- Email: [email protected]
Phase behavior and gelation of solutions of poly(vinylchloride)
ELSEVIER
Polymer Gels and Networks 2 (1994) 159-172 ~) 1994 Elsevier Science Limited Printed in Northern Ireland. All rights reserved 0966-7822/94/$07.00
Phase Behavior and Gelation of Solutions of Poly(vinylchloride)
H. Soenen & H. Berghmans Laboratory of Polymer Research, Catholic University Leuven, Celestijnenlaan 200F, 3001 Heverlee, Belgium
ABSTRACT The phase behavior and the thermoreversible gelation of solutions of PVC in nitrobenzene has been studied. Two concentration domains have to be considered. At low polymer concentration isothermal annealing results in a relatively high degree of crystallinity without the formation of a gel network. At higher polymer content, cooling results in the formation of a thermoreversible gel of low crystallinity. This crystallinity increases after isothermal annealing. The influence of polymer content and annealing temperature is investigated. A model, based on the formation of cooperative intermolecular associations, is proposed in order to explain this complex phase behavior.
INTRODUCTION Commercial poly(vinylchloride) (PVC) has a very low degree of stereoregularity, 12 ° but is nevertheless capable of crystallizing from the melt and from solution. 3 It is well known that in moderately concentrated solutions of commercial PVC transparent thermoreversible gels can be obtained upon cooling. This gel formation can take place in a large variety of solvents. 4-7 The formation mechanisms of these gels as well as the network structure are still under discussion. Different models have been proposed in the literature to explain the behavior of PVC in solution and in dry form. In many reports it has been proposed that the physical cross-links consist of small crystals, resulting from a crystallization of the syndiotactic sequences, in spite of the low degree of stereoregularity in PVC. This view has been supported by extensive discussions on gel melting behavior in terms of the thermodynamics of melting point depression. In order to account for the low degree of stereoregularity, Juijn et al. 8 have proposed the possibility of a 159
160
H. Soenen, H. Berghmans
co-crystallization between iso- and syndiotactic units. Keller et al. reported the existence of two different crystal populations in a PVC-gel particle. 9 Both crystallites show a different orientation when the gel is stretched. Type-A crystals, which are believed to be lamellar in origin, arise from the longer syndiotactic sequences and orient with the molecular axis perpendicular to the stretching direction. Type-B crystals, on the other hand, are supposed to possess a micellar morphology. They orient with their molecular axis parallel to the stretching direction. Only B-crystals are responsible for the network formation while A-crystals are either loosely connected with the polymer network or not at all. Guenet et al.10 have pointed out that two types of link exist in a gel particle: strong links, resulting from a crystallization between syndiotactic sequences and, depending on the solvent structure, weak links between less stereoregular sequences. The strong interactions are primarily responsible for the network formation while the weak interactions stiffen the gel. Yang and Geil tl suggested that PVC-gelation, at least in its early stage, is due to the formation of interchain hydrogen bonds. Afterwards crystals can develop, but these are not responsible for the network. EXPERIMENTAL The PVC used was prepared by a suspension polymerization. The average molecular weights, measured by gel permeation chromatography, are M, = 76 × 103 and Mw = 136 × 103. The tacticity, determined by SaC NMR and expressed in tryads, is 19% isotactic, 50% heterotactic and 30% syndiotactic. Calorimetric measurements are performed with a Perkin Elmer Differential Scanning Calorimeter, DSC 2C, at scanning rates reported in the text. Large volume sample pans, containing 30 mg sample are used. Gels in nitrobenzene, up to 30 wt % PV, are prepared by dissolving the polymer at 150°C under stirring. Once a transparent, homogeneous solution is obtained, this mixture is allowed to cool to room temperature and is transferred to a DSC-pan. Higher concentrated PVC-nitrobenzene solutions are prepared directly in a DSC-pan, and homogenized at 240°C for a short time. The temperature at the end of the melting endotherm is taken as the melting temperature, Tin. Gel melting temperatures are measured optically. A piece of gel is sealed in a glass tube. These tubes are suspended upside down in an oil bath and heated. The temperature at which flow sets in is taken as the gel-sol temperature. 12 Wide angle X-ray Diffraction (WAXD) patterns are measured in the reflection mode with a Phillips PW 1729 generator. The scanning speed was 1 (20) per 8 min. The angular range between 8° < 20 < 50° is investigated. RESULTS Dry PVC In this part, the data obtained from experiments that wcrc alrcady discussed in the literature 13-18 and from some new type of experiments will be reported.
Phase behavior and gelation of solutions of poly(vinylchloride)
161
__.....-.
50
70
c u r v ~
~ ~
90
190
110
130
150
170
210
230
temperature (°C) IFig. 1. DSC-curvesfor unplasticized PVC, recorded at2~C/min. Curve l:heatingscan;curve 2:coolmg scan.
The literature experiments were repeated in order to dispose of a homogeneous set of data obtained with the same polymer sample under well controlled experimental conditions.
Dynamic observations Solvent-free PVC is subjected to a heating and cooling cycle in the DSC. Curve 1 in Fig. 1 represents the thermogram obtained on heating, while curve 2 represents the thermogram obtained on cooling. The scanning rate is 20°C/min. The shift in the baseline of curve 1 between 80 and 100°C is due to a glass transition. This glass transition is followed by a broad endothermic transition extending from 120 to 230°C. At higher temperatures, degradation sets in. Similar DSC-heating curves for PVC have been reported in the literature. 13-18 In these reports the broad endothermic transition is always attributed to a gradual melting of PVC-crystals. Because of a distribution in crystal size and crystal perfection the melting process of the crystals takes place in a very broad temperature region. The opposite transitions are observed to take place on cooling. A broad exothermic signal sets in at 175°C and extends up to the glass transition, which manifests itself also as a shift in the baseline. In Fig. 2 DSC-cooling curves measured at different scanning rates are presented. In all these curves an exothermic transition is present, even at scanning rates as high as 40°C/min. In Fig. 3 the onset-temperature of these exotherms is plotted as a function of the cooling rate. The cooling rate has only a limited effect on the position of the exotherm on the temperature scale, and leads to 185°C for zero scanning rate. In Fig. 4 two DSC-curves are presented. Curve 1 is measured after cooling the sample in the DSC from 240°C to room temperature at a programmed scanning rate of 320°C/min. It is evident that the sample will not exactly follow this high cooling rate because of the lack of thermal equilibrium. But in this
162
H. Soenen, H. Berghmans
~,,.5 ° Clmin.
~ O ° C / m i n . f
o
50
7'0
"
.
_
li0
90
~
140
150
1'70
190
temperature (eC)
Fig. 2. DSC-cooling curves, measured at different scanning rams for unplasticizcd PVC. The scanning rate is indicated in the figure.
case the DSC is only used in order to prepare a sample that is cooled at a scanning rate intermediate between 40°C/min and the cooling rate of a sample quenched in liquid nitrogen. Curve 2 is measured after quenching the sample from 240°C in liquid nitrogen. Both curves exhibit at lower temperatures a baseline shift due to a glass transition and at higher temperatures an endotherm. Curve 1 shows the expected broad endotherm. This proves that the structure formation takes place already at a cooling rate of 320°C/rain. 200
I
I
,
I
490
~" 180
,= +,
o
170
r'~
E 160
150
140
I
I
10
20
cooling Fig. 3.
I
I
30
40
50
rate ( ° C / m i n . )
Onset temperatures of the exotherms measured in Fig. 2 as a function of scanning rate.
Phase behavior and gelation of solutions of poly(vinylchloride)
163
curve I
t •
0
210
230
temperature (°C) Fig. 4. DSC-heating curves of unplasticized PVC recorded at 20*C/min after different cooling treatments. Curve 1: after cooling at 320*C/rain; curve 2: after quenching in liquid nitrogen.
Quenching in liquid nitrogen seems to transform the sample into an amorphous glass. Heating just above Tg (curve 2) introduces very quickly a supra molecular structure by an exothermic process and this is followed by the endothermic melting.
Isothermal annealing The behavior of PVC after isothermal annealing has also been investigated. Figure 5 represents DSC-heating scans recorded after isothermal annealing at
10-1 i
•
i
10.
m
60
8b
lbO
lio
1~o
leo
~i3o
2bo
21o
temperature (*C) Fig. 5. DSC-curves recorded at 20*C/min after isothermal annealing at 120"C for different annealing times: (1) no annealing, (2) 70s, (3) 90s, (4) 2 rain, (5) 5 min, (6) 12 rain, (7) 21 rain, (8) 60 rain, (9) 104 rain, (10) 225 rain.
164
H. Soenen, H. Berghmans
120°C for different lengths of time. These DSC-curves show that isothermal annealing induces an additional and rather sharp endotherm, superimposed on the broad endotherm. This superposition of two endotherms is clearly illustrated by the differential curve, obtained by subtracting curve 1 from curve 10. The structures responsible for the sharp endotherm are therefore due to an additional structure formation and not to a transformation of the already existing structures into new ones. We ascribe these additional endotherms to a melting process, therefore we define the temperature at the end of the endotherm as a melting temperature and the corresponding enthalpy as a melting enthalpy. The corresponding melting temperature and melting enthalpy increase logarithmically with increasing annealing time. The increase in crystallinity of the sample is also reflected in the change of the WAXD patterns. These patterns illustrate the formation of crystals by the syndiotactic units. A typical sequence is represented in Fig. 6, representing WAXD patterns recorded after different thermal treatments, reported in the legend. An increase in the annealing time results in the appearance of diffraction signals characteristic of the crystal structure of syndiotactic PVC. It is clear that at least 5 min are needed to introduce a recognizable crystal texture. The influence of the annealing temperature is also investigated. The melting point of the sharp melting endotherm increases linearly with increasing temperature. The behavior of PVC after isothermal annealing as described in
3.
1. 40 38 3,6 34 3,2 30 2,8 2~6 24 2,2 2,0 1,8 1,6 14 12 1=0
•
diffraction angle 2 0
Fig. 6. WAXD patterns of solvent-free PVC after different thermal treatments. (1) Quenched from 240°C; (2) quenched from 24ff'C, heated to 100°C and again quenched; (3) quenched from 240°C, 2 rain at 100°C; (4) quenched from 240°C, 5 rain at 100°C; (5) quenched from 240°C, 60 rain at 100°C.
Phase behavior and gelation of solutions of poly(vinylchloride)
165
literature 13-18 agrees well with these measurements, and in many reports endotherms developed after annealing are ascribed to the melting of crystals formed during annealing.
PVC-nitrobenzene mixtures
The same series of experiments has been performed in the presence of nitrobenzene. Transparent gels are formed in the low concentration region.
Dynamic observations PVC-nitrobenzene mixtures have been investigated by DSC. The experimental approach, used for the solvent-free PVC, has been used. The data are similar to those obtained in absence of the solvent. Some DSC-cooling curves are shown in Fig. 7. Cooling results in a broad exotherm and the temperature at the onset of this exotherm decreases with increasing solvent content. The endotherm, observed on heating, shifts to lower temperatures when the solvent content is increased. These corresponding temperatures are reported in Fig. 8. Nitrobenzene plasticizes PVC. This results in a decrease in the glass transition with increasing nitrobenzene content. This concentration dependence of Tg is also reported in Fig. 8 as a function of solvent content. The temperature at which the gel-sol transition takes place was measured at PVC concentrations between a mass fraction of PVC of 0.05-0.30. The temperature at which
t o
30
Fig. 7.
50
70
90 110 130 temperature (°C)
150
170
DSC-cooling curves of some PVC-nitrobenzene samples recorded at 20°C/min. The mass fraction of PVC is indicated.
166
H. Soenen, H. Berghmans 250
200
'
'
o/"
'
150 tO o
D
-~
.
mm
100
o (3-
E ~
-'
50
/
/
I
-50 0.0
0.2
i
I
I
I
0.4
0.6
0.8
d
i
.0
mass froction PVC Fig. g. Temperature-concentration diagram for PVC-nitrobenzene. Dynamic measurements: O, temperature at the end of the broad endotherm; A, gel-sol transition measured on beating; 41,, crystallization temperature of nitrobenzene; II, temperature at the onset of the exotherm; O, glass transition.
traces of liquification are first observed is taken as the gel melting temperature. This temperature increases as the mass fraction of PVC increases. Isothermal experiments PVC-nitrobenzene mixtures were annealed at five different temperatures for one week. They were subjected to a DSC-heating procedure after this thermal treatment. This annealing results in the appearance of a supplementary endotherm as was also observed in the solvent-free samples. The corresponding melting temperatures are presented in Fig. 9, together with the transition temperatures reported in Fig. 8. Their concentration dependence can be subdivided in two parts: at low polymer concentration, an important increase of the melting temperature with increasing polymer content is observed; at higher polymer concentration, this melting point is almost independent of the overall concentration. The plateau temperature increases with increasing annealing temperature. The limit between the two concentration regions is made up by the gel melting temperature-concentration relationship. These two concentration domains also differ by the degree of crystallinity that can be obtained after annealing. These crystallinity-concentration relationships are represented in Fig. 10, and the DSC-curves in Fig. 11. The difference in melting enthalpy observed in the two concentration regions is most pronounced at low annealing temperatures. At a mass fraction smaller than 0-10, a maximum value of 23 J/g is measured. Once the gel can be formed, this melting enthalpy decreases to almost zero and, on increasing the mass fraction
Phase behavior and gelation of solutions of poly(vinylchloride) 250
i
i
i
200
167
i
/o o~ _
_
~.... ¢%e.
150 (..) o
**/*
* +
;.',-:/
m*
/
-6 loo Q.
E ~
50
,
--50
¢
/6
I
0.0
0.2
i
I
0.4
,
l
i
0.6
I
0.8
o/
i
.0
mass fraction PVC Fig. 9. Temperature-concentration diagram for PVC-nitrobenzene after isothermal annealing at different temperatures. Dynamic measurements: ©, temperature at the end of the broad endotherm; A, gel-sol transition measured on heating; 0 , crystallization temperature of nitrobenzene; O, glass transition. Isothermal observations: temperatures at the end of extra endotherms alter isothermal annealing at: [] = 20°C, • = 40*C, A = 60°C, + = 80*C, • = 100°C.
of PVC, rises then again to a plateau value of 1.5 J/g. The maximum in the lower concentration region decreases as the annealing temperature is raised. DISCUSSION General considerations
The endothermic phenomena, observed on heating an unplasticized PVC sample, have been reported in the literature and are always ascribed to the melting of crystals. This study shows that the broad endothermic transition observed in a DSC-heating scan can be related to a broad exothermic process that takes place on cooling. This exothermic process occurs even at very high cooling rates and can only be suppressed by quenching the DSC-pan in liquid nitrogen. After this quenching, an exothermic transition takes place in a subsequent heating scan, as soon as the glass transition region is reached. Isothermal annealing on the other hand induces sharp, additional endotherms. This results in a development of crystallinity as shown by X-ray scattering observations. This proves that a three dimensional crystalline structure develops by isothermal annealing. The formation of a three dimensional crystalline structure during a fast cooling procedure is much less evident, especially in the presence of a solvent.
H. Soenen, H. Berghmans
168
25
i
i
i
.
=
25
i
= =
80°C
20
20
15
15 Q.
Q_
0
0
r-
10
g
10
E O~
÷
\
A + ~
+ " " ' ~ + "Jf'÷~ ÷ ~
+~
0 0.0
,
.+÷ I
i
0.2 mess
25
~f~
I
,
0.4
i
0.6
fraction
i
=
I
0.8
o o.o
,
1.0
1
~=...,~.....--AA~tA-A--~
0.2
0.4
25
i
,
,
moss
PVC
i
.
i
,
0.6
fraction
¢
,
0.8
1.0
PVC
i
=
o
40°C
i
20°C
20
20
3
60°C
15
3
15
CL 0
t-
lO
c-
t.-
Io
E
l. . If"''--, 0 . . 0.0 0.2
"-* "--'*~ "-~ 0.4
0.6
"-;f 0.8
moss f r o c t i o n PVC
1.0
0 0.0
0.2 moss
0.4
0.6
fraction
0.8
.0
PVC
Fig. 10. Melting-enthalpy-concentration relationship for the system nitrobenzene-PVC. This melting enthalpy is expressed in Joules per gram PVC. The annealing temperature is indicated.
The syndiotacticity of PVC is very low and these syndiotactic units are expected to be responsible for the structure formation. It has been shown, by a statistical treatment of N M R data, that only a very low quantity of syndiotactic sequences capable of forming a stable crystal is present. 8 The high final melting temperature of the broad endotherm, supposes the formation of very stable crystals, while the statistical treatment, mentioned above, suggests that the formation of crystals of this dimension is almost to be excluded. The rate at
Phase behavior and gelation of solutions of poly(vinylchloride)
169
.61
0.13
--~.o3~
20
40
gO
80
temperature (°C) Fig. 11. DSC-hcating curves for different PVC-nitrobcnzcn¢ mixtures recorded at 20°C/rain after isothermal annealing at 20°C for ] week. The mass fraction of PVC is indicated.
which a structure formation takes place on cooling is also surprising in view of the poor stereoregularity of the polymer chains. Isothermal annealing, however, results in the rather slow formation of well defined crystals that melt in a narrow temperature region. An additional problem arises when this annealing takes place in the presence of nitrobenzene. The phase diagram has to be subdivided into two concentration regions each with their own concentration dependence of the melting point and crystallinity. It has been reported in literature 19 - 2 2 that domains are formed 100-800 A apart on cooling a dry PVC sample as well as a plasticized sample. It is difficult to explain this complex behavior of a polymer chain with limited stereoregularity by the general principles of polymer crystallization. Therefore, an alternative explanation will be given. It is based on the formation of intermolecular associations, reported in the literature, 23"24 for chemical complementary macromolecules like polyelectrolytes. Chemically complementary macromolecules (e.g. polyacid and polybase) associate in solution to form intermolecular complexes. These intermolecular associations can also occur between different polymer chains by the formation of, e.g. hydrogen bonds. The basic condition is the presence, along the polymer chain, of complementary chemical functions so that cooperative interaction can take place over a certain distance. These problems have been treated extensively in the literature and were compiled some years ago in excellent review papers. 23'24 The reader is referred to these publications and other ones in this field. This intermolecular complex formation is characterized by an equilibrium constant.
H. Soenen, H. Berghmans
170
$1 $2 K C polymer sequence 1 + polymer sequence 2,~ polymer complex The temperature dependence of K is given by: d In K dO/T)
- AH ° =
n
where A/-/~ represents the change in standard enthalpy upon complex formation. This complex formation is exothermic and an increase in temperature will shift the equilibrium to the left. This shift will lead to the decomposition of the complex (decrease in K). Cooling will promote complex formation and shift the equilibrium to the right (increase in K). The change in K will proceed over a certain temperature range and the width of this temperature range will decrease as the cooperativity increases. The degree of cooperativity increases with increasing length of the syndiotactic sequence. Therefore, long syndiotactic sequences will have this equilibrium shifted over a narrow temperature range while shorter sequences will need a broader range. An increase in the sequence length will also shift the position of the equilibrium on the temperature scale to higher temperatures. This is schematically represented in Fig. 12. This model postulates that a stereoregular sequence as a whole takes part in the association-dissociation process. The experimentally observed transition will correspond to the summation of the transitions of the different sequences taking into account their populations.
C 0
1
o in
== LO~ oJ
"o 0 temperature
B
temperature Fig. 12. ':,chematical presentation of: (A) Influence of the sequence length on the equilibrium transition between mr" associated and non-associated situation. (The number of chain units in the sequence is indicated in the figure.) (B) Heat transfer as a consequence of sequence association.
Phase behavior and gelation of solutions of poly(vinylchloride)
171
Interactions in PVC
In PVC, this type of intermolecular, exothermic association can take place between the hydrogen bonds of the polymer chain. Cooperativity between subsequent C1--C--H groups of a PVC-chain is possible for the association between two syndiotactic sequences, and an equilibrium of the following type is formed: \ \ \ \ H--C--C1 H--C---C1 H--C--CI--H--C--CI / / / / CH2 CH2 CH2 CH2 \ \ \ \ CI--C--H CI--C--H CI---C--H--CI--C--H / / K / / CH2 CH2 ~CH2 CH2 \ \ \ \ H--C--C1 H--C--C1 H--C--C1--H--C--C1 / / / /
CH2 \
CH2
CH2
\
\
CH2 \
These associations are responsible for the gel formation and also act as nucleation points for further crystallization during isothermal annealing. The presence of the solvent has an important influence. In the presence of an excess of nitrobenzene, this fast intermolecular association can not take place and no gel is formed. Isothermally crystallization can proceed to a high degree of crystallinity. At higher polymer concentration, an elastic gel is quickly formed by the occurrence of these associations. This is reflected in the broad exo- and endothermic signals, observed on cooling and heating, respectively. The presence of this gel network and the osmotic pressure that results from this network formation will limit further crystallization and keep the crystallinity very low. The additional endotherm, observed on melting the sample, represents only a few J/g (PVC). The concentration independence of the melting point of these crystals suggests that they are formed in almost solvent-free polymer domains. CONCLUSION The investigation of the structure formation in the system PVC-nitrobenzene suggests that two mechanisms of structure formation are operative. Cooling results in a very fast formation of intermolecular associations which induce thermoreversible gelation in the presence of an excess of solvent. This process is very fast and can only be suppressed by quenching to a very low temperature. Isothermal annealing of these associated samples induces the formation of a crystalline phase. ACKNOWLEDGEMENT The authors wish to thank the IWONL for a fellowship and for financial support. Financial support by the National Fund for Scientific Research and the Ministry of Scientific Programmation through the IUAP-16 is gratefully acknowledged. The authors also thank Professor R. Koningsveld for his interest and constructive advice.
172
H. Soenen, H. Berghmans REFERENCES
1. Liu, N. I., Tong, S. N. & Koenig, J. L., Stereoregularity studied by C 13 NMR. J. Appl. Polym. Sci., 25 (1980) 2205-10. 2. Nakajima, A., Hamada, H. & Hayashi, S., Structure and some physical properties of PVC polymerized at different temperatures. Die Makromol. Chem., 95 (1966) 40-51. 3. Dawson, P. C., Gilbert, M. & Maddams, W. F., Comparison of X-ray diffraction and thermal analysis methods for assessing order in PVC. J. Polym. Sci. Polym. Phys., 29 (1991) 1407-18. 4. Kawanishi, K., Takeda, Y. & Inoue, T., The sol-gel transition and the liquid-liquid phase separation in PVC solutions. Polym. J., 18 (1986) 411-6. 5. Kawanishi, K., Komatsu, M. & Inoue, T., Thermodynamic considerations of the sol-gel transition in polymer solutions. Polymer, 28 (1987) 980-4. 6. Dorrestijn, A. & te Nijenhuis, K., Viscoelastic behaviour during gelation of various molar-mass polyvinylchloride solutions and polymerization temperature. Coll. Polym. Sci., 268 (1990) 895-900. 7. Jackson, R. S., Bower, D. I. & Maddams, W. F., Ageing in wet PVC gels studied by gel-sol transition temperature and raman spectroscopy. Polymer, 31 (1990) 875-60. 8. Jujin, J. A., Gisolf, ,l. H. & de .long, W. A., Crystallinity in atactic PVC. Kolloid-Z.u.Z.Polymere, 251 (1973) 456-473. 9. Guerrero, S. J., Keller, A., Soni, P. L. & Geil, P. H., A study of texture in PVC gels by infrared dichro'ism and X-ray diffractions. J. Macromol. Sci.-Phys., B20(2) (1981) 161-66. 10. Najeh, M., Munch, J. P. & Guenet, J. M., Physical gels from PVC: effect of solvent type. Macromolecules, 25 (1992) 7018-23. 11. Yang, Y. C. & Geil, P. H., Morphology and properties of PVC/solvent gels. J. Macromol. Sci.-Phys. B22(3) (1983) 463-89. 12. Matsuzawa, S., Yamaura, K., Maeda, R. & Ogasawara, K., Melting behavior of syndiotacticity-rich poly(vinylalcohol) gel. Macrmol. Chem., 180 (1979) 229-39. 13. Illers, K. H., Influence of thermal history upon physical properties of PVC. J. Macromol. Sci.-Phys., B14(4) (1977) 471-82. 14. Brown, H. R., Musundi, G. M. & Stachurski, Z. H., Effect of plasticizer concentration and annealing on the structure of PVC. Polymer, 23 (1982) 1508-14. 15. Gray, A. & Gilbert, M., Structural order in heat treated vinyl chloride polymers. Polymer, 17 (1976) 44-50. 16. Ohta, S., Kajiyama, T. & Takayamagi, H., Annealing effect on the microstructure of PVC. Polymer Engng Sci., 16 (1976) 465-72. 17. Wilski, H., Die spezifische W~m des Polyvinylchlorids II. Kolloid-Z.u.Z.Polymere, 238 (1970) 426--49. 18. Miinstedt, H., Relationship between rheological properties and structure of PVC. J. Macromol. Sci.-Phys., B14(2) (1977) 195-212. 19. Blundell, D. J., Small-angle X-ray study of microdomains in rigid PVC. Polymer, 20 (1979) 934-8. 20. Neilson, G. F. & Jabarin, S. A., Small-angle X-ray scattering study of microstructure of PVC. J. Appl. Phys., 46 (1975) 1175-83. 21. Soni, P. L., Geil, P. H. & Collins, E. A., Microdomain structure in plasticized PVC. J. Macromol. Sci. Phys., B20(4) (1981) 479-503. 22. Brown, H. R., Kasakevich, M. & Wigmall, G. D., A SANS study of the structure of plasticized PVC. Polymer, 27 (986) 1345-8. 23. Kabanov, V. A. & Papisov, I. M., Formation of complexes between complementary synthetic polymers and oligomers in dilute solution: review. Polymer Sci. USSR, 21 (1979) 261-307. 24. Tsuchida, E. & Abe, K., Interactions between macromolecules in solution and intermolecular complexes. Adv. Polym. Sci., 45 (1982) 1-130.
Polymer Gels and Networks 2 (1994) 159-172 ~) 1994 Elsevier Science Limited Printed in Northern Ireland. All rights reserved 0966-7822/94/$07.00
Phase Behavior and Gelation of Solutions of Poly(vinylchloride)
H. Soenen & H. Berghmans Laboratory of Polymer Research, Catholic University Leuven, Celestijnenlaan 200F, 3001 Heverlee, Belgium
ABSTRACT The phase behavior and the thermoreversible gelation of solutions of PVC in nitrobenzene has been studied. Two concentration domains have to be considered. At low polymer concentration isothermal annealing results in a relatively high degree of crystallinity without the formation of a gel network. At higher polymer content, cooling results in the formation of a thermoreversible gel of low crystallinity. This crystallinity increases after isothermal annealing. The influence of polymer content and annealing temperature is investigated. A model, based on the formation of cooperative intermolecular associations, is proposed in order to explain this complex phase behavior.
INTRODUCTION Commercial poly(vinylchloride) (PVC) has a very low degree of stereoregularity, 12 ° but is nevertheless capable of crystallizing from the melt and from solution. 3 It is well known that in moderately concentrated solutions of commercial PVC transparent thermoreversible gels can be obtained upon cooling. This gel formation can take place in a large variety of solvents. 4-7 The formation mechanisms of these gels as well as the network structure are still under discussion. Different models have been proposed in the literature to explain the behavior of PVC in solution and in dry form. In many reports it has been proposed that the physical cross-links consist of small crystals, resulting from a crystallization of the syndiotactic sequences, in spite of the low degree of stereoregularity in PVC. This view has been supported by extensive discussions on gel melting behavior in terms of the thermodynamics of melting point depression. In order to account for the low degree of stereoregularity, Juijn et al. 8 have proposed the possibility of a 159
160
H. Soenen, H. Berghmans
co-crystallization between iso- and syndiotactic units. Keller et al. reported the existence of two different crystal populations in a PVC-gel particle. 9 Both crystallites show a different orientation when the gel is stretched. Type-A crystals, which are believed to be lamellar in origin, arise from the longer syndiotactic sequences and orient with the molecular axis perpendicular to the stretching direction. Type-B crystals, on the other hand, are supposed to possess a micellar morphology. They orient with their molecular axis parallel to the stretching direction. Only B-crystals are responsible for the network formation while A-crystals are either loosely connected with the polymer network or not at all. Guenet et al.10 have pointed out that two types of link exist in a gel particle: strong links, resulting from a crystallization between syndiotactic sequences and, depending on the solvent structure, weak links between less stereoregular sequences. The strong interactions are primarily responsible for the network formation while the weak interactions stiffen the gel. Yang and Geil tl suggested that PVC-gelation, at least in its early stage, is due to the formation of interchain hydrogen bonds. Afterwards crystals can develop, but these are not responsible for the network. EXPERIMENTAL The PVC used was prepared by a suspension polymerization. The average molecular weights, measured by gel permeation chromatography, are M, = 76 × 103 and Mw = 136 × 103. The tacticity, determined by SaC NMR and expressed in tryads, is 19% isotactic, 50% heterotactic and 30% syndiotactic. Calorimetric measurements are performed with a Perkin Elmer Differential Scanning Calorimeter, DSC 2C, at scanning rates reported in the text. Large volume sample pans, containing 30 mg sample are used. Gels in nitrobenzene, up to 30 wt % PV, are prepared by dissolving the polymer at 150°C under stirring. Once a transparent, homogeneous solution is obtained, this mixture is allowed to cool to room temperature and is transferred to a DSC-pan. Higher concentrated PVC-nitrobenzene solutions are prepared directly in a DSC-pan, and homogenized at 240°C for a short time. The temperature at the end of the melting endotherm is taken as the melting temperature, Tin. Gel melting temperatures are measured optically. A piece of gel is sealed in a glass tube. These tubes are suspended upside down in an oil bath and heated. The temperature at which flow sets in is taken as the gel-sol temperature. 12 Wide angle X-ray Diffraction (WAXD) patterns are measured in the reflection mode with a Phillips PW 1729 generator. The scanning speed was 1 (20) per 8 min. The angular range between 8° < 20 < 50° is investigated. RESULTS Dry PVC In this part, the data obtained from experiments that wcrc alrcady discussed in the literature 13-18 and from some new type of experiments will be reported.
Phase behavior and gelation of solutions of poly(vinylchloride)
161
__.....-.
50
70
c u r v ~
~ ~
90
190
110
130
150
170
210
230
temperature (°C) IFig. 1. DSC-curvesfor unplasticized PVC, recorded at2~C/min. Curve l:heatingscan;curve 2:coolmg scan.
The literature experiments were repeated in order to dispose of a homogeneous set of data obtained with the same polymer sample under well controlled experimental conditions.
Dynamic observations Solvent-free PVC is subjected to a heating and cooling cycle in the DSC. Curve 1 in Fig. 1 represents the thermogram obtained on heating, while curve 2 represents the thermogram obtained on cooling. The scanning rate is 20°C/min. The shift in the baseline of curve 1 between 80 and 100°C is due to a glass transition. This glass transition is followed by a broad endothermic transition extending from 120 to 230°C. At higher temperatures, degradation sets in. Similar DSC-heating curves for PVC have been reported in the literature. 13-18 In these reports the broad endothermic transition is always attributed to a gradual melting of PVC-crystals. Because of a distribution in crystal size and crystal perfection the melting process of the crystals takes place in a very broad temperature region. The opposite transitions are observed to take place on cooling. A broad exothermic signal sets in at 175°C and extends up to the glass transition, which manifests itself also as a shift in the baseline. In Fig. 2 DSC-cooling curves measured at different scanning rates are presented. In all these curves an exothermic transition is present, even at scanning rates as high as 40°C/min. In Fig. 3 the onset-temperature of these exotherms is plotted as a function of the cooling rate. The cooling rate has only a limited effect on the position of the exotherm on the temperature scale, and leads to 185°C for zero scanning rate. In Fig. 4 two DSC-curves are presented. Curve 1 is measured after cooling the sample in the DSC from 240°C to room temperature at a programmed scanning rate of 320°C/min. It is evident that the sample will not exactly follow this high cooling rate because of the lack of thermal equilibrium. But in this
162
H. Soenen, H. Berghmans
~,,.5 ° Clmin.
~ O ° C / m i n . f
o
50
7'0
"
.
_
li0
90
~
140
150
1'70
190
temperature (eC)
Fig. 2. DSC-cooling curves, measured at different scanning rams for unplasticizcd PVC. The scanning rate is indicated in the figure.
case the DSC is only used in order to prepare a sample that is cooled at a scanning rate intermediate between 40°C/min and the cooling rate of a sample quenched in liquid nitrogen. Curve 2 is measured after quenching the sample from 240°C in liquid nitrogen. Both curves exhibit at lower temperatures a baseline shift due to a glass transition and at higher temperatures an endotherm. Curve 1 shows the expected broad endotherm. This proves that the structure formation takes place already at a cooling rate of 320°C/rain. 200
I
I
,
I
490
~" 180
,= +,
o
170
r'~
E 160
150
140
I
I
10
20
cooling Fig. 3.
I
I
30
40
50
rate ( ° C / m i n . )
Onset temperatures of the exotherms measured in Fig. 2 as a function of scanning rate.
Phase behavior and gelation of solutions of poly(vinylchloride)
163
curve I
t •
0
210
230
temperature (°C) Fig. 4. DSC-heating curves of unplasticized PVC recorded at 20*C/min after different cooling treatments. Curve 1: after cooling at 320*C/rain; curve 2: after quenching in liquid nitrogen.
Quenching in liquid nitrogen seems to transform the sample into an amorphous glass. Heating just above Tg (curve 2) introduces very quickly a supra molecular structure by an exothermic process and this is followed by the endothermic melting.
Isothermal annealing The behavior of PVC after isothermal annealing has also been investigated. Figure 5 represents DSC-heating scans recorded after isothermal annealing at
10-1 i
•
i
10.
m
60
8b
lbO
lio
1~o
leo
~i3o
2bo
21o
temperature (*C) Fig. 5. DSC-curves recorded at 20*C/min after isothermal annealing at 120"C for different annealing times: (1) no annealing, (2) 70s, (3) 90s, (4) 2 rain, (5) 5 min, (6) 12 rain, (7) 21 rain, (8) 60 rain, (9) 104 rain, (10) 225 rain.
164
H. Soenen, H. Berghmans
120°C for different lengths of time. These DSC-curves show that isothermal annealing induces an additional and rather sharp endotherm, superimposed on the broad endotherm. This superposition of two endotherms is clearly illustrated by the differential curve, obtained by subtracting curve 1 from curve 10. The structures responsible for the sharp endotherm are therefore due to an additional structure formation and not to a transformation of the already existing structures into new ones. We ascribe these additional endotherms to a melting process, therefore we define the temperature at the end of the endotherm as a melting temperature and the corresponding enthalpy as a melting enthalpy. The corresponding melting temperature and melting enthalpy increase logarithmically with increasing annealing time. The increase in crystallinity of the sample is also reflected in the change of the WAXD patterns. These patterns illustrate the formation of crystals by the syndiotactic units. A typical sequence is represented in Fig. 6, representing WAXD patterns recorded after different thermal treatments, reported in the legend. An increase in the annealing time results in the appearance of diffraction signals characteristic of the crystal structure of syndiotactic PVC. It is clear that at least 5 min are needed to introduce a recognizable crystal texture. The influence of the annealing temperature is also investigated. The melting point of the sharp melting endotherm increases linearly with increasing temperature. The behavior of PVC after isothermal annealing as described in
3.
1. 40 38 3,6 34 3,2 30 2,8 2~6 24 2,2 2,0 1,8 1,6 14 12 1=0
•
diffraction angle 2 0
Fig. 6. WAXD patterns of solvent-free PVC after different thermal treatments. (1) Quenched from 240°C; (2) quenched from 24ff'C, heated to 100°C and again quenched; (3) quenched from 240°C, 2 rain at 100°C; (4) quenched from 240°C, 5 rain at 100°C; (5) quenched from 240°C, 60 rain at 100°C.
Phase behavior and gelation of solutions of poly(vinylchloride)
165
literature 13-18 agrees well with these measurements, and in many reports endotherms developed after annealing are ascribed to the melting of crystals formed during annealing.
PVC-nitrobenzene mixtures
The same series of experiments has been performed in the presence of nitrobenzene. Transparent gels are formed in the low concentration region.
Dynamic observations PVC-nitrobenzene mixtures have been investigated by DSC. The experimental approach, used for the solvent-free PVC, has been used. The data are similar to those obtained in absence of the solvent. Some DSC-cooling curves are shown in Fig. 7. Cooling results in a broad exotherm and the temperature at the onset of this exotherm decreases with increasing solvent content. The endotherm, observed on heating, shifts to lower temperatures when the solvent content is increased. These corresponding temperatures are reported in Fig. 8. Nitrobenzene plasticizes PVC. This results in a decrease in the glass transition with increasing nitrobenzene content. This concentration dependence of Tg is also reported in Fig. 8 as a function of solvent content. The temperature at which the gel-sol transition takes place was measured at PVC concentrations between a mass fraction of PVC of 0.05-0.30. The temperature at which
t o
30
Fig. 7.
50
70
90 110 130 temperature (°C)
150
170
DSC-cooling curves of some PVC-nitrobenzene samples recorded at 20°C/min. The mass fraction of PVC is indicated.
166
H. Soenen, H. Berghmans 250
200
'
'
o/"
'
150 tO o
D
-~
.
mm
100
o (3-
E ~
-'
50
/
/
I
-50 0.0
0.2
i
I
I
I
0.4
0.6
0.8
d
i
.0
mass froction PVC Fig. g. Temperature-concentration diagram for PVC-nitrobenzene. Dynamic measurements: O, temperature at the end of the broad endotherm; A, gel-sol transition measured on beating; 41,, crystallization temperature of nitrobenzene; II, temperature at the onset of the exotherm; O, glass transition.
traces of liquification are first observed is taken as the gel melting temperature. This temperature increases as the mass fraction of PVC increases. Isothermal experiments PVC-nitrobenzene mixtures were annealed at five different temperatures for one week. They were subjected to a DSC-heating procedure after this thermal treatment. This annealing results in the appearance of a supplementary endotherm as was also observed in the solvent-free samples. The corresponding melting temperatures are presented in Fig. 9, together with the transition temperatures reported in Fig. 8. Their concentration dependence can be subdivided in two parts: at low polymer concentration, an important increase of the melting temperature with increasing polymer content is observed; at higher polymer concentration, this melting point is almost independent of the overall concentration. The plateau temperature increases with increasing annealing temperature. The limit between the two concentration regions is made up by the gel melting temperature-concentration relationship. These two concentration domains also differ by the degree of crystallinity that can be obtained after annealing. These crystallinity-concentration relationships are represented in Fig. 10, and the DSC-curves in Fig. 11. The difference in melting enthalpy observed in the two concentration regions is most pronounced at low annealing temperatures. At a mass fraction smaller than 0-10, a maximum value of 23 J/g is measured. Once the gel can be formed, this melting enthalpy decreases to almost zero and, on increasing the mass fraction
Phase behavior and gelation of solutions of poly(vinylchloride) 250
i
i
i
200
167
i
/o o~ _
_
~.... ¢%e.
150 (..) o
**/*
* +
;.',-:/
m*
/
-6 loo Q.
E ~
50
,
--50
¢
/6
I
0.0
0.2
i
I
0.4
,
l
i
0.6
I
0.8
o/
i
.0
mass fraction PVC Fig. 9. Temperature-concentration diagram for PVC-nitrobenzene after isothermal annealing at different temperatures. Dynamic measurements: ©, temperature at the end of the broad endotherm; A, gel-sol transition measured on heating; 0 , crystallization temperature of nitrobenzene; O, glass transition. Isothermal observations: temperatures at the end of extra endotherms alter isothermal annealing at: [] = 20°C, • = 40*C, A = 60°C, + = 80*C, • = 100°C.
of PVC, rises then again to a plateau value of 1.5 J/g. The maximum in the lower concentration region decreases as the annealing temperature is raised. DISCUSSION General considerations
The endothermic phenomena, observed on heating an unplasticized PVC sample, have been reported in the literature and are always ascribed to the melting of crystals. This study shows that the broad endothermic transition observed in a DSC-heating scan can be related to a broad exothermic process that takes place on cooling. This exothermic process occurs even at very high cooling rates and can only be suppressed by quenching the DSC-pan in liquid nitrogen. After this quenching, an exothermic transition takes place in a subsequent heating scan, as soon as the glass transition region is reached. Isothermal annealing on the other hand induces sharp, additional endotherms. This results in a development of crystallinity as shown by X-ray scattering observations. This proves that a three dimensional crystalline structure develops by isothermal annealing. The formation of a three dimensional crystalline structure during a fast cooling procedure is much less evident, especially in the presence of a solvent.
H. Soenen, H. Berghmans
168
25
i
i
i
.
=
25
i
= =
80°C
20
20
15
15 Q.
Q_
0
0
r-
10
g
10
E O~
÷
\
A + ~
+ " " ' ~ + "Jf'÷~ ÷ ~
+~
0 0.0
,
.+÷ I
i
0.2 mess
25
~f~
I
,
0.4
i
0.6
fraction
i
=
I
0.8
o o.o
,
1.0
1
~=...,~.....--AA~tA-A--~
0.2
0.4
25
i
,
,
moss
PVC
i
.
i
,
0.6
fraction
¢
,
0.8
1.0
PVC
i
=
o
40°C
i
20°C
20
20
3
60°C
15
3
15
CL 0
t-
lO
c-
t.-
Io
E
l. . If"''--, 0 . . 0.0 0.2
"-* "--'*~ "-~ 0.4
0.6
"-;f 0.8
moss f r o c t i o n PVC
1.0
0 0.0
0.2 moss
0.4
0.6
fraction
0.8
.0
PVC
Fig. 10. Melting-enthalpy-concentration relationship for the system nitrobenzene-PVC. This melting enthalpy is expressed in Joules per gram PVC. The annealing temperature is indicated.
The syndiotacticity of PVC is very low and these syndiotactic units are expected to be responsible for the structure formation. It has been shown, by a statistical treatment of N M R data, that only a very low quantity of syndiotactic sequences capable of forming a stable crystal is present. 8 The high final melting temperature of the broad endotherm, supposes the formation of very stable crystals, while the statistical treatment, mentioned above, suggests that the formation of crystals of this dimension is almost to be excluded. The rate at
Phase behavior and gelation of solutions of poly(vinylchloride)
169
.61
0.13
--~.o3~
20
40
gO
80
temperature (°C) Fig. 11. DSC-hcating curves for different PVC-nitrobcnzcn¢ mixtures recorded at 20°C/rain after isothermal annealing at 20°C for ] week. The mass fraction of PVC is indicated.
which a structure formation takes place on cooling is also surprising in view of the poor stereoregularity of the polymer chains. Isothermal annealing, however, results in the rather slow formation of well defined crystals that melt in a narrow temperature region. An additional problem arises when this annealing takes place in the presence of nitrobenzene. The phase diagram has to be subdivided into two concentration regions each with their own concentration dependence of the melting point and crystallinity. It has been reported in literature 19 - 2 2 that domains are formed 100-800 A apart on cooling a dry PVC sample as well as a plasticized sample. It is difficult to explain this complex behavior of a polymer chain with limited stereoregularity by the general principles of polymer crystallization. Therefore, an alternative explanation will be given. It is based on the formation of intermolecular associations, reported in the literature, 23"24 for chemical complementary macromolecules like polyelectrolytes. Chemically complementary macromolecules (e.g. polyacid and polybase) associate in solution to form intermolecular complexes. These intermolecular associations can also occur between different polymer chains by the formation of, e.g. hydrogen bonds. The basic condition is the presence, along the polymer chain, of complementary chemical functions so that cooperative interaction can take place over a certain distance. These problems have been treated extensively in the literature and were compiled some years ago in excellent review papers. 23'24 The reader is referred to these publications and other ones in this field. This intermolecular complex formation is characterized by an equilibrium constant.
H. Soenen, H. Berghmans
170
$1 $2 K C polymer sequence 1 + polymer sequence 2,~ polymer complex The temperature dependence of K is given by: d In K dO/T)
- AH ° =
n
where A/-/~ represents the change in standard enthalpy upon complex formation. This complex formation is exothermic and an increase in temperature will shift the equilibrium to the left. This shift will lead to the decomposition of the complex (decrease in K). Cooling will promote complex formation and shift the equilibrium to the right (increase in K). The change in K will proceed over a certain temperature range and the width of this temperature range will decrease as the cooperativity increases. The degree of cooperativity increases with increasing length of the syndiotactic sequence. Therefore, long syndiotactic sequences will have this equilibrium shifted over a narrow temperature range while shorter sequences will need a broader range. An increase in the sequence length will also shift the position of the equilibrium on the temperature scale to higher temperatures. This is schematically represented in Fig. 12. This model postulates that a stereoregular sequence as a whole takes part in the association-dissociation process. The experimentally observed transition will correspond to the summation of the transitions of the different sequences taking into account their populations.
C 0
1
o in
== LO~ oJ
"o 0 temperature
B
temperature Fig. 12. ':,chematical presentation of: (A) Influence of the sequence length on the equilibrium transition between mr" associated and non-associated situation. (The number of chain units in the sequence is indicated in the figure.) (B) Heat transfer as a consequence of sequence association.
Phase behavior and gelation of solutions of poly(vinylchloride)
171
Interactions in PVC
In PVC, this type of intermolecular, exothermic association can take place between the hydrogen bonds of the polymer chain. Cooperativity between subsequent C1--C--H groups of a PVC-chain is possible for the association between two syndiotactic sequences, and an equilibrium of the following type is formed: \ \ \ \ H--C--C1 H--C---C1 H--C--CI--H--C--CI / / / / CH2 CH2 CH2 CH2 \ \ \ \ CI--C--H CI--C--H CI---C--H--CI--C--H / / K / / CH2 CH2 ~CH2 CH2 \ \ \ \ H--C--C1 H--C--C1 H--C--C1--H--C--C1 / / / /
CH2 \
CH2
CH2
\
\
CH2 \
These associations are responsible for the gel formation and also act as nucleation points for further crystallization during isothermal annealing. The presence of the solvent has an important influence. In the presence of an excess of nitrobenzene, this fast intermolecular association can not take place and no gel is formed. Isothermally crystallization can proceed to a high degree of crystallinity. At higher polymer concentration, an elastic gel is quickly formed by the occurrence of these associations. This is reflected in the broad exo- and endothermic signals, observed on cooling and heating, respectively. The presence of this gel network and the osmotic pressure that results from this network formation will limit further crystallization and keep the crystallinity very low. The additional endotherm, observed on melting the sample, represents only a few J/g (PVC). The concentration independence of the melting point of these crystals suggests that they are formed in almost solvent-free polymer domains. CONCLUSION The investigation of the structure formation in the system PVC-nitrobenzene suggests that two mechanisms of structure formation are operative. Cooling results in a very fast formation of intermolecular associations which induce thermoreversible gelation in the presence of an excess of solvent. This process is very fast and can only be suppressed by quenching to a very low temperature. Isothermal annealing of these associated samples induces the formation of a crystalline phase. ACKNOWLEDGEMENT The authors wish to thank the IWONL for a fellowship and for financial support. Financial support by the National Fund for Scientific Research and the Ministry of Scientific Programmation through the IUAP-16 is gratefully acknowledged. The authors also thank Professor R. Koningsveld for his interest and constructive advice.
172
H. Soenen, H. Berghmans REFERENCES
1. Liu, N. I., Tong, S. N. & Koenig, J. L., Stereoregularity studied by C 13 NMR. J. Appl. Polym. Sci., 25 (1980) 2205-10. 2. Nakajima, A., Hamada, H. & Hayashi, S., Structure and some physical properties of PVC polymerized at different temperatures. Die Makromol. Chem., 95 (1966) 40-51. 3. Dawson, P. C., Gilbert, M. & Maddams, W. F., Comparison of X-ray diffraction and thermal analysis methods for assessing order in PVC. J. Polym. Sci. Polym. Phys., 29 (1991) 1407-18. 4. Kawanishi, K., Takeda, Y. & Inoue, T., The sol-gel transition and the liquid-liquid phase separation in PVC solutions. Polym. J., 18 (1986) 411-6. 5. Kawanishi, K., Komatsu, M. & Inoue, T., Thermodynamic considerations of the sol-gel transition in polymer solutions. Polymer, 28 (1987) 980-4. 6. Dorrestijn, A. & te Nijenhuis, K., Viscoelastic behaviour during gelation of various molar-mass polyvinylchloride solutions and polymerization temperature. Coll. Polym. Sci., 268 (1990) 895-900. 7. Jackson, R. S., Bower, D. I. & Maddams, W. F., Ageing in wet PVC gels studied by gel-sol transition temperature and raman spectroscopy. Polymer, 31 (1990) 875-60. 8. Jujin, J. A., Gisolf, ,l. H. & de .long, W. A., Crystallinity in atactic PVC. Kolloid-Z.u.Z.Polymere, 251 (1973) 456-473. 9. Guerrero, S. J., Keller, A., Soni, P. L. & Geil, P. H., A study of texture in PVC gels by infrared dichro'ism and X-ray diffractions. J. Macromol. Sci.-Phys., B20(2) (1981) 161-66. 10. Najeh, M., Munch, J. P. & Guenet, J. M., Physical gels from PVC: effect of solvent type. Macromolecules, 25 (1992) 7018-23. 11. Yang, Y. C. & Geil, P. H., Morphology and properties of PVC/solvent gels. J. Macromol. Sci.-Phys. B22(3) (1983) 463-89. 12. Matsuzawa, S., Yamaura, K., Maeda, R. & Ogasawara, K., Melting behavior of syndiotacticity-rich poly(vinylalcohol) gel. Macrmol. Chem., 180 (1979) 229-39. 13. Illers, K. H., Influence of thermal history upon physical properties of PVC. J. Macromol. Sci.-Phys., B14(4) (1977) 471-82. 14. Brown, H. R., Musundi, G. M. & Stachurski, Z. H., Effect of plasticizer concentration and annealing on the structure of PVC. Polymer, 23 (1982) 1508-14. 15. Gray, A. & Gilbert, M., Structural order in heat treated vinyl chloride polymers. Polymer, 17 (1976) 44-50. 16. Ohta, S., Kajiyama, T. & Takayamagi, H., Annealing effect on the microstructure of PVC. Polymer Engng Sci., 16 (1976) 465-72. 17. Wilski, H., Die spezifische W~m des Polyvinylchlorids II. Kolloid-Z.u.Z.Polymere, 238 (1970) 426--49. 18. Miinstedt, H., Relationship between rheological properties and structure of PVC. J. Macromol. Sci.-Phys., B14(2) (1977) 195-212. 19. Blundell, D. J., Small-angle X-ray study of microdomains in rigid PVC. Polymer, 20 (1979) 934-8. 20. Neilson, G. F. & Jabarin, S. A., Small-angle X-ray scattering study of microstructure of PVC. J. Appl. Phys., 46 (1975) 1175-83. 21. Soni, P. L., Geil, P. H. & Collins, E. A., Microdomain structure in plasticized PVC. J. Macromol. Sci. Phys., B20(4) (1981) 479-503. 22. Brown, H. R., Kasakevich, M. & Wigmall, G. D., A SANS study of the structure of plasticized PVC. Polymer, 27 (986) 1345-8. 23. Kabanov, V. A. & Papisov, I. M., Formation of complexes between complementary synthetic polymers and oligomers in dilute solution: review. Polymer Sci. USSR, 21 (1979) 261-307. 24. Tsuchida, E. & Abe, K., Interactions between macromolecules in solution and intermolecular complexes. Adv. Polym. Sci., 45 (1982) 1-130.