Clay modification for the production of polystyrene nanocomposites
Transcription
Clay modification for the production of polystyrene nanocomposites
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, χand φare 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 Figure 8.5. TEM pictures of PS1220/ Dylark 10 % with phosphonium organoclays (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 Alkyl phosphonium (C17) 2.87 Zuh et al, 2001 Alkyl ammonium (C18) 2.32 Trans-2-butene-1,4270 1.70 Takana et al, 2006 bis(triphenylphosphonium) Alkyl phosphonium (C16) 270 1.95 Hartwig et al, 2003 Alkyl ammonium (C16) 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- MW: 532 g/mol; MP 42 C Ph4: Cyphos IL 164, Tetra n-butyl-P+Cl- MW: 294 g/mol; MP 82 C 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 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 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 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. 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 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. 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 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 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 Figure 8.4. TEM pictures of PS1510/ Dylark 10 % with phosphonium organoclays (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 Figure 8.5. TEM pictures of PS1220/ Dylark 10 % with phosphonium organoclays (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 Dylark Concentration, % 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 Dylark Concentration, % 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. 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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, such as reinforcement, HDT, CLTE and barrier. Typical Properties: Treatment/Properties Organic 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. 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 10.79 1.66 18.64 X-Ray Results: d 001= 31.5 Å 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, such as reinforcement, HDT, CLTE and barrier. Typical Properties: Treatment/Properties Organic Cation % % Weight Loss Modifier Exchange Moisture on Ignition Capacity Cloisite Na+ None 92.6 meq/100 g < 2% 7% clay 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 12.45 20.95 2.86 X-Ray Results: d 001= 11.7 Å 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