843
Macromolecules 1989,22, 843-848
Physical Gels from PVC: Aging and Solvent Effects on Thermal Behavior, Swelling, and Compression Modulus P. H. Mutin' and J. M. Guenet*p* Znstitut Charles Sadron (CNRS), 6 rue Boussingault, 67083 Strasbourg Cedex, France. Received April 11, 1988; Revised Manuscript Received July 14, 1988 ABSTRACT: The thermal response as well as the swelling behavior and the mechanical properties of gels prepared with different PVC samples (molecular weight and tacticity) have been studied as a function of preparation solvent and aging time. It is shown that the solvent type has an important influence and, particularly, can play a role similar to an increase of syndiotacticity. These results, which support the existence of a second type of physical link, are briefly discussed in terms of possible polymer-solvent molecular interactions.
Introduction Although the propensity of PVC to produce thermoreversible gels in a large variety of solvents has been known for over fourty the formation mechanism as well as the network structure are still under discu~sion.~-" Originally, the formation mechanism was thought to arise from the crystallization of the more syndiotactic sequences in spite of their rather low content.l-* Yet, recently Yang and Geil have suggested that PVC gelation might be due, a t least in its early stage, to the formation of interchain hydrogen bondsag In addition, Mutin et al. have pointed out,ll on the basis of investigations on PVC aggregates formed in dilute solutions, that another type of link must exist. Similarly, He et al.I2 have observed by studying the mechanical properties of thermoreversible gels prepared with a semicrystalline multiblock copolymer that PVC gel moduli could only be compared to a copolymer sample containing at least 50% of a crystallizable sequence. They accordingly conclude that the possibility of additional links, different from those that could form between the syndio or %ear syndio" sequences, should be seriously considered. The purpose of this paper is to gain some understanding about these links by investigating the role of the solvent. Particularly, we shall distinguish between the solvents wherein gelation interferes with a liquid-liquid phase separation and those where it does not. The thermal behavior, the swelling, and the mechanical properties will be examined. Experimental Section 1. Materials. The PVC samples, of commercial origin, were provided by RhBne-Poulenc SA and were used without further purification. One type of samples consisted of PVC synthesized at +50 "C; another sample was PVC synthesized at -40 "C. They will be designated as HTPVC (synthesized at +50 "C) and LTPVC (synthesized a t -40 "C). Molecular weights and molecular weight distribution were obtained by means of gel permeation chromatography in THF at 25 "C using the universal calibration method. The results are as follows: HTPVCl M, = 1.2 X lo6, M w / M n 2.3; HTPVCP,M, = 1.6 X 106,Mw/Mnu 2.2; HTPVC3 M, = 2.1 x ~O~V,./M, 2.3; LTPVC M, = 2 x 105 M,/M, = 3.3. The tacticity of these samples was determined by 13C NMR in a cyclohexanone/perdeuteriated benzene mixture a t a concentration of about 10% (w/w). The following results were obtained for the triads: HTPVCs i = 0.18, s = 0.33, h = 0.49; LTPVC i = 0.12, s = 0.39, h = 0.49. All the solvents were freshly distilled before use. 2. Gel Preparation. The samples were prepared following a standard procedure which allowed PVC degradation to be avoided. The polymer was dissolved at 150 "C under vigourous Present address: Laboratoire mixte RP/CNRS/USTL, Place E. Bataillon, 34060 Montpellier Cedex, France. Present address: Laboratoire de Spectromgtrie, et d'Imagerie Ultrasonore, Universitg Louis Pasteur, UA 851 CNRS, 4 rue Blaise Pascal, 67070 Strasbourg Cedex, France.
*
0024-9297/89/2222-0843$01.50/0
stirring in tubes sealed from atmosphere. Once a transparent, homogeneous solution was obtained, the tube was quenched into water held a t 20 "C. The concentrations are expressed in weight /volume. For swelling experiments and compression modulus measurements cylindrical gel samples (diameter of the tube 1.8 cm and typical height 1.8 cm) were prepared by cutting the ends of the gel with a hot razor blade guided by a piece of metal so as to produce reasonably flat, parallel surfaces. The samples were weighed and then immersed into a large excess of their preparation solvent for aging at 20 OC. 3. Mechanical Testing. Compression modulus determination and stress relaxation experiments were performed with a device described e1se~here.I~The gels were kept immersed in the preparation solvent, whose temperature was thermostatically monitored by an outer water circulation. As usual the compression modulus E was calculated from the relation UR
= E ( h - 1/h2)
(1)
where UR is the reduced stress and X the strain (X = l/lo, lo being the sample's initial height). h was never lower than 0.8. 4. Thermal Analysis. Thermal analysis was carried out with two differential scanning calorimeters from Perkin-Elmer, DSC-I1 and DCS-4, respectively, equipped with the TADS system (thermal analysis data station). Approximately 10 mg of freshly prepared gel was placed into a "volatile sample" pan. Heating rates were ranging from 5 to 40 "C/mn and cooling rates limited to -20 "C/mn. 5. Morphology. Gel morphology was essentially investigated by the phase contrast method in optical microscopy using a Zeiss photomicroscope 11.
Results and Discussion 1. Thermal Properties. Whatever the gel type, the thermal behavior is generally the same. To some extent, the results reported in the literature as well as their interpretation seem contradictory. Early work by Guerrero and Keller7 report the existence of two endotherms, Le., a low-melting temperature endotherm and a high-melting one. These authors, after considering an analogy with the thermal behavior of isotactic polystyrene gels,14 conclude that the low-melting endotherm corresponds to the gel melting while the high melting one represents the fusion of chain-folded crystals. However, simple experiments in a test tube show that the gel actually melts a t the high melting-endotherm. In a recent paper, Yang and Geilg have shown that there is no endotherm on reheating a piece of gel that has been molten in the DSC pan and then quenched to 20 "C. They accordingly conclude that gelation does not arise from crystallization, at least in its early stage, and put forward the notion of hydrogen bonding. There are strong evidences that crystallization must be involved as suggested by electric birefringence experiments carried out on PVC ~rege1s.l~ As a matter of fact, these experiments can only be accounted for with the existence of strongly cooperative dipoles, that is, with highly ordered links. Clearly, additional experiments are needed to throw 0 1989 American Chemical Society
844 Mutin and Guenet
Macromolecules, Vol. 22, No. 2, 1989
0.U
.
I
I
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-50 O0
10GSL II DEMcrystols 20
30
Io'xc(p c")
L
20
.
1
I
60
I
I
loo
I
I
140 T(OC)
Figure 1. (A, Top) DSC thermograms on a HTPVCl/diethyl malonate gel (C = 0.175) at 20 OC/m: (a) first heating at a gel aged 30 min at 20 "C; (b) second heating after melting the gel in the DSC pan and cooling to 20 "C; (c) heating after cutting into small pieces the gel of b and placing it into a new pan. (B, Bottom) DSC thermograms on a HTPVCl/diethyl malonate gel (C = 0.175) at 20 OC/m: (a) first heating of a gel aged 24 h at 20 "C. (b) second heating of the same gel after melting in the pan and then cooling to 20 OC and aging 24 h at this temperature.
some light on this puzzling question. We have repeated these experiments, and, depending on the sample preparation, the thermograms are quite different. (i) The thermogram of a freshly prepared gel always contains one endotherm (endotherm a in Figure 1A) which corresponds to the gel melting as measured independently by the ball drop method (Figure 2). This is observed independent of the solvent. However, extrapolation of the melting area at zero heating rate gives a zero melting enthalpy. (ii) Reheating a gel that has been molten and cooled to 20 OC in the DSC pan shows the absence of endotherm a. Further, this endotherm does not reappear even after several days of ageing a t room temperature (Figure 1B). In addition, nosformationexotherm can be detected, unlike other gelling systems.16J7 (iii) These results do not depend on the type of pan. We tried gold-coated pans, graphite pans, and perforated-top pans without noticing any difference. (iv) If the piece of gel is taken out from the DSC pan, cut into several pieces, and then rescanned in another pan, the high melting endotherm reappears (Figure 1A). It would then seem that the magnitude of this endotherm is merely due to a gel-pan contact effect. While we partially confirm Yang and Geil's results, we cannot put forward any firm interpretation. Two explanations might be worth considering a t the moment: the absence of endotherm might be due to the crystals' imperfection or to the crystals' dimensionality. The crystals' imperfection can entail a broad range of melting temperatures. As a result, the melting endotherm is spread over a large domain and is no longer detectable by DSC. The crystals are certainly not as large as in crystalline polymers. Currently, it is conjectured that low-dimen-
Figure 2. Temperature-concentration phase diagram for HTPVCl/diethyl malonate gels aged 24 h at 20 "C. Temperatures determined by DSC, 0,at 20 OC/m (bars indicate width of the melting endotherm) and visually by the ball drop method, 0,at a heating rate of 2 OC/m.
sionality systems (d < 3) should cause an absence of an endotherm.18 For all the solvents we studied, aging led to the appearance of a second endotherm (endotherm B) at always a lower temperature (typically 50 "C, see Figure 1B). The magnitude of this endotherm and the rate at which it appears are solvent dependent. Unlike endotherm A, endotherm B depends on neither the heating rate nor the sample preparation. In other words, endotherm B is perfectly reproducible in spite of its low value (generally = 0.1 cal/g). A similar endotherm has already been observed for platicized PVC.lg It has generally been attributed to a secondary crystallization of lower order which is regarded as responsible for the stiffening of plasticized PVC as a function of time.8 Our recent results obtained on PVC aggregates from dilute solutions do agree with the existence of two types of links." One type is said to be stronger and due to the more syndiotactic sequences while the second type is thought to arise from the ordering of the less stereoregular sections. The temperature location of endotherm B does not significantly vary with polymer concentration. Leharne et al. have reported a similar behavior of PVC plasticized with a series of phthalate plasticizers (down to 70% W / W ) . ~ They suggeated that the decrease of polymer concentration might simultaneously lead to an increase of crystallite size: hence a compensation between the melting point lowering due to increasing amounts of diluent and the melting point enhancement due to crystal thickening. They mentioned the possibility of a phase transition but dismissed it. While the former hypothesis, although rather far-fetched, might be worth considering in the high polymer concentration range, it cannot hold any longer a t lower polymer concentrations. Crystal thickening is usually a matter of a few degrees for highly crystalline polymers, which is not the case with PVC. We therefore estimate that the phase transition hypothesis is more appropriate. The temperature-concentration phase diagram drawn in Figure 2 for PVC/diethyl malonate gels clearly shows this first-order transition near 50 "C that we shall designate as a gel I-gel I1 transition. As will be seen in the following, this thermal transition is correlated to mechanical changes. 2. Morphology. The morphology, as revealed through optical microscopy, depends strongly on whether liquid-
Macromolecules, Vol. 22, No. 2, 1989
Physical Gels from PVC 845 I
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Figure 3. Optical micrographs ohtained nn ( a , top) HTPVCl/benzyl alcohol gels and (b, bottom) HTPVCl/cyclohexanone-hexanol (40/60) (C = 0.125).
liquid phase separation was involved in the early stage of the gelation process or not. For instance, gels prepared either in benzyl alcohol (0 e 150 "C) or a 40/60 (v/v) cyclohexanone-hexanone mixture exhibit a coarse fibrillike structure (see Figure 3). These fibrils from a network whose mesh size is of the order of 5 pm. This explains why the solvent is so readily expelled from these gels making mechanical measurements difficult. Conversely, gels for which liquid-liquid phase separation did not interfere with gelation do not display the same morphology. Instead a faint salt-and-pepper morphology can sometimes be seen (PVC/diethyl malonate gels for instance) which is reminiscent of what was observed with isotactic polystyrene gels?' We accordingly suggest that the morphology is also fibrillar but with far smaller fibrils. This suggestion meets recent conclusions by Yang and GeiP deduced from electron microscope observations. 3. Swelling Behavior. The swelling behavior of gels that form through a liquid-liquid phase separation and those that do not is quite different. The former exhibits a large syneresis while the latter may either slightly deswell or swell abundantly. In Figure 4 is given the evolution of concentration as a function of time for the latter systems immersed in an excess of their preparation solvent. Depending on the solvent and the polymer tacticity, various types of behavior are observed. For HTPVC/diethyl malonate gels, one observes a slight deswelling while for HTPVC/bromonaphthalene gels a large swelling occurs. However, if LTPVC is used instead of HTPVC with bromonaphthalene, a deswelling similar to that observed for HTPVC/diethyl malonate gels is seen. All happens as if the solvent played a role similar to tacticity, that is, promoting the formation of additional physical links. Accordingly, it is quite possible that the slight deswelling observed in diethyl malonate is not due to a lower solvent
I
Figure 4. Evolution BS a function of time of the polymer concentration for gels immeraed in their preparation solvent (a, top) HTPVCl/bromonaphthalenegel- (C = 0.093(0). 0.135 (0).0.175 (A).0.21 (vj,0.28 10);(h, middle) HTPVCl/diethyl malonate g d n IC = 0.093 (Ob,0.135 (01,0.175 (A),0.25 (VI,0.31 ( 0 ) ;(C. hottom) LTPVC/bromonaphthalene gels (C = 0.093 (0). 0.135 101. 0.175 (AI.0.21 ( 0 ) .0.28 (01. quality t o w d PVC but to the capability of promoting the formation of additional links with the less stereoregular sequences. Gels prepared from HTPVCs of higher molecular weights show only a slight deviation from what is obtained with HTPVCl gels. For instance, samples prepared in diethyl malonate a t 13.5% (w/w) deswell to C = 15.7% for HTPVCl and to C = 17.3% for HTPVC3. The difference is still smaller in bromonaphthalene, where HTPVC1 gels swell up to 9.3% while HTPVC3s swell up to 9.6690 when both prepared at 13.5%. The solvent effect on the swelling ratio C (G = P / P , where Po is the sample's initial weight) is worth examining closer. We studied a series of diesters where alcohol
Macromolecules, Vol. 22, No. 2, 1989
846 Mutin and Guenet
'r
7
T
Table I Reduced Swelling Ratio GB = P/P,- 1 for Gels Prepared in Various Solvents and Aged at Different Times in Their Preparation Solvent" solvent aging time GR = P/Po - 1 diethyl oxalate 20 h 0.09 4 days 9 days
B
20 h
dibutyl oxalate
0.12 1.49 1.64
4 days 9 days 20 h 4 days 9 days
dimethyl adipate
It
0.07 0.05
0.13 0.16 0.13 0.43 0.78 0.83 1.03 1.02 0.99 0.78 0.44 0.41
20 h 4 days 9 days 20 h 4 days 9 days 20 h 4 days 9 days
diethyl adipate
isoamyl acetate
ethyl heptanoate
a The absence of swelling corresponds to GR = 0 (gel sample diameter: 0.8 cm).
GR 02,
IO-
-0 I t I
i
L-_-i-
20
30
L40
05-
i---J 50
T(T)
60
Figure 5. (a, Top) Reduced swelling ratio GR for HTPVCl/ bromonaphthalene gels aged at different temperatures in their preparation solvent for different times (0,1 day; 0 , 4 days; A, 11 days) (C = 0.135). (b, Bottom) Reduced swelling ratio GR for HTPVCl/diethyl malonate gels aged at different temperatures 1 day; 0,3 in their preparation solvent for different times (0, days; A, 7 days; V, 14 days) (C = 0.135).
substituents are changed. For instance, we compared diethyl oxalate and dibutyl oxalate. Also, we examined whether the use of a diester or a monoester would have any influence. Results are reported in Table I. As can be seen, the shorter the alcohol substituent, the lower the swelling ratio. Furthermore, we only observe a significant swelling with the monesters. Admittedly, more tests are needed to confirm or invalidate the above "rules". The effect of temperature has been assessed in the light of DSC results. In Figure 5, the reduced swelling ratio GR (GR = P / P o - 1) is plotted as a function of temperature for HTPVCl/diethyl malonate gels and for HTPVCl/ bromonaphthalene gels. For t S 40 "C the variation seems to be linear. Deviation from linearity occurs for T > 40 "C. This transition seems to be associated with the aforementioned low-melting endotherm observed by DSC experiments. Moreover, this "swelling transition" is also independent of the polymer concentration. It is worth mentioning, however, that although there is a low-melting endotherm in bis(2-ethylhexyl) phthalate, the reduced swelling ratio does not exhibit a marked transition as with the other solvents (see Figure 6). 4. Mechanical Testing. We first investigated the
Figure 6. Reduced swelling ratio GR for HTPVCl/bis(2ethylhexyl)phthalate gels aged at different temperatures in their 1 day; 0,3 days; A, preparation solvent for different times (0, I days; V, 14 days).
-
5 1
I
I
I
1
I
1
'E
3.5
I
t
i
j 5
3~
2
3
loglOI(SI
Figure 7. Stress relaxation for X = 0.8: 0,HTPVCl/diethyl malonate gels; 0,HTPVCl/diethyl adipate gels (in both gels C = 0.175).
stress relaxation of gels submitted to constant deformation. In Figure 7 are given typical curves. As usual the variation
Physical Gels from PVC 847
Macromolecules, Vol. 22, No. 2, 1989
I
heating'al 65% for ldor
E(Pa'
I
.) _ _ _ - -4-
1
1041
~
,
-
I -
I 15
DEMlHTPVC 1 DEO/HTPVC 1 DESlHTPVCl
time (days1 1
, DEA/ HI PVC 1 BN/HT PVC 1 I
co
I 20
I
1
~dr~(gcm-?
Figure 8. Evolution of both the polymer concentration and the compression modulus of gels aged in their preparation solvent (DEM = diethyl malonate, DE0 = diethyl oxalate, DES = diethyl succinate, DEA = diethyl adipate, BN = bromonaphthalene). Aging times: 0 , l h; 0 , 1 day; A, 7 days; V, 14 days; O , 2 1 days.
Ob
40
20
Figure 9. Schematic evolution of the compression modulus as a function of time and treatment. Points are experimental results obtained for HTPVCl/diethyl malonate gels for C = 0.175.
"I 4.0
is of the type log OR cv m log t , where t is the time and m a parameter the value of which is about 0.008-0.015 for short times. Such values correspond to what is found for chemically cross-linked gels22or crystalline physical ge1s,l2 where the cross-links are supposed to be unlabile. For gels that are said to be noncrystalline, such as thermoreversible isotactic polystyrene gels, this parameter is far larger ( m N 0.1-0.15).23 We accordingly conclude that PVC gels do arise from crystallization. Yet, two results are worth emphasizing: (i) While for longer times the PVC/diethyl adipate gels always show the same behavior, gels in diethyl malonate relax more rapidly as shown by an increase of m up to m 0.05. (ii) For the same concentration the modulus in diethyl malonate is about 3.5 times larger than that of gels in diethyl adipate. The latter remark indicates that there are probably more links in PVC/diethyl malonate gels than are in diethyl adipate gels. The former might indicate that these links are more mobile than crystalline links. These links may thus correspond to what was designated as weak links in a previous publication." The evolution of the compression modulus as a function of time, solvent type, and swelling degree can shed some light on this question. These results are summarized in one figure (Figure 8). We have compared the evolution of gels prepared in different solvents at the same polymer concentration (C,)and then immersed into their preparation solvent. The modulus and the concentration reached after a certain time were simultaneously determined. As can be seen, gels of HTPVC in bromonaphthalene and diethyl adipate possess nearly the same modulus and the same swelling degree. Of further note is the invariance of the compression modulus in spite of a significant swelling. In these solvents are obtained the lowest values for the compression modulus. Evidently, there is a correlation between the propensity to swell and the modulus value. The lesser the gels swell the larger is the modulus value. Eventually, a LTPVC/ bromonaphthalene gel exhibits a large modulus increase while deswelling. It is worth noticing that the solvent type can play a role similar to an increase of tacticity as far as the mechanical properties are concerned. As a matter of fact, using diethyl malonate instead of bromonaphthalene with HTPVCl allows a fourfold increase of the modulus and using LTPVC an eightfold increase for C = 0.175. As the increase of syndiotacticity is supposed to produce more physical links, we also conclude
I
30
I
I
I
I
I
loo 200 M* Figure 10. Variation of the compression modulus as a function of PVC molecular weight: . , gels in diethyl malonate (Co = 0.136); 0,gels in bromonaphthalene (Co = 0.175).
that diethyl malonate helps form more links too. From what has been said above, however, we infer that these links are weaker than those purely due to regular syndiotactic sequences and are those that melt in the vicinity of 50 OC. This can be checked by annealing aged gels above 50 "C. Under these conditions, we do retrieve the modulus determined on a freshly prepared gel, which is supposed to contain a lower amount, if any, of these weak links. This behavior is schematically represented in Figure 9. The effect of molecular weight on the compressive modulus has been examined too (see Figure 10). For gels prepared in bromonaphthalene there is approximately a twofold increase of the compression modulus when doubling the molecular weight. Since the larger the PVC molecular weight the higher gel melting pointg we therefore come to the conclusion that the average length of the syndiotactic sequences increases as well. Curiously enough, the increase of modulus with increasing molecular weight is not that marked in diethyl malonate gels. This may stem from the fact that additional links already exist in diethyl malonate gels, unlike bromonaphthalene gels. We also notice that for the highest molecular weight the discrepancy between either gel has considerably been reduced.
Concluding Remarks In this paper we show that the solvent plays a role in the gelation process of PVC on two different levels. First, the interference of a liquid-liquid phase separation with gelation modifies drastically the gel structure. Second, in the absence of a liquid-liquid phase separation we observe another solvent effect: some solvents promote the formation of additional links. What seems common independent of the solvent is the formation of strong links. These links are certainly due to the crystallization of the more regular syndiotactic sequences, an assumption al-
848
Macromolecules 1989,22, 848-852
ready discussed b y several a ~ t h o r s Conversely, . ~ ~ ~ ~ ~ the formation of the weaker links is solvent-dependent. Tabb and Koenig26 have shown in PVC plasticized b y bis(2ethylhexyl) phthalate that there is a strong interaction between C=O and C1-C, which (according to these authors) form a complex. A similar result is reported for PVC in methyl ehter ketone.27 We m a y accordingly infer that the same t y p e of interaction takes place with diethyl malonate and that while one carbonyle interacts with one chain, the other one can interact with another chain, thus forming a bridge. This m a y help the ordering of the less stereoregular parts and form the weak links. Solvents possessing only one site of interaction such as bromo-
naphthalene or isoamyl acetate should therefore not be able to bridge two chains together. T h i s does agree with the results detailed above. A negative influence of the solvent is also possible. The solvation of the PVC chains b y large molecules would take them apart, thus impeding the formation of the weakest links. Additional experiments a r e now needed to confirm or invalidate the above explanations. Registry No. PVC, 9002-86-2; DES,123-25-1; diethyl malonate, 105-53-3;cyclohexanone, 108-94-1;hexanol, 111-27-3;benzyl alcohol, 100-51-6; bromonaphthalene, 27497-51-4; bis(2-ethylhexyl) phthalate, 117-81-7; diethyl oxalate, 95-92-1; dibutyl oxalate, 2050-60-4; dimethyl adipate, 627-93-0; diethyl adipate, 141-28-6; isoamyl acetate, 123-92-2; ethyl heptanoate, 106-30-9.
References and Notes (1) Aiken, W.; Alfrey, Jr., T.; Janssen, A,; Mark, H. J. Polym. Sci. 1947, 2, 178. (2) Alfrey, Jr., T.; Wiederhorn, N.; Stein, R. S.; Tobolsky, A. Znd. Eng. Chem. 1949,41, 701. (3) Walter, A. T. J. Polym. Sci. 1954, 13, 207.
(4) Takahashi, A.; Nakamura, T.; Kagawa, I. Polym. J. 1972, 3, 207. ( 5 ) Haas, H. C.; McDonald, R. L. J. Polym. Sci., Polym. Chem. Ed. 1973, 11, 1133. (6) Guerrero, S. J.; Keller, A.; Soni, P. L.; Geil, P. H. J. Polym. Sci., Polym. Phys. Ed. 1980, 18, 1533. (7) Guerrero, S. J.; Keller, A. J. Macromol. Sci., Phys. 1981, B20(2), 167. (8) Dorrestijn, A.; Keijzers, A. E.; te Nijenhuis, K. Polymer 1981, 22, 305. (9) Yang, Y. C.; Geil, P. H. J.Macromol. Sci., Phys. 1983, B22(2), 463. (IO) Leharne, S. A.; Park, G. S. Eur. Polym. J. 1985,21, 383. (11) Mutin, P. H.; Guenet, J. M.; Hirsch, E.; Candau, S. J. Polymer 1988, 29, 31. (12) He, X.; Herz, J.; Guenet, J. M. Macromolecules 1988, 21, 1757-1763. (13) Belkebir-Mrani, A.; Herz, J.; Rempp, P. Makromol. Chem. 1977, 178, 485. (14) Girolamo. M.: Keller. A.: Mivasaka. K.: Overbergh. - , N. J. Polym. Sci.,'Polym. Phys. Ed. i976, i4,39. (15) Candau, S. J.; Dormoy, Y.; Mutin, P. H.; Debeauvais, A.; Guenet, J. M. Polymer 1987, 28, 1334. (16) Berghmans, H. et al. Polymer 1987,28, 97. (17) Guenet, J. M.; McKenna, G . B. Macromolecules 1988,21,1752. (18) Halperin, B. I.; Nelson, D. R. Phys. Rev. Lett. 1978, 41, 121. (19) Juijn, J. A.; Gisolf, A.; de Jong, W. A. Kolloid Z. Z. Polym. 1969,235, 1157. (20) Leharne, S. A.; Park, G. S.; Norman, R. H. Br. Polym. J. 1979, 11, 7. (21) Guenet, J. M.; Lotz, B.; Wittmann, J. C. Macromolecules 1985, 18, 420. (22) Thirion, P.; Chasset, R. Chim. Znd., Glnie Chim. 1967,97,617. (23) Guenet, J. M.; McKenna, G. B. J.Polym. Sci., Phys. Ed. 1986, 24, 2499. (24) Lemstra, P. J.; Keller, A.; Cudby, M. J.Polym. Sci., Polym. Phys. Ed. 1978,16, 1507. (25) Juijn, J. A.; Gisolf, A.; de Jong, W. A. Kolloid Z. Z. Polym. 1973, 251, 456. (26) Tabb, D. L.; Koenig, J. L. Macromolecules 1975, 8, 929. (27) Monteiro, E. E. C.; Mano, E. B. J. Polym. Sci., Polym. Phys. Ed. 1984, 22, 533.
Polyelectrolyte Tracer Diffusion in a Thermoreversible Gel: Structural Probe across the Gelation of Gelatin Hichang Yoon, Hongdoo Kim,+and Hyuk Yu* Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706. Received June 27, 1988; Revised Manuscript Received August 16, 1988
ABSTRACT Structural changes in gelatin gel across its gel temperature are probed by examining the diffusion coefficient D, of a tracer polyelectrolyte with the technique of forced Rayleigh scattering. The tracer polymer was poly(2-vinylpyridine), quaternized partially with bromoethane and labeled randomly with 4 4 (bromomethy1)azolbenzene. The temperature dependence of Dt, exhibited a significant retardation below the gel temperature, which spans over 3 orders of magnitude, and the retardation is interpreted as a consequence of the reduced effective channel size due to larger crystallite formation at lower temperatures. Further Dtr was found to be free of temperature hysteresis, provided a sufficient time was allowed for the gel to equilibrate at a given temperature. There appeared two components of D, below the gel temperature. The slow component becomes increasingly prominent at lower temperatures and the corresponding diffusion coefficient was found to be 5-10 times smaller than that of the fast component over the temperature range 0-30 "C. A kinetic study at 5.7 *C showed that the gelation process is of second order and it reached an equilibrium state within 10 h after quenching. The observed second order of the reaction is tentatively ascribed to the crystallites growth and annealing processes in the context of the mechanism proposed by Hauschka and Harrington.
Introduction In t h i s study, the forced Rayleigh scattering (FRS) technique was used to d e t e r m i n e the tracer diffusion coefficient of a linear polyelectrolyte diffusing in a 10% solution of gelatin and the corresponding gel which is 'Present address: Polymer Science and Standards Division, National Institute of Standards and Technology, Gaithersburg, MD 20899. 0024-9297/89/2222-0848$01.50/0
thermally induced upon cooling below the gel temperature (33 "C). T h e FRS technique has an unique advantage of following only the tagged molecule, a linear polyelectrolyte in this study, and this was exploited to probe the dynamic processes taking place in the matrix of gelatin solution and its thermoreversible gel b y tracer diffusion coefficient D,. Gelatin, derived from naturally occurring collagen, has been well-known for i t s gelation b e h a ~ i o r . l - ~T h i s is att r i b u t e d to the renaturation of gelatin molecules i n t o
0 1989 American Chemical Society
