- Journal de Chimie Physique et de Physico

J. Chirn. Phys. (1 998) 95, 1964-1990
Q EDP Sciences. Les Ulis
Glass transition of adsorbed stereoregular PPMA
by inverse gas chromatography at infinite dilution
T. ~amieh',M. Rezzaki, Y. Grohens and J. Schultz
Institut de Chimie des Surfaces et Interfaces (/CS/-CNRS), UPR 9069,
15 rue Jean Starcky, BP. 2488, 68057 Mulhouse cedex, France
(Received 22 Septernber 1997; accepted 28 May 1998)
'Correspondence and reprints.
Dans cet article, nous montrons que la chromatographie gazeuse inverse (CGI) à
dilution infinie se révèle être une technique très intéressante pour la détermination de
la transition vitreuse de polymères stéréoréguliers adsorbés sur des substrats solides
tels que l'alumine. Nous avons mis en évidence des transitions attribuées aux
phénomènes de relaxation béta, transition vitreuse et autres transitions des systèmes
PMMA/A1203de tacticité définie à différents taux de recouvrement. Ainsi, la Tg du
PMMA isotactique adsorbé augmente de façon significative par rapport a celle du
polymère massique.
L'étude des propriétés physico-chimiques du système PMMA/A1203,révèle une
différence importante dans le comportement acido-basique, au sens de Lewis, de
l'alumine pour de taux de recouvrement en PMMA variables. Il apparaît qu'il y a
stabilisation des propriétés physico-chimiques de PMMA/A120; pour un taux de
recouvrement en PMMA supérieur à 50 %. Cette étude a montré également une
influence importante de la tacticité du polymère sur le caractère acido-basique du
couple PMMA/A1203.
Mots-clés : Chromatographie gazeuse inverse, transition vitreuse, adsorption,
Constantes acide-base.
ABSTRACT
In this paper, we used inverse gas chromatography (IGC) at infinite dilution that
proved to be a powerful technique to determine glass transition and other transitions
of PMMA adsorbed on a-alumina. We highlighted the glass transition temperature
Glass transitions of stereoregular PMMA by IGC
1965
of the system PMMAIa-A120; with defined polymer tacticity at various covered
surface fractions. Thus, the Tg of the adsorbed isotactic PMMA increases strongly as
compared to the bulk value.
The study of the physical chemical properties of PMMA/a-alumina revealed an
important difference in the acidic and basic behaviour, in Lewis terms, of aluminium
oxide covered by various concentrations of PMMA. It appears that there is a
stabilisation of the physical chemical properties of PMMAIa-A1203 for a surface
coverage above 50 %. This study also highlighted an important effect of the tacticity
of the polymer on the acid-base character of the system PMMA/A1203.
Keywords : Inverse gas chromatography, glass transition, adsorption, acidic and
basic constants
1. INTRODUCTION
Polyrner properties strongly
depend on temperature. Polymers can be easily
affected by abrupt variations of the temperature. In fact, such modifications would
induce modifications in the chain segment mobility of polymers. These changes in
mobility arising at the glass transition temperatures (Tg) of bulk polymers can be
determined by using thermal methods, like differential thermal analysis or
differential scanning calorimetry [l].
dWdt
( arbitrary unity)
Tg
Temperature (K)
Figure 1: Examples of thermogramrnes obtained by DSC with bulk P M M
(atactic, curve 1) and PMMA (atactic) adsorbed on alumina (curve 2).
1966
T. Harnieh et al.
However, when adsorbed on aluminium oxide, the glass transitions of PMMA
could not be detected by these thermal methods. Indeed, the amount of polymer
involved in this kind (less than 2% in weight) of experiments is too low to provide a
significant signal. On figure 1, we give two examples of thermograms obtained by
DSC on bulk PMMA (cunle 1) and PMMA adsorbed on a-alumina (curve 2).
In the recent years, there has been considerable work done on the characterisation of
the glass transition temperature of polymers on flat surfaces [2,3]. It has been shown
that the changes in Tg are related to the film thickness, the molecular weight of the
polymer and also the nature of the polymer/substrate interactions. An empirical
relation has been proposed :
T,(h)
= T,
(bulk) [ l - (a/h)']
Where T,(h) is the measured glass transition for a film of thickness h and a and 6
areadjustable parameters.
Subsequent ellipsometry measurements of Tg for PMMA films [4] revealed the
strong influence of the substrate on the glass transition of polymer thin films. When
decreasing the PMMA film thickness: an increase of the T, values was observed on
Si and in contrast, a decrease for PMMA films on Au. It was suggested that the
observed behaviour for PMMA films on Si was due to hydrogen bonding between
the polymer and the SiOl surface.
No mention is made in the literature to attempts made for the direct determination
of the glass transition of the specifically adsorbed polymer on powder. Nevertheless,
works pointing out the effect on Tg of fillers have been found [5] which take into
account the fiee and the adsorbed polymer since the ratio of polymer to filler is high.
No spectroscopic technique such as ellipsometry or X-ray reflectomeny can be used
for the characterisation of polymer adsorbed ont0 rough surfaces. However, it is of
great interest to know if the same effect as those pointed out on flat surfaces can
happen on spherical particles. Therefore, we used Inverse Gas Chromatography
J. Chim. Phys.
Glass transitions of stereoregular PMMA by IGC
1967
(IGC) on a P W a l u m i n a system which is know to interact strongly through acidbase interactions and ionic bonds [6]. Moreover, stereoregularity of the polymer has
been varied as well as the molecular weight and the their effects investigated.
IGC technique was used, for thirty years, to characterise glass transitions of
polymers [7]. We applied this technique in order to veri@ and determine the change,
as a function of temperature, of the properties of the polymer adsorbed on metallic
oxide like alumina. The next section gives certain theoretical details of IGC
technique.
II. INVERSE GAS CHROMATOGRAPHY
2.1. Principle and methods
Inverse gas chromatography technique at infinite dilution [SI was used to
characterise the surface characteristics of various oxides and polymers. Probes of
known properties are injected in the column containing the solid. The retention times
of these probes, measured at infinite dilution, allow us to determine the interactions
between the organic molecules and the solid, in the absence of interactions between
the probe molecules themselves.
Measurements were carried out with a DELSI GC 121 FB Chromatograph
equipped with a flame ionisation detector of high sensitivity. The retention data were
obtained with a stainless steel column of length 30 cm and 2 mm interna1 diameter
packed with 1 to 2 g of polymer powders.
The net retention volume Vn was calculated from :
where t~ is the retention time of the probe, to the zero retention reference time
measured with a non adsorbing probe such as methane, D the flow rate and j a
correction factor taking into account the compression of the gas [3].
1968
T. Hamieh et al.
The free energy of adsorption AG0 of n-alkanes is given by :
AG0 = RT ln Vn + C
(3 )
where R is the ideal gas constant, T the absolute temperature and C a constant
depending on the reference state of adsorption . In the case of n-alkanes, AG0 is
equal to the free energy of adsorption corresponding to dispersive interactions
AG^
only .
2.2. Determination of specific interactions and of acid-base constants
The method used to obtain specific enthalpy of interaction between a probe and a
solid is that developed by Papirer et al. [9-111 who obtained a straight line when
plotting RTlnV, against In Po where Po is the vapor pressure of the probes.
for a homologous series of n-alkanes, whatever the nature of the solid substrates:
where A and B are constants depending of the nature of the solid substrate.
If polar probes are injected into the colurnn, specific interactions are established
between these probes and the solid surface and AG0 is now given by
where AGSP refers to specific interactions as shown in figure 1.
Polar molecules used to detennine the specific interactions with the solid
substrates are characterised by their donor (DN) and acceptor (AN) numbers [12].
The concept of donor-acceptor interactions is an extension of the Lewis acid-base
reactions, dealing with coordinate bonds which are formed by sharing a pair of
electrons between donor and acceptor species.
J. Chim. Phys.
Glass transitions of stereoregular PMMA by IGC
1969
Polar m olecule
I
I
I
I
Figure 2. I.,ariation of the logarithm of the retention volume
of n-aikanes versus the logarithm of the vapor pressure
By plotting AGSP of the polar molecules as a function of the temperature, we can
deduce the spkcific enthalpy (AI-ISP) from :
Knowing AHSP of the various polar molecules, we will be able to determine the
acidic constant KA and basic constant KD, the two constants characterising the solid
substrate, using the following relationship [13] :
- AHSP = (KA DN
+
KD AN)
(7)
The knowledge of the AHSP of various polar molecules thus allows to us to
determine the two constants KA et KD of the solid by plotting (AHSPIAN) as a
function of (NDNA), according to equation (6).
In a previous study [13], by studying some metallic oxides, we corrected the
T. Hamieh et al.
1970
relation (7) and proposed a neur relationship by adding a third parameter K reflecting
the amphoteric character of the oxide according to :
Another relationship was also proposed [15] :
(-AHSP) = w (KA DN
+ KD AN)
(9)
where w is the weighing factor of the exchanging interactions between adsorbed
molecule and solid substrate.
In the case of polymer powder, we proved that the results obtained by eqn. 7 and
eqns. 8 and 9 are the sarne. In this article, we only used equation 7 to calculate KA
and KD.
2.3. Determination of the dispersive component ofthe surface energy of solids
When non-polar , such as n-alkanes , are used , AG0 can be related to the London
interactions by using the well-known relationship of Fowkes [16] expressing the
geometric mean of the dispersive components (exponent d) of the surface energy of
the probe
and the solid ysd :
where Wa is the energy of adhesion, N is Avogadro's number and a the surface area
of one adsorbed molecule of the probe.
f
(figure 3), we can
By plotting RTlnVn as a function of 2 ~ a R o n-alkanes
deduce, from the slope of the straight line, the value of dispersive component of the
surface energy of the solid [17].
J. Chim. Phys.
Glass transitions of stereoregular PMMA by IGC
1971
Figure 3. Variation of the logarithm ofthe retention volume
However, the surface area of a molecule adsorbed on a solid substrate is not known
with a good accuracy. In a previous study [17], we proposed various models giving
the molecular areas of n-alkanes : geometrical model, cylindrical molecular model,
liquid density model, BET method, Kiselev results and the model using the twodimensional Van der Waals' constant b that depends on the critical temperature and
pressure of the liquid. Redlich-Kwong equation transposed fiom three-dimensional
space to two-dimensional space was also used to calculate the areas of organic
molecules.
To obtain qualitative results conceming the various polymers used in this study,
we applied the different models above giving the surface area of the molecule in
5
order to determine the variations of y of the solid as a function of the temperature
T. Hamieh ef al.
1972
and possibly deduce some properties conceming the transition phenornena in
PMMA.
III. MATERIALS AND SOLVENTS
3.1. a-Alumina
The a-alumina (a-A1203), used in this work, was obtained, by Bayer method, in
powder form with particle sizes comprised between O and 200 Pm.It contains small
quantities of Na20 (about 200 ppm relatively to A1203) and of other minera1
impurities (Si02 : 8 10 ppm and C a 0 : 450 ppm). It exhibits a specific surface area
equal to 1.5 rn2lg.
3.2. PMMA
We used three types of poly(methy1 methacrylate) : atactic PMMA (a), isotactic
PMMA (iso) and syndiotactic PMMA (s) al1 synthesised by anionic polymerisation
are purchased from Polymer Source. W e also studied another PMMA (Iw) having
lower molecular weight. Table 2 gives some chemical data of the various PMMA
that we experimentally obtained.
Table II experimental values of the tacticiîy (in %), the molecular weight
(Mn), polydispersity (1), the glass transition temperature (Tg) and the
intrinsic viscosiîy ([il]) of the various PMMA.
Polymer
tacticity (%)
i : a :s
Mn
1
(10~~/rnol)
Tg
[ri]
(OC)
l/mg
35.7
i-PMMA
97 : 03 : O
37
1.2 1
60.6
a-PMMA
7 : 39 :54
28.5
1.O9
104
s - P M
O : 20 : 80
33
1.O5
130.8
21
J. Chim. Phys.
Glass transitions of stereoregular PMMA by IGC
1973
hl.B.The tacticity of the different PMMA was determined by 'HNMR spectroscopy,
their molecular weight and polydispersity were determined by size exclusion
chromatography, the glass transition temperatures by DSC and the intrinsic viscosity by
capillary viscosimetry.
3.3. Mode1 organic molecules
Organic molecules are characterised by their donor and acceptor numbers [12, 181.
In this paper, we used the corrected acceptor number AN'
=
AN- AN^, given by Riddle
and Fowkes [19] \vho subtracted the contribution of Van der Waals interactions
(or dispersion forces). Donor number was normalised and used like a dimensionless
number DN' [19] according to the following relationship :
Where 2.5 is a conversion factor.
Table 3 gives donor and acceptor numbers of probe solvents used in this study.
Table III Values of donor and acceptor nurnbers of probe solvents [12,18,19].
J. Chim. Phys.
SOLVENT
DN'
AN' = AN- AN^
cc14
O
2.3
CH3N02
6.8
14.8
CH2C12
3
13.5
CHC13
O
18.7
Ether
48
4.9
THF
5O
1.9
Ethy 1 acetate
42.8
5.3
Acetone
42.5
8.7
CH3CN
35.3
16.3
T. Hamieh et al.
1974
IV.EXPEMMENTAL RESULTS
4.1. Adsorption of PMMA on a-A1203
4.1.1. Method
The polymer was dissolved in a chloroform solution containing a-alumina
powder. The solution obtained was stirred for 48 hours and the residue decanted was
washed three times with 25 ml of CHCI; to suppress any trace of PMMA non
directly adsorbed on A1203. The surface coverage 8 was determined by DRIFT
(Diffuse Reflectance Infrared Spectroscopy). The following formula was applied :
where A is the absorbance of the carbonyl groups for a given sample and AM, the
maximum absorbance recorded for a totally saturated alumina surface.
4.1.2. Adsorption isotherm of PMMA
Three types of PMMA were studied : syndiotactic (s), atactic (a) and atactic
having a low rnolecular weight (lw). These polymers were adsorbed ont0 alumina
following the sarne procedure
.
In figure 4, we represented the variations of the surface coverage (in %) of PMMA
adsorbed on aluminium oxide. We observed a difference in the behaviour of the
various polymers. If the molecular weight of the polymer decreases, then the polymer
quantity adsorbed on the oxide decreases. We 'also noted an important effect of the
tacticity of the polymer on its adsorption on a-alumina. The more the polymer is
syndiotactic, the more the surface coverage is high. Indeed, starting fiom a 2g/l
concentration of the PMMA solution, the surface coverage is 70 % for s-PMMA, 55
% for the a-PMMA and only 25 % for the i-PMMA.
This result is assumed to be due to the low surface area occupied by a syndiotactic
chain as compared to an isotactic one. Therefore, the low spreading out of
J. Chim. Phys.
Glass transitions of stereoregular PMMA by IGC
1975
syndiotactic chains enables the adsorption of more PMMA chains at the alumina
surface area. Figure 4 shows that if the concentration of the polymer is higher than 3
g/l a layer saturated in polymer is obtained whatever the polymer tacticity or
molecular weight. These differences in the chain conformation and in the chain
packing at the interface may lead to differences in the chain mobility that are
expected to be detected by Inverse Gas Chromatography.
Concentration (gll)
Figure 1.Evolution ofsurface coverage of the various PMMA adsorbed
on a-Al203 versus the concentration of PMMA in the solution.
4.2. Results of the IGC technique
4.2.1. Results concerning the dispersive component of the surface energy
The study of y: of or-alumina as a function of temperature allowed us to plot the
curves given in figure 5 for al1 the models of molecular areas given in table 1.
T. Hamieh et al.
1976
-Céom6trique
+Cylindrique
+Van
Der Waals
+Kiselev
++ Redlich-Kwong
*Sphérique
Figure 5. Evolution of the dispersive component of the surface energy of a-AZ203
versus the temperaturefor every molecular mode1 of surface area.
Al1 the curves of figure 5 show that the variation of
of a-alumina is
monotonous and decreases with increasing temperature. This result obtained with
aluminium oxide is in good agreement with the results obtained with other metallic
oxides analysed in a previous study [14]. These curves do not show any transition
phenomena. However, when PMMA is adsorbed on a-alumina, figures 6,7 and 8
giving the variation of the dispersive component of the surface energy of atactic,
isotactic and syndiotactic PMMA, respectively, adsorbed on a-alumina, as a function
of temperature, show a change in the variation of
versus temperature. The
maxima of the curves showed correspond to transition temperatures of the polymer
adsorbed on to the solid.
Figure 6 shows that three maxima are obtained when absorbing atactic PMMA on
a-alumina. This corresponds to the apparition of three characteristic temperatures
that may represent, the first at 60°C,a p-relaxation temperature, the second at 120°C,
Glass transitions of stereoregular PMMA by IGC
1977
a glass transition temperature and the third at 170°C, a liquid-liquid transition
temperature. These transitions are known to occur in bulk polymers and are generally
ascribed to changes in the vibrational, rotational or conformational mobility of
polymer chains segments. These transitions are also expected to occur in the
adsorbed layer at temperature which can be shifted because of the restriction in chain
mobility resulting from the interactions with the substrate.
+Van
Der Waals
Kiselev
-Redlich-Kwong
Figure 6. Evolurion of the dispersive componenr of the surface energy of faractic. wirh 2g.11
as concentration) P M M adsorbed on a-A1203 versus temperature, for the
diferent molecular models of surface area.
The results obtained with isotactic PMMA adsorbed on a-alumina (figure 7) prove
the presence of a glass transition temperature equal to 1 10°C and liquid-liquid
transition temperature equal to 150°C.
Figure 8 shows three maxima with the adsorption of PMMA syndiotactic on aalumina, the first at 70°C,a 0-relaxation temperature, the second at 135"C, a glass
transition temperature and the third at 170°C, a liquid-liquid transition temperature.
T. Hamieh et al.
1978
+Cylindrique
+Van
Der Waals
+Kiselev
-Redlich-Kwong
Figure 7. Evolution ofthe dispersive component ofthe surface energy of (isotactic,with 2g/l
as concenrration) PMMA adsorbed on a-Ah03 versus
temperature, for the
dzzerenr molecular models of surface area.
-.
+Géométrique
-Cylindrique
+Van
Der Waals
*Kislev
-Redlich-Kwong
+Sphérique
I
Figure 8. Evolution of the dispersive component of the surface energy of syndiotactic
PMMA (Ze'l concentration) adsorbed on a-A1203 versus temperarure, for the
dzfferent molecular models of surface area.
Glass transitions of stereoregular PMMA by IGC
1979
*Cylindrique
+Van
Der Waals
-CKielev
eRedlich-Kwong
Figure 9. Evolution of the dispersive component ofthe surface energy of atactic PMMA
adsorbed on Chromosorb W versus the temperature.
Since IGC is difficult to perform on bulk polymers, (atactic, isotactic and
syndiotactic) PMMA were adsorbed on Chromosorb W. This silanized silica is
relatively non adsorptive and is expected to develop only dipersive van der WaIls
interactions with the polymer. Therefore, since no specific interactions take place
between the polymer an the surface, the bulk behaviour of the polymer can
mimicked. We obtained the curves of figure 9, figure 10 and figure 11.
1980
T. Hamieh et al.
-Géométrique
+Cylindrique
+Van
Der Waals
*Kislev
*Redlich-Kwong
+Sphérique
Figure 1O. Evolution of the dispersive component of the surface energy of isoracric PMMA
adsorbed on Chromosorb W versus the temperature.
Cylindrique
+Van
Der Waals
Kiselev
eRedlich-Kwong
Figure 1I . Evolution of the dispersive component of the surface enero of syndiotacric PMW
adsorbed on Chromosorb W versus the temperature.
Grass transitions of stereoregular PMMA by IGC
1981
The results obtained by IGC conceming the transition temperatures, surnrnarised
in table 4, are in good agreement with the values measured by DSC.
Table IV. Values of transition temperatures of the various PMMA.
Table IV shows that three transition temperatures can be observed on a- and sPMMA but only two transitions on the isotactic PMMA. This observation is
consistent with the conclusions drawn by other workers [2 1,221 who show that the j3relaxation and the glass transition are partially merged for the highly isotactic
isomer. It is worth noting that the glass transition temperature increases when the
PMMA is adsorbed on reactive polar surface as alumina as compared to the Tg value
recorded for the sarne polymer adsorbed on Chromosorb W. Moreover, this increase
of Tg ranges from 5 for an s-PMMA to 50 degrees for the i-PMMA. The same
behaviour is observed for the liquid-liquid transition temperature.
4.2.2. ResuIts obtained by IGC with alkane probes
The result obtained with studying the evolution of RTlnVn of the various alkanes
as a function of the reciprocal temperature (11T) is in good agreement with the results
concerning the dispersive component of the surface energy.
For example, we show in figures 12, 13 and 14 the curves obtained with atactic,
isotactic and syndiotactic PMMA adsorbed on a-alumina.
These transition temperatures are consistent with the results obtained ffom
dielectric analysis [21] and thermally stimulated current (TSC) studies [22].
1982
T. Hamieh et al.
Figure 12. Evolution ofRTlnVn ofsome alkanes versus ( I R ) of(atactic, with 2g/l. as,
concenrration)PMU4 adsorbed on a-AI1O3 versus the temperature.
Figure 13. Evolution o f RTlnVn ofsome alkanes versus (I/T)of fi.soractic,with 2g/l. as.
concentration)PMMA adsorbed on a-A1203versus the temperature.
Glass transitions of stereoregular PMMA by IGC
1983
Figure II. Evolurion of RTIn Vn of some alkanes ilersus (I/T) of (syndiotactic, with 2g/l. as
concen1ratlon)PMU4adsorbed on a-A1203versus rhe temperature.
4.2.3. Discussion
The study of the adsorption of the PMMAs of various tacticity on a-alumina
shows that for a surface coverage above 20%, the variation of
and RTlnVn as a
function of the temperature are not monotonous. The maxima of
indicate the
presence of transition temperatures (glass or local transition). In general, we
observed with PMMA, two principal maxima that reflect the changes in motions
leading to reorganisation and rearrangement of the various groups or chain segments
of the polymer.
The change in the retention mechanism of the probes at the transition temperatures
is attributed to an increased molecular mobility of the polymer segments, allowing
for the penetration of the probes into the polymer layer. Indeed, for a polymer below
T,, the penetration of the solute molecules in the bulk polymer is precluded and
retention proceeds only by surface adsorption. Then, at glass transition, the
J Chlm Phys
1984
T. Hamieh et al.
penetration of the probe molecules begins, however, due to an initially slow rate of
diffusion of the solute into and out of the polymer, non equilibrium conditions
prevail. This can explain the decrease in the retention volume with temperature at
Tg. As the temperature is increased, the diffusion coefficient rises sharply which
leads to equilibrium between the different possibilities of probe diffusion.
It can be observed that the maxima of
y
correspond to the minima of the curves
of the evolution of RTlnVn as a function of (11T). Two or three transitions
temperatures are present in the plot of
versus reciprocal temperature, depending
on the PMMA tacticity. Glass transition and liquid-liquid transition are always
detected whereas p-relaxation is only observed for a certain degree of
syndiotacticity. The results obtained by IGC concerning the transition temperatures
are surnrnarised in table 4.
IGC has been shown to be a powerhl technique to detect the presence of PMMA
transitions which are iikely to be ascribed to B-relaxation, glass transition and other
liquid-liquid transitions. Differences in the structure of the adsorbed layer are
detected by the variations in temperature of the transitions between the bulk
(or adsorbed on Chromosorb W) polymer and the adsorbed one (on alumina).
Although the adsorbed polymer layers are likely to be non homogeneous over the
total thickness, (with for example train segments having highly restricted mobility
and loops and tail segments with higher mobility) the different transitions
temperatures seem to be an average value the motion of the chain segments.
Looking at the glass transition, the adsorbed i-PMMA exhibits a large shift of T,
of about 50°C as compares to the bulk Tg (From 60°C for bulk PMMA to 1 10°C for
the adsorbed polymer). In contrast, no effect of such magnitude of the adsorption on
T, seems to occur for the a- and s-PMMA. The Tgincrease of adsorbed i-PMMA, is
supported by the assumption of the spreading out of isotactic chain on the surface
plane, maximising their contact area with the surface (figure 15). This has already
been studied by Our group and published elsewhere [23]. This spreading out allows
J. Chim. Phys.
Glass transitions of stereoregular PMMA by IGC
1985
the establishment of numerous interactions of the ester groups, along the chain, with
the surface, which may highly restrict the chain backbone motions. Therefore, the
restriction of chain mobility of macromolecules in the interfacial region may
originate in the interactions of the polymer with the substrate. Thus, strong
P M W a l u m i n a interactions may be responsible for the Tgincrease of the adsorbed
i-PMMA. However, as the nature of the polymer/substrate interactions remains
constant whatever the PMMA tacticity, there must be other effects acting on the Tg
of thin layers. Thus, the level of side-chain acid-base interactions with the substrate,
depending on PMMA tacticity, may have a significant contribution to the increase of
glass transition temperature Tg.
Figure 15. Scheme of rhe configuration of the adsorbed PMUA layer
as afirnction ofthe polymer tacricity(fiom diluted solutions).
4.3.Acid-base constants
IGC at infinite dilution allows to obtain the variation of the specific free energy
AGSPby using the method developed in paragraph 2.1. By applying of eqn. (6), we
1986
T. Hamieh et al.
can deduce AHSPand bSSPof the specific interaction between polar molecules and
the solid substrate. Figure 16 gives an example of the evolution of (AGsP/T)as a
function of (l/T) in the case of the adsorption of PMMA (a) on a-alumina with polar
molecules probes.
CC14
iCHCI3
A CH2C12
+ Ether
Figure 16. Evolution of AGSP/Tversus (I/T) in the case of the adsorption
of atactic P M U on a-alurnina.forsome polar molecules.
The study of (AGSP/T)of the various polar molecules adsorbed on the different
solid substrates (PMMA/A1203) leads to values of AH" of specific interactions
between molecules and solid. By plotting of (AHSP/AN')as a function of (DN/ AN'),
equation (6) gives the acid KA and base KDconstants of the solid substrates. The
results obtained by this procedure are listed in table 4.
Table 4 shows that acid and base character depends on both the tacticity and
surface coverage of PMMA adsorbed on a-alumina. Figure 17 shows that the acid
and basic character of PMMA atactic adsorbed on alumina become stable for surface
coverage (0) above 50 %. The acidity of this adsorbed polymer remains constant for
al1 values of the surface coverage, whereas, its basicity decreases from 8
8 =.40 % and then remains constant for 8 > 50 %.
=
O to
Glass transitions of stereoregular PMMA by IGC
Table IV. Values of acidic and basic constants of the
various PMMA adsorbed on c(-A12O3
K.4 and KD
(Jlmoi)
2,s
2
Surface recoverage (in %)
Figure 17. Variations ofacidic
and basic KD constants of a?actic PMU4
adsorbed on aluminium oxide as a funciion of the surface coverage.
On the other hand, concerning the effect of the tacticity of the polymer, we can
classi@ the various adsorbed PMMA by decreasing order of this basic character :
PMMA atactic > PMMA syndiotactic > PMMA isotactic
1988
T. Harnieh et al.
It seems that the isotactic polymer strongly interacts with the alumina surface by
its basic ester groups [24]. This decreases the basicity of the P W a l u m i n a system.
In the case of syndiotactic PMMA, some basic groups don't interact with alumina
and this confers to the covered surface a more pronounced character basic than for
isotactic PMMA but less than that of the atactic polymer.
The explanations for these results are that interfacial conformational changes of
PMMA chain segments arise with a level which depends on tacticity and that the
level of the acid-base interactions of the PMMA side-chain with the alumina also
depends on tacticity. Indeed, i-PMMA undergoes more interfacial conformational
changes than the other isomers and most of the functional groups of the adsorbed
isotactic PMMA are involved in interfacial interactions. Hence, the free functional
groups (not involved in acid-base interaction with alumina) probed by CG1 are more
important for the a- and s-PMMA than for the isotactic isomer.
V. CONCLUSION
Inverse gas chromatography at infinite dilution allowed us, in this study, to
examine the acidic and basic behaviour of the various PMMA adsorbed on
a-alumina as a function of surface coverage and tacticity. This last parameter has
also shown to be a significant parameter on the increase of the glass transition
temperature at the interface. Finally, it has been demonstrated that IGC is a powerful
tool for the determination of glass transition temperatures and other transition
temperatures of tactic PMMA adsorbed on alumina surfaces.
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