Thermodynamics of the metastability of aqueous solutions of polyvinyl alcohol

THERMODYNAMICS OF THE METASTABILITY OF AQUEOUS SOLUTIONS OF POLYVINYL ALCOHOL* V. I. KLEN1N, O. V. KLEIn'IliA, V. A. KOLCHAXOV, B. I. SHVARTSBURD a n d S. YA. FRENKEL' N. G. Chernyshevskii State University, Saratov Institute of High Molecular Weight Compounds, U.S.S.R. Academy of Sciences

(Received 8 May 1973) Two approaches have been adopted to solve problems concerning the thermodynamics of the metastability of aqueous polyvinyl alcohol (PVA) solutions: a structural method, such as the method using the turbidity spectrum (determining dimensions of supermolecular particles (SMP) during isothermal formation over a wide range of solution temperature (20-150 °) and concentration (1-20°/o) and the thermodynamic method--determining the second virial coefficient A2 over the range of 20-90 °. The type of kinetic curves of turbidity r=r(t) and dimensions ~=~x(t) and the extremal relationship between the rate of formation of supermolccular order and temperature at constant concentration and concentration at constant temperature indicates that SMP are crystalline. The complex structure of supermolecular organization (SMO) has three levels. A system of SMP remaining in the solution after dissolving the main mass of the polymer under given conditions forms the first level. After preparing the solution the second level of SMO is formed, of which the degree of development depends on solution temperature and concentration. SMP formed in solution at high temperature (~ 80°C) while cooling the solution aggregate into anisodiametric structures. These aggregates are very sensitive to temperature variation and determine; the third level of SMO. The positive value of A~ in the range studied confirms the stability of re]atively amorphous solutions. Consequently, supermolecular order in aqueous solutions of PVA should be related to the instability of the system in relation to crystalline equilibrium. THERMODYNAMICS of the m e t a s t a b i l i t y of aqueous solutions of p o l y v i n y l alcohol (PVA) [1, 2] are still u n d e r discussion. Various scientists have proposed different physical a n d chemical conditions for the P V A - w a t e r system, which conflict w i t h each other. A c o n s t i t u t i o n a l d i a g r a m has been p r o p o s e d [3, 4] for a m o r p h o u s s e p a r a t i o n w i t h a n u p p e r critical p o i n t of mixing (UCPT) at ~ 80 +. E x p e r i m e n t a l results concerning the deterioration of the t h e r m o d y n a m i c q u a l i t y of w a t e r as solvent on increasing t e m p e r a t u r e are incompatible with the c o n s t i t u t i o n a l d i a g r a m with UCPM. N e g a t i v e e n t r o p y a n d e n t h a l p y p a r a m e t e r s of mixing calculated f r o m viscosity m e a s u r e m e n t s of solutions [5] a n d swelling of crosslinked p o l y m e r gels [6], a r e d u c t i o n of the second virial coefficient A 2 * Vysokomol. soyed. A16: :No. 10, 2351 2359, 1974. 2731

2732

V.I. KLENINet al.

on increasing temperature [1, 7] and negative heats of dilution [8] are among the results. The lower critical point of mixing (LCPM) in the region of ~ 240 ° was observed in the PVA-water system [9]. Further, there are indications in the literature concerning the possible crystalline (paracrystalline) nature of supermoleeular particles (SMP) which exist in aqueous solutions of PVA [1, 5, 10] and prevent, in particular, the measurefi~ent of polymer molecular weight by light scattering [1, 5]. However, the identification of crystalline phase even in concentrated aqueous solutions and PVA gels by X-ray analysis involves serious limitations of sensitivity due to the low proportion of paracrystalline sections and structural imperfection [11]. The fact that morphologically perfect crystalline structures of PVA cannot be formed in an aqueous solution was explained by the internal diphyl properties of macromolecules resulting in coiled chain conformation [12]. The metastability of aqueous PVA solutions therefore remains essentially unexplained, particularly for dilute solutions of PVA. Progress in this question could be expected on using new methods of investigation to determine parameters (mainly dimensions) of SMP directly in polymer solution under different physical and chemical conditions of treating solutions. The variation of parameters of SMP with controlled conditions should provide the necessary information about the type and properties of these particles and consequently about the metastable conditions of the polymer-solvent system as a whole. The turbidity spectrum method [13] opened up interesting possibilities for the study of SMP structure; by this method temperature-concentration ranges were detected in particular for the formation and decomposition (melting) of SMP in aqueous solutions of PVA [14-18]. The effects detected confirm the paracrystallinity of SMP formed in an aqueous solution of PVA. In previous studies [14-18] formation and decomposition of supermolecular order (SMOr) were studied under conditions when solutions were heated and hydrodynamically treated, and SMP were measured at room temperature. For a more detailed study of properties of SMOr it was advisable to examine the formation of SMOr under isothermal conditions over a wide range of temperature and solution concentration. The direct determination of thermodynamic parameters of the system is important and may be decisive in elucidating the type of SMOr. Such a parameter m a y for example be A S [19]. Results in the literature concerning A S in the PVA-water system are very limited [1, 7]. This is, apparently, due to the difficulty of freeing the solution from impurity particles and SMP [1, 5] and the fact that even in a solution purified to the molecular state SMP are again formed

[1, 17, is, 20]. We were able to overcome these difficulties and measure by light scattering A S in an aqueous solution of PVA over the temperature range of 20-90 °. To solve the problem concerning the metastability of aqueous solutions of PVA two methods were used: properties of SMP were examined using the method of turbidity spectrum which in this case functions as a structural method of

Thermodynamics of metastability of aqueous solutions of polyvinyl alcohol

2733

analysing the system and the method of determining thermodynamic parameter A~. It is important that determining A~ by light scattering specifies a molecular degree of solution dispersion (absence of SMP), whereas results concerning A~ enable us to draw conclusions about the thermodynamics of SMP. EXPERIMENTAL *

Two industrial P V A specimens with the following properties were examined: P V A - I [t/]25o~0"81 dl/g (in water), content of acetate groups [ A G ] ~ 0 . 4 3 % , Mw~140,000, ~ s l ) =60,000, /17/sv----72,000 [15]; PVA-2:[~]25o=0"76 dl/g, [ A G ] : 1 . 2 5 % , /~w=85,000. SMOr was characterized by the average equivalent radius of SMP ~x [13-17], and by solution t u r b i d i t y v when 2 = 540 nm. The value of ~x was calculated for a relative refractive index m----1.10. The value of ~z being determined for the extreme case of m = l . 1 5 (perfect P V A crystals) does not result in a marked change of results owing to small particle dimensions [14]. Most interesting effects of forming and melting SMOr in aqueous solutions of P V A were observed [14-18] over the t e m p e r a tu r e range of 60-150°; therefore, to study the formation of SMOr under isothermal conditions (measuring the t u r b i d i t y spectrum at the temperature of formation Tf of SMOr) a special apparatus was developed and assembled to measure the t u r b i d i t y spectrum of solutions at temperatures above the boiling point o f solvent. The solutions were prepared by controlling the temperature of a p o l y m e r - w a t e r mixture at 80 ° for 30 min, the flask being slightly tilted. This method ensures a relatively low level of SMOr over a wide range of concentration [15, 17]. To determine As, solutions of concentration c ~ l g/dl were filtered through a bacteriological mat, original E K (German Democratic Republic) and a system of bacteriological membrane filters in a ]3 14-NZh hermetic box. W a t e r was freed from minerals by ion exchange resin and distilled in a q u a r t z apparatus. Light scattering intensity was measured in an FPS-2 apparatus in the angular range of 30-135 ° for the blue line of mercury (2-~435.8 nm). To determine the t em p er at u r e relationship between A2 [A2 (T)] a solution of given concentration was heated from ~ 20 to 90% the index of scattering being measured at several fixed temperatures. Past 90 ° the solution was cooled to room t e m p e r a tu r e and the reversibility of scattering intensity checked. T h e solution was then diluted and the procedure of measuring the temperature dependence of light scattering intensity repeated. The refractive index increment d n / d c was measured for 2----435.8 and 546.1 n m (OI-18 condenser with a SVD-120 mercury arc lamp) with an accuracy of up to two to three units of the third sign in a I R F - 2 3 refractometer with an imp r o v e d system of thermostatic control over the range of 10 to 80 °. The apparatus was calibrated for light scattering using benzene (R~oo=48 × 10-6cm -1 for 2=435.8 nm [21]). A 2 was determined by a conventional m e t h o d [ 19] by plotting K × c / R o ~ o against c, where K is the optical constant, R0 ~0-- Rayleigh ratio extrapolated to 0 : 0 °, c-- solution concentration, g/ml. I t should be emphasized t h a t measurements of light scattering intensity had to be made within a short period of time to avoid " r e t r o g r a d a t i o n " of the solution (i.e. during the induction period [18]). RESULTS A N D D I S C U S S I O N

Kinetics of isothermal formation of SMOr. Isothermal formation of SMOr over wide ranges of temperature and solution concentration takes place in two stages: a relatively rapid stage and then a slow increase in the level of SMOr * Z. A. Gavrilova took part in the experimental study.

V . I . KLENIN et al.

2734

(Fig. 1, 80 ° a n d :Fig. 2, 20°). T h e r a t e o f f o r m a t i o n o f SMOr was c h a r a c t e r i z e d b y t h e v a l u e o f k, t h e a v e r a g e increase in t u r b i d i t y of t h e solution during u n i t t i m e at t h e first stage o f f o r m a t i o n : k=ziv/voAt, w h e r e ¢0 is initial t u r b i d i t y o f t h e solution, z t t - - d u r a t i o n of t h e first stage, hours.

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FIG. 1. Kinetics of formation (a, b) and melting (c, d) SMOr in 5~o aqueous solutions of PVA-I: a, b: 1--T~=87 (1, 1') and 80 ° (1", 3); 1,/'--independent experiments; 2--values Of and rx after Cooling to 20°; 3--the solutions were thermostatically controlled in special autoclaves [17]; to measure T and r~, the solution was cooled to room temperature after every thermostatic control cycle; c, d: /--melting of SMOr under conditions of increasing temperature (10 rain at each temperature); the turbidity spectrum was measured at the appropriate temperature; 3--melting of SMOr in special autoclaves [17] by sequentially raising temperature (30 rain at each temperature) with cycles of cooling to room temperature, at which the turbidity spectrum was measured. F o r c o m p a r i s o n , Fig. 1 (curves 3) shows e x p e r i m e n t a l results [17] o b t a i n e d u n d e r conditions w h e n t h e s y s t e m was r e t a i n e d in a n a u t o c l a v e a t a given t e m p e r a t u r e for a certain p e r i o d o£ t i m e , t h e n cooled to r o o m t e m p e r a t u r e a n d t h e

Thermodynamics of metastability of aqueous solutions of polyvinyl alcohol 2735 level of SMOr measured. As shown by Fig. 1, a more highly developed Sl~IOr is formed under these conditions, a p p a r e n t l y more satisfactory structurally, which follows from the existence of a limited melting range (Fig. le, curve 3). During isothermal formation of SMOr no restrictions are imposed on the melting range; solution t u r b i d i t y decreases i m m e d i a t e l y when T>Tf. This process of reduction of r continues gradually up to 160-170 ° (Fig. lc, curve 1; Fig. 2e), above which chemical displacement occurs in the macromolecular chain of PVA (e.g. d e h y d r a t i o n [18]). The average dimension of SMP during melting m a y first increase (Fig. ld, curve 3, Fig. 2d, curve 1) a n d t h e n decrease (Fig. ld, curve 3). An increase in r~ is, apparently, due to the enrichment of the s y s t e m of SMP by coarser particles as a consequence of melting of fine, structurally more imperfect particles. An increasing n u m b e r of particles t h e n melts a n d i x suddenly decreases with a further increase of temperature. During melting of SMP formed by an isothermal process these two effects are superimposed a n d particle size varies slightly (Fig. ld, curve 1 a n d Fig. 2f, curve 1). "U~ Crn -I

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FIG. 2. Formation kinetics at 20° (a, b) and melting (c-f) SMOr in 5% PVA-1 solutions: a, b--different signs on the curves refer to independent experiments; c-f--temperature variation with 10 min thermostatic control at each temperature: /--heating; 2--cooling. Measurements of the turbidity spectrum in a FEK-56 (c, d) apparatus and a high-temperature photometer (e, f). On increasing t e m p e r a t u r e individual paracrystalline sections inside the particles m a y also melt which reduces their relative refractive index a n d consequently, causes t u r b i d i t y in t h e dispersion. Dependence of SMOr on solution concentration. Since there is a graded organi-

2736

V . I . KLEMN et al.

z a t i o n o f levels of SMO in a q u e o u s solutions o f P V A , t h e m i n i m u m SMOr w h i c h e x i s t s in solution a f t e r p r e p a r a t i o n , was a s s u m e d to be t h e first level. F i g u r e s 3a a n d b (curves 1) i n d i c a t e t h a t this level of SMO is i n d e p e n d e n t of p o l y m e r conc e n t r a t i o n w h e n c > 3 % a n d its p r e h i s t o r y can be t r a c e d b a c k to a f r a c t i o n o f p e r f e c t c r y s t a l l i t e s existing in t h e original p o l y m e r specimen. I n t h e region o f dilute solutions ( < 3 % ) m i n i m u m SMOr s u d d e n l y decreases on r e d u c i n g t h e c o n c e n t r a t i o n of t h e solution to be p r e p a r e d a n d a t a certain characteristic conc e n t r a t i o n t h e solution is p r a c t i c a l l y free f r o m SMOr a n d is a m o l e c u l a r dispersed s o l u t i o n [22].

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FIG. 3. Relationship between T/c (a); ~ (b) and k (c) and the concentration of PVA-I: a, b: /--initial solution; /'--experimental and /"--calculated values of ~/c [22]; 2--after the first stage of formation at 87 ° and 2"--20°; 3 - - a t 20 ° after isothermal formation at 87°; c--T~=20 (1) and 87 ° (2). FIG. 4. Relationship between T (a); ~a (b) and k (c) and the value of Tf in 5% aqueous solutions of PVA-1. Numbers on the curves show the level of SMOr.

Thermodynamics of metastability of aqueous solutions of polyvinyl alcohol 2737 A second level of SMO is formed under isothermal conditions (Figs. 1 and 2), of which the degree of formation depends on the concentration and temperature of formation. Temperature dependence of SMO. Figure 4 shows results of isothermal formation of SMOr for 50//0 solutions of PVA-1 in the range of 20-150 °. Straight line 1 of Fig. 4 determines the first order of SMOr and the variation of points characterizes the reproducibility of this level. The temperature dependence of the rate of formation of the second level of SMO (Fig. 4c) is extrcmal. For 5% solutions of PVA-1 at 110 ° and inversion is observed in the course of kinetic curves T=T (t): above 110 ° solution turbidity decreases during thermostatic control which is due to the partial melting of the initial level of SMO (Fig. 4a). Average dimensions fx----f~ (t) increase (Fig. 4b) since the finer SMP melt first. dr, O.f6

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FIG. 5. Relationship between dn/dc and temperature for ~=435.8 (1) and 546.1 nm (2). Here and in Fig. 7 points indicate results of parallel experiments. FIG. 6. Indices of scattering with a concentration of PVA-2 solution of 1-02 g/dl and at temperatures of 20 (1) and 90° (2). During cooling solutions after isothermal formation of SMOr at high temperatures (80 °) the dimensions of SM-P and solution turbidity increase (Fig. 4a, b, curves 3). This effect is reversible with a relaxation time that cannot be recorded with the apparatus used. This sensitivity of solution turbidity (structural organization of SMOr) in relation to temperature, apparently, points to the existence of a labile level of SMO, of which structural units are aggregates of particles of a second level of SMO. These aggregates determine the third level of SMO. Strong gradient relations of the viscosity of aqueous PVA solutions subjected to thermostatic control at temperatures higher than 100 ° [16] are probably due to the third level of SMO. Special experiments show that this effect also decreases during subsequent heating of solutions and practically disappears at 50 ° [23]. Consequently, SMP aggregate into fairly asymmetrical structural formations. Temperature dependence of A 2. Absolute values of dn/dc for PVA-2 are somewhat lower than those given previously [1], b u t the temperature coefficient dn/dc coincides with that previously indicated [1] (Fig. 5). The Rayleigh t y p e

2738

V.I. KLENIb; et al.

index of scattering was maintained at all temperatures and concentrations studied (Fig. 6), therefore, extrapolation of Re~ 0 essentially resulted in averaging R e values. Figure 7 indicates that over the temperature range examined Mw remains constant, i.e. no macromolecular aggregation occurs during the measurements (retrogradation of solution [ 17, 18]). The variation of scattering intensity with temperature was reversible within the range of experimental error. The value of ~tw=85,000 coincided with 2tfv Calculated from [7] according to formula

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FIG. 7. Relationship between K/cRo and the concentration of PVA-2 at 20 (1, 5); 40 (2); 60 (3); 80 (4) and 90 (6); 5--after cooling. FIG. 8. Relationship between A 2 and temperature for an aqueous solution of PVA-2: 1 sequential heating; 2--cooling; 3--according to results previously obtained [1]. Figure 8 shows that the temperature range studied (A2) is positive, which proves the stability of the P V A - w a t e r system in relation to amorphous separation. Then, the supermolecular order formed in the solution in this temperature range should be regarded as being crystalline. This is not surprising since PVA is a crystallizing polymer independent of whether it has a regular or irregular structure [24]. Results confirm that the formation of SMOr in aqueous PVA solutions is a process of macromoleeular crystallization in the presence of a low molecular weight component. This is suggested b y kinetics of formation of SMOr (two stages: crystallization and post-crystallization). The extremal dependence of the rate of formation of SMOr on temperature is typical of crystallization (Fig. 4c).

Thermodynamics of metastability of aqueous solutions of polyvinyl alcohol 2739 The concentration dependence of the rate of formation of SMOr is understandable from this point of view. With a solution concentration lower than a certain threshold value c~cp (Fig. 3c) no substantial formation of SMOr takes place, which is probably due to the fact that the dimensions of SMP present in the solution do not reach a critical value. For the formation of SMOr when c~% an induction period is required the duration of which, other conditions remaining equal, depends on the dimensions of existing intermediate products [18]. On increasing concentration c ) % , the rate of formation of SMOr increases suddenly without a noticeable induction period, which is due to the increase in the dimension of SMP--structural intermediate products of SMOr (Fig. 3) and the increase in the mass of crystallizing material in unit volume. On further increasing c the increasing viscosity of the system hinders the mobility of macromolecular chains and the rate of formation decreases (Fig. 3c). For the same reason, in concentrated solutions dimensions of SMP increase slightly (Fig. 3b). Consequently, the microstructural heterogeneity of concentrated solutions is less marked and in this respect they are more stable over a period of time than solutions of average concentration. The imperfect morphological form of paracrystallites of PVA from an aqueous solution (particularly with isothermal crystallization) is reflected b y the uncertain melting range of these structures (Fig. lc, d). A reduction in A 2 proves a deterioration in the solvent power of water for PVA on increasing temperature in the range of 20-90 °, which is in agreement with results [6] concerning a reduction in the degree of swelling in water of an amorphous PVA network, crosslinked with chemical bonds. The conclusions of authors of previous papers [3, 7] about the amorphous separation of aqueous solutions in the temperature range of 20-80 ° and about the improvement in the solvent power of water with an increase in temperature, based on a study of swelling PVA in water and the absorption of steam, it seems to us, are due to ignoring the fact that the extent of swelling and the absorption of steam b y polyvinyl alcohol depend considerably on the crystallinity of the specimen [25]. Moreover, as shown previously [25], with high relative moisture content even at room temperature crystallites m a y melt partially; in another study it was established [26] that prolonged treatment of PVA with steam promotes crystallization. By some means or other, on retaining PVA in steam, crystallinity may vary and considerably affect absorption. Limited swelling of PVA in water and an increase in the degree of swelling with temperature [3, 4, 26] m a y be explained as follows. It is well known that PVA crystallites have a wide distribution according to size and crystallinity. During swelling they function as physical crosslinks [11], partially melting during swelling even at room temperature [27]. On increasing temperature an increasing proportion may melt, i.e. the degree of crosslinking decreases and the degree of swelling increases with a reduction in crosslinking. An increase in swelling of PVA in water with an increase in temperature, in spite of a deterioration of sol-

2740

V.I. KLEI~N et al.

v e n t power, m a y be due to the p r e v a l e n t effect of r e d u c t i o n in p o l y m e r crosslinking (crystallinity) o n increasing t e m p e r a t u r e . P l o t t i n g a c o n s t i t u t i o n a l d i a g r a m of liquid phase s e p a r a t i o n f r o m results o f swelling for a crystallizing p o l y m e r is, evidently, also incorrect. Translated by E. SEMERE REFERENCES 1. T. MATSUO and H. INAG~LK1, Makromo]ek. Chem. 53: 130, 1962; 55: 150, 1962

2. M. MATSUMOTO and Y. OHYANAGI, J. Polymer Sci. 26: 389, 1957 3. G. N. KORMANOVSKAYA, Problcmy fiziko-khimicheskoi mekhaniki (Problems of Physico-chemical Mechanics). Riga, 1967; G. N. KORMANOVSKAYA, E. I. YEVKO, V. V. CHURANOV, V. M. LUK'YANOVICH and I. I. VLODAVETS, Kolloidn. zh. 30: 696, 1968 4. S. I. MEYERSON, and Ye. M. SHAKHOVA, Nauchno-issled. trudy MTI 22: 337, 1969; Ye. M. SHAKHOVA and S. I. MEYERSON, Kolloidn. zh. 34: 589, 1972 5. A. DIEU, J. Polymer Sei. 12: 417, 1954 6. I. SAKURADA, A. NAKAJIMA and K. SHIBATANI, Makromolek. Chem. 87: 103, 1965 7. A. A. TAGER, A. A. AN1KEYEVA, L. V. ADAMOVA, V. M. ANDREYEVA, T. A. KUZ'MINA and ~1. V. TSILIPOTKINA, Vysokomol. soyed. A13: 659, 1971 (Translated in

Polymer Sci. U.S.S.R. 13: 3, 751, 1971) 8. K. AMAYA and P. FUJISHI~O, Bull. Chem. Soc. Japan 29: 361, 1953 9. V. M. ANDREYEVA, A. A. TAGER, A. A. ANI/£EYEVA and T. A. KUZ'MINA, Vysokotool. soyed. B l l : 555, 1969 (Not translated in Polymer Sci. U.S.S.R.) 10. S. PETER and H. FASBENDER, Kolloid-Z. und Z. fiir Polymere 196: 125, 1964 11. G. REHAGE, Kunststoffe 53: 605, 1963 12. V. G. BARANOV, T. I. VOLKOV and S. N. FRENKEL', Dokl. AN SSSR 172: 849, 1967 13. W. HELLER, H. L. BHATNAGAR and M. NAKAGAKI, J. Chem. Phys. 36: 1163, 1962; V. I. KLENIN, Trudy molodykh uchenykh (Studies of Young Scientists, Saratov). 1965; V. I. ](LENIN and O. V. KLENINA, J. Polymer Sei. C16: 1011, 1967; S. Yu. SHCHEGOLEV and V. I. ](LENIN, Vysokomol. soyed. A13: 2809, 1971 (Translated in Polymer Sci. U.S.S.R. 13: 12, 1971) 14. V. I. KLENIN, O. V. KLENINA and V. V. GALAKTIONOV, Vysokomol. soyed. 8: 1574, 1966 (Translated in Polymer Sci. U.S.S.R. 8: 9, 1734, 1966) 15. O. V. KI~NINA, V. I. KLENIN and S. Ya. FRENKEL', Vysokomol. soyed. A12: 1277, 1970 (Translated in Polymer Sei. U.S.S.R. 12: 6, 1448, 1970) 16. N. K. KOLNIBOLOTCHUK, V. I. KLENIN and S. Ya. FRENKEL', Vysokomol. soyed. A12: 2257, 1970 (Translated in Polymer Sei. U.S.S.R. 12: 10, 2558, 1970) 17. O. V. KLENINA, V. I. KLENIN, L. I. POLUBARINOVA and S. Ya. FRENKEL', Vysokomol. soyed. A14: 2192, 1972 (Translated in Polymer Sci. U.S.S.R. 14: 10, 2564, 1972) 18. V. I. KLENIN and O. V. KLENINA, Protsessy strukturoobrazovaniya v rastvorakh i gelyakh polimerov (Structure Formation in Solutions and Gels of Polymers). Saratov, 1971 19..V.N. TSVETKOV, V. Ye. ESKIN and S. Ya. FRENKEL', Struktura makromolekul v rastvorakh (Maeromolecular Structure in Solutions). Izd. "Nauka", 1964 20. A. STACEY and P. ALEXANDER, Ricerca Sei. 25: 889, 1955 21. I. L. FABELINSKII, Molekulyaxnoye rasseyanie sveta (Molecular Light Scattering). Izd. "Nauka", 1965

Plasticization of polymers with oligomers

2741

22. V. I. KLENIN, N. 7. UZUN and S. Ya. F R E N ~ E L ' , Vysokomol. soyed. B15: 601, 1973 (Not translated in Polymer Scl. U.S.S.R.) 23. V. I. I~LENIN, N. K. KOLNIBOLOTCHUK and S. Ya. FRENI~EL', Vysokomol. soyed. B15: 389, 1973 (Not translated in Polymer Sci. U.S.S.R.) 24. C. W. BUNN, Nature 161: 929, 1942 25. A. TAKIZAWA, T. NEGISHI and K. ISHIKAWA, J. Polymer Sci. 6, A - I : 475, 1968 26. W. I. PRIEST, J. Polymer Sci. 6: 699, 1951 27. H. TADOKORO, K. KOZAI, S. SEKII and I. N1TTA, J. Polymer Sci. 26: 379, 1957

PLASTICIZATION OF POLYMERS WITH OLIGOMERS AND LOW MOLECULAR WEIGHT SOLVENTS* A. YA. MALKIlg, G. ZH. ZHAlgGEREYEVA a n d M. P. ZABUGI1N~A A. V. Topchiev I n s t i t u t e of Petrochemical Synthesis, U.S.S.R. Academy of Sciences

(Received 14 May 1973) A study of the variation in viscosity of polymers plasticized with oligomer and low molecular weight substances revealed the existence of three ranges of molecular weight ratio of components of systems, in which plasticization has a varying effect. Over the range of systems of mixtures of different fractions of the same polymer, plasticization is simply determined b y the ratio of molecular weights. Over the range of systems consisting of polymer mixtures with oligomer products plasticization is generally independent of the molecular weight of the components to be mixed. Finally, over the range of solutions the effect of plasticization is independent of poly~mr molecular weight, b u t depends on solvent viscosity.

WH~¢ examining plasticization of polymers related problems of reducing the glass temperature or viscosity at a selected temperature are significant. In this connection it is of interest to explain the general relations which govern the viscosity variation of melts according to the polydispersion of polymers, particularly on adding low molecular weight fractions, oligomers or solvents to the polymer. A study of this problem using model systems--mixtures of polybutadienes with narrow molecular weight distribution (MWD)--is the subject of this paper. EXPERIMENTAL I n continuation of earlier studies [1-3] series of polybutadienes (PB) with narrow MWD were used. Components with different molecular weights were mixed in ratios, selected so as to obtain polydispersed specimens of known viscosity and average molecular weight. Mixing involved joint extrusion of a coarse mixture through a capillary of the viscometer. * Vysokomol. soyed. AI6: No. 10, 2360-2364, 1974.