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The properties of solutions of graft copolymers of poly(vinyl alcohol) with polyvinyl pyridine
2478
S. A. TASHMUKHAMEDOVetal.
12. A. L. BUCHACHENKO and A. I. VASSERMAN, Stabfl'nye radikaly (Stable Radicals). Izd. "Khimiya", 1973 13. Kh. S. BAGDASARYAN, Teoriya radikal'noi polimerizatsii (Theory of Radical Polymerization). Izd. "Nauka", 1966 14. B. R. SMIRNOV, T. V. SOSINA, V. B. STRYIfKOV, G. V. KOROLEV and R.V. VINOGRADOV, Dokl. Akad. Nauk SSSR 214: 615, 1974 15. P. P. KOBENKO, Amorfnye veshchestva (Amorphous Substances). Izd. Akad. Nauk SSSR, 1952 16. N.A. PLAT]~ and A. G. PONOMARENKO, Vysokomol soyed. AI6: 2635, 1974 (Translated in Polymer Sci. U.S.S.R. 16: 12, 3067, 1974) 17. D. KHARDI, K. NITRAI, N. FEDOROVA and G. KOVACH, VysokomoL soyed. 4: 1972, 1962 (Not translated in Polymer Sei. U.S.S.R.). 18. R. BUKISLAVOVICH, S. IVANOVICH and D. KOSANOVICH, Glasnik khem. druzhb Belgrade 35: 419, 1970 19. R. BORISLAVLEVICH, S. IVANOVIC and D. KASANOVIC, Hemijska Industr. Jugoslaviya 26: 45, 1972 20. R. E. LEGCHUNTES, Izv. AXad. Nauk Kaz. SSR, set. khim., 1~o. 2, 35, 1972 21. H. GEIL, Polimernye monokristally (Polymer Single Crystals). Izd. "Khimiya", 1968 (Russian translation)
THE PROPERTIES OF SOLUTIONS OF GRAFT COPOLYMERS OF POLY(VINYL ALCOHOL) WITH POLYVINYL PYRIDINE* S. A. TASttMUKHAMEDOV, SH. A. AZIZOV, A. SH. KARABAYEV, E. T. TSAGARAYEV, R. S. Tm~AYEv and KH. U. USMA~OV V. I. Lenin State University, Tashkent
(Received 30 January 1975) Graft copolymers of poly(vinyl alcohol) with ~btained by the radiation chemical method and the formic acid have boon studied. In solution in formic cules exhibit a number of specific properties, shown scopy and sorption measurements.
polyvinyl pyridine have been properties of their solutions in acid the graft copolymer moleby viscometry, electron micro-
I~ study of the properties of solutions of graft copolymers, systems in which specific interaction between segments of different chemical nature is possible, are of considerable interest. A system of this type is provided by graft copolymers of poly(vinyl alcohol) (PVA) with polyvinyl pyridine (PVP), since they contain hydroxyl groups on the main chain and nitrogen atoms in the side chains. This must obviously be reflected in the properties of their solutions, because * Vysokomol. soyed. A18: No. 10, 2171-2176, 1976.
Solutiotls of graft copolymors of poly(vinyl alcohol) with polyvinyl pyridino
2479
the conformation of graft copolymer molecules is strongly dependent on the presence of functional groups in their composition. For this reason in the present work we studied the properties of solutions of P V A - P V P graft copolymers by a number of physico-chemical methods. For preparation of the graft eopolymers we used PVA from the factory "Polyvinylacetate" at Erevan, with [t/]~0-74 dl/g in water at 25 ° and containing 4.27~o of acetate groups. The 2-vinylpyridine was purified b y redistillatioa i n v a c u o at 69-71°/18 torr, t/~= 1.5490, d 26 0.9827. Graft copolymerization was initiated b y radiation of a mixture of the polymer and monomer, in water or methanol, with 7-rays from a e°Co source. I n order to free the graft copolymer from the two homopolymers the product was extracted for m a n y hours with selective solvents (benzene for P V P and water for PVA). The increase in weight of the original polymer was found after extraction with benzene. I t is soon in Fig. 1 t h a t in the presence of water the increase in weight follows an S-shaped curve as the dosage increases from 0.1 to 3.0 Mrad, with a sharp rise in the 1.0-2-0 Mrad range. The same pattern is followed in methanol, b u t the grafting efficiency is very much lower, especially at low radiation dosages. I t would be expected that water, being a solvent for PVA, would loosen the structure of ~ho polymer to a greater extent, a n d thus increase the accessibility of the monomer molecules to the macroradicals of the polymer. This is
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1~o. 1. Effect of the radiation dosage D a n d the nature of the solvent on grafting of VP to P V A (PVA : V P = 1.0 : 2.5, concentration of solvent 30~o of weight of monomer, dosage 67 rad/sec): / - - w a t e r , 2 - - m e t h a n o l , 3--effect of the ratio of methanol to water on grafting of VP to PVA a t PV : P V A = 1.0 : 2.5, dosage 67 rad/sec, D = 1.5 Mrad. Fro. 2. Variation in It/] of P V A - P V P graft copolymers on the P V P content, in solution in formic acid. confirmed b y the evidence of Fig. 1 (curve 3), from which it is seen that as the proportion of methanol is increased in a water-methanol mixture, the increase in weight falls steadily. B y this means it is possible to prepare a n u m b e r of graft copolymers of different composition at the same radiation dosage. To eliminate the effect of dosage, samples produced in a series of experiments of this type were therefore used subsequently. These products were insoluble in water, dilute mineral acids, DMF, benzyl alcohol, etc. I t was found t h a t they are soluble only in formic acid a n d mixtures of formic acid with other solvents, with dioxan in particular.
2480
S. A. TAS~LrKHAMEDOVet
al.
Samples obtained by evaporation of the solvent from dilute solutions in formic aeid~ were examined in a "Tesla',' electron microscope. Intrinsic viscosities, the degree of swelling and adsorption isotherms were measured at 25°, by methods identical with those described in references [1] and [2]. In investigation of the properties of polymer solutions much attention is usually paid to intrinsic viscosity because this characteristic is closely related to the conformation of the polymer molecules and their effective dimensions, as well as to intermoleeular interaction. Figure 2 shows the dependence of [t/] on the proportion of P V P in the graft copolymer, measured in solution in formic acid. I t is seen that the curve contains a sharp maximum at 60% PVP. The high value of [~/] of homo-PVP and the still higher values for the graft copolymers, in comparison with the original PVA, indicates the specific nature of the interaction of the P V P side chains with formic acid molecules. According to refer-
FiG. 3. Electron photomicrographs, a--PYA, c=0.01; b--PVA-PVP (25"5~o), c=0.1; c--PVA-PV-P (47.5~/o), c----0-1. ences [3] a n d [4], in which the interaction of pyridine and vinyl pyridine with carboxylic acids was studied, it m a y be assumed that the conformation of P V A P V P is mainly controlled b y hydrogen bonding between P V P units and molecules of the acid, as well as b y possible transfer of a proton from the acid to the nitrogen atom of VP, corresponding to protonation of the nitrogen atom. This must promote uncoiling of the side chains and increase in the size of the macromolecules. This is exhibited as a sharp increase in the viscosity of the solution. These effects must increase as the P V P content increases, consequently [~/] of a graft copolymer containing 58'3~/o of P V P is almost ten times the value of It/] of the original PVA. Analysis of the I R spectrum of a copolymer swollen in formic acid in fact showed a marked increase in the intensity of absorption b y hydrogen bonded hydroxyl groups in the 3700-2400 cm -1 region. The absorption bonds of P V P a t 1572, 1480 and 1440 cm -1 shift to 1550, 1470 and 1400 cm -1 respectively.
Solutions of graft copolymers of poly(vinyl alcohol) with polyvinyl pyridiue
2481'
Consequently it m a y be eonduded t h a t the interaction involves both the h y d r o x y l groups of the main chain and the grafted PVP chain. The predominating effect of the grafted chain on the behaviour of the copolymer is also shown by the nature o f the structure of the P V A - P V P copolymer in solution in formic acid. I t is seen from Fig. 3 t h a t the original PVA has a typical globular structure, whereas t h e graft copolymers form large, anisodiametric structures t h a t increase in size a s the proportion of PVP is increased. ~btz /00 0 - ZOO - q00 -
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FIG. 4. Effect of temperature on the i~trinsic viscosity of PVA, P~P and their copolymers, in formic acid: 1--PVA, 2--29.3% PVP, 3--39"3~o PVP, 4--58"3% PVP, 5--PVP, 6-- mechanical mixture, PVA : PVP= 1 : 1. FIG. 5. Dependence of the Huggins conztar~t k' and the Zelinger (Ai/A[q]) and Krigbaum (Abl,) compatibility parameters on the composition of the PVA-PVP system in formic acid. Figure 4 shows t h a t [~/] of PVP and PVA homopolymers, and of mixtures o f the two, decrease slightly as the temperature is increased, whereas [~/] of graft eopolymers of different composition remain almost unchanged. Here a marked difference between [~] of a mechanical mixture and a graft copolymer can a l ~ .
2482
S.A. TASHMUK~DOV et al.
be seen. Consequently the copolymers do not undergo any conformational changes in the temperature interval between 15 ° and 45 °. In a number of papers ~5-7] the presence of extrema in the curves of the temperature dependence of the viscosity of graft copolymcrs is explained b y the suggestion of a conformational change from a "segregated" structure to the random coil structure characteristic of linear polymers. The fact that this effect does not occur in the present instance indicates that the solvating power of the solvent does not change as the temperature is raised and that the dimensions of the macromolecular coil remain constant. A segregated structure of graft copolymer molecules is explained b y the fact that chemically different polymers are usually incompatible in solution. Incompatibility of segments of different chemical nature within the same macromolecules leads to mutual repulsion and thus to aggregation of segments of the same ~hemical nature. It is therefore important to establish the degree of compatibil-
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:Fie. 6. Variation in the mean free energy of mixing on the composition of PVA-PVP systems in formic acid, at PVA : PVP ratios of: 1--PVA, 2--0-8 : 0.2, 3--0.6 : 0.4, 4--0"4 : 0.6; 5--0-2 : 0-8, 6--PVP. FIG. 7. Dependence of the Gibbs mean free energy of mixing on the ratio of the components in PVA-PVP mixtures. ity of polymers comprising the given graft copolymer. A convenient method for obtaining information about interaction in solution between macromolecules of different nature is b y studying the viscosity of polymer mixtures. This is based on the supposition [8] that repulsion interaction causes compression of the coils a n d consequently the viscosity of the mixture is lower than the value calculated on the basis of the additivity principle. When however the opposite effect is f o u n d it is considered that positive interaction is taking place.
Solutions of graft copolymers of poly(vinyl alcohol) with polyviayl pyridiuo
2483
According to our viscometrie measurements positive deviation from the additive values was obtained over the entire range of concentrations of mechanical mixtures studied (0.075-0.6~/o), which indicates predominant interaction between the molecules of this pair of polymers in solution in formic acid. The deviation increases as the proportion of P V P in the mixture increases. I t must be assumed that this behaviour indicates interaction between the macromolecules of different kinds. This qualitative assessment can however lead to erroneous conclusions, because compatibility is dependent on many, sometimes contradictory, factors [8, 9]. Therefore for quantitative determination of the degree of compatibility calculations were made according to the theoretical models of Krigbaum and Wall [10] and of Zelinger [11], based on the results of the measurements of the viscosity of mechanical mixtures. It is seen from Fig. 5 that according to the Krigbaum-Wall model this pair of polymers is incompatible, because the compatibility parameter Abl~ is negative at all ratios of the polymers and the curve of the variation of this parameter contains a sharp minimum at a content of P V P of a little above 0.8, indicating association of molecules of the same type. I t is characteristic that it is in this same region of compositions that extrema appear in the curve of the variation of Zelinger's compatibility parameter Ai/A[7] and the values of the Ituggins constant k' are low. I t is obvious that interaction of formic acid molecules with the hydroxyl groups of PVA and with the nitrogen atoms of P V P will favour contact between the two, especially in the course of preparation of a film b y evaporation of a solution, when penetration of molecules of the other kind into associations of one or other of the components is possible. We therefore measured the sorption of formic acid vapour b y films of mechanical mixtures of PVA and PVP, obtained from solutions in formic acid. The thermodynamic stability of polymer mixtures of different composition was calculated b y the method proposed b y Tager in reference [12]. It is seen from Fig. 6 t h a t the curves of the mean free energy of mixing of the mechanical mixtures lie between the curves for the homopolymers and that Agx, which characterizes the thermodynamic stability, is positive (Fig. 7). This shows that these two polymers are incompatible. Hence it can be said that association of macromolecules of the same type is characteristic of mixtures of PVA and PVP, both in solution and in the solid state. The side chain in a graft copolymer usually reduces the affinity of the products to some liquids and increases it to others. This can be shown b y measurements of the swelling of such copolymers. In the present instance as the proportion of P V P was increased the affinity to water (a solvent for PVA) was decreased and to methanol (a solvent for PVP) it was increased. In the case of formic acid the adsorption was very high, indicating strong interaction between the components of the graft copolymer and the solvent. The values of the Flory-Huggins interaction parameter Z [13], calculated from the sorption isotherms, confirms this. It is seen from Fig. 8 that for the original PVA X is positive and it decreases with increasing dilution. The picture is quite different in the case of the graft
2484
S. A. TAS~r~UKn~DOV e~ a/.
copolymers. F o r t h e m X is negative over t h e entire range o f compositions studied, w h i c h indicates t h e specific n a t u r e o f the i n t e r a c t i o n b e t w e e n the solvent molecules a n d t h e polymer. I t is evident t h a t this is a t t r i b u t a b l e to t h e g r a f t e d P V P ~f
g'q ~.x 0.6
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.-I
-3 FIQ. 8. Concentration dependence of the Flory-Huggins interaction parameter: 1--PVA, 2--PVA-PVP (25"5~o), 3--PVA-PVP (47.5%), 4--PVP. branches, because h o m o - P V P gives a large negative value of X. These results p r o v e t h a t the g r a f t e d c o m p o n e n t exerts a p r e d o m i n a t i n g effect on the b e h a v i o u r o f the P V A - P V P c o p o l y m e r molecules in solution. Transtatexl by E. O. PHILLIPS
REFERENCES
1. S. A. TASHMU~HAMEDOV, Kh. U. USMANOV and R. S. TILLAYEV, Vysokomol. soyed. A1O: 379, 1968 (Translated ill Polymer Sei. U.S.S.R. 10: 2, 444, 1968) 2. S. A. TASHMUKHAMEDOV, Kh. U. USMANOV and R. S. TII.T.&YEV, Izv. vuzov. Khimlya i khim. tekimol. 12: 490, 1969 3. G. V. KUSAKOVA, G. S. DENISOVA, A. L. SMOLYANSKII and V. M. SHRAIBER, Dokl. Akad. Nauk SSSR 198: 1065, 1970 4. I. V. SAVINOVA, V. P. ZUBOV and V. A. KABANOV, Vysokomol soyed. A14: 1847, 1972 (Translated in Polymer Sci. U.S.S.R. 14: 8, 2069, 1972) 5. A. DODOS, European Polymer J. 5: 767, 1969 6. A. DONDOS, P. REMPP and H. BENOIT, Makromolek. Chem. 130: 233, 1969 7. M. GIROLAMO and J. R. URWIN, European Polymer J. 7: 693, 1971 8. B. BOHMER, D. BEREK and S. FLORIAN, European Polymer J. 6: 472, 1970 9. C. VASILE and J. R. SCHM:IDER, Makromolek. Chem. 141: 127, 1971 10. W. R. KRIGBAUM and F. T. WALL, J. Polymer Sci. 5: 505, 1950 11. J. ZELINGER and V. HEIDINGSFELD, Sb. Vysoke. Chemiko-teehnologicke Praze, Organika chemie a technologie 9: 63, 1966 12. A. A. TAGER, T. I. SHOLOKHOVICH and M. V. TSILIPOTKINA, Vysokomol soyed. A14: 1423, 1972 (Translated in Polymer Sci. U.S.S.R. 14: 6, 1597, 1972) 13. P. FLORY, Principles of Polymer Chemistry, New York, 1953
S. A. TASHMUKHAMEDOVetal.
12. A. L. BUCHACHENKO and A. I. VASSERMAN, Stabfl'nye radikaly (Stable Radicals). Izd. "Khimiya", 1973 13. Kh. S. BAGDASARYAN, Teoriya radikal'noi polimerizatsii (Theory of Radical Polymerization). Izd. "Nauka", 1966 14. B. R. SMIRNOV, T. V. SOSINA, V. B. STRYIfKOV, G. V. KOROLEV and R.V. VINOGRADOV, Dokl. Akad. Nauk SSSR 214: 615, 1974 15. P. P. KOBENKO, Amorfnye veshchestva (Amorphous Substances). Izd. Akad. Nauk SSSR, 1952 16. N.A. PLAT]~ and A. G. PONOMARENKO, Vysokomol soyed. AI6: 2635, 1974 (Translated in Polymer Sci. U.S.S.R. 16: 12, 3067, 1974) 17. D. KHARDI, K. NITRAI, N. FEDOROVA and G. KOVACH, VysokomoL soyed. 4: 1972, 1962 (Not translated in Polymer Sei. U.S.S.R.). 18. R. BUKISLAVOVICH, S. IVANOVICH and D. KOSANOVICH, Glasnik khem. druzhb Belgrade 35: 419, 1970 19. R. BORISLAVLEVICH, S. IVANOVIC and D. KASANOVIC, Hemijska Industr. Jugoslaviya 26: 45, 1972 20. R. E. LEGCHUNTES, Izv. AXad. Nauk Kaz. SSR, set. khim., 1~o. 2, 35, 1972 21. H. GEIL, Polimernye monokristally (Polymer Single Crystals). Izd. "Khimiya", 1968 (Russian translation)
THE PROPERTIES OF SOLUTIONS OF GRAFT COPOLYMERS OF POLY(VINYL ALCOHOL) WITH POLYVINYL PYRIDINE* S. A. TASttMUKHAMEDOV, SH. A. AZIZOV, A. SH. KARABAYEV, E. T. TSAGARAYEV, R. S. Tm~AYEv and KH. U. USMA~OV V. I. Lenin State University, Tashkent
(Received 30 January 1975) Graft copolymers of poly(vinyl alcohol) with ~btained by the radiation chemical method and the formic acid have boon studied. In solution in formic cules exhibit a number of specific properties, shown scopy and sorption measurements.
polyvinyl pyridine have been properties of their solutions in acid the graft copolymer moleby viscometry, electron micro-
I~ study of the properties of solutions of graft copolymers, systems in which specific interaction between segments of different chemical nature is possible, are of considerable interest. A system of this type is provided by graft copolymers of poly(vinyl alcohol) (PVA) with polyvinyl pyridine (PVP), since they contain hydroxyl groups on the main chain and nitrogen atoms in the side chains. This must obviously be reflected in the properties of their solutions, because * Vysokomol. soyed. A18: No. 10, 2171-2176, 1976.
Solutiotls of graft copolymors of poly(vinyl alcohol) with polyvinyl pyridino
2479
the conformation of graft copolymer molecules is strongly dependent on the presence of functional groups in their composition. For this reason in the present work we studied the properties of solutions of P V A - P V P graft copolymers by a number of physico-chemical methods. For preparation of the graft eopolymers we used PVA from the factory "Polyvinylacetate" at Erevan, with [t/]~0-74 dl/g in water at 25 ° and containing 4.27~o of acetate groups. The 2-vinylpyridine was purified b y redistillatioa i n v a c u o at 69-71°/18 torr, t/~= 1.5490, d 26 0.9827. Graft copolymerization was initiated b y radiation of a mixture of the polymer and monomer, in water or methanol, with 7-rays from a e°Co source. I n order to free the graft copolymer from the two homopolymers the product was extracted for m a n y hours with selective solvents (benzene for P V P and water for PVA). The increase in weight of the original polymer was found after extraction with benzene. I t is soon in Fig. 1 t h a t in the presence of water the increase in weight follows an S-shaped curve as the dosage increases from 0.1 to 3.0 Mrad, with a sharp rise in the 1.0-2-0 Mrad range. The same pattern is followed in methanol, b u t the grafting efficiency is very much lower, especially at low radiation dosages. I t would be expected that water, being a solvent for PVA, would loosen the structure of ~ho polymer to a greater extent, a n d thus increase the accessibility of the monomer molecules to the macroradicals of the polymer. This is
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1~o. 1. Effect of the radiation dosage D a n d the nature of the solvent on grafting of VP to P V A (PVA : V P = 1.0 : 2.5, concentration of solvent 30~o of weight of monomer, dosage 67 rad/sec): / - - w a t e r , 2 - - m e t h a n o l , 3--effect of the ratio of methanol to water on grafting of VP to PVA a t PV : P V A = 1.0 : 2.5, dosage 67 rad/sec, D = 1.5 Mrad. Fro. 2. Variation in It/] of P V A - P V P graft copolymers on the P V P content, in solution in formic acid. confirmed b y the evidence of Fig. 1 (curve 3), from which it is seen that as the proportion of methanol is increased in a water-methanol mixture, the increase in weight falls steadily. B y this means it is possible to prepare a n u m b e r of graft copolymers of different composition at the same radiation dosage. To eliminate the effect of dosage, samples produced in a series of experiments of this type were therefore used subsequently. These products were insoluble in water, dilute mineral acids, DMF, benzyl alcohol, etc. I t was found t h a t they are soluble only in formic acid a n d mixtures of formic acid with other solvents, with dioxan in particular.
2480
S. A. TAS~LrKHAMEDOVet
al.
Samples obtained by evaporation of the solvent from dilute solutions in formic aeid~ were examined in a "Tesla',' electron microscope. Intrinsic viscosities, the degree of swelling and adsorption isotherms were measured at 25°, by methods identical with those described in references [1] and [2]. In investigation of the properties of polymer solutions much attention is usually paid to intrinsic viscosity because this characteristic is closely related to the conformation of the polymer molecules and their effective dimensions, as well as to intermoleeular interaction. Figure 2 shows the dependence of [t/] on the proportion of P V P in the graft copolymer, measured in solution in formic acid. I t is seen that the curve contains a sharp maximum at 60% PVP. The high value of [~/] of homo-PVP and the still higher values for the graft copolymers, in comparison with the original PVA, indicates the specific nature of the interaction of the P V P side chains with formic acid molecules. According to refer-
FiG. 3. Electron photomicrographs, a--PYA, c=0.01; b--PVA-PVP (25"5~o), c=0.1; c--PVA-PV-P (47.5~/o), c----0-1. ences [3] a n d [4], in which the interaction of pyridine and vinyl pyridine with carboxylic acids was studied, it m a y be assumed that the conformation of P V A P V P is mainly controlled b y hydrogen bonding between P V P units and molecules of the acid, as well as b y possible transfer of a proton from the acid to the nitrogen atom of VP, corresponding to protonation of the nitrogen atom. This must promote uncoiling of the side chains and increase in the size of the macromolecules. This is exhibited as a sharp increase in the viscosity of the solution. These effects must increase as the P V P content increases, consequently [~/] of a graft copolymer containing 58'3~/o of P V P is almost ten times the value of It/] of the original PVA. Analysis of the I R spectrum of a copolymer swollen in formic acid in fact showed a marked increase in the intensity of absorption b y hydrogen bonded hydroxyl groups in the 3700-2400 cm -1 region. The absorption bonds of P V P a t 1572, 1480 and 1440 cm -1 shift to 1550, 1470 and 1400 cm -1 respectively.
Solutions of graft copolymers of poly(vinyl alcohol) with polyvinyl pyridiue
2481'
Consequently it m a y be eonduded t h a t the interaction involves both the h y d r o x y l groups of the main chain and the grafted PVP chain. The predominating effect of the grafted chain on the behaviour of the copolymer is also shown by the nature o f the structure of the P V A - P V P copolymer in solution in formic acid. I t is seen from Fig. 3 t h a t the original PVA has a typical globular structure, whereas t h e graft copolymers form large, anisodiametric structures t h a t increase in size a s the proportion of PVP is increased. ~btz /00 0 - ZOO - q00 -
-
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FIG. 4. Effect of temperature on the i~trinsic viscosity of PVA, P~P and their copolymers, in formic acid: 1--PVA, 2--29.3% PVP, 3--39"3~o PVP, 4--58"3% PVP, 5--PVP, 6-- mechanical mixture, PVA : PVP= 1 : 1. FIG. 5. Dependence of the Huggins conztar~t k' and the Zelinger (Ai/A[q]) and Krigbaum (Abl,) compatibility parameters on the composition of the PVA-PVP system in formic acid. Figure 4 shows t h a t [~/] of PVP and PVA homopolymers, and of mixtures o f the two, decrease slightly as the temperature is increased, whereas [~/] of graft eopolymers of different composition remain almost unchanged. Here a marked difference between [~] of a mechanical mixture and a graft copolymer can a l ~ .
2482
S.A. TASHMUK~DOV et al.
be seen. Consequently the copolymers do not undergo any conformational changes in the temperature interval between 15 ° and 45 °. In a number of papers ~5-7] the presence of extrema in the curves of the temperature dependence of the viscosity of graft copolymcrs is explained b y the suggestion of a conformational change from a "segregated" structure to the random coil structure characteristic of linear polymers. The fact that this effect does not occur in the present instance indicates that the solvating power of the solvent does not change as the temperature is raised and that the dimensions of the macromolecular coil remain constant. A segregated structure of graft copolymer molecules is explained b y the fact that chemically different polymers are usually incompatible in solution. Incompatibility of segments of different chemical nature within the same macromolecules leads to mutual repulsion and thus to aggregation of segments of the same ~hemical nature. It is therefore important to establish the degree of compatibil-
0"2
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-50 Fro. 6
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FIG. 7
:Fie. 6. Variation in the mean free energy of mixing on the composition of PVA-PVP systems in formic acid, at PVA : PVP ratios of: 1--PVA, 2--0-8 : 0.2, 3--0.6 : 0.4, 4--0"4 : 0.6; 5--0-2 : 0-8, 6--PVP. FIG. 7. Dependence of the Gibbs mean free energy of mixing on the ratio of the components in PVA-PVP mixtures. ity of polymers comprising the given graft copolymer. A convenient method for obtaining information about interaction in solution between macromolecules of different nature is b y studying the viscosity of polymer mixtures. This is based on the supposition [8] that repulsion interaction causes compression of the coils a n d consequently the viscosity of the mixture is lower than the value calculated on the basis of the additivity principle. When however the opposite effect is f o u n d it is considered that positive interaction is taking place.
Solutions of graft copolymers of poly(vinyl alcohol) with polyviayl pyridiuo
2483
According to our viscometrie measurements positive deviation from the additive values was obtained over the entire range of concentrations of mechanical mixtures studied (0.075-0.6~/o), which indicates predominant interaction between the molecules of this pair of polymers in solution in formic acid. The deviation increases as the proportion of P V P in the mixture increases. I t must be assumed that this behaviour indicates interaction between the macromolecules of different kinds. This qualitative assessment can however lead to erroneous conclusions, because compatibility is dependent on many, sometimes contradictory, factors [8, 9]. Therefore for quantitative determination of the degree of compatibility calculations were made according to the theoretical models of Krigbaum and Wall [10] and of Zelinger [11], based on the results of the measurements of the viscosity of mechanical mixtures. It is seen from Fig. 5 that according to the Krigbaum-Wall model this pair of polymers is incompatible, because the compatibility parameter Abl~ is negative at all ratios of the polymers and the curve of the variation of this parameter contains a sharp minimum at a content of P V P of a little above 0.8, indicating association of molecules of the same type. I t is characteristic that it is in this same region of compositions that extrema appear in the curve of the variation of Zelinger's compatibility parameter Ai/A[7] and the values of the Ituggins constant k' are low. I t is obvious that interaction of formic acid molecules with the hydroxyl groups of PVA and with the nitrogen atoms of P V P will favour contact between the two, especially in the course of preparation of a film b y evaporation of a solution, when penetration of molecules of the other kind into associations of one or other of the components is possible. We therefore measured the sorption of formic acid vapour b y films of mechanical mixtures of PVA and PVP, obtained from solutions in formic acid. The thermodynamic stability of polymer mixtures of different composition was calculated b y the method proposed b y Tager in reference [12]. It is seen from Fig. 6 t h a t the curves of the mean free energy of mixing of the mechanical mixtures lie between the curves for the homopolymers and that Agx, which characterizes the thermodynamic stability, is positive (Fig. 7). This shows that these two polymers are incompatible. Hence it can be said that association of macromolecules of the same type is characteristic of mixtures of PVA and PVP, both in solution and in the solid state. The side chain in a graft copolymer usually reduces the affinity of the products to some liquids and increases it to others. This can be shown b y measurements of the swelling of such copolymers. In the present instance as the proportion of P V P was increased the affinity to water (a solvent for PVA) was decreased and to methanol (a solvent for PVP) it was increased. In the case of formic acid the adsorption was very high, indicating strong interaction between the components of the graft copolymer and the solvent. The values of the Flory-Huggins interaction parameter Z [13], calculated from the sorption isotherms, confirms this. It is seen from Fig. 8 that for the original PVA X is positive and it decreases with increasing dilution. The picture is quite different in the case of the graft
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S. A. TAS~r~UKn~DOV e~ a/.
copolymers. F o r t h e m X is negative over t h e entire range o f compositions studied, w h i c h indicates t h e specific n a t u r e o f the i n t e r a c t i o n b e t w e e n the solvent molecules a n d t h e polymer. I t is evident t h a t this is a t t r i b u t a b l e to t h e g r a f t e d P V P ~f
g'q ~.x 0.6
\
0;8
\
I0
.-I
-3 FIQ. 8. Concentration dependence of the Flory-Huggins interaction parameter: 1--PVA, 2--PVA-PVP (25"5~o), 3--PVA-PVP (47.5%), 4--PVP. branches, because h o m o - P V P gives a large negative value of X. These results p r o v e t h a t the g r a f t e d c o m p o n e n t exerts a p r e d o m i n a t i n g effect on the b e h a v i o u r o f the P V A - P V P c o p o l y m e r molecules in solution. Transtatexl by E. O. PHILLIPS
REFERENCES
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