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Experimental study of phase separation phenomena in swollen polyvinyl acetate and polystyrene gels near the critical solution temperature
Experimental Study of Phase Separation Phenomena in Swollen Polyvinyl Acetate and Polystyrene Gels near the Critical Solution Temperature M I K L O S Z R I N Y I AND E R V I N W O L F R A M [ Department of Colloid Science, Lordnd Ertvrs University, H-1445 Budapest 8, P.O. Box, Hungary
Received July 13, 1981; accepted November 10, 1981 DEDICATED TO THE MEMORY OF HERBERT FREUNDLICH Qualitative observations are reported of macroscopicchanges in polyvinylacetate gels that occur upon removal from swelling equilibrium with isopropylalcohol by lowering the temperature below the (upper) O point (54.7°C). In addition to syneresis, all systems exhibit turbidity, the extent of which depends on the degree of supercooling, the "curing," i.e., aging time (being different from the crosslinking time), and the network density. The development of turbidity is thought to be due either to microsyneresis, i.e., polymer-diluent incompatibility on microscale or local deformation of the network resulting in structural inhomogeneity.This is in most cases so pronounced that separation of a turbid "core" and a more transparent "crust" takes place. On the basis of phase diagrams constructed from cloud point data the network density was found to be of no detectable influence upon phase behavior being at variance with existing theory. Finally, phase diagrams of polystyrene gels swollen in methyl acetate were determined in the vicinity of both the upper and lower O temperatures (43 and 114°C, resp.). The shape of those phase diagram sections which are experimentally available for polyvinyl acetate and polystyrene gels was found to be similar to that of the solutions of these polymers in the same solvent. INTRODUCTION
consist of two distinct miscibility curves separating two heterogeneous regions from one homogeneous one. (For aqueous polymer solutions closed-looped miscibility behavior is more c o m m o n and Ou > OL as for lowmolecular liquid mixtures.) More details are to be found in (3). Much less investigated and understood is the phase behavior o f polymer gels. We define gels as chemically crosslinked three-dimensional elastic networks o f long-chain molecules with a certain a m o u n t of i m m o bilized solvent (diluent) molecules. U n d e r isothermal conditions, a polymer gel can be in t h e r m o d y n a m i c equilibrium with its pure diluent or its vapor at a well-defined polymer concentration. Otherwise, either syneresis or, if excess diluent is available, swelling occurs depending on whether the actual concentration of the polymer in the gel is lower or higher than the equilibrium value.
It is known that, unlike low-molecular binary liquid mixtures which exhibit, apart from a few exceptions, either an upper or a lower critical solution temperature (UCST and LCST, resp.), binary polymer solutions always have both an LCST and U C S T (1, 2). This is due to the fact that the Flory-Huggins interaction parameter, ×, which reflects the compatibility of the polymer segments with solvent molecules, depends on the temperature according to a m i n i m u m curve (2, 3). Therefore, its critical value, xc, at which phase separation necessarily occurs, can be attained at two different temperatures which are close to the O temperature(s) Ou and Or. It is to be noted that for nonpolar polymer solutions Ou is lower than OL and, hence, the complete phase diagram must To whom correspondence should be addressed. 34
0021-9797/82/110034-10502.00/0 Copyright © 1982 by Academic Press, Inc. All rights of reproduction in any form reserved.
Journal of Colloid and Interface Science, VoL 90, No. 1, November [982
35
PHASE SEPARATION IN GELS
According to the Flory-Huggins treatment (4, 6), the swelling equilibrium of a polymer network can be described as a balance of osmotic pressure due to mixing with a diluent and a mechanical pressure due to the elastic response of the network chains. The condition of equilibrium is the equality of the chemical potential inside and outside the swollen network phase. If the outside diluent is at activity a~, then A/A 1 = (A]Al)mi x q- ( A ~ l ) e I q- ( A u l ) c r
= R T l n al ,
[1]
with terms referring to contributions from mixing, chain elasticity and the constraint due to the presence of cross links. For isotropic swelling in a pure diluent (a~ = 1) of an ideal network, i.e., without any structural defects arising from unreacted functionalities, closed loops, and permanent entanglements, one obtains according to Dusek and Prins (6) the relation A ~ q / R T = ln(1 - ~2) + ~)2 ~- )(~(I72 + v g l ( A q o 2 / 3 q ~ 1/3 -
B ~ 2 ) = 0,
[2]
where u is the a m o u n t (expressed in moles) of network chains per unit dry polymer volume, ,b2 the volume fraction of the polymer in the gel, l~t the partial molar volume of the diluent, X the Flory-Huggins interaction parameter, and q0 the so-called memory parameter through which the system "remembers" those set of states at which the crosslinks have been introduced. The values A and B are constants the molecular meaning of which is not well understood. The best argumented values are A ~ 1 and B = 2If, f being the functionality of a branch point, i.e., in the most c o m m o n case B = 1/2. Now let us focus on the stability of gels. Swelling of gels corresponds to the miscibility of the network polymer in the swelling agent. According to the general stability criteflon of thermodynamic equilibrium, the Gibbs free energy of a stable system must be a convex function of the composition at con-
stant temperature and pressure. This implies that the negative free energy of swelling is a necessary and the 02X~l/0q~ < 0 inequality, hold over the entire composition range, is the sufficient condition for the stability of gels. If the inequality is not satisfied, the system becomes thermodynamically unstable and separates into two phases. Thus in good solvent condition (x ~ 1/2, where A]~ 1 is a monotonous decreasing function of composition) neither syneresis nor swelling can be considered as phase separation despite the fact that at syneresis a "new phase" appears, and at swelling the surrounding liquid phase may disappear. It is easy to verify that at given crosslinking density the stability conditions are satisfied at low and violated at high X values. At a certain Xc value which depends upon v itself (5) one observes a critical point corresponding to OA~ 0--~ = 0,
02A# 0~ 2 - 0.
[31
Above this Xc the existence of two gel phases has been predicted (5-7). On the basis of Eq. [2] the presence of the coherent network is expected to influence the properties, among them also the phase behavior of gels as compared with the corresponding polymer solutions of similar concentration. However, surprisingly few experimental work so far has been done in this respect and most efforts are concentrated on theoretical treatment of swelling of ideal networks. Some attention was focused on the collapse of gels, i.e., the sudden change of the swelling degree at a certain temperature or composition of the swelling fluid (7-9). Tanaka observed (7) that there is a sharp transition from swelling to collapse of polyacrylamide gels swollen in water-acetone mixtures within a narrow range of polymersolvent interaction if the crosslinking density exceeded a certain critical value. The same phenomenon was studied and interpreted by Janas et al. (I0) using a model proposed by Weiss et al. (11) but it turned out that the collapse had to be due to electrostatic interJournal of Colloid and Interface Science. Vol. 90, No. 1, November 1982
36
ZRINYI AND WOLFRAM
actions (12, 13). Candau et al. (14) provided evidence of an internal microphase separation of polystyrene gels swollen in cyclohexane below the O temperature by measuring the temperature dependence of the correlation time of laser light. In a recent paper (15) of this laboratory, experimental evidence was presented for a rather sophisticated behavior of some swollen gels under nonequilibrium conditions. The results were only partly indicative of the influence of network parameters upon macroscopic stability of the gels. The main purpose of the present work has, therefore, been to extend these investigations in a more systematic manner to obtain further information of the influence of the relevant parameters. In addition, taking into account the almost complete lack of quantitative experimental data as to the phase behavior of swollen gels, an attempt has been made to determine phase diagrams of gels over a temperature range comprising also OL. EXPERIMENTAL
Materials Polyvinyl acetate (PVAc) gels swollen in isopropyl alcohol (iPrOH) as well as polystyrene (PS) gels swollen in methyl acetate (MeAC) and isopropyl acetate (iPrAc), resp., have been investigated in form of films (23 mm thick) and cylinders (1 cm in diameter by 1 cm in height); the given dimensions refer to the initial state. The PVAc gels were obtained by polymer homologous acetylation of polyvinyl alcohol (PVA) gels in a mixture of pyridine-acetic anhydride-acetic acid at 90°C for 8 hours, whereafter the mixture was exchanged with acetone and finally with iPrOH. The PVA gels themselves were prepared by crosslinking with glutaric dialdehyde of a commercial product (PVA-420, Kuraray Poval, Japan) in aqueous solution at pH = 1.5. Prior to the crosslinking reaction, the acetate content (18-20 wt%) of the commercial polymer was reduced by alcalic hydrolysis in a 20:80 Journal of Colloid and Interface Science, Vol. 90, No. 1, November 1982
(v/v) methanol-water mixture below 1%, and homogeneity with respect to molecular mass was increased by fractionation with a mixture of acetone and n-heptane. Several series of the PVAc gels have been prepared, varying both the crosslinking density, L, i.e., the average number of crosslinks on a primary chain, and the initial PVA concentration, c, at which the crosslinks had been introduced, the samples being identified by the c/L figures, e.g., PVAc gel "6/40" means c = 6 wt% and L = 40 wt%. The PS gels were synthesized by crosslinking polymerization of styrene with divinyl benzene in toluene at about 80°C using azobis-(isobutyronitrile) as initiator. After gelling, the sol fraction was extracted and the toluene was exchanged against MeAc or iPrAc.
Methods Different kinds of experiments have been carried out. First, qualitative macroscopic properties (as transparency, shape, and size changes) of PVAc gel cylinders and film were observed upon cooling-warming cycles. The specimens, being in swelling equilibrium with iPrOH at 60°C (i.e., above the upper O temperature, Ov = 54.7°C) were cooled down to a temperature Ti less than Ou, and kept at this temperature ("curing") for different lengths of time, t (up to several months). In some cases, they were reheated again up to 60°C, to look at reversibility. Photographs were taken from sample series with different L at different Ti and t. In a second set of experiments, phase diagrams (or sections of them) were determined by measuring the cloud points of PVAc and PS gels with different L as well as of PVAc solutions of different polymer concentration. The turbidity measurements were carried out by a Brice-Phoenix universal light-scattering photometer at a wavelength of 546 nm. The intensity of the transparent light as a function of temperature was detected by an X - Y recorder, and the cloud point was taken as the
37
PHASE SEPARATION IN GELS
temperature corresponding to the sudden change of transparency upon cooling, obtained by graphical interpolation. The actual polymer concentration, % of the gels was determined by weighing the samples. The speed of cooling was fast enough to prevent deswelling of the gels, thus ensuring the polymer concentration to remain constant. The temperature was measured by a platinum resistance sensor connected to a Knauer universal temperature measuring instrument. When studying the region of the phase diagram above OL, difficulty arose from approaching the boiling point of the solvent present in the gel. This was avoided by placing the gel specimens into sealed glass tubes, in which boiling was prevented by the increased pressure upon raising the temperature. This, of course, might slightly alter the actual shape of the upper section of the phase diagram, but being unaware of the pressure dependence of the critical miscibility, the effect has been disregarded. In a third kind of experiments, the (mechanical) relaxation time of PVAc films upon swelling or syneresis was determined by measuring unidirectional deformation, i.e., changes of a preselected characteristic dimension (e.g., length or diameter) as a function of time at constant temperature. The relaxation time, r, was obtained according to Tanaka et al. (16) using the (simplified) relation Id, - d~o) Ido - d ~ l
- C expl-t/rl,
[41
where dr, do, and d~ is the selected dimension measured at the time indicated by the subscript, and C is a constant, characteristic of a given system. During the deformation no significant shape change has been observed. The specimens were placed in a glass cell, the mantle of which could be connected with each of two thermostats of the initial and, resp., the final temperature. After equilibrium at the initial temperature was reached, the water circulation was switched over to
the other thermostat of final temperature. The film dimension was determined using a measuring microscope. Instead of the relaxation time which depends not only on the mobility of the segments but also on the magnitude of the gel sample itself, it is useful to introduce the cooperative diffusion coefficient, Dc, Do = d~/r, which characterizes the collective motion of the network polymer in the gel liquid. RESULTS
Qualitative Observations
Upon cooling, all gels exhibit optical, and many of them also structural changes that can be observed by the naked eye. The originally transparent PVAc gels, swollen in iPrOH, became turbid, the extent of turbidity and its variation with time being different for gels of various crosslinldng density and also for different Ti values. From the great number of experimental data, Fig. 1 as a typical example shows the behavior of gel cylinders with L = I0, 20, 30, and 40 at 30, 36, and 42°C, resp., after having been "aged" at these temperatures for from 1 hour up to 14 days. As expected, the developed turbidity for gels with the same L increases with decreasing temperature, and, at first glance somewhat less trivially, with decreasing L at the same temperature. The latter dependence means that denser networks are comparatively more stable upon cooling than looser ones. It can also be seen that all gels undergo syneresis, the extent of which is, according to expectation, the greater, the higher the crosslinking density. In addition to (macro)syneresis in the common sense, i.e., shape-invariant shrinking, more profound structural changes were also observed: in all cylindrical specimens the turbidity was getting to be localized in an approximately sphere-like core, whereas the surrounding region became practically as transparent again as the original gel had been above Otj. The volume of the turbid core occupied inside of a gel of a given L is the larger, the Journal o f Colloid and Interface Science, VoL 90, No. 1, November 1982
38
ZRINYI AND WOLFRAM
lower the temperature and the shorter the time of storing the gel at the given temperature. With increasing L, the core becomes smaller, and after a sufficiently long time it may even disappear (see bottom right picture in Fig. 1) or else it gets stabilized without any further observable changes over years (bottom middle picture in Fig. 1), Figure 2a shows a more enlarged picture taken of a gel L
10
20
50
40
10
cylinder with the turbid sphere-like core and the transparent crust region. It is apparent that the cylinder is somewhat distorted, which is likely to be due to mechanical stress generated by the core. For comparison, Fig. 2b displays the surface of a gel film obtained under similar circumstances. Apparently, the structure is quite different: the film contains visible pores and bubbles distributed rather 20
30
40
10
20
30
40
T = 60°C
36°C
T. = 30°C I
42oC
0
......
I hour
1
..................
t
day
5 days
9 days
14. days
FIG, 1. The effect of cooling from 60°C (0u = 54.7°C) upon polyvinyl acetate gels swollen in isopropyl alcohol. (L = crosslinking density). Journal of Colloid and Interface Science. VoL 90, No. 1, November 1982
39
PHASE SEPARATION IN GELS
8
t't'--
IJ
"6 :a e~
o
t:L
v~
v~ 0)
¢a
Journal of Colloid and Interface Science, Vol. 90, N o . 1, N o v e m b e r t 9 8 2
40
ZRINYI AND WOLFRAM TABLE I
Effect of Cooling (60 ~ 40°C) and Heating (40 --~ 60 °) on the Relaxation Time (z) and the Cooperative Diffusion Coefficient (De) of Disk-Shaped Polyvinyl Acetate Gel Films with Different Network Parameters (c/L) Swollen in Isopropyl Alcohol z/103 sec
D f f l 0 -9 m 2. see -J
c/L
Heating
Cooling
Heating
Cooling
7.77/10 7.77/20 7.77/30 7.77/40 9.60/10 9.60/20 9.60/30 9.60/40
4.78 (4.62a) 4.52 (4.28a) 3.11 3.57 4.52 (4.68b) 2.96 2.20 3.21 (1.90b)
-19.8 (322b) 16.5 19.4 - - (250c) 24.1 14.9 12.8 (367c)
19.3 (19.2a) 20.5 (20.0a) 33.1 37.7 29.5 (27.4b) 31.1 36.6 40.0 (42.56)
-1.21 (0.150c) 1.48 1.50 - - (0.152c) 1.67 2.77 2.74 (0.2109
Strips. b 25 °C ~ 60°C. 60oc -~ 25°C. uniformly over the film. Nevertheless, n e a r the periphery a thin transparent region (A) and a continuous turbid region (B) can clearly be distinguished. Collapse o f gels has not been observed.
that for swelling. Fourth, the temperature range around Ou, over which the processes were taking place, influences the mechanical relaxation only in case of syneresis. Fifth, the shape of the specimen has no substantial effect upon speed o f relaxing.
Relaxation Time Two temperature programs were used to determine the values (according to Eq. [4]) of P V A c / i P r O H gels with different c/L parameters both for swelling (upon heating the samples) and for syneresis (upon cooling). For swelling the initial temperatures were 25 and 40°C, resp., and the final t e m p e r a t u r e was 60°C, whereas with the syneresis experiments 60°C was the initial and 25 and 40°C, resp., were the final ones. The m a i n results are s u m m a r i z e d in Table I. Even if taking the a p p r o x i m a t e character of these m e a s u r e m e n t s into account, some interesting features are apparent f r o m the data of Table I. First, the initial p o l y m e r concentration at which crosslinking was introduced has practically no effect on the r values o f either process. Second, the increasing crosslinking density slightly reduces the relaxation time in b o t h processes. Third, the rate of attaining syneresis equilibrium is by one or two orders of magnitude smaller than Journal of Colloid and Interface Science, Vol. 90, No. 1, November 1982
Phase Diagrams Phase diagrams were constructed from the cloud-point vs temperature dependence o f both PVAc gels and solutions with i P r O H as swelling agent and, resp., solvent. The gels were of different crosslinking densities and the dissolved polymers of different molecular mass. The results are plotted in Fig. 3. The most i m p o r t a n t result is that, within experimental accuracy, no difference has been found between the two gels with different network density. It is also apparent that the cloud-point curves for the gels exhibit neither a m a x i m u m , nor an increasing section, and, furthermore, that they appear to represent a continuation of the corresponding curves for the solutions. On the other hand, the curves for the two solutions with polymers of different molecular mass differ from each other in the sense as expected: the curve for higher M goes higher than that for the lower M. The concentration m a r k e d on the abscissa rep-
41
P H A S E S E P A R A T I O N IN GELS J
g-
5O
~*C a. oo o o ~ ,oo%~
• •o o•
L.,O
• @ A
AO °
30
•
c~i °
c~io
I
cp/w*/,
20
25
50
75
FiG. 3. The dependence o f the cloud point u p o n polym e r concentration (co) for polyvinyl acetate gels and solutions in isopropyl alcohol. Gels: A, c/L = 6/10; O, c/L = 6/40. Solutions: [:1, Mw = 130,000; O, M,~ = 35,000.
resents the minimum possible concentration at which the gels can exist at all. This roughly corresponds to the overlap concentration of the equivalent solution, i.e., the solution of the same polymer with a molecular mass which is identical to that of the network chains in the gel; below this concentration the polymer coils are separated and thus no permanent network can form. To get the complete phase diagram, it would have been necessary to also investigate systems above OL. Unfortunately, PVAc gels exhibit rather a high lower critical temperature (235 °C), above which degradation of the polymer (taking place over 170°C) can hardly be avoided. Therefore, PS gels swollen in MeAC and iPrAc, resp., were chosen having OL values of 114 and 107°C, resp., allowing measurements to be made without any difficulty. Turbidity development was observed for the PS/MeAc gels above 140°C and for the PS/iPrAc gels above 161 °C. With an increase of the gel concentration at a given network density (which in this case was characterized by the styrene-divinyl benzene ratio) also the temperature increased at which
turbidity occurred (see upper curve of Fig. 4). When lowering the temperature below OL (being 43°C for the PS/MeAc system), the concentration dependence of the cloud point gets reversed: the higher the polymer concentration in the gel, the lower the cloud point (see lower curve of Fig. 4). It should be noted again that the data for systems above the OL are to be considered with some criticism due to the inherent difficulty, as mentioned before, of not knowing the pressure dependence of the cloud points, for the measurements had necessarily to be performed using sealed tubes. It is worth mentioning that the core-andcrust structure also appeared after keeping the samples at a given temperature at least for a month. DISCUSSION
Let us first briefly summarize our main results. (a) All the gel systems studied were capable of undergoing a phase transition which results in the development of turbidity, independently of the preparation conditions
(c/L). (b) Within experimental accuracy the PVAc gels of different crosslinking densities have the same cloud-point concentration dependence which seems to coincide with that of the uncrosslinked concentrated solutions. (c) All the cloud point vs concentration diagrams measured for gels were found to be monotonous; extremum points have not been observed. (d) Phase separation of gels always has been found to be followed by syneresis, and in some cases it produced the total disappearance of the developed turbidity. (e) No collapse of the gels has been observed. (f) A very pronounced effect of the phase transition on the kinetic behavior of swollen gels, characterized by r or Do, has been found. Most of the results do not support the prediction of the model calculations in which Journal of Colloid and Interface Science, V o L 9 0 , N o . 1, N o v e m b e r 1982
42
ZRINYI A N D W O L F R A M
all the three parts of the chemical potential of the diluent have been taken into account. To overcome this discrepancy we suppose that the location of the coexistence (binodal) and spinodal points and consequently the cloud point is predominantly determined by the mixing contribution (first three terms in Eq. [2]). The spinodals by means of which the general character of a phase diagram can be deduced are defined by OA/-t I
0~2 -
0A/.tmi x j_ 0 A # e l
0~2
[.
150
140
II.
OA/2cr
- ~ 2 + 0~2 - 0
[5]
and the solution of Eq. [5] depends on the concentration dependence of the terms in Eq. [1]. Thus, the absence of the expected crosslinking density dependence can be explained if the concentration dependence of the terms &gel and A#cr is negligibly small. If we consider that B = 0, the crosslinking term vanishes and the concentration dependence of the other two terms must be taken into account. A comparison of the concentration dependence of the mixing and elastic parts of the overall chemical potential shows that usually 3A#JO~ is much smaller than OAu,~ix/0~ (A#~ oc ~1,1/3and Aumix oc ~ m , where 2 _< m < 3). This was supported by recent findings (17, 18) according to which the constant B and, consequently, the crosslinking term in Eq. [2] vanishes for PVAc gels. It was also found that AtL~ is a slightly increasing function of the concentration, independent of the temperature. This leads to the conclusion that phase separation of gels is very likely to be highly similar to that occurring in concentrated polymer solutions, without any essential influence of the crosslinking density. In other words, gels are expected to have approximately the same critical parameters as the corresponding solutions. This is not at variance with the results obtained in the supercooled state in a far from equilibrium situation. In this case, the increase of the interaction parameter above its critical value is not only incident to the phase separation Journal o f Colloid and Interface Science,
yoc
Vol. 90, No. 1, November 1982
10o
III.
Cp/W% 25
50
75
FIG. 4. The dependence of the cloud point upon polymer concentration (cp) of polystyrene gels swollen in methyl acetate. The crosslinking (styrene: divinylbenzene) ratio was 400:1. Ou = 43°C, OL = 114°C. I, III: turbid regions; II: transparent region.
of gels but also to the change in the equilibrium concentration, the value of which is largely determined by the crosslinking density. As far as collapse is concerned, one has to consider that its occurrence would require that the overall concentration of gels ought to be simultaneously smaller and higher than the only critical value. If one accepts that the critical concentration for gels is the same as that for the corresponding solution, no collapse in the entire gel volume can be expected to occur since the critical concentration of a polymer solution is roughly equal to the overlap concentration below which no permanent gel system can exist. It is to be noted that it was not our intention to give a comprehensive and quantitative account for the results presented here. The main purpose of this paper has been to report on experiments which, to the best of our knowledge, had not been done before.
PHASE SEPARATION IN GELS REFERENCES 1. Freeman, P. I., and Rowlinson, J. S., Polymer 1, 20 (1959). 2. Patterson, D., and Robard, A., Macromolecules 11, 690 (1978). 3. Patterson, D., Rubber. Chem. Tech. 40, 1 (1967). 4. Flory, P. J., "Principles of Polymer Chemistry." Cornell Univ. Press, Ithaca, N. Y., 1953. 5. Dusek, K., and Patterson, D., J. Polym. Sci. A-2 6, 1209 (1968). 6. Dusek, K., and Prins, W., Advan. Polym. Sci. 6, 1 (1969). 7. Tanaka, T., Phys. Rev. Lett. 40, 820 (1978). 8. Khoklov, A. R.,, Polymer 21, 376 (1980). 9. Khoklov, A. R., PhysicaA 105, 357 (1981). 10. Janas, F. V., Rodrigez, F., and Cohen, C., Macromolecules 13, 977 (1980).
43
11. Weiss, N., Van Vliet, T., and Silberberg, A., J. Polym. Sci. A-2, 17, 2229 (1979). 12. Stejskal, J., Gordon, M., and Torkington, A. A., Polym. Bull 3, 621 (1980). 13. Tanaka, T., Fillmore, D., Sun, S. T., Nishio, I., Swislow, G., and Shah, A., Phys. Rev. Lett. 45, 1636 (1980). 14. Candau, S., Munch, J. P., and Hild, G., J. Phys. Paris 41, 103l (1980). 15. Zrinyi, M., Molnhr, T., and Horv~th, E., Polymer 22, 429 (1981). 16. Tanaka, T., and Fillmore, D. J. Chem. Phys. 70, 1 (1979). 17. Horkay, F., Nagy, M., and Zrinyi, M., 9th Europhys. Conf. Macromol. Phys. Jablonna, 3C II-7 (1979). 18. Horkay, F., and Nagy, M., Acta. Chim. Acad. Sci. Hung, in press.
Journal of Colloid and Interface Science, ~¢oI. 90, No. 1, November 1982
Received July 13, 1981; accepted November 10, 1981 DEDICATED TO THE MEMORY OF HERBERT FREUNDLICH Qualitative observations are reported of macroscopicchanges in polyvinylacetate gels that occur upon removal from swelling equilibrium with isopropylalcohol by lowering the temperature below the (upper) O point (54.7°C). In addition to syneresis, all systems exhibit turbidity, the extent of which depends on the degree of supercooling, the "curing," i.e., aging time (being different from the crosslinking time), and the network density. The development of turbidity is thought to be due either to microsyneresis, i.e., polymer-diluent incompatibility on microscale or local deformation of the network resulting in structural inhomogeneity.This is in most cases so pronounced that separation of a turbid "core" and a more transparent "crust" takes place. On the basis of phase diagrams constructed from cloud point data the network density was found to be of no detectable influence upon phase behavior being at variance with existing theory. Finally, phase diagrams of polystyrene gels swollen in methyl acetate were determined in the vicinity of both the upper and lower O temperatures (43 and 114°C, resp.). The shape of those phase diagram sections which are experimentally available for polyvinyl acetate and polystyrene gels was found to be similar to that of the solutions of these polymers in the same solvent. INTRODUCTION
consist of two distinct miscibility curves separating two heterogeneous regions from one homogeneous one. (For aqueous polymer solutions closed-looped miscibility behavior is more c o m m o n and Ou > OL as for lowmolecular liquid mixtures.) More details are to be found in (3). Much less investigated and understood is the phase behavior o f polymer gels. We define gels as chemically crosslinked three-dimensional elastic networks o f long-chain molecules with a certain a m o u n t of i m m o bilized solvent (diluent) molecules. U n d e r isothermal conditions, a polymer gel can be in t h e r m o d y n a m i c equilibrium with its pure diluent or its vapor at a well-defined polymer concentration. Otherwise, either syneresis or, if excess diluent is available, swelling occurs depending on whether the actual concentration of the polymer in the gel is lower or higher than the equilibrium value.
It is known that, unlike low-molecular binary liquid mixtures which exhibit, apart from a few exceptions, either an upper or a lower critical solution temperature (UCST and LCST, resp.), binary polymer solutions always have both an LCST and U C S T (1, 2). This is due to the fact that the Flory-Huggins interaction parameter, ×, which reflects the compatibility of the polymer segments with solvent molecules, depends on the temperature according to a m i n i m u m curve (2, 3). Therefore, its critical value, xc, at which phase separation necessarily occurs, can be attained at two different temperatures which are close to the O temperature(s) Ou and Or. It is to be noted that for nonpolar polymer solutions Ou is lower than OL and, hence, the complete phase diagram must To whom correspondence should be addressed. 34
0021-9797/82/110034-10502.00/0 Copyright © 1982 by Academic Press, Inc. All rights of reproduction in any form reserved.
Journal of Colloid and Interface Science, VoL 90, No. 1, November [982
35
PHASE SEPARATION IN GELS
According to the Flory-Huggins treatment (4, 6), the swelling equilibrium of a polymer network can be described as a balance of osmotic pressure due to mixing with a diluent and a mechanical pressure due to the elastic response of the network chains. The condition of equilibrium is the equality of the chemical potential inside and outside the swollen network phase. If the outside diluent is at activity a~, then A/A 1 = (A]Al)mi x q- ( A ~ l ) e I q- ( A u l ) c r
= R T l n al ,
[1]
with terms referring to contributions from mixing, chain elasticity and the constraint due to the presence of cross links. For isotropic swelling in a pure diluent (a~ = 1) of an ideal network, i.e., without any structural defects arising from unreacted functionalities, closed loops, and permanent entanglements, one obtains according to Dusek and Prins (6) the relation A ~ q / R T = ln(1 - ~2) + ~)2 ~- )(~(I72 + v g l ( A q o 2 / 3 q ~ 1/3 -
B ~ 2 ) = 0,
[2]
where u is the a m o u n t (expressed in moles) of network chains per unit dry polymer volume, ,b2 the volume fraction of the polymer in the gel, l~t the partial molar volume of the diluent, X the Flory-Huggins interaction parameter, and q0 the so-called memory parameter through which the system "remembers" those set of states at which the crosslinks have been introduced. The values A and B are constants the molecular meaning of which is not well understood. The best argumented values are A ~ 1 and B = 2If, f being the functionality of a branch point, i.e., in the most c o m m o n case B = 1/2. Now let us focus on the stability of gels. Swelling of gels corresponds to the miscibility of the network polymer in the swelling agent. According to the general stability criteflon of thermodynamic equilibrium, the Gibbs free energy of a stable system must be a convex function of the composition at con-
stant temperature and pressure. This implies that the negative free energy of swelling is a necessary and the 02X~l/0q~ < 0 inequality, hold over the entire composition range, is the sufficient condition for the stability of gels. If the inequality is not satisfied, the system becomes thermodynamically unstable and separates into two phases. Thus in good solvent condition (x ~ 1/2, where A]~ 1 is a monotonous decreasing function of composition) neither syneresis nor swelling can be considered as phase separation despite the fact that at syneresis a "new phase" appears, and at swelling the surrounding liquid phase may disappear. It is easy to verify that at given crosslinking density the stability conditions are satisfied at low and violated at high X values. At a certain Xc value which depends upon v itself (5) one observes a critical point corresponding to OA~ 0--~ = 0,
02A# 0~ 2 - 0.
[31
Above this Xc the existence of two gel phases has been predicted (5-7). On the basis of Eq. [2] the presence of the coherent network is expected to influence the properties, among them also the phase behavior of gels as compared with the corresponding polymer solutions of similar concentration. However, surprisingly few experimental work so far has been done in this respect and most efforts are concentrated on theoretical treatment of swelling of ideal networks. Some attention was focused on the collapse of gels, i.e., the sudden change of the swelling degree at a certain temperature or composition of the swelling fluid (7-9). Tanaka observed (7) that there is a sharp transition from swelling to collapse of polyacrylamide gels swollen in water-acetone mixtures within a narrow range of polymersolvent interaction if the crosslinking density exceeded a certain critical value. The same phenomenon was studied and interpreted by Janas et al. (I0) using a model proposed by Weiss et al. (11) but it turned out that the collapse had to be due to electrostatic interJournal of Colloid and Interface Science. Vol. 90, No. 1, November 1982
36
ZRINYI AND WOLFRAM
actions (12, 13). Candau et al. (14) provided evidence of an internal microphase separation of polystyrene gels swollen in cyclohexane below the O temperature by measuring the temperature dependence of the correlation time of laser light. In a recent paper (15) of this laboratory, experimental evidence was presented for a rather sophisticated behavior of some swollen gels under nonequilibrium conditions. The results were only partly indicative of the influence of network parameters upon macroscopic stability of the gels. The main purpose of the present work has, therefore, been to extend these investigations in a more systematic manner to obtain further information of the influence of the relevant parameters. In addition, taking into account the almost complete lack of quantitative experimental data as to the phase behavior of swollen gels, an attempt has been made to determine phase diagrams of gels over a temperature range comprising also OL. EXPERIMENTAL
Materials Polyvinyl acetate (PVAc) gels swollen in isopropyl alcohol (iPrOH) as well as polystyrene (PS) gels swollen in methyl acetate (MeAC) and isopropyl acetate (iPrAc), resp., have been investigated in form of films (23 mm thick) and cylinders (1 cm in diameter by 1 cm in height); the given dimensions refer to the initial state. The PVAc gels were obtained by polymer homologous acetylation of polyvinyl alcohol (PVA) gels in a mixture of pyridine-acetic anhydride-acetic acid at 90°C for 8 hours, whereafter the mixture was exchanged with acetone and finally with iPrOH. The PVA gels themselves were prepared by crosslinking with glutaric dialdehyde of a commercial product (PVA-420, Kuraray Poval, Japan) in aqueous solution at pH = 1.5. Prior to the crosslinking reaction, the acetate content (18-20 wt%) of the commercial polymer was reduced by alcalic hydrolysis in a 20:80 Journal of Colloid and Interface Science, Vol. 90, No. 1, November 1982
(v/v) methanol-water mixture below 1%, and homogeneity with respect to molecular mass was increased by fractionation with a mixture of acetone and n-heptane. Several series of the PVAc gels have been prepared, varying both the crosslinking density, L, i.e., the average number of crosslinks on a primary chain, and the initial PVA concentration, c, at which the crosslinks had been introduced, the samples being identified by the c/L figures, e.g., PVAc gel "6/40" means c = 6 wt% and L = 40 wt%. The PS gels were synthesized by crosslinking polymerization of styrene with divinyl benzene in toluene at about 80°C using azobis-(isobutyronitrile) as initiator. After gelling, the sol fraction was extracted and the toluene was exchanged against MeAc or iPrAc.
Methods Different kinds of experiments have been carried out. First, qualitative macroscopic properties (as transparency, shape, and size changes) of PVAc gel cylinders and film were observed upon cooling-warming cycles. The specimens, being in swelling equilibrium with iPrOH at 60°C (i.e., above the upper O temperature, Ov = 54.7°C) were cooled down to a temperature Ti less than Ou, and kept at this temperature ("curing") for different lengths of time, t (up to several months). In some cases, they were reheated again up to 60°C, to look at reversibility. Photographs were taken from sample series with different L at different Ti and t. In a second set of experiments, phase diagrams (or sections of them) were determined by measuring the cloud points of PVAc and PS gels with different L as well as of PVAc solutions of different polymer concentration. The turbidity measurements were carried out by a Brice-Phoenix universal light-scattering photometer at a wavelength of 546 nm. The intensity of the transparent light as a function of temperature was detected by an X - Y recorder, and the cloud point was taken as the
37
PHASE SEPARATION IN GELS
temperature corresponding to the sudden change of transparency upon cooling, obtained by graphical interpolation. The actual polymer concentration, % of the gels was determined by weighing the samples. The speed of cooling was fast enough to prevent deswelling of the gels, thus ensuring the polymer concentration to remain constant. The temperature was measured by a platinum resistance sensor connected to a Knauer universal temperature measuring instrument. When studying the region of the phase diagram above OL, difficulty arose from approaching the boiling point of the solvent present in the gel. This was avoided by placing the gel specimens into sealed glass tubes, in which boiling was prevented by the increased pressure upon raising the temperature. This, of course, might slightly alter the actual shape of the upper section of the phase diagram, but being unaware of the pressure dependence of the critical miscibility, the effect has been disregarded. In a third kind of experiments, the (mechanical) relaxation time of PVAc films upon swelling or syneresis was determined by measuring unidirectional deformation, i.e., changes of a preselected characteristic dimension (e.g., length or diameter) as a function of time at constant temperature. The relaxation time, r, was obtained according to Tanaka et al. (16) using the (simplified) relation Id, - d~o) Ido - d ~ l
- C expl-t/rl,
[41
where dr, do, and d~ is the selected dimension measured at the time indicated by the subscript, and C is a constant, characteristic of a given system. During the deformation no significant shape change has been observed. The specimens were placed in a glass cell, the mantle of which could be connected with each of two thermostats of the initial and, resp., the final temperature. After equilibrium at the initial temperature was reached, the water circulation was switched over to
the other thermostat of final temperature. The film dimension was determined using a measuring microscope. Instead of the relaxation time which depends not only on the mobility of the segments but also on the magnitude of the gel sample itself, it is useful to introduce the cooperative diffusion coefficient, Dc, Do = d~/r, which characterizes the collective motion of the network polymer in the gel liquid. RESULTS
Qualitative Observations
Upon cooling, all gels exhibit optical, and many of them also structural changes that can be observed by the naked eye. The originally transparent PVAc gels, swollen in iPrOH, became turbid, the extent of turbidity and its variation with time being different for gels of various crosslinldng density and also for different Ti values. From the great number of experimental data, Fig. 1 as a typical example shows the behavior of gel cylinders with L = I0, 20, 30, and 40 at 30, 36, and 42°C, resp., after having been "aged" at these temperatures for from 1 hour up to 14 days. As expected, the developed turbidity for gels with the same L increases with decreasing temperature, and, at first glance somewhat less trivially, with decreasing L at the same temperature. The latter dependence means that denser networks are comparatively more stable upon cooling than looser ones. It can also be seen that all gels undergo syneresis, the extent of which is, according to expectation, the greater, the higher the crosslinking density. In addition to (macro)syneresis in the common sense, i.e., shape-invariant shrinking, more profound structural changes were also observed: in all cylindrical specimens the turbidity was getting to be localized in an approximately sphere-like core, whereas the surrounding region became practically as transparent again as the original gel had been above Otj. The volume of the turbid core occupied inside of a gel of a given L is the larger, the Journal o f Colloid and Interface Science, VoL 90, No. 1, November 1982
38
ZRINYI AND WOLFRAM
lower the temperature and the shorter the time of storing the gel at the given temperature. With increasing L, the core becomes smaller, and after a sufficiently long time it may even disappear (see bottom right picture in Fig. 1) or else it gets stabilized without any further observable changes over years (bottom middle picture in Fig. 1), Figure 2a shows a more enlarged picture taken of a gel L
10
20
50
40
10
cylinder with the turbid sphere-like core and the transparent crust region. It is apparent that the cylinder is somewhat distorted, which is likely to be due to mechanical stress generated by the core. For comparison, Fig. 2b displays the surface of a gel film obtained under similar circumstances. Apparently, the structure is quite different: the film contains visible pores and bubbles distributed rather 20
30
40
10
20
30
40
T = 60°C
36°C
T. = 30°C I
42oC
0
......
I hour
1
..................
t
day
5 days
9 days
14. days
FIG, 1. The effect of cooling from 60°C (0u = 54.7°C) upon polyvinyl acetate gels swollen in isopropyl alcohol. (L = crosslinking density). Journal of Colloid and Interface Science. VoL 90, No. 1, November 1982
39
PHASE SEPARATION IN GELS
8
t't'--
IJ
"6 :a e~
o
t:L
v~
v~ 0)
¢a
Journal of Colloid and Interface Science, Vol. 90, N o . 1, N o v e m b e r t 9 8 2
40
ZRINYI AND WOLFRAM TABLE I
Effect of Cooling (60 ~ 40°C) and Heating (40 --~ 60 °) on the Relaxation Time (z) and the Cooperative Diffusion Coefficient (De) of Disk-Shaped Polyvinyl Acetate Gel Films with Different Network Parameters (c/L) Swollen in Isopropyl Alcohol z/103 sec
D f f l 0 -9 m 2. see -J
c/L
Heating
Cooling
Heating
Cooling
7.77/10 7.77/20 7.77/30 7.77/40 9.60/10 9.60/20 9.60/30 9.60/40
4.78 (4.62a) 4.52 (4.28a) 3.11 3.57 4.52 (4.68b) 2.96 2.20 3.21 (1.90b)
-19.8 (322b) 16.5 19.4 - - (250c) 24.1 14.9 12.8 (367c)
19.3 (19.2a) 20.5 (20.0a) 33.1 37.7 29.5 (27.4b) 31.1 36.6 40.0 (42.56)
-1.21 (0.150c) 1.48 1.50 - - (0.152c) 1.67 2.77 2.74 (0.2109
Strips. b 25 °C ~ 60°C. 60oc -~ 25°C. uniformly over the film. Nevertheless, n e a r the periphery a thin transparent region (A) and a continuous turbid region (B) can clearly be distinguished. Collapse o f gels has not been observed.
that for swelling. Fourth, the temperature range around Ou, over which the processes were taking place, influences the mechanical relaxation only in case of syneresis. Fifth, the shape of the specimen has no substantial effect upon speed o f relaxing.
Relaxation Time Two temperature programs were used to determine the values (according to Eq. [4]) of P V A c / i P r O H gels with different c/L parameters both for swelling (upon heating the samples) and for syneresis (upon cooling). For swelling the initial temperatures were 25 and 40°C, resp., and the final t e m p e r a t u r e was 60°C, whereas with the syneresis experiments 60°C was the initial and 25 and 40°C, resp., were the final ones. The m a i n results are s u m m a r i z e d in Table I. Even if taking the a p p r o x i m a t e character of these m e a s u r e m e n t s into account, some interesting features are apparent f r o m the data of Table I. First, the initial p o l y m e r concentration at which crosslinking was introduced has practically no effect on the r values o f either process. Second, the increasing crosslinking density slightly reduces the relaxation time in b o t h processes. Third, the rate of attaining syneresis equilibrium is by one or two orders of magnitude smaller than Journal of Colloid and Interface Science, Vol. 90, No. 1, November 1982
Phase Diagrams Phase diagrams were constructed from the cloud-point vs temperature dependence o f both PVAc gels and solutions with i P r O H as swelling agent and, resp., solvent. The gels were of different crosslinking densities and the dissolved polymers of different molecular mass. The results are plotted in Fig. 3. The most i m p o r t a n t result is that, within experimental accuracy, no difference has been found between the two gels with different network density. It is also apparent that the cloud-point curves for the gels exhibit neither a m a x i m u m , nor an increasing section, and, furthermore, that they appear to represent a continuation of the corresponding curves for the solutions. On the other hand, the curves for the two solutions with polymers of different molecular mass differ from each other in the sense as expected: the curve for higher M goes higher than that for the lower M. The concentration m a r k e d on the abscissa rep-
41
P H A S E S E P A R A T I O N IN GELS J
g-
5O
~*C a. oo o o ~ ,oo%~
• •o o•
L.,O
• @ A
AO °
30
•
c~i °
c~io
I
cp/w*/,
20
25
50
75
FiG. 3. The dependence o f the cloud point u p o n polym e r concentration (co) for polyvinyl acetate gels and solutions in isopropyl alcohol. Gels: A, c/L = 6/10; O, c/L = 6/40. Solutions: [:1, Mw = 130,000; O, M,~ = 35,000.
resents the minimum possible concentration at which the gels can exist at all. This roughly corresponds to the overlap concentration of the equivalent solution, i.e., the solution of the same polymer with a molecular mass which is identical to that of the network chains in the gel; below this concentration the polymer coils are separated and thus no permanent network can form. To get the complete phase diagram, it would have been necessary to also investigate systems above OL. Unfortunately, PVAc gels exhibit rather a high lower critical temperature (235 °C), above which degradation of the polymer (taking place over 170°C) can hardly be avoided. Therefore, PS gels swollen in MeAC and iPrAc, resp., were chosen having OL values of 114 and 107°C, resp., allowing measurements to be made without any difficulty. Turbidity development was observed for the PS/MeAc gels above 140°C and for the PS/iPrAc gels above 161 °C. With an increase of the gel concentration at a given network density (which in this case was characterized by the styrene-divinyl benzene ratio) also the temperature increased at which
turbidity occurred (see upper curve of Fig. 4). When lowering the temperature below OL (being 43°C for the PS/MeAc system), the concentration dependence of the cloud point gets reversed: the higher the polymer concentration in the gel, the lower the cloud point (see lower curve of Fig. 4). It should be noted again that the data for systems above the OL are to be considered with some criticism due to the inherent difficulty, as mentioned before, of not knowing the pressure dependence of the cloud points, for the measurements had necessarily to be performed using sealed tubes. It is worth mentioning that the core-andcrust structure also appeared after keeping the samples at a given temperature at least for a month. DISCUSSION
Let us first briefly summarize our main results. (a) All the gel systems studied were capable of undergoing a phase transition which results in the development of turbidity, independently of the preparation conditions
(c/L). (b) Within experimental accuracy the PVAc gels of different crosslinking densities have the same cloud-point concentration dependence which seems to coincide with that of the uncrosslinked concentrated solutions. (c) All the cloud point vs concentration diagrams measured for gels were found to be monotonous; extremum points have not been observed. (d) Phase separation of gels always has been found to be followed by syneresis, and in some cases it produced the total disappearance of the developed turbidity. (e) No collapse of the gels has been observed. (f) A very pronounced effect of the phase transition on the kinetic behavior of swollen gels, characterized by r or Do, has been found. Most of the results do not support the prediction of the model calculations in which Journal of Colloid and Interface Science, V o L 9 0 , N o . 1, N o v e m b e r 1982
42
ZRINYI A N D W O L F R A M
all the three parts of the chemical potential of the diluent have been taken into account. To overcome this discrepancy we suppose that the location of the coexistence (binodal) and spinodal points and consequently the cloud point is predominantly determined by the mixing contribution (first three terms in Eq. [2]). The spinodals by means of which the general character of a phase diagram can be deduced are defined by OA/-t I
0~2 -
0A/.tmi x j_ 0 A # e l
0~2
[.
150
140
II.
OA/2cr
- ~ 2 + 0~2 - 0
[5]
and the solution of Eq. [5] depends on the concentration dependence of the terms in Eq. [1]. Thus, the absence of the expected crosslinking density dependence can be explained if the concentration dependence of the terms &gel and A#cr is negligibly small. If we consider that B = 0, the crosslinking term vanishes and the concentration dependence of the other two terms must be taken into account. A comparison of the concentration dependence of the mixing and elastic parts of the overall chemical potential shows that usually 3A#JO~ is much smaller than OAu,~ix/0~ (A#~ oc ~1,1/3and Aumix oc ~ m , where 2 _< m < 3). This was supported by recent findings (17, 18) according to which the constant B and, consequently, the crosslinking term in Eq. [2] vanishes for PVAc gels. It was also found that AtL~ is a slightly increasing function of the concentration, independent of the temperature. This leads to the conclusion that phase separation of gels is very likely to be highly similar to that occurring in concentrated polymer solutions, without any essential influence of the crosslinking density. In other words, gels are expected to have approximately the same critical parameters as the corresponding solutions. This is not at variance with the results obtained in the supercooled state in a far from equilibrium situation. In this case, the increase of the interaction parameter above its critical value is not only incident to the phase separation Journal o f Colloid and Interface Science,
yoc
Vol. 90, No. 1, November 1982
10o
III.
Cp/W% 25
50
75
FIG. 4. The dependence of the cloud point upon polymer concentration (cp) of polystyrene gels swollen in methyl acetate. The crosslinking (styrene: divinylbenzene) ratio was 400:1. Ou = 43°C, OL = 114°C. I, III: turbid regions; II: transparent region.
of gels but also to the change in the equilibrium concentration, the value of which is largely determined by the crosslinking density. As far as collapse is concerned, one has to consider that its occurrence would require that the overall concentration of gels ought to be simultaneously smaller and higher than the only critical value. If one accepts that the critical concentration for gels is the same as that for the corresponding solution, no collapse in the entire gel volume can be expected to occur since the critical concentration of a polymer solution is roughly equal to the overlap concentration below which no permanent gel system can exist. It is to be noted that it was not our intention to give a comprehensive and quantitative account for the results presented here. The main purpose of this paper has been to report on experiments which, to the best of our knowledge, had not been done before.
PHASE SEPARATION IN GELS REFERENCES 1. Freeman, P. I., and Rowlinson, J. S., Polymer 1, 20 (1959). 2. Patterson, D., and Robard, A., Macromolecules 11, 690 (1978). 3. Patterson, D., Rubber. Chem. Tech. 40, 1 (1967). 4. Flory, P. J., "Principles of Polymer Chemistry." Cornell Univ. Press, Ithaca, N. Y., 1953. 5. Dusek, K., and Patterson, D., J. Polym. Sci. A-2 6, 1209 (1968). 6. Dusek, K., and Prins, W., Advan. Polym. Sci. 6, 1 (1969). 7. Tanaka, T., Phys. Rev. Lett. 40, 820 (1978). 8. Khoklov, A. R.,, Polymer 21, 376 (1980). 9. Khoklov, A. R., PhysicaA 105, 357 (1981). 10. Janas, F. V., Rodrigez, F., and Cohen, C., Macromolecules 13, 977 (1980).
43
11. Weiss, N., Van Vliet, T., and Silberberg, A., J. Polym. Sci. A-2, 17, 2229 (1979). 12. Stejskal, J., Gordon, M., and Torkington, A. A., Polym. Bull 3, 621 (1980). 13. Tanaka, T., Fillmore, D., Sun, S. T., Nishio, I., Swislow, G., and Shah, A., Phys. Rev. Lett. 45, 1636 (1980). 14. Candau, S., Munch, J. P., and Hild, G., J. Phys. Paris 41, 103l (1980). 15. Zrinyi, M., Molnhr, T., and Horv~th, E., Polymer 22, 429 (1981). 16. Tanaka, T., and Fillmore, D. J. Chem. Phys. 70, 1 (1979). 17. Horkay, F., Nagy, M., and Zrinyi, M., 9th Europhys. Conf. Macromol. Phys. Jablonna, 3C II-7 (1979). 18. Horkay, F., and Nagy, M., Acta. Chim. Acad. Sci. Hung, in press.
Journal of Colloid and Interface Science, ~¢oI. 90, No. 1, November 1982