content for PS-Dylark
nanocomposites prepared with different phosphonium organoclays. a) PS1510Dylark, and b) PS1220-Dylark.
Both strength and maximum deformation of the composites decreased with
increasing MMT content, due to the incorporation of the stiff particles (Figure 8.12 and
8.13).
The observed reduction in these two mechanical properties due to clay
incorporation has been reported by many researchers, especially for systems containing
stiff polymers, such as PS, SAN or PMMA (Fu and Naguib, 2006, Hwu et al, 2002, Park
169
and Jana, 2003, Tanoue et al, 2006,).
Ductile polymer systems such as nylon,
polyethylene or polypropylene, could exhibit, in some cases, significant improvements in
all three mechanical properties by incorporating clay and/or compatibilizers in the
nanocomposite formulation. (Burmistr et al, 2005, Gyoo et al, 2006, Lee et al, 2005a,
Liang et al, 2004, Su et al, 2004b).
170
58
Ph1
Ph2
Ph3
Ph4
56
Strenght, MPa
54
52
50
48
46
44
42
40
0
1
2
3
4
5
MMT, % w/w
a
70
Ph1
Ph2
Ph3
Ph4
Strenght, MPa
65
60
55
50
45
40
0
1
2
3
4
5
MMT, % w/w
b
Figure 8.12. Variation of strength with MMT content for PS-Dylark nanocomposites
prepared with different phosphonium organoclays. a) PS1510-Dylark, and b)
PS1220-Dylark.
171
1.9
Deformation, %
1.8
1.7
1.6
1.5
1.4
Ph1
Ph2
Ph3
Ph4
1.3
0
1
2
3
4
5
3
4
5
MMT, % w/w
a
3.0
Deformation, %
2.5
2.0
1.5
1.0
Ph1
Ph2
Ph3
Ph4
0.5
0
1
2
Clay content, % w/w
b
Figure 8.13. Variation of maximum deformation with MMT content for PS-Dylark
nanocomposites prepared with different phosphonium organoclays. a) PS1510Dylark, and b) PS1220-Dylark.
172
8.8 Summary
The incorporation of SMA in PS-phosphonium organoclay nanocomposites
improved the clay dispersion and promoted, in some cases, a certain degree of MMT
delamination. Phosphonium organoclays were dispersed into swollen clay aggregates
which exhibited a mixture of exfoliated and unintercalated structures. The observations
indicated that the clays were directly exfoliated without undergoing intermediate
intercalation. The improvements in dispersion depended on the surface properties and
basal spacing of organoclays, and the melt flow index of the PS resin. Phosphonium
organoclay (Ph1), with high surface energy and intermediate basal spacing, appeared to
be dispersed in the form of partially exfoliated structures. WAXD showed that the
intensity peak disappeared for this system, in the presence of SMA. The low thermal
stability of SMA and its intra-molecular and/or intermolecular reactivity could limit the
potential benefits of SMA incorporation. An optimum content of maleic anhydride
content would need to be established. SMA, at low concentrations (5-10 % w/w),
improved clay dispersion, with benefits in both modulus and barrier properties. Thermal
stability of SMA-PS nanocomposites depends not only on the SMA copolymer
concentration, but also on the thermal stability of the phosphonium organoclay.
173
Chapter 9 Conclusions and Recommendations
174
Conclusions
1. Thermal stability and organoclay basal spacing of the phosphonium
organoclays varied according to the degree of cation exchange and
molecular structure of surfactant. The phosphonium organoclays were
more thermally stable in the range of processing temperatures and their
basal spacing was comparable or superior to corresponding properties of
the tested ammonium organoclays.
2. The presence of additives, such as zinc stearate, modifies the surface
energy of the resin.
Consequently, the polymer-organoclay surface
interactions are modified by the presence of such additives.
3. In some cases, the interfacial tension between the clay and the resin was
higher at the processing temperature than at room temperature. This fact
has a profound effect on clay dispersion. Processing temperature should
be optimized, with due consideration to this effect.
4. Clay dispersion correlated with the Hamaker constant (A131) and A11 at the
processing temperature. Interestingly, the correlation of basal spacing in
the nanocomposite with that in the organoclay was weak.
5. Thermal stability and mechanical, and barrier properties improved with
organoclay concentration. The degree of improvement depended strongly
dependent on the surface energy of organoclays.
6. The flexural modulus corelated with the work of adhesion at room
temperature, while permeability to oxygen correlated with initial basal
175
spacing of the organoclay and the Hamaker constant (A131) at the
processing temperature.
7. Overall, in assessing the performance of nanocomposites, it is necessary to
take into consideration the following three factors: thermal stability of the
organoclay and polymer, the quality of dispersion, and the surface energy
interactions.
8. The incorporation of SMA improved clay dispersion, promoting the
development of delaminated or intercalated structures. Additionally, SMA
produced higher values of work of adhesion.
However, the overall
influence on properties was marginal, possibly due to the reactivity of the
anhydride groups.
176
Original Contributions to Knowledge
1. The first reported surface property data based on measurement of surface
properties of organoclays and organoclay-PS systems.
2. The first reported application of quantitative correlations between clay dispersion
and nanocomposite properties, on one hand, and interfacial properties and basal
spacing of organoclays, on the other hand.
3. .The approach employed in this work represents the first integrated effort to
conduct a comprehensive quantitative and experimental analysis of the
contributions of surface energy interactions, basal spacing, and thermal stability to
nanocomposite behavior.
4. Two of the phosphonium organoclays were not reported by other researchers. A
new process was developed for producing organoclays incorporating these waterinsoluble surfactants.
177
Recommendations
1. Similar work should be applied to other commercially interesting systems,
such as nanocomposites based on PA-6, PET, PP, PE, and related blends.
2. The study of SMA incorporation in PS nanocomposites should be
extended.
3. More work is needed to study molecular interactions and to understand the
influence of molecular weight and molecular architecture of surfactants on
the surface energy of corresponding organoclays.
178
References
Adao, M., Saramago, B., Fernández, A., 1999. Estimation of the Surface Properties of
Styrene-Acrylonitrile Random Copolymers from Contact Angle Measurements. J.
Colloid Interf. Sci., 217,94-106
Akanni, S., Burrows, H., 1987. Surface Tensions of Some Molten Divalent Metal
Carboxylates. Thermochim. Acta 115,351-7.
Akelah, A., Moet A., 1996. Polymer-Clay nanocomposites: Free-Radical Grafting of
Polystyrene on to Organophilic Montmorillonite Interlayers. J. Mater. Sci., 31,3589-3596
Akelah, A., Rehab, A., Agag, T., Betiha, M., 2007. Polystyrene Nanocomposite Materials
by In Situ Polymerization into Montmorillonite–Vinyl Monomer Interlayers. J. Appl.
Polym. Sci., 103,3739-3750
Alam, K., 1999. Surface and Interfacial Tension Measurements of Polymer Melts with
Pendant Drop Apparatus: Effect of Structural and Material Properties on the Surface
Tension of LLDPE, an Improved Experimental Method. M. Thesis. McGill University.
Alexandre, M., Dubois, P., 2000. Polymer-Layered Silicate Nanocomposites:
Preparation, Properties and Uses of a New Class of Materials. Mater. Sci. Eng., 28,1-63
Angles, M., Dufresne, A., 2000. Plasticized starch/Tunicin Whiskers Nanocomposites. 1.
Structural Analysis. Macromolecules 33,8344-8353
Ansaria, M., Price, G., 2004. Chromatographic Estimation of Filler Surface Energies and
Correlation with Photodegradation of Kaolin Filled Polyethylene. Polymer, 45,1823-1831
Aphiwantrakul, S., Srikhirin, T., Triampo, D., Putiworanat, R., Limpanart, S., Osotchan,
T., Udomkichdecha, W., 2005. Role of the Cation-Exchange Capacity in the Formation
of Polystyrene–Clay Nanocomposites by In Situ Intercalative Polymerization. J. Appl.
Polym. Sci., 95,785-789
Arroyo M., Suarez R., López-Manchado M., Fernández J., 2006. Relevant Features of
Bentonite Modification with a Phosphonium Salt. J. Nanosci. Nanotechnol., 7,2151-2154
Askin, A, Yazici, T., 2005. Surface Characterization of Sepiolite by Inverse Gas
Chromatography. Chromatographia, 61,625-631
Aulagner, P., Ainser, A., Carrot, C., Guillet, J., 2000. Free Radical Degradation of
Polypropylene in the Bulk: Coupling of Kinetic and Rheological Models. J. Polym. Eng.,
20,381-401
Balazs, A., Huh, J., Ginburg, V., 2000. Thermodynamic Behavior of Particle/Diblock
Copolymer Mixtures: Simulation and Theory. Macromolecules, 33,8085-8096
179
Balazs, A., Lyatkaya, Y., 1998. Modeling the Phase Behavior of Polymer-Clay
Composites. Macromolecules, 31, 6676-6680
Balazs, A., Singh, C., Zhulina E., 1998. Modeling the Interactions between Polymers and
Clay Surfaces through Self-Consistent Field Theory. Macromolecules, 31,8370-8381
Bartholomea, C., Beyou, E., Bourgeat-Lamib, E., Cassagnau, P., Chaumont, P., David,
L., Zydowicz N., 2005. Viscoelastic Properties and Morphological Characterization of
Silica/Polystyrene Nanocomposites Synthesized by Nitroxide-Mediated Polymerization.
Polymer, 46,9965-9973
Beyer, F., Tan, N., Dasgupta, A., Galván, M., 2002. Polymer-Layered Silicate
Nanocomposites from Model Surfactants. Chem. Mater., 14,2983-2988
Bharadwaj, R., 2001. Modeling the Barrier Properties of Polymer-Layered Silicate
Nanocomposites. Macromolecules, 34,9189-9192
Bhiwankar, N., Weiss R., 2005. Melt-Intercalation of Sodium-Montmorillonite with
Alkylamine and Quarternized Ammonium Salts of Sulfonated Polystyrene Ionomers.
Polymer, 46,7246-254
Biresaw, G., Carriere, C., 2004. Surface Energy Parameter of Polymer from Directly
Measured Interfacial Tension with Probe Polymer. J. Adhes. Sci. Technol., 18,1675-1685
Borse, N, Kamal, M., 2006. Melt Processing Effects on the Structure and Mechanical
Properties of PA-6/Clay Nanocomposites. Polym. Eng. Sci., 46,1094-1103
Borse, N., 2006. Melt Processing of Thermoplastic/Cay Nanocomposites. Ph D. Thesis.
McGill University
Bottino, F., Fabbri, E., Fragala, I., Malandrina, G., Orestano, A., Pilati, F., Pollicino, A.,
2003. Polystyrene-Clay Nanocomposites Prepared with Polymerizable Imidazolium
Surfactants. Macromol. Rapid. Commun. 24,1079–1084
Bourbigot, S., Vanderhart, D., Gilman, J., Awad, W., Davis, R., Morgan, A., Wilkie, C.,
2003. Investigation of Nanodispersion in Polystyrene–MMT Nanocomposites by SolidState NMR”, J. Polym. Sci. Part B: Polym .Phys., 41,3188–3213
Bourbigota, S., Gilman, J., Wilkie, C., 2004. Kinetic Analysis of the Thermal
Degradation of Polystyrene-Montmorillonite Nanocomposite. Polym. Degrad. Stab.,
84,483-492
Brune, D., Bicerano, J., 2002. Micromechanics of Nanocomposites: Comparison of
Tensile and Compressive Elastic Moduli, and Prediction of Effects of Incomplete
Exfoliation and Imperfect Alignment on Modulus. Polymer, 43,369-387
Bruzaud, S., Grohens, Y., Ilinca, S., Carpentier, J., 2005. Syndiotactic PolystyreneOrganoclay Nanocomposites: Synthesis via In Situ Coordination-Insertion
180
Polymerization and Preliminary Characterization. Macromol. Mater. Eng., 290,1106–
1114
Burmistr, M., Sukhyy, K., Shilov, V., Pissis, P., Spanoudaki, A., Sukha, I., Tomilo, V.,
Gomza, Y., 2005. Synthesis, Structure, Thermal and Mechanical Properties of
Nanocomposites Based on Linear Polymers and Layered Silicates Modified by Polymeric
Quaternary Ammonium Salts (Ionenes). Polymer, 46,12226-122
Carastan, D., Demarquette N., 2006. Microstructure of Nanocomposites of Styrenic
Polymers. Macromol. Symp., 233,152-160
Cervantes-Uc, J., Cauich-Rodriguez, J., Vazquez-Torres, H., Garfias-Mesias, L., Paul, D.,
2007. Thermal Degradation of Commercially Available Organoclays Studied by TGAFTIR. Thermochim. Acta 457,92-102.
Chae, D., Kim, B., 2005. Characterization on Polystyrene/Zinc Oxide Nanocomposites
Prepared from Solution Mixing. Polym. Adv. Technol., 16, 846-850
Chaiko, D., Leyva, A., 2005. Thermal Transitions and Barrier Properties of Olefinic
Nanocomposites. Chem. Mater., 17,13-19
Chang, J, Mun, M, Kim, J., 2006. Poly(trimethylene terephthalate) nanocomposite fibers
comprising different organoclays: Thermomechanical properties and morphology. J.
Appl. Polym. Sci., 102,4535-4545
Chang, J., Park, K., Cho, D., Yang, H., Ihn, K., 2001. Preparation and Characterization of
Polyimide Nanocomposites with Different Organo-Montmorillonites. Polym. Eng. Sci.,
41,1514-1520
Chen, B., 2004. Polymer–Clay Nanocomposites: an Overview with Emphasis on
Interaction Mechanisms. Br. Ceramic. Trans., 103,241-249
Chen, C., Ma, Y., Qi, Z., 2001. Preparation and Morphological Study of an Exfoliated
Polystyrene-Montmorillonite Nanocomposite. Scripta Mater., 44,125-128
Chen, G., Liu, S., Chen, S., Qi, Z., 2001. FTIR Spectra, Thermal Properties, and
Dispersibility of a Polystyrene/Montmorillonite Nanocomposite. Macromol. Chem.
Phys., 202,1189-1193
Chen, G., Lui, S., Zhang, S., Qi, Z., 2000. Self-Assembly in a PolystyreneMontmorillonite Nanocomposite”, Macromol. Rapid Commun., 21,746-749
Chen, G., Qi, Z, 2000. Shear-Induced Ordered Structure in Polystyrene/Clay
Nanocomposite. J. Mater Res., 15, 351-356
Chen, K., Vyazovkin, S., 2006. Mechanistic Differences in Degradation of Polystyrene
and Polystyrene-Clay Nanocomposite: Thermal and Thermo-Oxidative Degradation.
Macromol. Chem. Phys., 207,587-595
181
Chen, Y., He, B., Lee, J., Patankar, N., 2005. Anisotropy in the Wetting of Rough
Surfaces. J. Colloid Interface Sci. 281,458-464
Chigwada, C., Wang, D., Wilkie, C., 2006. Polystyrene Nanocomposites Based on
Quinolinium and Pyridinium Surfactants. Polym. Degrad. Stab., 91,848-855
Chigwada, G., Jash, P., Jiang, D., Wilkie, C., 2005. Synergy between Nanocomposite
Formation and Low Levels of Bromine on Fire Retardancy in Polystyrenes. Polym.
Degrad. Stab., 88,382-393
Chigwada, G., Jiang, D., Wilkie, C., 2005. Polystyrene Nanocomposites Based on
Carbazole-Containing Surfactants. Thermochim. Acta, 436,113-121
Chigwada, G., Wang, D., Jiang, D., Wilkie, C., 2006. Styrenic Nanocomposites Prepared
Using a Novel Biphenyl-Containing Modified Clay. Polym. Degrad. Stab., 91,755-762
Chigwada, G., Wilkie, C., 2003. Synergy between Conventional Phosphorus Fire
Retardants and Organically-Modified Clays Can Lead to Fire Retardancy of Styrenics.
Polym. Degrad. Stab., 80,551-557
Chitanu, G., Bumbu, G., Carpov, A., Vasile, C., 1998. Analysis and Characterization of
Maleic Anhydride Copolymers. Part 2. Some Aspects of Thermo-oxidative
Decomposition. Int. J. Polym. Anal Charact., 4,479-500
Cho, Y., Kamal, M., 2004. Estimation of Stress for Separation of Platelets by Melt
Processing. Polym. Eng. Sci., 44,1187-1195
Chu, L., Anderson, S., Harris, J., Beach, M., Morgan A., 2004. A Styrene-Acrylonitrile
(SAN) Layered Silicate Nanocomposites Prepared by Melt Compounding. Polymer,
45,4051-4061
Ciprari, D., Jacob, K., Tannenbaum, R., 2006. Characterization of Polymer
Nanocomposite Interphase and Its Impact on Mechanical Properties. Macromolecules,
39,6565-6573
Clegg. W., Editor , 2001. Crystal Structure Analysis, Principle and Practice, Oxford
Science Publications. N.Y.
Comyn, J., Blackley, D.and Harding, L., 1993. Contact Angles of Liquids on Films from
Emulsion Adhesives, and Correlation with the Durability of Adhesive Bonds to
Polystyrene. Int. J. Adhes. Adhes., 13,163-171
Dai J, Huang J., 1999. Surface Modification of Clays and Clay–Rubber Composite. Appl.
Clay Sci., 15,51-65
Dazhu, C., Haiyang, Y., Pingsheng, H., Weian, Z., 2005. Rheological and Extrusion
Behavior of Intercalated High-Impact Polystyrene-Organomontmo rillonite
Nanocomposites. Composites Sci. Technol., 65,1593-1600
182
Demarquette, N., 1993. Interfacial Tension in Polymer Blends: Measurements and
Analysis. PhD Thesis. McGill University.
Demarquette, N., Kamal, M., 1994. Interfacial Tension in Polymer Melts. I: an Improved
Pendant Drop Apparatus. Polym. Eng. Sci., 34,1823-1833
Dennis, H., Hunter D., Chang, D., Kim, J., White, J., Cho, J., Paul, D., 2001. Effect of
Melt Processing Conditions on the Extent of Exfoliation in Organoclay-Based
Nanocomposites. Polymer, 42,329-336
Dharaiya, D., Jana, S., 2005. Thermal Decomposition of Alkyl Ammonium ions and its
Effects on Surface Polarity of Organically Treated Nanoclay. Polymer, 46,10139-10147
Ding, C., Guo, B., He, H., Jia, D., Hong, H., 2005. Preparation and Structure of Highly
Confined Intercalated Polystyrene/MMT Nanocomposite via a Two-Step Method. Eur.
Polym. J., 41,1781-1786
Ding, P., Qu, B., 2000. Nanocomposites via In Situ Emulsion and Suspension
Polymerization. J. Appl. Polym. Sci., 101,3758-3766
Doh, J., Cho I., 1998. Synthesis and Properties of Polystyrene-Organoammonium Hybrid.
Polym. Bull., 41,511-518
Dolgovskij, M., Lortie, F, Macosko, C. 2004. Effects of Stress on the Exfoliation of
Polystyrene Nanocomposites, ANTEC- SPE 1336-1340
Dong, Z., Liu, Z., Zhang, J., Han, B., Sun, D., Wang, Y., Huang, Y., 2004. Synthesis of
Montmorillonite/Polystyrene Nanocomposites in Supercritical Carbon Dioxide. J. Appl.
Polym. Sci., 94,1194-1197
Drelich, J., Miller, J., 1994. Examination of Newman’s Equation of State of Surface
Tensions. J. Colloid Interf. Sci., 167,217-220
Essawy, H., Badran, A., Youssef, A., El-Hakim, A., 2004. Polystyrene-Montmorillonite
Nanocomposites Prepared by In Situ Intercalative Polymerization: Influence of the
Surfactant Type. Macromol. Chem. Phys., 205,2366-2370
Fan, J., Liu, S., Chen, G., Qi, Z., 2002. SEM Study of a Polystyrene/Clay
Nanocomposite. J. Appl. Polym. Sci., 83,66-69
Fan, X., Xia, C., Advincula, R., 2003.Grafting of Polymers from Clay Nanoparticles via
In Situ Free Radical Surface-Initiated Polymerization: Monocationic vs. Bicationic
Initiators. Langmuir, 19,4381-4389
Fan, X., Xia, R., Advincula, R., 2003. Intercalation of Polymerization Initiators into
Montmorillonite Nanoparticle Platelets: Free Radical vs. Anionic Initiator Clays,
Colloids Surf. A, 219,75-86
183
Favre, H., Lagaly, G., 1991. Organo-Bentonites with Quaternary Alkylammonium Ions.
Clay Miner. 26,19-32
Fischer, H., Gielgens, H., Koster, T., 1999. Nanocomposites from Plymers and Layered
Minerals. Acta Polym., 50,122-126
Foldes, E., Szigeti-Erdei, A., 1997. Migration of Additives in Polymers. J. Vinyl Addit.
Technol., 3,220-224
Fox, T., Flory, P., 1950. Second-Order Transition Temperatures and Related Properties of
Polystyrene. I. Influence of Molecular Weight. J. Appl. Phys., 21, 581-591
Fu, J., Naguib, H., 2006. Effect of Nanoclay on the Mechanical Properties of PMMA
/Clay Nanocomposite Foams. J. Cell. Plast., 42,325-342
Fu, X., Qutubuddin, S., 2000. Synthesis of Polystyrene–Clay Nanocomposites. Mater.
Lett. 42,12–15
Fu, X., Qutubuddin, S., 2002. Polymer-Clay Nanocomposites: Exfoliation of
Organophilic Montmorillonite Nanolayers in Polystyrene. Polymer, 42,807-813
Fu, X., Qutubuddin, S., 2005. Swelling Behavior of Organoclays in Styrene and
Exfoliation in Nanocomposites. J. Colloid Interface Sci., 283,373-379
Garmabi, H., Demarquette, N., Kamal, M., 1998. Effect of Temperature and
Compatibilizer on Interfacial Tension of PE/PA-6 and PP/EVOH. Intern. Polym.
Process., 13,183-191
Gennes, P., 1985. Wetting: Statics and Dynamics. Rev. Mod. Phys., 57, 827-863.
Giannelis, E., 1998. Review: Polymer-Layered Silicate Nanocomposites: Synthesis,
Properties and Applications. Appl. Organomet. Chem., 12,675-680
Giese, R., Wu, W., van Oss, C., 1998. Surface Thermodynamic Properties of Micas and
Related Layer Silicate Mineral J. Dispersion Sci. Technol., 19,775-783
Gilman, J., 1999. Flammability and Thermal Stability of Polymer Layered-Silicate (clay)
Nanocomposites. Appl. Clay Sci., 15,31-49
Gilman, J., Awad, W., Davis, R., 2002. Polymer/Layered Silicate Nanocomposites from
Thermally Stable Trialkylidazolium-Treated Montmorillonite. Chem. Mater., 14,37763785
Gilman, J., Awad, W., Davis, R., Shields, J., Harris, R., Davis, C., Morgan, A., Sutto, T.,
Callahan, T., Trulove, P. DeLongr, H., 2002. Polymer/Layered Silicate Nanocomposites
from Thermally Stable Trialkylimidazolium-Treated Montmorillonite. Chem. Mater.,
14,3776-3785
184
Gilman, J., Jackson, C., Morgan, A., Harris, R., Manias, E., Giannelis, E., Wuthenow,
M., Hilton, D., Phillips, S., 2000. Flammability Properties of Polymer-Layered-Silicate
Nanocomposites. Polypropylene and Polystyrene Nanocomposites. Chem. Mater.,
12,1866-1873
Ginzburg, V., Singh, C., Balazs, A., 2000. Theoretical Phase Diagrams of Polymer/Clay
Composites: the Role of Grafted Organic Modifiers. Macromolecules, 23,1089-1099
Girish G., Ramesh, C., Ashish, L., 2001. A Rheological Study on the Kinetics of Hybrid
Formation in Polypropylene Nanocomposites. Macromolecules, 34,852-858
GOBI INTERNATIONAL, www.gobi.co.uk, March 2007
Govindaiah, P., Mallikarjuna, S., Ramesh, C., 2006. Preparation and Characterization of
Sulfonated
Syndiotactic
Polystyrene
Ionomers/Organoclay
Nanocomposites.
Macromolecules, 39,7199-7203
Gu, Q., Lin, Q., Hu, C., Yang, B., 2005. Study on Emulsion and Suspension in Situ
Polymerization. J. Appl. Polym. Sci., 95,404-412
Gusev, A, Lusti, H., 2001. Rational Design of Nanocomposites for Barrier Applications.
Adv. Mater., 13,1641-1643
Gyoo, P., Venkataramani, S., Kim, S., 2006. Morphology, Thermal, and Mechanical
Properties of Polyamide 66/Clay Nanocomposites with Epoxy-Modified Organoclay. J.
Appl. Polym. Sci., 101,1711-1722
Ha, Y., Kwon, Y., Breiner, T., Chan, E., Tzianetopoulou, T., Cohen, T., Óbice, M.,
Thomas, L., 2005. An Orientationally Ordered Hierarchical Exfoliated Clay-Block
Copolymer Nanocomposite. Macromolecules, 38,5170-5179
Hadjiev, V., Mitchell, C., Bahr, J., Tour, J., Krishnamoorti, R., 2005. Thermal Mismatch
Strains in Sidewall Functionalized Carbon Nanotube/Polystyrene Nanocomposites. J.
Chem. Phys., 122,124708,1-6
Han, X., Zeng, C., Lee, J., Koelling, K., Tomasko, D., 2003. Extrusion of Polystyrene
Nanocomposite Foams with Supercritical CO2. Polym. Eng. Sci., 43,1261-1275 .
Hartwig, A., Putz, D., Schartel, M., Wendschuh-Josties M., 2003. Combustion Behaviour
of Epoxide Based Nanocomposites with Ammonium and Phosphonium Bentonites.
Macromol. Chem. Phys., 204,2247–2257
Hasegawa, N., Okamoto, H., Kawasumi, M., Usuki, A., 1999. Preparation and
Mechanical Properties of Polystyrene-Clay Hybrids. J. Appl. Poly. Sci., 74,3359-3364
He, F., Zhang, L., Yang, F., Chen, L., Wu Q., 2006. New Nanocomposites Based on
Syndiotactic Polystyrene and Organo-Modified ZnAl Layered Double Hydroxide. J.
Polym. Res., 13,483-493
185
He, H., Duchet, J., Galy, J., Gérard, J-F., 2006. Influence of cationic surfactant removal
on the thermal stability of organoclays. J. Colloid Interface Sci., 295,202-208
Hedley, C., Yuan, G., Theng, B., 2007. Thermal analysis of montmorillonites modified
with quaternary phosphonium and ammonium surfactants. Appl. Clay Sci., 35,180-188
Helmy, A., Ferreiro, E., Bussetti, S., 1999. Surface Area Evaluation of Montmorillonite. J
Colloid Interface Sci., 210,167–171
Hoffmann, B., Dietricha, C., Thomann, R., Friedrich, C., Mulhaupt, R., 2000.
Morphology and Rheology of Polystyrene nanocomposites Based upon Organoclay.
Macromol. Rapid Commun., 21,57-61
Hrobarikova, J., Robert, J.-L., Calberg, C., Jerome, R., Grandjean, J., 2004. Solid-State
NMR Study of Intercalated Species in Poly(E-caprolactone)/Clay Nanocomposites.
Langmuir, 20,9828-9833
Hui, Y., Shia, D., 2001. Simple formulae for the effective moduli of directional aligned
composites. Polym. Eng. Sci., 38,774-781
Hussain, F., Hojjati, M., Okamoto, M., Gorga, R., 2006. Review Article: Polymer-Matrix
Nanocomposites, Processing, Manufacturing, and Application: an Overview. J.
Composite Mater.,. 40,1511-1575
Hwu, J., Ko, T., Yang, W., Lin, J., Jiang, G., Xie, W., 2004. Synthesis and Properties of
Polystyrene–Montmorillonite Nanocomposites by Suspension Polymerization. J. Appl.
Polym. Sci., 91,101–109
Ioan, S., Cosutchi, A., Hulubei, C., Macocinschi, D., Ioanid, G., 2007. Surface and
Interfacial Properties of Poly(amic acid)s and Polyimides. Polym. Eng. Sci., 47,381-389
Ishida, H., Campell, S., Blackwell, J., 2000. General Approach to Nanocomposites
Preparation. Chem. Mater., 12,1260-1267
Janek, M., Lagaly, G., 2003. Interaction of a Cationic Surfactant with Bentonite: a
Colloid Chemistry Study. Colloid Polym. Sci. 281,293-301
Jang, B., Costache, M., Wilkie, C., 2005. The Relationship Between Thermal
Degradation Behavior of Polymer and the Fire Retardancy of Polymer/Clay
Nanocomposites. Polymer, 46,10678-10687
Jang, B., Wang, D., Wilkie, C., 2005. Relationship between the Solubility Parameter of
Polymers and the Clay Dispersion in Polymer/Clay Nanocomposites and the Role of the
Surfactant. Macromolecules, 38,6533-6543
Jang, B., Wilkie, C., 2005. The thermal Degradation of Polystyrene Nanocomposites.
Polymer, 46.2933-2942
186
Jang, L., Kang, C., Lee, D., 2001. A New Hybrid Nanocomposite Prepared by Emulsion
Copolymerization of ABS in Presence of Clay. J. Polym. Sci. part B: Polym. Physic.,
39,719-727
Jeong, H., Choi, J., Ahn, Y., Kwon, K., 2006. Characteristics of Polystyrene/Organoclay
Nanocomposites Prepared by In-Situ Polymerization with Macroazoinitiator Containing
Poly(dimethylsiloxane) Segment. J. Appl. Polym. Sci., 99,2841–2847
Ji, X., Jing, J., Jiang. W., Jiang, B., 2002. Tensile Modulus of Polymer Nanocomposites.
Polym. Eng. Sci., 42,983-993
Ji, Y., Li, B., Ge, S., Sokolov, J, Rafailovich, M., 2006. Structure and Nanomechanical
Characterization of Electrospun PS/Clay Nanocomposite Fibers. Langmuir, 22,13211328
Jiang, L., Kim J., 2006. A New Nonhydrolytic Synthesis of Magnetite Nanocrystallites in
the Presence of Functionalized Polystyrene Matrix. J. Appl. Polym. Sci., 101,186-191
Jordan, J., Jacob, K., Tannenbaum, R., Sharaf, M., Jasiuk, I., 2005. Experimental Trends
in Polymer Nanocomposites—a Review. Mater. Sci. Eng., A 393,1–11
Kamal, M., Lai-Fook, R., Demarquette, N., 1994. Interfacial tension in polymer melts.
Part II: Effects of temperature and molecular weight on interfacial tension. Polym. Eng.
Sci., 34,1834-1839
Kamal, M., Uribe, J., 2006. Polystyrene/Clay Nanocomposites by Melt Intercalation.
ANTEC-SPE Conference proceedings 64,357-361
Kamusewitz, H., Possart, W., 2003. Wetting and Scanning Force Microscopy on Rough
Polymer Surfaces: Wenzel's Roughness Factor and the Thermodynamic Contact Angle.
Appl. Phys. A 76,899–902
Khayankarn, O., Magaraphan, R., Schwank, J., 2003. Adhesion and Permeability of
Polyimide–Clay Nanocomposite Films for Protective Coatings. J. Appl. Polym. Sci.,
89,2875-2881
Kim, K., Utracki, L., Kamal, M., 2004. Numerical Simulation of Polymer
Nanocomposites Using a Self-Consistent Mean-Field Model. J. Chem. Phys., 121,1076610777
Kim, M., Park, C., Choi, W., Lee, J., Lim, J., Park, O., Kim J., 2004. Synthesis and
Material Properties of Syndiotactic Polystyrene/Organophilic Clay Nanocomposites. J.
Appl. Polym. Sci., 92,2144–2150
Kim, S., Jo, W., Lee, M., Ko, M., Jhon, J., 2001. Preparation of Clay Dispersed
Poly(Styrene-co-Acrylonitrile) Nanocomposites using Poly(ε-Caprolactone) as a
Compatibilizer. Polymer, 42,9837-9842
187
Kim, T., Jang, L., Lee, D., Choi, H., Jhon, M., 2002. Synthesis and Rheology of
Intercalated Polystyrene/Na+-Montmorillonite Nanocomposites. Macromol. Rapid
Commun., 23,191-195
Kim, T., Lim, S., Lee, C., Choi, H., Jhon, M., 2003. Preparation and Rheological
Characterization
of
Intercalated
Polystyrene/Organophilic
Montmorillonite
Nanocomposite. J. Appl. Polym. Sci., 87,2106–2112
Kong, Q., Hu, Y., Lu. H., Chen, Z., Fa, W., 2005. Synthesis and Properties of
Polystyrene/Fe-Montmorillonite Nanocomposites using Synthetic Fe-Montmorillonite by
Bulk Polymerization. J. Mater. Sci., 40,4505-4509
Kotoky, T., Dolui, S., 2006. Synthesis of Polystyrene/Silica Hybrid Composites by the
Sol–Gel Method: Effect of Introduction of a Flexible Component (Butyl Acrylate) into
the Silylated Polystyrene Backbone. Colloid Polym. Sci., 284,1163-1169
Kovacevic, V., Leskovac, M., Blagojevic, S., Vrsaljko, D., 2005. Complex Adhesion
Effects of Inorganic Nanofillers vs Microfillers in Polymer Composites. Macromol.
Symp., 221,11-22.
Krishnamoorti, R., Vaia, R., Giannelis, E., 1996. Structure and Dynamics of PolymerLayered Silicate Nanocomposites. Chem. Mater., 8,1728-1734
Krook, M., Albeeitsson, A., Gedde, U., Hedenqvist, M., 2002. Barrier and Mechanical
Properties of Montmorillonite/Polyesteramide Nanocomposites. Polym. Eng. Sci.,
42,1238-1246
Kubilay, S., Gurban, R., Savran, A., Yalcinkaya, Z., 2006. Determination of the Surface
Properties of Untreated and Chemically Treated Kaolinites by Inverse Gas
Chromatography. Colloid J., 68,274-284
Kurian, M., Dasgupta, A., Galvin, M., Ziegler, C., Beyer, C., 2006. A Novel Route to
Inducing Disorder in Model Polymer-Layered Silicate Nanocomposites.
Kurian, M.. Dasgupta, A., Beyer, F., Galván, F., 2004. Investigation of the Effects of
Silicate Modification on Polymer-Layered Silicate Nanocomposite Morphology. J.
Polym. Sci. Part B: Polym. Phys., 42,4075-4083
Lagaly, G., 1986. Interaction of Alkylamines with Different Types of Layered
Compounds. Solid State Ionics, 22,43-51.
Lagaly, G., Ziesmer, S., 2005. Surface Modification of Bentonites. iii. Sol-Gel
Transitions of Na-Montmorillonite in the Presence of Trimethylammonium-End-Capped
Poly(ethylene oxides). Clay Miner. 40,523-536
Lape, N., Nuxoll, E., Cussler, E., 2004. Polydisperse Flakes in Barrier Films. J. Membr.
Sci., 236,29–37
188
Le Baron, P., Wang, Z., Pinnavaia, T., 1999. Polymer-layered Silicate Nanocomposites:
an Overview. Appl. Clay Sci., 15,11-29
LeBaron, P., Pinnavaia, T., 2001. Clay Nanolayer Reinforcement of a Silicone Elastomer.
Chem. Mater., 13,3760-3765
Lee, C., Kim, H., Lim, S., Kim, H., Kwon, Y., Choi, H., 2006. Ordering Behavior of
Layered Silicate Nanocomposites with a Cylindrical Triblock Copolymer. Macromol.
Chem. Phys., 207,444–455
Lee, J., Kontopoulou, M., Parent, J., 2004. Time and Shear Dependent Rheology of
Maleated Polyethylene and its Nanocomposites, Polymer, 45,6595-6600
Lee, J., Kontopoulou, M., Parent, J., 2005. Synthesis and Characterization of
Polyethylene-Based Ionomer Nanocomposites. Polymer, 46,5040-5049
Lee, S., Kim, J., 2004. Surface Modification of Clay and Its Effect on the Intercalation
Behavior of the Polymer/Clay Nanocomposites. J. Polym. Sci. Part B: Polym. Phys.,
42,2367–2372
Lee, S., Lee, H., Kim, M., Kwak, S., Park, M., Lim, S., Choe, C., Kim, J., 2002. Specific
Interaction Governing the Melt Intercalation of Clay with Poly(Styrene-co-Acrylonitrile)
Copolymer. J. Appl. Polym. Sci. part B: Polym. Physic, 39,2430-2435
Lee, Y. H., Zheng, W. G., Park, C. B., Kontopoulou, M., 2005. Effects of Clay
Dispersion on the Mechanical Properties and Flammability of Polyethylene/Clay
Nanocomposites. SPE ANTEC Tech. Papers, 63,1428-1432
Leroux, F., Meddar, L., Mailhot, B., Morlat-Thérias, S., Gardette. J., 2005.
Characterization and Photooxidative Behaviour of Nanocomposites Formed with
Polystyrene and LDHs Organo-Modified by Monomer Surfactant. Polymer, 46,35713578
Lewin, M., Mey-Marom, A., Frank1, R., 2005. Surface Free Energies of Polymeric
Materials, Additives and Minerals. Polym. Adv. Technol., 16,429-441
Li, C., Huang, C., Hsieh, M., Wei, K., 2005. Properties of Covalently Bonded LayeredSilicate/Polystyrene Nanocomposites Synthesized via Atom Transfer Radical
Polymerization. J. Polym .Sci. Part A: Polym. Chem., 43,534-542
Li, D., Newman, A., 1992. Contact Angles on Hydrophobic Solid Surfaces and their
Interpretation. J. Colloid Interf. Sci., 148, 190-200
Li, H., Yu, Y., Yang, Y., 2005. Synthesis of Exfoliated Polystyrene-Montmorillonite
Nanocomposite by Emulsion Polymerization using a Zwitterion as the Clay Modifier.
Eur. Polym. J., 41,2016-2022
189
Li, Y., Ishida, H., 2003. Solution Intercalation of Polystyrene and the Comparison with
Poly(ethyl methacrylate). Polymer, 44,6571–6577
Li, Y., Ishida, H., 2005. A Study of Morphology and Intercalation Kinetics of
Polystyrene-Organoclay Nanocomposites. Macromolecules, 38, 6513-6519
Liang, G., Xu, J., Bao, S., Xu, W., 2004. Polyethylene/Maleic Anhydride Grafted
Polyethylene/Organic-Montmorillonite Nanocomposites. I. Preparation, Microstructure,
and Mechanical Properties. J. Appl. Polym. Sci., 91,3974-3980
Lim, Y., Park, O., 2000. Rheological Evidence for the Microstructure of Intercalated
Polymer/Layered Silicate Nanocomposites. Macromol. Rapid Commun. 21,231–235
Lim, Y., Park, O., 2001. Phase Morphology and Rheological Behavior of
Polymer/Layered Silicate Nanocomposites. Rheol Acta, 40,220-229
Limpanarta, S., Khunthon, S., Taepaiboon, P., Supaphol, P., Srikhirin, T.,
Udomkichdecha, W., Boontongkong Y., 2005. Effect of the Surfactant Coverage on the
Preparation of Polystyrene–Clay Nanocomposites Prepared by Melt Intercalation. Mater.
Lett., 59,2292-2295
Litina, K., Miriouni, A., Gournis, D., Karakassides, M., Georgiou, N., Klontzas, E.,
Ntoukas, E., Avgeropoulos A., 2006. Nanocomposites of Polystyrene-b-Polyisoprene
Copolymer with Layered Silicates and Carbon Nanotubes”, Eur. Polym. J., 42,2098-2107
Liu, G., Zhang, L., Zhao, D., Qu, X., 2005. Bulk Polymerization of Styrene in the
Presence of Organomodified Montmorillonite. J. Appl. Polym. Sci., 96,1146–1152
Liu, J., Xu, Q., Peng, Q., Pang, M., He, S., Zhu, C., 2006. Supercritical CO2-Assisted
Synthesis of Polystyrene/Clay Nanocomposites via in situ Intercalative Polymerization. J.
Appl. Polym. Sci., 100,671–676
Liu, W., Guo, Z., Yu J., 2005. Preparation of Crosslinked Composite Nanoparticles. J.
Appl. Polym. Sci., 97,1538-1544
Liu, Z., Chen, K., Yan, D., 2004. Nanocomposites of Poly(trimethylene terephthalate)
with Various Organoclays: Morphology, Mechanical and Thermal Properties. Polym.
Test., 23,323–331
Loos, J., Alexeev, A., Grossiord, N., Koning, C., Regev, O., 2005. Visualization of
Single-Wall Carbon Nanotube (SWNT) Networks in Conductive Polystyrene
Nanocomposites by Charge Contrast Imaging. Ultramicroscopy, 104,160-167
López-Quintanilla, M., Sánchez-Valdés, S., Ramos de Valle, L., Guedea Miranda, R.,
2006. Preparation and Mechanical Properties of PP/PP-g-MA/Org-MMT
Nanocomposites with Different MA Content. Polymer Bull., 57,385–393
190
Ma, C., Chen, Y., Kuan, H., 2005. Polystyrene Nanocomposite Materials: Preparation,
Morphology, and Mechanical, Electrical, and Thermal Properties. J. Appl. Polym. Sci.,
98,2266-2273
Maiti, P., 2003. Influence of Miscibility on Viscoelasticity, Structure and Intercalation of
Oligo-poly(caprolactone)/Layered Silicate Nanocomposites. Langmuir, 19,5502-5510
Maiti, P., Yamada, K., Okamoto, M., Ueda, K., Okamoto, K., 2002. New
Polylactide/Layered Silicate Nanocomposites: Role of Organoclays. Chem. Mater.,
14,4654-4661
Maiti, S., Singh, K., 1986. Influence of Wood Flour on the Mechanical Properties of
Polyethylene. J. Appl. Polym. Sci., 32,4285-4289
Martos, C., Rubio, F., Rubio, J., Oteo, J., 2001. Surface Energy of Silica-TEOS-PDMS
Ormosils. J. Sol-Gel Sci. Technol., 20,197-210
Médout-Marère, J., 2000. A Simple Experimental Way of Measuring the Hamaker
Constant A11 of Divided Solids by Immersion Calorimetry in Apolar Liquids. V. Colloid
Interface Sci., 228, 434-4
Minnikanti, V., Archer, L., 2006. Entropic Attraction of Polymers toward Surfaces and
Its Relationship to Surface Tension Macromolecules 39,7718-7728
Mitsuya, M., Taniguchi, Y., Akagi, M., 1983. The Structure Of Vacuum-Deposited Zinc
Stearate Film. J. Colloid Interface Sci., 92,291-2
Moet, A., Akelah, A., 1993. Polymer-Clay Nanocomposites: Polystyrene Grafted onto
Montmorillonite Interlayers. Mater. Lett., 18,97-102
Mollet, V., Kamal, M., 2006. Quantitative Characterization of Exfoliation in Polyamide6/Layered Silicate Nanocomposites. J. Polym. Eng., 26,757-782.
Moreira, J., Demarquette, N., 2001. Influence of Temperature, Molecular weight, and
Molecular Weight Dispersity on the Surface Tension of PS, PP and PE. I. Experimental J.
Appl. Polym. Sci., 82,1907-1920
Morgan, A., Chu, L., Harris J., 2005. A flammability Performance Comparison between
Synthetic and Natural Clays in Polystyrene Nanocomposites. Fire Mater., 29,213–229
Morgan, A., Harris J., 2004. Exfoliated Polystyrene-Clay Nanocomposites Synthesized
by Solvent Blending with Sonication. Polymer, 45,8695-8703
Morgan, A., Harris, R., Kashiwagi, T., Chyall, L., Gilman, J., 2002. Flammability of
Polystyrene Layered Silicate (Clay) Nanocomposites: Carbonaceous Char Formation.
Fire Mater., 26,247–253
Murray, H., 2000. Traditional and New Applications for Kaolin Smectite, and
Palygorskite: a General Overview. Appl. Clay Sci., 17,207-221
191
Nakae, H., INRI, R., Hirata, Y., Saito, H., 1998. Effect of Surface Roughness on
Wettability. Acta Mater., 46,2313-2318
Nassar, N., 2003. Melt Exfoliation in Montmorillonite/Polystyrene Nanocomposites. M.
Eng. Thesis. McGill University
Nassar, N., Utracki, L., Kamal, M., 2005. Melt Intercalation in MontmorillonitePolystyrene Nanocomposites. Int. Polym. Process., 20,423-431
Neumann, A., Li, D., 1990. A Reformulation of the Equation of State for Interfacial
Tensions. J. Colloid Interf. Sci., 137,304-307
Neumann, A., Omenyi, S., van Oss, C., 1979. Negative Hamaker coefficients I. Particle
engulfment or rejection at solidification fronts. Colloid Polym. Sci., 257, 413-419
Nguyen, T., Schwartz, B., Schaller, R., Johnson, J., Lee, L., Haber, L., Saykally, R.,
2001. Near-Field Scanning Optical Microscopy (NSOM) Studies of the Relationship
between Interchain Interactions, Morphology, Photodamage, and Energy Transport in
Conjugated Polymer Films. J. Phys. Chem. B 105,5153-5160
Nielsen, L. E.: Mechanical Properties of Polymers, Reinhold, New York (1962).
Noh, M., Dong ,C., 1999. Comparison of Characteristics of SAN-MMT Nanocomposites
Prepared by Emulsion and Solution Polymerization. J. Appl. Polym. Sci., 74,2811-2819
Noh, M., Jang, L., Kang, C., Lee, D., 1999. Intercalation of Styrene-Acrylonitrile
Copolymer in Layered Silicate by Emulsion Polymerization. J. Appl. Polym. Sci.,
74,179-188
Noh, M., Lee, D., 1999. Synthesis and Characterization of PS-Clay Nanocomposite by
Emulsion Polymerization. Polym. Bull., 42,619-626
Norris, J., Giese, R., van Oss, C., Costazo, P., 1992. Hydrophobic nature of organoclays
as a Lewis Acid/base phenomenon. Clays Clay Miner., 40,327-334
Okamoto, K., Ray, S., Okamoto, M., 2003. New Poly(Butylene Succinate)/Layered
Silicate Nanocomposites. II. Effect of Organically Modified Layered Silicates on
Structure, Properties, Melt Rheology, and Biodegradability. J. Polym. Sci. Part B: Polym.
Phys., 41,3160–3172
Okamoto, M., Morita, S., Kotaka, T., 2001. Dispersed Structure and Ionic Conductivity
of Smectic Clay/Polymer Nanocomposites. Polymer, 42,2685–2688
Okamoto, M., Morita, S., Taguchi, H., Kim, Y., Kotaka, T., Tateyama, H., 2000.
Synthesis and Structure of Smectic Clay/Poly(methyl methacrylate) and Clay/Polystyrene
Nanocomposites via in situ Intercalative Polymerization. Polymer, 1,3887-3890
192
Osman, M., Mittal, V., Morbidelli, M., Suter, U., 2004. Epoxy-Layered Silicate
Nanocomposites and Their Gas Permeation Properties. Macromolecules, 37,7250-7257
Osman, M., Rupp, J., Suter, U., 2005. Gas Permeation Properties of PolyethyleneLayered Silicate Nanocomposites. J. Mater. Chem., 15,1298-1304
Owens, D., 1964. Friction of Polymer Films. I. Lubrication. J. Appl. Polym. Sci. 8,14651475
Owens, D., 1970. Friction of Polymer Films. III. Migration of Lubricants. J. Appl.
Polym. Sci. 14,185-192
Park, C., Park, O., Lim, J., Kim, H., 2001. Fabrication of Sindiotactic PolystyreneOrganophilic Clay Nanocomposites and their Properties. Polymer, 42,7465-7475
Ponsonnet, L., Reybier, K., Jaffrezi, N., Comte, V., Lagneau, C., Lissac, M., Martelet, C.,
2003. Relationship between Surface Properties (Roughness, Wettability) of Titanium and
Titanium Alloys and Cell Behaviour. Mater. Sci. Eng. C 23,551–560
Porter, D., Metcalfe, E., Thomas, J., 2000. Nanocomposites Fire Retardants- a Review.
Fire Mater., 24,45-52
Price, G, Ansari, D., 2003. An Inverse Gas Chromatography Study of Calcination and
Surface Modification of Kaolinite Clays. Phys. Chem. Chem. Phys., 5,5552-5557
Qi, R., Jin, X., Nie, J., Yu, W., Zhou, C., 2005. Synthesis and Properties of Polystyrene–
Clay Nanocomposites via In Situ Intercalative Polymerization. J. Appl. Polym. Sci.,
97,201–207
Qiu, L., Qu, B., 2006. Preparation and Characterization of Surfactant-Free
Polystyrene/Layered Double Hydroxide Exfoliated Nanocomposite via Soap-Free
Emulsion Polymerization. J. Colloid Interface Sci., 301,347–351
Qutubuddin, Q., Fu, X., Tajaddin, Y., 2002. Synthesis of Polystyrene-Clay
Nanocomposites via Emulsion Polymerization Using a Reactive Surfactant. Polym. Bull.,
48,143-149
Ragosta, G., Abbate, M., Musto, P., Scarinzi, G., Mascia, L., 2005. Epoxy-Silica
Particulate Nanocomposites: Chemical Interactions, Reinforcement and Fracture
Toughness. Polymer 46,10506-10516
Ray, S., Okamoto, K., Okamoto, M., 2003. Structure-Property Relationship in
Biodegradable
Poly(butylenes
succinate)/Layered
Silicate
Nanocomposites.
Ray, S., Yamada, K., Okamoto, M., Fujimoto, Y., Ogami, A., Ueda, K., 2006. New
Polylactide/Layered Silicate Nanocomposites. 5. Designing of Materials with Desired
Properties. Polymer, 44,6633-6646
193
Ren, J., Silva, A., Krishnamoorti R., 2000. Linear Viscoelasticity of Disordered
Polystyrene-Polyisoprene Block Copolymer Based Layered-Silicate Nanocomposite.
Ren, J., Silva, A., Krishnamoorti R., 2001. Shear Response of Layered Silicate
Nanocomposites. J. Chem. Phys., 114,4968-4973
Rogers, K., Takacs, E. Thompson, M., 2005. Contact Angle Measurement of Selected
Compatibilizers for Polymer-Silicate Layer Nanocomposites. Polym. Test., 24,423-427
Rong, Y., Chen, H., Wu, G., Wang M., 2005. Preparation and Characterization of
Titanium Dioxide Nanoparticle/Polystyrene Composites via Radical Polymerization.
Mater. Chem. Phys., 91,370–374
Ryul, J., Lee, P., Kim, H., Lee J., 2001. Development of PP-based Nanocomposites via
in-situ Copolymerization and Melt Intercalation with the Power Ultrasonic Wave. Annu.
Tech. Conf. Soc. Plast. Eng., 59,2135-2139
Saito, R., Kobayashi, S., Hosoya T., 2005. Effects of Hydroxyl Groups and Architecture
of Organic Polymers on Polystyrene/Silica Nanocomposites. J. Appl. Polym. Sci.,
97,1835-1847
Sakane, Y., Inomata, K., Morita, H., Kawakatsu, T., Doi, M., Nose, T., 2001. Effects of
Low-Molecular-Weight Additives on Interfacial Tension of Polymer Blends:
Experiments for Poly(dimethylsiloxane) /Poly(tetramethyldi siloxanylethylene) +
Oligo(dimethylsiloxane), and Comparison with Mean-Field Calculations. Polymer,
42,3883-3891
Salaniwal, S., Kumar, S., Douglas, J., 2002. Amorphous Solidification in PolymerPlatelet Nanocomposites. Phys. Review lett. 89,258301
Sauer, B., Dee, G., 2002. Surface Tension and Melt Cohesive Energy Density of Polymer
Melts Including High Melting and High Glass Transition Polymers Macromolecules,
35,7024-7030
Schleidt, G., Wolkenhauer, Q., Spiess, H., Jeschke G., 2006. Heterogeneity of the
Surfactant Layer in Organically Modified Silicates and Polymer/Layered Silicate
Composites. Macromolecules, 39,2191-2200
Sepehr, M., Utracki, L., Zheng, X., Wilkie, C., 2005. Polystyrenes with Macrointercalated Organoclay. Part II. Rheology and Mechanical Performance. Polymer,
46,11569-11581
Shang, W., Williams, J., Soderholm, K., 1994. How the Work of Adhesion Affects the
Mechanical Properties of Silica-Filled Polymer Composites. J. Mater. Sci., 29,2406-2416
Shen, J., Cao, X., Lee, L., 2006. Synthesis and Foaming of Water Expandable
Polystyrene Clay Nanocomposites. Polymer, 47,6303-6310
194
Shen, Z., Zhu, F., Zeng, D., Lin, S., 2005. Preparation of Syndiotactic
Polystyrene/Montmorillonite Nanocomposites via In Situ Intercalative Polymerization of
Styrene with Monotitanocene Catalyst. J. Appl. Polym. Sci., 95,1412–1417
Shia, D, Hui, Y., 1998. An Interface Model for the Prediction of Young’s Modulus of
Layered Silicate-Elastomer Nanocomposites. Polym. Composites, 19,608-617
Shimizu, R., Demarquette, N., 2000. Evaluation of Surface Energy of Solid Polymers
Using Different Models. J Appl. Polym. Sci., 76,1831-1845
Sikka, M., Cerini, M., Ghosh, S., Winey, K., 1996. Melt Intercalation of Polystyrene in
Layered Silicates. J. Polym. Sci. Part B: Polym. Phys., 34,1443-1449
Singh, C., Balazs, A., 2000. Effect of Polymer Architecture on the Miscibility of
Polymer/Clay Mixtures. Polym. Inter., 49,469-471
Smedberg, A., Hjertberg, T., Gustafsson, B., 2003. Effect of Molecular Structure and
Topology on Network Formation in Peroxide Crosslinked Polyethylene. Polymer, 44,
3395–3405
Sohn, J., Lee, C., Lim, S., Kim, T., Choi, H., Jhon. M., 2003. Viscoelasticity and
Relaxation Characteristics of Polystyrene/Clay Nanocomposite. J. Mater. Sci., 38,18491852
Spelt J, Vargha-Butler E. Contact Angle and Liquid Surface tension Measurments:
general Prodecures abd Techniques. In: Neumann A, Spelt J, editors. Applied Surface
Thermodynamics. New York: M. Dekker, 1996. pp 379-412
Steinert, V., Ratzsch, M., 1998. Investigations on The Decomposition of Lateral Perester
Groups in Maleic Anhydride Copolymers, Makromol. Chem., 190,2325-2328
Stoeffler, K., Lafleur, P., Denault, J., 2006. ANTEC-SPE Conference Proceedings
64,263-267
Su, S., Jiang, D., Wilkie, C., 2004. Novel Polymerically-Modified Clays Permit the
Preparation of Intercalated and Exfoliated Nanocomposites of Styrene and Its
Copolymers by Melt Blending. Polym. Degrad. Stab., 83,333-346
Su, S., Jiang, D., Wilkie, C., 2004. Polybutadiene-Modified Clay and Its Polystyrene
Nanocomposites. J. Vinyl Addit. Technol., 10,44-51
Su, S., Wilkie, C., 2003. Exfoliated Poly(Methyl Methacrylate) and Polystyrene
Nanocomposites Occur When the Clay Cation Contains a Vinyl Monomer. J. Polym.
Sci.: Part A: Polym. Chem., 41,1124–1135
Suchocka-Gałas, K., Kowalonek, J., 2006. The Surface Properties of Ionomers Based on
Styrene-co-Acrylic Acid Copolymers. Surf. Sci., 600,1134-1139
195
Sukpirom, N., Lerner. M., 2003. Rapid Syntheses of Nanocomposites with Layered
Tetratitanate Using Ultrasound. Mater. Sci. Eng. A 354,180-187
Tanaka, Y., Okada, T., Ogawa, M., 2006. Preparation and Properties of trans-2-butene1,4-bis (triphenylphosphonium)-saponite. J. Porous Mater. 13,157–161
Tanoue, S., Hasook, A., Itoh, T, Yanou, M., Iemoto, Y., Unryu, T., 2006. Effect of Screw
Rotation Speed on the Properties of Polystyrene/Organoclay Nanocomposites Prepared
by a Twin-Screw Extruder. J. Appl. Polym. Sci., 101,1165–1173
Tanoue, S., Utracki, L., Garcia-Rejon, A., Tatibouët, J., Cole, K., Kamal, M., 2004. Melt
Compounding of Different Grades Of Polystyrene with Organoclay. Part 1:
Compounding and Characterization. Polym. Eng. Sci. 44,1046–1060
Tanoue, S., Utracki, L., Garcia-Rejon, A., Tatibouet, J., Kamal, M., 2005. Melt
Compounding of Different Grades Polystyrene with Organoclay. Part 3: Mechanical
Properties. Polym. Eng. Sci., 45,827–837
Thostenson, E., Li, C., Chou, T., 2005. Nanocomposites in Context. Comp. Sci. Technol.,
65,491–516
Tokihisa, M., Yakemoto, K., Sakai, T., Utracki, L., Sepehr, M., Li, J., Simard, Y., 2006.
Extensional Flow Mixer for Polymer Nanocomposites. Polym. Eng. Sci., 46,1040–1050,
2006.
Tong, Z., Deng, Y., 2006. Synthesis of Water-Based Polystyrene-Nanoclay Composite
Suspension via Miniemulsion Polymerization. Ind. Eng. Chem. Res., 45,2641-2645
Torok, B., Bartok, M., Dekany, I., 1999.The Structure of Chiral Phenyl-Ethylammonium
MMT in Ethanol-Tolune Mixture. Colloid Polym. Sc., 277, 340-346
Torre, L., Lelli, G., Kenny J., 2006. Synthesis and Characterization
sPS/Montmorillonite Nanocomposites. J. Appl. Polym. Sci., 100,4957-4963
of
Tseng, C., Lee, H., Chang, F., 2001. Crystallization Kinetics and Crystallization Behavior
of Syndiotactic Polystyrene/Clay Nanocomposites. J. Polym. Sci. Part B: Polym. Phys.,
39,2097-2107
Tseng, C., Wu, J., Lee, H., Chang, F., 2001. Preparation and Crystallization Behavior of
Syndiotactic Polystyrene-Clay Nanocomposites. Polymer, 42,10063-10070
Tseng, C., Wu, J., Lee, H., Chang, F., 2002. Preparation and Characterization of
Polystyrene-Clay Nanocomposites by Free Radical Polymerization. J. Appl. Polym. Sci.,
85,1370-1377
Tseng, C., Wu, S., Wu, J., Chang, F., 2002. Crystallization Behavior of Syndiotactic
Polystyrene Nanocomposites for Melt- and Cold-Crystallizations. J. Appl. Polym. Sci.,
86,2492–2501
196
Uhl, F., Wilkie, C., 2002. Polystyrene/Graphite Nanocomposites: Effect on the Thermal
Stability. Polym. Degrad. Stab., 76,111-122
Uribe Calderon, J., 2003. Melt Intercalation and/or Exfoliation of PolystyreneMontmorillonite Nanocomposites. Master Thesis. McGill University.
Uribe, J, Lennox, B, Kamal, M., 2007. Influence of Interfacial Tension on Dispersion and
Flexural Properties of Ps/Phosphonium Organoclay Nanocomposites. SPE ANTEC Tech.
Papers, 85-89
Uribe, J., Lennox, B., Kamal, M., 2007. Influence of Surface Energy on Dispersion and
Flexural Properties of PS/Phosphonium Organolcay Nanocomposites. SPE ANTEC Tech.
Papers, 65,85-89
Uthirakumar, P., Song, M., Nah, C., Lee, Y., 2005. Preparation and Characterization of
Exfoliated Polystyrene/Clay Nanocomposites Using a Cationic Radical Initiator-MMT
Hybrid. Eur. Polym. J., 41,211–217
Utracki L.A., 2004. Clay-Containing Polymeric Nanocomposites. Shrewsbury. Rapra
Technology Limited.
Utracki, L.A. editor, 1989. Multiphase polymers: Blends and ionomers. Washington,
ACS
Vaia, R., Giannelis, E., 1997. Lattice Model of Polymer Melt Intercalation in
Organically-Modified Layered Silicates. Macromolecules, 30,8000-8009
Vaia, R., Giannelis, E., 1997. Polymer Melt Intercalaltion in Organically-Modified
Layered Silicates: Model Predictions and Experiment. Macromolecules 30,7990-7999
Vaia, R., Ishii, H., Giannelis, E., 1993. Synthesis and Properties of Two-Dimensional
Nanostructures by Direct Intercalation of Polymer Melts in Layered Silicates. Chem.
Mater., 5,1694-1696
Vaia, R., Jandt, K., Kramer, E., Giannelis, E., 1995. Kinetics of Polymer Melt
Intercalation. Macromolecules, 28,8080-8085
Vaia, R., Jandt, K., Kramer, E., Giannelis, E., 1996. Microstructural Evolution of Melt
Intercalated Polymer-Organically Modified Layered Silicates Nanocomposites. Chem.
Mater., 8,2628-2635
Vaia, R., Teukolsky, R., Giannelis, E., 1994. Interlayer Structure and Molecular
Environment of Alkylammonium Layered Silicates. Chem. Mater., 6,1017-1022
Vyazovkin, S., Dranca, I., Fan, X., Advincula, R., 2004. Kinetics of the Thermal and
Thermo-Oxidative Degradation of a Polystyrene–Clay Nanocomposite. Macromol. Rapid
Commun., 25,498–503
197
Wang, C., Chen, X., 2005. Intercalated PS/Na-MMT Nanocomposites Prepared by
Ultrasonically Initiated in situ Emulsion Polymerization. Macromol. Mater. Eng., 290,
920–926
Wang, D., Echols, K., Wilkie, C., 2005. Cone Calorimetric and Thermogravimetric
Analysis Evaluation of Halogen-Containing Polymer Nanocomposites. Fire Mater.,
29,283-294
Wang, D., Wilkie, C., 2003. A stibonium-Modified Clay and its Polystyrene
Nanocomposite. Polym. Degrad. Stab., 82,309-315
Wang, D., Zhu, J., Yao, Q., Wilkie, C., 2002. A Comparison of Various Methods for
Preparation of Polystyrene and Poly(Methyl Methacrylate) Clay Nanocomposites. Chem.
Mater., 14,3837-3843
Wang, H., Chang, K., Chu, H., 2005. Effect of Clay on the Properties of Poly(Styrene-coAcrylonitrile)–Clay Nanocomposites. Polym. Int., 54,114-119
Wang, J., Hu, Y., Wang, S., Chen, Z., 2005. Sonochemical One-Directional Growth of
Montmorillonite–Polystyrene Nanocomposite. Ultrason. Sonochem., 12,165–168
Wang, L., Huang, T., Kamal, M., Rey, A., Teh, J., 2000. Surface Topography and Gloss
of Polyolefibe Blown Films. Polym. Eng. Sci., 40,747-760
Wang, X., Lin, J., Wang, X., Wang, Z., 2004. A Novel Modification on Melt
Intercalation via Exothermal MMT. J. Appl. Polym. Sci., 93,2230–2236
Wang, Z., Chung, T., Gilman, J., Manias, E., 2003. Melt-Processable Syndiotactic
Polystyrene/Montmorillonite Nanocomposites. J. Polym. Sci. Part B: Polym. Phys.,
41,3173–3187
Wang, Z., Xie, G., Wang, X., Zhang, Z., 2006. Rheology and Dispersion Behavior of
High-Impact Polystyrene/Ethylene–Vinyl Acetate Copolymer/TiO2 Nanocomposites. J.
Appl. Polym. Sci., 100,4434-4438
Wu, H., Tseng, C., Chang, F., 2001. Chain Conformation and Crystallization Behavior of
the Syndiotactic Polystyrene Nanocomposites Studied using Fourier Transform Infrared
Analysis. Macromolecules, 34,2992-2999
Wu, T., Hsu, S., Chien, C., 2004. Isothermal and Nonisothermal Crystallization Kinetics
of Syndiotactic Polystyrene/Clay Nanocomposites. J. Polym. Eng. Sci., 44,2288–2297
Wu, T., Hsu, S., Wu, J., 2002. Polymorphic Behavior in Syndiotactic Polystyrene/Clay
Nanocomposites. J. Appl. Polym. Sci. part B: Polym. Phys., 40,736-746
Xi, F., Zhou, Q., Frost, R., He, H., 2007. Thermal Stability of Octadecyltrimethyl
Ammonium Bromide Modified Montmorillonite Organoclay. J. Colloid Interface Sci.,
311, 347–353
198
Xi, Y., Frost, R., He, H., Kloprogge, T., Bostrom, T., 2005. Modification of Wyoming
Montmorillonite Surfaces Using a Cationic Surfactant. Langmuir,21,8675-8680
Xie, W., Gao, Z., Pan, W., Hunter, D., Singh, A., Vaia, R., 2001. Thermal Degradation
Chemistry of Alkyl Quaternary Ammonium MMT. Chem. Mater., 13,2979-2990
Xie, W., Hwu, J., Jiang, G., Buthelezi, T., Pan, W., 2003. A Study of the Effect of
Surfactants on the Properties of Polystyrene-Montmorillonite Nanocomposites. Polym.
Eng. Sci., 43,214-222
Xie, W., Xie, R., Pan, W., Hunter, D., Koene, B., Tan, L., Vaia, R., 2002. Thermal
Stability of Quaternary Phosphonium Modified Montmorillonites. Chem. Mater.,
14,4837-4845
Xinping, Z., Sirong, Y., Zhenming, H., Yaoxin, M., 2004. Wetting of Rough Surfaces.
Surf. Rev. Lett., 11,7-13
Xu, L., Reeder, S., Thopasridharan, M., Ren, J., Shipp, D., Krishnamoorti, R., 2005.
Structure and Melt Rheology Polystyrene-Based Layered Silicate Nanocomposites.
Nanotechnology, 16,514–S521
Xu, Y., Higgins, B., Brittain, W., 2005. Bottom-up Synthesis of PS–CNF
Nanocomposites. Polymer, 46799-810
Yamaguchi, T., Yamada E., 2006. Preparation and Mechanical Properties of
Clay/Polystyrene-block-Polybutadieneblock-Polystyrene Triblock Copolymer (SBS)
Intercalated Nanocomposites using Organoclay Containing Stearic acid. Polym. Int.,
55,662-667
Yan, C., Ma, L., Yang, J., 2005. Preparation of Polystyrene/Montmorillonite
Nanocomposites in Supercritical Carbon Dioxide. J. Appl. Polym. Sci., 98,22–28
Yang, F., Nelson G., 2006. Polymer/Silica Nanocomposites Prepared via Extrusion.
Polym. Adv. Technol., 17,320-326
Yano, K., Usuki, A., Okada, A., 1997. Synthesis and Properties of Polyimide-Clay
Hybrid Films, J. Polym. Sci. A: Polym.Chem. 35,2289-2294.
Yao, H., Zhu, J., Morgan, A., Wilkie, C., 2002. Crown Ether-Modified Clays and Their
Polystyrene Nanocomposites. J. Appl. Polym. Sci., 86,2492-2501
Yei, D., Kuo, S., Fu, H., Chang, F., 2005. Enhanced Thermal Properties of PS
Nanocomposites Formed from MMT Treated with a Surfactant/Cyclodextrin Inclusion
Complex. Polymer, 46,741-750
Yei, D., Kuo, S., Su, Y., Chang, F., 2004. Enhanced Thermal Properties of PS
Nanocomposites Formed from Inorganic POSS-Treated Montmorillonite. Polymer,
45,2633–2640
199
Yilmazer, U., Ozden, G., 2006. Polystyrene–Organoclay Nanocomposites Prepared by
Melt Intercalation, In Situ, and Masterbatch Methods. Polym. Comps., 27,249-255
Yoon, J., Jo, W., Lee, M., Ko M., 2000. Effect of Comonomers and Shear on the Melt
Intercalation of Styrenics/Clay Nanocomposites. Polymer, 42,329-336
Yurekli, K., Karim, A., Amis, E., Krishnamoorti, R., 2004. Phase Behavior of PS-PVME
Nanocomposites. Macromolecules, 37,507-515
Zeliazkow, M., 2001. Thermal Degradation of Copolymers of Styrene with Dicarboxylic
Acids Alternating Styrene-Maleic Acid Copolymer. Polym. Degrad. Stab., 74,579-584
Zeng, C., Lee, J., 2001. Poly(methyl methacrylate) and Polystyrene/Clay
Nanocomposites Prepared by in-Situ Polymerization. Macromolecules, 34,4098-4103
Zha, W., Choi, S., Lee, K., Han, C., 2005. Dispersion Characteristics of Organoclay in
Nanocomposites Based on End-Functionalized Homopolymer and Block Copolymer.
Zhang, J., 2005. Study on the Flammability of HIPS–Montmorillonite Nanocomposites
Prepared by Static Melt Intercalation. J. Fire Sci., 23,193-208
Zhang, J., Jiang, D., Wang, D., Wilkie, C., 2005. Mechanical and Fire Properties of
Styrenic Polymer Nanocomposites Based on an Oligomerically-Modified Clay. Polym.
Adv. Technol., 16,800–806
Zhang, J., Jiang, D., Wang, D., Wilkie, C., 2006. Styrenic Polymer Nanocomposites
based on an Oligomerically-Modified Clay with high Inorganic Content. Polym. Degrad.
Stab., 91,2665-2674
Zhang, J., Wilkie, C., 2004. A Carbocation Substituted Clay and its Styrene
Nanocomposite. Polym. Degrad. Stab., 83,301-307
Zhang, W., Chen, D., Xu, H., Shen, X., Fang, Y., 2003. Influence of Four Different
Types of Organophilic Clay on the Morphology and Thermal Properties of
Polystyrene/Clay Nanocomposites Prepared by using the γ-Ray Irradiation Technique.
Eur. Polym. J., 39,2323–2328
Zhang, W., Liu, L., Fang, Y., 2004. A Novel Property of Styrene–Butadiene–
Styrene/Clay Nanocomposites: Radiation Resistance. J . Mater. Chem., 14,209–213
Zhang, W., Shen, X., Lie, M., Fang, Y., 2003. Synthesis and Thermal Properties of
Polystyrene/Montmorillonite Nanocomposites by γ-Ray Radiation Polymerization. J.
Appl. Polym. Sci., 90,1692–1696
Zhang, W., Zeng, J., Liu, L., Fang, Y., 2004. A Novel Property of Styrene–Butadiene–
Styrene/Clay Nanocomposites: Radiation Resistance. J. Mater. Chem., 14,209-213
200
Zhao, H., Argoti, S., Farrell, B., Shipp, D., 2004. Polymer–Silicate Nanocomposites
Produced by in Situ Atom Transfer Radical Polymerization. J. Polym. Sci. Part A: Polym.
Chem., 42,916–924
Zhao, J., Morgan, A., Harris, J., 2005. Rheological Characterization of Polystyrene–Clay
Nanocomposites to Compare the Degree of exfoliation and Dispersion. Polymer,
46,8641-8660
Zhao, Q., Samulski, E., 2006. A Comparative Study of Poly(methyl methacrylate) and
Polystyrene/Clay Nanocomposites Prepared in Supercritical Carbon Dioxide. Polymer,
47,663-671
Zheng, X., Jiang, D., Wilkie, C., 2006. Polystyrene Nanocomposites based on an
Oligomerically-Modified Clay Containing Maleic Anhydride. Polym. Degrad. Stab.,
91,108-113
Zheng, X., Wilkie C., 2003. Flame Rretardancy of Polystyrene Nnanocomposites Bbased
on an Oligomeric Organically-Modified Clay Containing Phosphate. Polym. Degrad.
Stab., 81,539-550
Zheng, X., Wilkie, C., 2003. Nanocomposites based on poly(E-caprolactone) (PCL)/Clay
Hybrid: Polystyrene, High Impact Polystyrene, ABS, Polypropylene and Polyethylene.
Polym. Degrad. Stab. 82,441-450
Zhong, Y., Zhu, Z., Wang, S., 2005. Synthesis and Rheological Properties of
Polystyrene/Layered Silicate Nanocomposite. Polymer, 46,3006-3013
Zhu, J., Morgan, A., Lamelas, F., Wilkie, C., 2001. Fire Properties of Polystyrene-Clay
Nanocomposites. Chem. Mater., 13,3774-3780
Zhu, J., Uhl, F., Morgan, A., Wilkie, C., 2001. Studies on the Mechanism by Which the
Formation of Nanocomposites Enhances Thermal Stability. Chem. Mater., 13,4649-4654
Zhu, J., Wilkie, C., 2000. Thermal and Fire Studies on Polystyrene-Clay
Nanocomposites. Polym. Int., 49,1159-1163
Zhulina, E., Singh, C., Balazs, A., 1999. Attraction between Surfaces in Polymer Melt
Containing Telechelic Chains: Guidelines for Controlling the Surface Separation in
Itercalated Polymer-Clay Composites. Langmuir, 15,3935-3943
201
Appendix A
Phosphoinium surfactants
202
203
Polymer resins
204
205
206
207
Clays
CLOSITE 10-A: Typical Physical Properties Bulletin.
Description: Cloisite 10-A is a natural Montmorillonite modified with a quaternary
ammonium salt.
Designed Use:
Cloisite 10-A is an additive for plastics to improve various plastic physical properties,
such as reinforcement, HDT, CLTE and barrier.
Typical Properties:
Treatment/Properties Organic
Modifier
%
% Weight Loss
Modifier
concentration
Moisture
on Ignition
Cloisite 10-A
2MBHT
125 meq/100 g < 2%
39 %
clay
CH3
CH3 N
CH2
HT
Where HT is Hydrigenated Tallow (~ 65% C18: ~30% C16; ~5% C14)
Anion: Chloride
2MBHT: dimethyl, benzyl, hydrogenatedtallow, quaternary ammonium.
Typical Dry Sizes: (microns, by volume)
10 % Less than 50 % Less than 90 % Less than
2μ
6μ
13 μ
Color: Off White.
Density:
Lose Bulk, lb/ ft3 Packed Bulk, lb/ ft3
Specific Gravity, g/cc
10.21
1.9
16.52
X-Ray Results: d 001= 19.2 Å
208
CLOSITE 15-A: Typical Physical Properties Bulletin.
Description: Cloisite 15-A is a natural Montmorillonite modified with a quaternary
ammonium salt.
Designed Use:
Cloisite 15-A is an additive for plastics to improve various plastic physical properties,
Typical Properties:
Modifier
%
% Weight Loss
Modifier
concentration
Moisture on Ignition
Cloisite 15-A
2M2HT
125 meq/100 g < 2%
43 %
clay
CH3
CH3 N
HT
HT
Where HT is Hydrigenated Tallow (~ 65% C18: ~30% C16; ~5% C14)
Anion: Chloride
2M2HT: dimethyl, dihydrogenatedtallow, quaternary ammonium.
2μ
6μ
13 μ
Color: Off White.
Density:
Lose Bulk, lb/ ft3 Packed Bulk, lb/ ft3
10.79
1.66
18.64
209
CLOSITE NA+: Typical Physical Properties Bulletin.
Description: Cloisite NA+ is a natural Montmorillonite.
Designed Use:
Cloisite Na+ is an additive for plastics to improve various plastic physical properties,
Typical Properties:
Cation
%
% Weight Loss
Modifier
Exchange
Moisture
on Ignition
Capacity
Cloisite Na+
None
92.6 meq/100 g
< 2%
7%
clay
2μ
6μ
13 μ
Color: Off White.
Density:
Lose Bulk, lb/ ft3
Packed Bulk, lb/ ft3
12.45
20.95
2.86
210
SW 25/1/2 LI
250
ZB 15/2/10 LI
SW 7.5/0.5/2 RI
Blister 7.5/24.5
Zone 3
Vacuum vent
400
450
Zone 4
500
550
600
650
SW 25/1/2 LI
ZB 37.5/5/10 L
700
SW 25/1/2 LI
KB 37.5/5/45 LI
SE 25/1/2 LI
SE 25/1/2 LI
KB 37.5/5/90 NI
KB 37.5/5/90 NI
Blister 7.5/24.5
SE 25/1/2 LI
SE 37.5/1/2 LI
SW 25/1/2 LI
SE 37.5/1/2 LI
SW 37.5/1/2 LI
SW 25/1/2 LI
SW 37.5/1/2 LI
SW 37.5/1/2 LI
SE 37.5/1/2 LI
Zone 2
SE 37.5/1/2 LI
350
SW 25/1/2 LI
300
ZB 37.5/5/10 LI
Blister 7.5/24.5
KB 37.5/5/45 RI
Zone 1
SW 25/1/2 LI
200
KB 37.5/5/90 NI
150
SW 25/1/2 LI
Feeding zone
KB 37.5/5/45 LI
100
SE 25/1/2 LI
0 mm 50
SE 25/1/2 LI
SE 37.5/1/2 LI
SE 37.5/1/2 LI
SE 25/1/2 LI
Appendix B
Screw configuration
SW 37.5/5/45 RI
Zone 5
750
211
Appendix C
Slit die sketch
Taken from Lohfink, G. Ph. Thesis: Morphology and Permeability in Extruded PP/EVOH
Blends. McGill University, 1994
212

Clay modification for the production of polystyrene nanocomposites

Transcription

Documents pareils

Saturday October 12th 2013

Sculptris - Ramsey County Libraries

Study of heat sealing of polymer multilayers Study of heat

Article - Max Planck Institut für Festkörperforschung

Communiqué de Presse AdVestigo

English

domaine du salvard - Kermit Lynch Wine Merchant

Vacuum Casting to Manufacture a Plastic Biochip for Highly