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Segmental adsorption energy and phase behaviour of filled polymer blends
Eur. Polym. J. Vol. 27, No. 4/5, pp. 455~,59, 1991 Printed in Great Britain
0014-3057/91 $3.00+ 0.00 PergamonPress plc
SEGMENTAL ADSORPTION ENERGY A N D PHASE BEHAVIOUR OF FILLED POLYMER BLENDS O. T. GRITSENKOand A. E. NESTEROV* Institute of Macromolecular Chemistry, Academy of Sciences of the Ukrainian SSR, Kiev, 252160, U.S.S.R. (Received 25 April 1990 received for publication 4 June 1990)
A~tract--The parameters of segmental adsorption energy for poly(methyl methacrylate) and poly(vinyl acetate) in solvents of various thermodynamic qualities have been determined. Selectiveadsorption from PMMA-PVAc blend and its influence on the phase behaviour of filled films have been investigated. It is supposed that the shift of cloud-point curves correlates with the magnitude of adsorption of one of the components on the filler surface, which is in turn determined by the magnitude of the segmentaladsorption energy.
INTRODUCTION Studying the phenomenon of displacement of adsorbed polymer from a filler surface by low-molecular materials is interesting from the standpoint of predicting the adsorption behaviour of polymers and solving practical problems of the adsorption chromatography, stabilization of colloidal systems and adhesion. It is also of great importance for advancing the theory of polymer adsorption, making possible experimental determination of the segmental adsorption energy parameter Zs, as defined by Silberberg in 1968 [1]. It should be noted that experimental estimation of this parameter has until recently not been achieved [2]. Estimation of the segmental adsorption energy can also clarify the solution of the problem of the effect of the thermodynamic quality of solvent on the magnitude of adsorption, since the character of the concentration dependence of adsorption is primarily determined by energy factors [3]. Of special interest is study of adsorption from solutions of polymer blends and establishment of correlation between Xs values and the component adsorption selectivity, since the latter substantially affects the phase behaviour of filled binary polymer blends obtained from a common solvent [4]. We have earlier [5] for the first time determined the values of the segmental adsorption energy Zs for poly(methyl methacrylate) (PMMA) in solvents of various thermodynamic qualities with the use of a proposed procedure [6]. Note that similar results have recently been published [7] when adsorption of polystyrene from cyclohexane and carbon tetrachloride was studied. The authors suggested a simplified Xs calculation procedure requiring no additional study of adsorption of low-molecular displacers on the adsorbent surface. In the present study, we used the suggested procedure to obtain energy characteristics of PMMA adsorption on the surface of modified aerosil. The data for PMMA together with those obtained for poly(vinyl acetate) (PVAc) enabled us to *To whom all correspondence should be addressed.
establish relation between the polymer adsorption magnitude and the values of thermodynamic interaction parameters, polymer-solvent X and ~ , between the parameters g~ and the selectivity of adsorption of polymers from solutions of their blends, and also to trace the influence of these processes on the phase behaviour of filled P M M A PVAc polymer blends, for which purpose the corresponding cloud-point curves were obtained. EXPERIMENTAL PROCEDURES
Materials
Polymers (PMMA, ~¢, = 1.1 • 105;PVAc, .g:]~= 1.4. 105), were purified by reprecipitation from acetone into hexane. Chloroform and carbon tetrachloride were used as solvents; low-molecular displacers, acetone and 1,4-dioxane, were purified by standard procedures. Adsorbents were aerosil A-175 with a specific surface of 175m2/g and aerosil modified with diethylene glycol (ADEG) and with dimethyldichlorosilane (ADDCS), treated by vacuum at 100° for 6 hr. PMMA and PVAc displacement by displacers
Experiments on the displacement were conducted as follows. A 200 mg sample of adsorbent were placed in 20 cm3 test tubes and then a solution of 0.1 g of polymer in a corresponding amount of solvent was added. After the onset of the adsorption equilibrium displacer was added to the test tubes and they were held for 48 hr with stirring. Next, the solution was separated from the adsorbent by centrifuging at 10,000rpm for 10rain and the polymer concentration was determined by i.r. spectroscopy using the carbonyl valence vibration band; for this purpose, 2 ern3 of the solution in mixed solvent were dried to constant weight and dissolved in chloroform. Isotherms of adsorption of displacers were determined as follows. Solvent and displacer were mixed in various proportions, covering a suitable concentration range. A I g sample of aerosil was added to 20 em3 of the mixture and shaken for 3 hr. Next, aerosil was removed by centrifuging and the change in the mixture composition was estimated interferometrically. Estimation of the selective adsorption from the polymer blend was carried out by the NMR method.
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0.5
-1.2
-0.8
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Fig. 1. Isotherms of displacement of PMMA (a) and PVAc (b) by dioxane (1, 3) and acetone (2, 4) after preliminary adsorption from CCi4 (1, 2) and from CHCI3 (3, 4).
Obtaining cloud-point curves The cloud-point curves for P M M A - P V A c binary blends were obtained by the light scattering method [8]. Samples for study were prepared by immersing glass plates into polymer blend solutions and removing the solvent at room temperature. With filled blends, adsorbent (flier) had been added beforehand to the solutions and then they were held with stirring until the adsorption equilibrium was attained. The film from one side o f a plate was removed by dissolution in chloroform and the remaining film was covered with another glass plate (so that a sandwich-type sample was formed) to prevent any high-temperature oxidation o f polymer. The high-temperature oxidation was checked by the TG method. The sample weight loss on heating at 220 ° for 2 hr amounted to ~ 5%. The procedure of determining the cloud points was similar to that described [4]. The cloud-point curves were taken at a heating rate of 1.3° per minute and selected as the average from 3-4 parallel experiments.
The ~0crvalue was employed to calculate the segmental adsorption energy parameter X, using the equation [9] X~ = In ~O~r+ In s + X,~,- ,lj Xp° + {(1 - ~oo,)0 - ,h) - 4, } A z do,
where s is the dimensionless initial slope of the isotherm of displacer adsorption from the solution on aerosil, (2)
S .~- F - I ( V d / V o ) S ,
and F = 2.33. 106mol/m 2 is the surface density of isolated silanols on aerosii [10]; S is the initial slope of the displacer adsorption isotherm; V~, Vo are the displacer and solvent molar volumes; X,¢r is the critical parameter of adsorption energy; Xp° is the Flory-Huggins polymer-solvent thermodynamic interaction parameter; and AXd° is the combined interaction parameter
RESULTS AND DISCUSSION
AXd° = ~
Figure 1 shows the displacement isotherms, i.e. the adsorption values A (mg/m ~) as a function of the logarithm of the volume fraction of displacer. At low displacer concentrations, the polymer adsorption is positive. As the displacer fraction in the blend is increased, the polymer adsorption value decreases, reaching zero at some critical displacer concentration ~ r and assuming negative values at ~0 > rpcr (i.e. adsorption of the displacer on the filler surface predominates).
(l)
+ Xd° - X~°,
(3)
where Xp°, Xd° are the polymer--displacer and displacer-solvent interaction parameters respectively. The In s values (see Table 1) were calculated from the initial slopes of isotherms of adsorption of displacers from solvents on aerosil (Fig. 2). For X,~r a value of 0.288 was taken, calculated from the formula X,cr = - l n ( l - gt ) for a hexagonal lattice [2]. In terms of classical adsorption theories [2], the critical free energy is the minimum free energy needed to compensate for an unfavourable contribution from the loss
Table I. Values of X~ of polymers, critical volume fraction of displacer and dimensionless initial slope of displacer adsorption isotherms In s Solvent CCI4 CHCI 3
Acetone 6.4 4.7
PVAc Dioxane 6.0 5.7
Acetone 0.50 0.33
PMMA Dioxane 0.56 0.21
Acetone 0.35 0.24
Z~ Dioxane 0.47 0.17
PVAc 4.0 1.3
PMMA 2.9 0.6
Filled polymer blends
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Fig. 2. Isotherms of adsorption of displacers: of dioxane from CHCI 3 (1) and from CC14 (3); of acetone from CHC13 (2) and from COl, (4). O
by a macromolecule of part of the conformational enthropy on adsorption at the solid-liquid interface. Since the parameter Afr ° cannot be derived directly experimentally, its values were calculated by solving a set of four equations with four unknowns for two solvents and two displacers [5]. The Flory-Huggins thermodynamic interaction parameters for the PMMA-chloroform system, zP°=0.402, and the PVAc-chioroform system, Zp°= 0.377, were taken [11], and for the P M M A CC14 system, Zp° = 0.497, and P V A c - C C I 4 system, Z~o = 0.538, were determined by the light scattering method [12]. The Z~ values are presented in Table 1, where it is seen that the Xs values for both polymers in a poor solvent (CC14) are about four times as high as those in a good solvent (chloroform). A correlation betwecn the Z and the Zs values is observed in this case. The adsorption value is higher in the poor solvent than in the good one. Note also that, judging from the in s values (see Table 1), the affinity of CC14 itself for the aerosil surface is lower than that of CHC13 (displacers are sorbed from CC14 better than from CHC13). It follows that the two factors (the thermodynamic quality of solvent with respect to polymer and its affinity for the surface) act in the same direction, so that the magnitude of adsorption of polymers is higher when CC14 is used as solvent. Comparing the results of estimation of the parameters Zs for PVAc and P M M A (Table 1) leads to an important conclusion, summarized as follows: PVAc adsorption from P V A c - P M M A blend solutions in any of the solvents used by us should predominate on the aerosil surface since Zs for PVAc is higher than for PMMA. Direct experimental data on estimation of the selective adsorption indicate that PVAc is indeed adsorbed on the aerosil surface in this case (Fig. 3). The difference in parameters for PVAc and P M M A interaction with the filler surface is also essential for understanding the effect of the filler on the phase behaviour of filled P M M A - P V A c blends. As can be seen from Fig. 4, addition of aerosil to the P M M A - P V A c blend leads to a sharp shift of the cloud-point curve (system with LCST) towards lower temperatures, the phase separation temperature decreasing more sharply in regions with a prevailing
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2
3
4
5
6
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PVAc : PMMA Fig. 3. Change in ratio of polymeric components as a result of PVAc adsorption from solution of PMMA-PVAc blend in chloroform.
content of either PVAc or PMMA. If a filled P V A c - P M M A blend is now treated as a "ternary" system (like a three-polymer blend), then the substantial change in the phase separation temperatures in this case can be [13-16], ascribed to the difference in the binary interactions. Figure 4 shows also the cloud-point curves of P M M A - P V A c blends filled with 10% modified aerosiis A D E G and ADDCS. As can be seen, the phase separation temperatures for these blends decrease the amounts of the decrease being in the order of A D E G < ADDCS < A-175. However, as also for the addition of unmodified aerosil, this shift is not the same for different blend compositions and so changes 520
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Fig. 4. Cloud-point curves for films of PMMA-PVAc blends: unfilled (1) filled with aerosil A-175 (2); filled with ADEG (3); and filled with ADDCS (4).
458
O. T. GRITSENKOand A. E. NESTr_ROV
the shape of the cloud-point curve (it acquires the form of a "diagram with LCST" although the phase separation, as before, occurs in heating). We tried to correlate the amount of decrease of the phase separation temperature of a blend on addition of filler and the parameter of segmental energy of adsorption of polymer on the filler surface. To this end, we determined the parameters ~, for PVAc and PMMA adsorption from chlorophorm on the ADEG surface, using a procedure [7] which allows Xs to be calculated without detailed characteristics of the adsorbent surface and knowledge of the nature of adsorption centres, needed for another procedure [6]. The essence of the chosen procedure [7] lies in successful selection of a pair of solvents so that adsorption would be observed even in a pure displacer and in calculation of X~ from: Z ~ = In (Per + Zscr -- ~'IZ pd - (1 -- Z~r)(1 -- 2,)(Z p° - Z p d - Za°),
(4)
where ~p. can be found by extrapolation o f the displacement isotherm to the region q~cr> 1, as shown in Fig. 5, and the parameter Z d° can be calculated from the Hildenbrand-Scatchard theory with the use of solubility parameters
Zd° = (Vo/RT)(6 o
-
-
(~d)2.
(5)
Here Vo is the molar volume of the solvent and 60, 6d are the parameters of solubility of the solvent and displacer. Table 2 presents the Xs values for PMMA and PVAc adsorption from chloroform on the ADEG surface as well as X~calculated [7] for adsorption of these polymers on the surface of A-175. As can be seen, the X~values for the two methods of calculation are in a good accord, and therefore we believe it possible to compare the X~ values obtained with the use of the two procedures [6 and 7]. It should be 1.2
(a) 0.8
'N\ X\ NX I -1.2
I -0.8
I -o.4
o
0.4
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0.4
0.2 I -0.4
I -o.2 too ~a
Adsorbent
PVAc P M M A
Aerosil A-175 2.70 ADEG 1.51
1.58 1.23
PVAc P M M A
0.1 0.1
0.02 0.02
PVAc P M M A
1.33 0.64
0.65 0.40
pointed out that the Z, values for adsorption of the two polymers on the ADEG surface are ~ 1.5 times lower than for adsorption on the surface of A-175. Moreover, PVAc interacts with the surface of A-175 twice, and with the surface of ADEG, 1.5 times as strongly as PMMA, i.e. the asymmetry of interactions is in the latter case less pronounced than in the former. Turning now to Fig. 4, it can be noticed that the shift of the cloud-point curve is smallest when ADEG is used as the filler. As regards the silanized aerosil, the shift in this case should at first sight be the minimum, but Fig. 4 shows that the cloud-point curve of an ADDCS-filled polymer blend occupies an intermediate position between the curves for ADEG- and A-175-filled blends. In this case, ADDCS can apparently be considered as a filler; the nature of its surface is similar to that of unmodified aerosil, but has a lower density of adsorption centres, since [10], the aerosil treatment with dimethyldichlorosilane involves the participation in the reaction primarily of surface silanols, bonded to one another by hydrogen bonds while the proportion of free hydroxyl groups undergoing the reaction is small. In adsorption of polymers on aerosil, hydrogen bonds with macromolecules form mainly because of free hydroxyl groups [17]. It appears that therefore also the magnitude of adsorption of polymers on the ADDCS surface does not become zero. Thus, Zs should not change in this case, since the replacement of aerosil with a silanized aerosil changes only the number of adsorption centres, not their nature. (This means that only the proportion of polymer disturbed by interaction with the filler is changed.) Thus, the shift of cloud-point curve correlates with the magnitude of adsorption, which increases in the order ADEG < ADDCS < A-175. CONCLUSIONS
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Table 2. Valuesof segmentalenergyof adsorptionof polymers,Z,~, from chloroform, combined interaction parameters and critical volumefractionof displacer(CHCI3). CC14has beentakenas solvent
The experimental data indicate that addition of mineral fillers leads to a shift of cloud-point curves for polymer blends obtained from solutions in a common solvent. The shift correlates with the magnitude of adsorption of one of the components on the filler surface, which is in turn determined by the magnitude of the segmental adsorption energy of the polymer for fillers with different natures of adsorption centres. The segmental adsorption energy depends also on the nature of the solvent from which the filled polymer blends are obtained.
""\~'\N I 0
0.2
Fig. 5. Isotherms of displacement of PMMA and PVAc, adsorbed from CC14, from surface of aerosil A-175 (a) and of ADEG (b) by chloroform.
REFERENCES
1. A Silberberg. Y. phys. Chem. 46, 1105 (1967). 2. G. Fleer and J Liclema. Adsorption from Solution at the Solid~Liquid Interface (edited by G. D. Parfit and C. H. Rochester). Academic Press, New York (1983).
Filled polymer blends 3. A. E. Nesterov and O. T. Gritsenko. V~sokomolek. Soedin. A29, 1252 (1987). 4. A. E. Nesterov, V. V. Gorichko and Yu. S. Lipatov. Dokl. Akad. Naurk. SSSR 229, 656 (1988). 5. O. T. Gritsenko and A. E. Nesterov. Vfisokomolek. Soedin. B29, 611 (1987). 6. M. A, Cohen Stuart, G. J. Fleer and J. M. H. M. Sheutjens. J. Colloid Interface Sci. 97, 515 (1984). 7. M. Kawaguchi, M. Chikazawa and A. Takahashi. Macromolecules 22, 2195 (1989). 8. S. Reich and J. Cohen. J. Polym. Sci.; Polym. Phys. Edn 19, 1255 (1981). 9. M. A. Cohen Stuart, G. J. Fleer and J. M. H. M. Sheutjens. J. Colloid Interface Sci. 97, 526 (1984).
459
10. A. V. Kiselev and V. I. Lygin. Infrared Spectra of Surface Compounds. Halsted Press/Wiley, New York (1975). 11. A. E. Nesterov. Handbook on Physical Chemistry of
Polymers. V. L Properties of Solutions and Blends of Polymers. Naukova Dumka, Kiev (1984). 12. Th. G. Scholte. Eur. Polym. J. 6, 1063 (1970). 13. A. C. Su and J. R. Fried. Polym. Engng Sci. 27, 1657 (1987). 14. W. H. Christiansen, D. R. Paul and J. W. Barlow. J. appl. Polym. Sci. 34, 537 (1987). 15. V. S. Shah, J. D. Keitz, D. R. Paul and J. W. Barlow. J. appL Polym. Sci. 32, 3863 (1986). 16. J. W. Barlow and D. R. Paul. Polym. England Sci. 27, 1482 (1987). 17. A. V. Kiselev, V. I. Lygin and I. N. Solomonova. Kolloid. J. 30, 386 (1968).
0014-3057/91 $3.00+ 0.00 PergamonPress plc
SEGMENTAL ADSORPTION ENERGY A N D PHASE BEHAVIOUR OF FILLED POLYMER BLENDS O. T. GRITSENKOand A. E. NESTEROV* Institute of Macromolecular Chemistry, Academy of Sciences of the Ukrainian SSR, Kiev, 252160, U.S.S.R. (Received 25 April 1990 received for publication 4 June 1990)
A~tract--The parameters of segmental adsorption energy for poly(methyl methacrylate) and poly(vinyl acetate) in solvents of various thermodynamic qualities have been determined. Selectiveadsorption from PMMA-PVAc blend and its influence on the phase behaviour of filled films have been investigated. It is supposed that the shift of cloud-point curves correlates with the magnitude of adsorption of one of the components on the filler surface, which is in turn determined by the magnitude of the segmentaladsorption energy.
INTRODUCTION Studying the phenomenon of displacement of adsorbed polymer from a filler surface by low-molecular materials is interesting from the standpoint of predicting the adsorption behaviour of polymers and solving practical problems of the adsorption chromatography, stabilization of colloidal systems and adhesion. It is also of great importance for advancing the theory of polymer adsorption, making possible experimental determination of the segmental adsorption energy parameter Zs, as defined by Silberberg in 1968 [1]. It should be noted that experimental estimation of this parameter has until recently not been achieved [2]. Estimation of the segmental adsorption energy can also clarify the solution of the problem of the effect of the thermodynamic quality of solvent on the magnitude of adsorption, since the character of the concentration dependence of adsorption is primarily determined by energy factors [3]. Of special interest is study of adsorption from solutions of polymer blends and establishment of correlation between Xs values and the component adsorption selectivity, since the latter substantially affects the phase behaviour of filled binary polymer blends obtained from a common solvent [4]. We have earlier [5] for the first time determined the values of the segmental adsorption energy Zs for poly(methyl methacrylate) (PMMA) in solvents of various thermodynamic qualities with the use of a proposed procedure [6]. Note that similar results have recently been published [7] when adsorption of polystyrene from cyclohexane and carbon tetrachloride was studied. The authors suggested a simplified Xs calculation procedure requiring no additional study of adsorption of low-molecular displacers on the adsorbent surface. In the present study, we used the suggested procedure to obtain energy characteristics of PMMA adsorption on the surface of modified aerosil. The data for PMMA together with those obtained for poly(vinyl acetate) (PVAc) enabled us to *To whom all correspondence should be addressed.
establish relation between the polymer adsorption magnitude and the values of thermodynamic interaction parameters, polymer-solvent X and ~ , between the parameters g~ and the selectivity of adsorption of polymers from solutions of their blends, and also to trace the influence of these processes on the phase behaviour of filled P M M A PVAc polymer blends, for which purpose the corresponding cloud-point curves were obtained. EXPERIMENTAL PROCEDURES
Materials
Polymers (PMMA, ~¢, = 1.1 • 105;PVAc, .g:]~= 1.4. 105), were purified by reprecipitation from acetone into hexane. Chloroform and carbon tetrachloride were used as solvents; low-molecular displacers, acetone and 1,4-dioxane, were purified by standard procedures. Adsorbents were aerosil A-175 with a specific surface of 175m2/g and aerosil modified with diethylene glycol (ADEG) and with dimethyldichlorosilane (ADDCS), treated by vacuum at 100° for 6 hr. PMMA and PVAc displacement by displacers
Experiments on the displacement were conducted as follows. A 200 mg sample of adsorbent were placed in 20 cm3 test tubes and then a solution of 0.1 g of polymer in a corresponding amount of solvent was added. After the onset of the adsorption equilibrium displacer was added to the test tubes and they were held for 48 hr with stirring. Next, the solution was separated from the adsorbent by centrifuging at 10,000rpm for 10rain and the polymer concentration was determined by i.r. spectroscopy using the carbonyl valence vibration band; for this purpose, 2 ern3 of the solution in mixed solvent were dried to constant weight and dissolved in chloroform. Isotherms of adsorption of displacers were determined as follows. Solvent and displacer were mixed in various proportions, covering a suitable concentration range. A I g sample of aerosil was added to 20 em3 of the mixture and shaken for 3 hr. Next, aerosil was removed by centrifuging and the change in the mixture composition was estimated interferometrically. Estimation of the selective adsorption from the polymer blend was carried out by the NMR method.
455
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-1.2
-0.8
-0.4
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-0.4
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Fig. 1. Isotherms of displacement of PMMA (a) and PVAc (b) by dioxane (1, 3) and acetone (2, 4) after preliminary adsorption from CCi4 (1, 2) and from CHCI3 (3, 4).
Obtaining cloud-point curves The cloud-point curves for P M M A - P V A c binary blends were obtained by the light scattering method [8]. Samples for study were prepared by immersing glass plates into polymer blend solutions and removing the solvent at room temperature. With filled blends, adsorbent (flier) had been added beforehand to the solutions and then they were held with stirring until the adsorption equilibrium was attained. The film from one side o f a plate was removed by dissolution in chloroform and the remaining film was covered with another glass plate (so that a sandwich-type sample was formed) to prevent any high-temperature oxidation o f polymer. The high-temperature oxidation was checked by the TG method. The sample weight loss on heating at 220 ° for 2 hr amounted to ~ 5%. The procedure of determining the cloud points was similar to that described [4]. The cloud-point curves were taken at a heating rate of 1.3° per minute and selected as the average from 3-4 parallel experiments.
The ~0crvalue was employed to calculate the segmental adsorption energy parameter X, using the equation [9] X~ = In ~O~r+ In s + X,~,- ,lj Xp° + {(1 - ~oo,)0 - ,h) - 4, } A z do,
where s is the dimensionless initial slope of the isotherm of displacer adsorption from the solution on aerosil, (2)
S .~- F - I ( V d / V o ) S ,
and F = 2.33. 106mol/m 2 is the surface density of isolated silanols on aerosii [10]; S is the initial slope of the displacer adsorption isotherm; V~, Vo are the displacer and solvent molar volumes; X,¢r is the critical parameter of adsorption energy; Xp° is the Flory-Huggins polymer-solvent thermodynamic interaction parameter; and AXd° is the combined interaction parameter
RESULTS AND DISCUSSION
AXd° = ~
Figure 1 shows the displacement isotherms, i.e. the adsorption values A (mg/m ~) as a function of the logarithm of the volume fraction of displacer. At low displacer concentrations, the polymer adsorption is positive. As the displacer fraction in the blend is increased, the polymer adsorption value decreases, reaching zero at some critical displacer concentration ~ r and assuming negative values at ~0 > rpcr (i.e. adsorption of the displacer on the filler surface predominates).
(l)
+ Xd° - X~°,
(3)
where Xp°, Xd° are the polymer--displacer and displacer-solvent interaction parameters respectively. The In s values (see Table 1) were calculated from the initial slopes of isotherms of adsorption of displacers from solvents on aerosil (Fig. 2). For X,~r a value of 0.288 was taken, calculated from the formula X,cr = - l n ( l - gt ) for a hexagonal lattice [2]. In terms of classical adsorption theories [2], the critical free energy is the minimum free energy needed to compensate for an unfavourable contribution from the loss
Table I. Values of X~ of polymers, critical volume fraction of displacer and dimensionless initial slope of displacer adsorption isotherms In s Solvent CCI4 CHCI 3
Acetone 6.4 4.7
PVAc Dioxane 6.0 5.7
Acetone 0.50 0.33
PMMA Dioxane 0.56 0.21
Acetone 0.35 0.24
Z~ Dioxane 0.47 0.17
PVAc 4.0 1.3
PMMA 2.9 0.6
Filled polymer blends
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Fig. 2. Isotherms of adsorption of displacers: of dioxane from CHCI 3 (1) and from CC14 (3); of acetone from CHC13 (2) and from COl, (4). O
by a macromolecule of part of the conformational enthropy on adsorption at the solid-liquid interface. Since the parameter Afr ° cannot be derived directly experimentally, its values were calculated by solving a set of four equations with four unknowns for two solvents and two displacers [5]. The Flory-Huggins thermodynamic interaction parameters for the PMMA-chloroform system, zP°=0.402, and the PVAc-chioroform system, Zp°= 0.377, were taken [11], and for the P M M A CC14 system, Zp° = 0.497, and P V A c - C C I 4 system, Z~o = 0.538, were determined by the light scattering method [12]. The Z~ values are presented in Table 1, where it is seen that the Xs values for both polymers in a poor solvent (CC14) are about four times as high as those in a good solvent (chloroform). A correlation betwecn the Z and the Zs values is observed in this case. The adsorption value is higher in the poor solvent than in the good one. Note also that, judging from the in s values (see Table 1), the affinity of CC14 itself for the aerosil surface is lower than that of CHC13 (displacers are sorbed from CC14 better than from CHC13). It follows that the two factors (the thermodynamic quality of solvent with respect to polymer and its affinity for the surface) act in the same direction, so that the magnitude of adsorption of polymers is higher when CC14 is used as solvent. Comparing the results of estimation of the parameters Zs for PVAc and P M M A (Table 1) leads to an important conclusion, summarized as follows: PVAc adsorption from P V A c - P M M A blend solutions in any of the solvents used by us should predominate on the aerosil surface since Zs for PVAc is higher than for PMMA. Direct experimental data on estimation of the selective adsorption indicate that PVAc is indeed adsorbed on the aerosil surface in this case (Fig. 3). The difference in parameters for PVAc and P M M A interaction with the filler surface is also essential for understanding the effect of the filler on the phase behaviour of filled P M M A - P V A c blends. As can be seen from Fig. 4, addition of aerosil to the P M M A - P V A c blend leads to a sharp shift of the cloud-point curve (system with LCST) towards lower temperatures, the phase separation temperature decreasing more sharply in regions with a prevailing
I
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2
3
4
5
6
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PVAc : PMMA Fig. 3. Change in ratio of polymeric components as a result of PVAc adsorption from solution of PMMA-PVAc blend in chloroform.
content of either PVAc or PMMA. If a filled P V A c - P M M A blend is now treated as a "ternary" system (like a three-polymer blend), then the substantial change in the phase separation temperatures in this case can be [13-16], ascribed to the difference in the binary interactions. Figure 4 shows also the cloud-point curves of P M M A - P V A c blends filled with 10% modified aerosiis A D E G and ADDCS. As can be seen, the phase separation temperatures for these blends decrease the amounts of the decrease being in the order of A D E G < ADDCS < A-175. However, as also for the addition of unmodified aerosil, this shift is not the same for different blend compositions and so changes 520
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0.8
PMMA
Fig. 4. Cloud-point curves for films of PMMA-PVAc blends: unfilled (1) filled with aerosil A-175 (2); filled with ADEG (3); and filled with ADDCS (4).
458
O. T. GRITSENKOand A. E. NESTr_ROV
the shape of the cloud-point curve (it acquires the form of a "diagram with LCST" although the phase separation, as before, occurs in heating). We tried to correlate the amount of decrease of the phase separation temperature of a blend on addition of filler and the parameter of segmental energy of adsorption of polymer on the filler surface. To this end, we determined the parameters ~, for PVAc and PMMA adsorption from chlorophorm on the ADEG surface, using a procedure [7] which allows Xs to be calculated without detailed characteristics of the adsorbent surface and knowledge of the nature of adsorption centres, needed for another procedure [6]. The essence of the chosen procedure [7] lies in successful selection of a pair of solvents so that adsorption would be observed even in a pure displacer and in calculation of X~ from: Z ~ = In (Per + Zscr -- ~'IZ pd - (1 -- Z~r)(1 -- 2,)(Z p° - Z p d - Za°),
(4)
where ~p. can be found by extrapolation o f the displacement isotherm to the region q~cr> 1, as shown in Fig. 5, and the parameter Z d° can be calculated from the Hildenbrand-Scatchard theory with the use of solubility parameters
Zd° = (Vo/RT)(6 o
-
-
(~d)2.
(5)
Here Vo is the molar volume of the solvent and 60, 6d are the parameters of solubility of the solvent and displacer. Table 2 presents the Xs values for PMMA and PVAc adsorption from chloroform on the ADEG surface as well as X~calculated [7] for adsorption of these polymers on the surface of A-175. As can be seen, the X~values for the two methods of calculation are in a good accord, and therefore we believe it possible to compare the X~ values obtained with the use of the two procedures [6 and 7]. It should be 1.2
(a) 0.8
'N\ X\ NX I -1.2
I -0.8
I -o.4
o
0.4
E
0.4
0.2 I -0.4
I -o.2 too ~a
Adsorbent
PVAc P M M A
Aerosil A-175 2.70 ADEG 1.51
1.58 1.23
PVAc P M M A
0.1 0.1
0.02 0.02
PVAc P M M A
1.33 0.64
0.65 0.40
pointed out that the Z, values for adsorption of the two polymers on the ADEG surface are ~ 1.5 times lower than for adsorption on the surface of A-175. Moreover, PVAc interacts with the surface of A-175 twice, and with the surface of ADEG, 1.5 times as strongly as PMMA, i.e. the asymmetry of interactions is in the latter case less pronounced than in the former. Turning now to Fig. 4, it can be noticed that the shift of the cloud-point curve is smallest when ADEG is used as the filler. As regards the silanized aerosil, the shift in this case should at first sight be the minimum, but Fig. 4 shows that the cloud-point curve of an ADDCS-filled polymer blend occupies an intermediate position between the curves for ADEG- and A-175-filled blends. In this case, ADDCS can apparently be considered as a filler; the nature of its surface is similar to that of unmodified aerosil, but has a lower density of adsorption centres, since [10], the aerosil treatment with dimethyldichlorosilane involves the participation in the reaction primarily of surface silanols, bonded to one another by hydrogen bonds while the proportion of free hydroxyl groups undergoing the reaction is small. In adsorption of polymers on aerosil, hydrogen bonds with macromolecules form mainly because of free hydroxyl groups [17]. It appears that therefore also the magnitude of adsorption of polymers on the ADDCS surface does not become zero. Thus, Zs should not change in this case, since the replacement of aerosil with a silanized aerosil changes only the number of adsorption centres, not their nature. (This means that only the proportion of polymer disturbed by interaction with the filler is changed.) Thus, the shift of cloud-point curve correlates with the magnitude of adsorption, which increases in the order ADEG < ADDCS < A-175. CONCLUSIONS
,q
I -o.6
Table 2. Valuesof segmentalenergyof adsorptionof polymers,Z,~, from chloroform, combined interaction parameters and critical volumefractionof displacer(CHCI3). CC14has beentakenas solvent
The experimental data indicate that addition of mineral fillers leads to a shift of cloud-point curves for polymer blends obtained from solutions in a common solvent. The shift correlates with the magnitude of adsorption of one of the components on the filler surface, which is in turn determined by the magnitude of the segmental adsorption energy of the polymer for fillers with different natures of adsorption centres. The segmental adsorption energy depends also on the nature of the solvent from which the filled polymer blends are obtained.
""\~'\N I 0
0.2
Fig. 5. Isotherms of displacement of PMMA and PVAc, adsorbed from CC14, from surface of aerosil A-175 (a) and of ADEG (b) by chloroform.
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