Relaxation properties of rubber-like polymer mixtures over a wide range of temperatures

1152

YU, V. ZELENEV a n d G. M. BARTEI~EV

7, P. V, KOZLOV, O. P. KOZ'MINA, VAN NAI-CHAN, P. A. SLAVETSKAYA and CHZHOU EN-LO, Dokl. Akad. Nauk SSSR 139: 1149, 1961 8. S. SADRON, J. Physique 9: 381, 1938 9. V. N. TSVETKOV, Vysokomol, soyed. 5: 740, 747, 1963 10. V. N. TSVETKOV, E. V. FRISMAN and L. S. MUKI~NA, Zh. eksperim, i teor. fiz. 30: 649, 1956 11. A. PETERLIN, Bull. Sci. Youg. 1: 40, 1953 12. R. CERF, J. Polymer Sci. 20: 216, 1956; 23: 125, 1957; 25: 247, 1957 13. V. N. TSVETKOV a n d S. M. SAWON, Zh. tekhn, fiz. 26: 348, 1956 14. V. N. TSVETKOV, N. N. BOITSOVA and A. E. GRISHCHENKO, Vestn. LGU 4: 59, 1962 15. V. G. BARANOV a n d S. Ya. MAGARIK, Vysokomol. soyed. 5: 1072, 1963 16. V. N. TSVETKOV and I. N. SHTENNIKOVA, Sb, tselluloza i e e proizvodnye. (Cellulose and its Derivatives.) Izd. Akad. Nauk SSSR, 1963

RELAXATION PROPERTIES OF RUBBER-LIKE POLYMER MIXTURES OVER A WIDE RANGE OF TEMPERATURES* Y u . V. ZELENEV and G. M. ]3ARTENEV Moscow V. I. Lenin State I n s t i t u t e of Pedagogics

(Received 5 June 1963) To IMPROVE the processing properties of polymers with rigid and semi-rigid molecular chains and reduce brittleness, two- or three-component mixtures are normally obtained which in addition to polymers, contain plasticizers, Low-molecular-weight plasticizers can "bleed" to the surface a n d evaporate, changing the properties of polymers. I n addition, low-molecularweight plasticizers reduce strength therefore, high-molecular-weight plastieizers, rubber-like polymers with flexible molecular chains have been recently introduced. To extend the complex of desirable properties, two or throe rubbers are often added to the mixture therefore, not only the compatibility of rubbers with hard polymer, b u t also the compatibility of various rubbers with one another is of great importance, On the other hand, rubber mixtures are often used for other purposes t h a n as plasticizers. E.g. the cold resistance of rubber-like polymers can be increased without marked reduction of strength as a result of obtaining a compatible rubber mixture. Normally, with a successful combination of certain polymers with others, new materials can be obtained with better properties t h a n the initial ones. The study of polymer compatibility showed t h a t ; polymers will be compatible when their dielectric constants are similar [1]; in compatibility, a decisive role is played b y interaction between the molecules (thermal effect of mixing) and the entropy variation of polymers having chain molecules of varying size and flexibility, is u n i m p o r t a n t [2, 3]; packing density of molecules a n d chemical structure of polymers [4] are of great importance from the point of view of compatibility. With rubber-like liquid-phase polymers, the properties of combined rubbers are not determined b y the components introduced b u t only b y the rubbers mixed, * Vysokomol. soyed. 6: No. 6, 1047-1053, 1964.

Properties of rubber-like polymer mixtures

1153

the mixing of two rubbers being a m u t u a l solution of two liquid phases [5]. On mixing the liquid rubbers or elastic ones mechanically, macroscopic compatibility m a y be achieved which is characterized b y macro-homogeneity and no separation even in the case of incompatible rubbers. Micro-compatibility, however which is characterized b y m u t u a l solubility of components of the mixture, close to the thermodynamically equilibrium state, will be absent from the various types of rubbers. The viseometrie method [6] is used most frequently for studying the compatibility of rubbers. I f the viscosity of the mixture is lower than the additive value, the polymers are incompatible: if higher, they are compatible and in the first case a folding while, in the second case, an extended configuration of molecules of both components is observed. For the study of compatibility, mechanical methods and dilatometric and thermodynamic methods of determining mixing heats are used [2, 3, 7, 8]. :Normally, non-polar polymers are compatible with non-polar, and polar with polar polymers. I t is considered that on mixing two polar polymers the nature of polarity is u n i m p o r t a n t [5], however, in paper [8] for a mixture of polychloroprene and nitrile rubbers, two glass temperatures were detected which proves incompatibility. Neither are two non-polar rubber-like polymers always compatible. Thus, on mixing rubbers SKS-30 and SKB only a moderate solubility will be observed and the micro-separation will be inhibited by the vulcanized structure. Owing to the loose packing of the molecular structure of SKS-30 rubber, when mixing with SKB, the volume of the mixture will be less t h a n the additive value, mixing heat will be positive and true compatibility will not be observed [9]. I n the reduction of cold resistance of several synthetic rubbers, natural rubber is used as plasticizer. I f the mixtures are compatible, both for low- and high-molecular weighv plasticizers, in the ease of polar a n d non-polar components, a mechanism [8--10] is in action which is subject to the law of volume [11] and not molar [12] fractions. The mixing mechanism have now been studied both for solid and rubber-like polymers and incompatible and compatible polymers classified. The mechanical and electrical relaxation properties of polymer mixtures have been very little studied and the investigations of temperature dependence of mechanical and dielectric losses have been carried out to a greater extent on hard and, to a less extent, on rubber-like polymers [13, 14]. All the available data relate to the transition of polymers from the high-elastic to the glass-like state. The relaxation properties of rubber-like polymer mixtures have not, in general, been studied over a wide range of temperatures. I n view of this, the purpose of this paper was to study the relaxation properties of compatible and incompatible rubbers in mechanical, electrical and magnetic fields at various frequencies over a wide range of temperatures. F r o m literature data, mixtures of compatible (NK-r SKB, :NK+SKS-30, SKN-18+SKN*-40) and incompatible ( N K + S K N - 1 8 , SKS-30+SKN-26, P K h P K + S K N - 4 0 ) rubbers were chosen. A standard method of preparing moderately crosslinked sulphur vulcanizatcs was used 100 parts by weight, polymer being incorporated in two various rubbers in appropriate proportions. The investigations were carried out by dynamic and mechanical methods under conditions of forced resonance and non-resonance oscillations over a frequency range from 10-3 to 102 c/s. The dynamic properties of rubber-like polymer mixtures have been determined in the temperature range, -- 170 to + 140° C. The devices and methods of measurements have been described earlier [15, 16]. To compare the processes of molecular relaxation in rubber mixtures for mechanical and electrical fields, dielectric losses were measured over a frequency range of 50 to 106 c/s, in a temperature range of -- 170 to + 100°C. Molecular mobility has also been studied in rubber mixtures at low temperatures by a NMR method. * ~ q K = n a t u r a l rubber; S K B = p o l y b u t a d i e n e - B u n a ; S K S = b u t a d i e n e - s t y r e n e rubber; S K N = b u t a d i e n e acrylonitrile rubber; (Translator's note).

1154

YU- V. ZELENE¥ and G. M. BARTENEY

We have observed earlier [15, 17] that for unfilled rubbers based on polar and non-polar rubbers and obtained b y sulphur vulcanization over a wide temperature range, three mechanical loss m axima were observed. At low temperatures (--130, --160°C) a low-temperature maximum (LM) is observed which is due to the mobility of side chains and methylene groups in the main chains. In the range of transition from the glass-like to the high-elastic state, a principal maximum of mechanical losses (PM) is observed which is due to the variation of segment mobility. Finally, at elevated temperatures (Jr 110, ~-120°C) for sulphur vulcanizates of various rubbers of high-temperature maximum (HM) is observed caused by the reversible breakdown of polysulphide bonds, independent of the type of rubber. Near the glass-transition range in a structurally liquid phase dielectric loss maxima have been observed for polar and non-polar rubbers. Below T~ barely distinguishable dielectric loss maxima have been detected only for polar nitrile rubbers [18]. In the high temperature range the investigations carried out in electric fields have not revealed an analogous high-temperature mechanical loss maximum because, at elevated temperatures, the sharply increasing electric conductivity losses begin to play a decisive role. When studying the relaxation properties of rubber mixtures, to define more accurately the mechanism of molecular interactions in compatible and incompatible polymers, it was of interest to elucidate which variations in the character of molecular mobility in the LM, PM and HI~ ranges will be effected by mixing. Investigations in mechanical and electric fields showed that a frequency increase displaces the maximum to high temperatures i.e. the processes, in the case of mixture, have a relaxation on character. The study of temperature dependence of mechanical and dielectric losses of compatible SKN-18 and SKN-40 rubbers (Fig. 1) shows that one principal maximum is observed for the mixture which occupies an intermediate position between the maxima of components, both in respect of the temperature, position and height of the maximum and in respect of its half-width. Gradual increase of the content in the mixture of rubber SKN-40, having a large number of polar nitrile groups per unit volume, displaces the maximum to the high temperatures, increases the height and halfwidth. In addition, the area of the maximum increases somewhat, which depends on the number of kinetic units responsible for this process of molecular relaxation and on the environment which determines the character of segment mobility. Since, in rubber of the same type, variation of the number of nitrile groups cannot alter the nature of kinetic units which determine the given process of relaxation, the effect observed only can be related to the increase of molecular interaction impeding the motion of segments and increasing internal friction. It should be noted that, in the case of mixing polar nitrile rubbers, no preference should be given to any method for determining mechanical or dielectric losess. The picture is different for mixtures of non-polar rubbers NK-~SKB and NK -~SKB-30. The dielectric losses of these mixtures are basically caused by the

Properties of rubber-like polymer mixtures

1155

presence of sulphur cross-links but, since the sulphur content in the vulcanizates of mixtures is low (2 parts b y wt. per 100 parts b y wt. of rubbers), even at maxima, the dielectric losses are low and observation of variation of the effect, on changing the content of components in the mixture, is difficult. In this case the method of determining mechanical losses is obviously preferable. The following relation A T~ J•MT• = Cn ,

established for the case of plasticized rubbers [19] is also correct for mixtures of compatible rubbers. Constants C in the case of mixing various polar rubbers are not different in practice. A similar circumstance is also observed in the case of mixtures of various non-polar rubbers. Therefore, T~, KM and AT~ being experimentally determined, the content of components in the rubber mixture can be evaluated. A single mean maximum of mechanical or dielectric loss is not observed for incompatible rubbers (SKB-30~-SKN-26). Two maxima, less clearly marked than for each seperate component, are observed on the curve of temperature dependence of mechanical and dielectric loss (Fig. 2). The maximum which coresponds to nitrile rubber is observed for the mixture because a considerable part of SKN-26 rubber remains in the form of macroscopic formations incorporated in the styrene rubber mass. The temperature position of the maxima of pure rubbers and the respective components of the mixture, are not different in practice. This proves that each component of the mixture retains its inherent kinetic properties. Determination of the temperature position of maxima for compatible and incompatible rubber mixtures in mechanical and electric fields b y changing the contents of components showed (Fig. 3a) that an increase in the SK-N-18 ~ SKN40 mixture of the content of SKN-18 rubber causes a linear decrase of T~I of both mechanical and dielectric losses. For a mixture of S K N - 4 0 ~ - P K h P K *, considered to be incompatible [8] we observed only one maximum in the glass-transition range which occupied an intermediate position between the maxima of the components. Increasing the amount of P K h P K in the mixture causes a more marked reduction of T~ of the mixture, if the increase of the share of P K h P K exceeds 50 parts b y weight. This results in a non-linear relation (Fig. 3b). Thus, the mixture of nitrile and polychloroprene rubbers, in which P K h P K content predominates, reveals properties which differ from those of compatible and incompatible rubbers. This is due to the different nature of polar nitrile and chloroprene groups. At low chloroprene group concentration, a different character of polarity is not observed in practice, b u t when the number of chloroprene groups predominates, molecular interaction decreases and T~ falls more sharply. With a high content of more polar nitrile groups no specific character of interaction with * P K h P H = p o l y c h l o r o p r e n e rubber (Translator's note).

1156

Y u . V. ZELEI~EV a n d G. M. BARTEI~EV

polychloroprene groups is observed and the mixture develops quasi-compatibility. When the number of chloroprene groups begins to predominate, microheterogenei t y develol~s caused by the energy losses in mixing different molecules or by steric hindrances of mixing. Thus, according to the measurements of mechanical and dielectric losses, the rubber mixtures investigated can be classified as compatible, incompatible and quasi-compatible (SKN-~PKhPK) mixtures. I t appeared that, in the case of compatible rubber mixtures, the variation of T M of mechanical and dielectric a

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FIG. 1. Temperature dependences of mechanical (a) and dielectric (b) losses of compatible rubbers in the region of transition from high-elastic to glass-like state: 1, 4--SKN-18; 2, 5--SKN-40; 3, 6--mixture of SKN-18+SKN-40 (50 parts by weight in each case). losses with the variation of parts b y weight n, the component of the mixture corresponds to the relation T~mix=nT£+(1--n) T~. For an incompatible rubber mixture a similar relation is not followed. I n the case of quasi-compatible rubbers this correlation is found only in the case of certain relations of the mixture components. To elucidate more accurately the character of molecular mobility variation in compatible and incompatible rubber-like polymers, temperature dependence of a NMR line width are cited. At low temperatures NMR line width for the mixture (Fig. 4) has an intermediate value compared with the zlH values for N K and S K B components. I f for pure N K at temperature of -- 140, -- 110°C a transition caused b y the mobility variation of methyl groups CHa was observed, in the mixture this transition was absent due to the reduction of the number of CH3 groups in unit mixture volume. The range of sharp narrowing of line width caused by the variation of segment mobility, occupies, for the rubber mixture an

Properties of rubber-llke polymer mixtures

1157

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FIG. 2. Temperature dependence of mechanical (a) and dielectric (b) loss of incompatible rubbers: 1, 4 - - S K S = 3 0 , 2, 5 - - S K N = 2 6 , 3, 6--mixture of S K S = 3 0 + S K N - 2 6 (50 parts by weight in each case). Fro. 3. Dependence of the temperature of the maxima of mechanical (1, 3) and dielectric (2, 4) losses on the variation of the amounts of components for: a--SKN-18 and SKN-40 compatible rubbers; b--SKN-40 and P K h P K incompatible rubbers.

intermediate position between the narrowing ranges AH for certain N K and SKB components. For incompatible rubber mixtures no particular difference is observed in the signal shape either at low temperatures or in the range of transition from glass-like to high-elastic state. This is due to the small difference in molecular mobility in various rubbers at low temperatures because, for an incompatible polymer and a low-molecular-weight liquid [20], in addition to the low wide component, a narrow high component of the NMR line is observed. 12" AH,gs

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1158

Y u . V. ZELENEV a n d G. M. BAttTENEV

The study of the temperature dependence of mechanical loss at low temperatures shows (Fig. 5a, b, I) that, in compatible N K and SKS-30 rubbers, a maximum is observed which occupies an intermediate position between the component maxima. An increase of I~K content in the mixture results in higher molecular ordering and a reduction of free volume. This reduces the maximum of the mixture and displaces it in the direction of high temperatures as compared with pure SKS-30 rubber. A similar relation is also observed for other compatible rubbers. For a mixture of incompatible rubbers SKS-30+SKN-40 a certain increase of mechanical losses is observed compared with pure SKIq-40 rubber but a clear determination of the two maxima cannot be made, which is partly 0.2

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FIe. 5. Temperature dependences of mechanical losses in the range of low-temperature (1) and high-temperature (11) maxima for compatible (a) and incompatible (b) rubbers with varying content of components in the mixture: 1 -- SKS-30 (80 parts by wt.)+NK (20 parts by wt.); 2--SKS-30 (50 parts by wt.)+NK (50 parts by wt.); 3 -- SKS-30 (20 parts by wt.) + N K (80 parts by wt.); 4-- SKN-40 (80 parts by wt.) + SKS-30 (20 parts by wt.); 5--SK1~-40 (50 parts by wt.)+SKS-30 (50 parts by wt.); 6--SKN-40 (20 parts by wt. )+SKS-30 (80 parts by wt.). due to the low mechanical losses in the glass-like state of polymers. The measurement of dielectric losses shows t h a t for mixtures of compatible nitrile rubbers a small maximum is observed, the height of which is less t h a n for pure SKbI-40 rubber and the temperature of maximum is higher t h a n for SKN-40. Mechanical loss measurements in the high temperature range for mixtures of rubbers vulcanized with diphenylguanidine also display marked differences in both cases. I f for pure rubbers, clear maxima are observed, in the case of compatible rubber mixtures, a weak maximum of mechanical loss is observed (Fig. 5a, b, I I ) or the maximum is hardly observable for identical amounts of components. Apparently, conditions of three-dimensional structure formation v a r y in the mixture and the number of polysulphide bonds decreases. I n the

Properties of rubber-like polymer mixtures

1159

case of i n c o m p a t i b l e rubbers, the mechanical loss m a x i m a do n o t s u b s t a n t i a l l y v a r y as c o m p a r e d w i t h pure r u b b e r s a n d v a r y little on c h a n g i n g the p r o p o r t i o n s of c o m p o n e n t s of various r u b b e r s in the mixture. CONCLUSIONS (l) The effect of the degree of compatibility and proportions of c o m p o n e n t s in the m i x t u r e on the mechanical a n d dielectric loss of polar a n d n o n - p o l a r r u b b e r m i x t u r e s has been i n v e s t i g a t e d a t frequencies of 10 - a - 106 H z over a wide r a n g e of t e m p e r a t u r e s , including the range of three mechanical loss m a x i m a observed in rubber-like crosslinked polymers. (2) I n the r a n g e of t r a n s i t i o n f r o m high-elastic to glass-like state one m a x i m u m o c c u p y i n g i n t e r m e d i a t e position between the m a x i m a of c o m p o n e n t s is o b s e r v e d for c o m p a t i b l e r u b b e r mixtures. F o r a m i x t u r e of incompatible rubbers two m a x i m a are observed w h i c h Correspond to the m a x i m a of the c o m p o n e n t s of the mixture. (3) A t t e m p e r a t u r e below Tg no significant variations are observed for inc o m p a t i b l e rubbers, c o m p a r e d w i t h pure rubbers, whereas in t h e case of a m i x t u r e of SKS-30 a n d N K c o m p a t i b l e rubbers, the l o w - t e m p e r a t u r e m a x i m u m decreases a n d is shifted to high t e m p e r a t u r e s . (4) N o r are a n y m a r k e d variations observed a t elevated t e m p e r a t u r e s for i n c o m p a t i b l e r u b b e r mixtures. I n the case of a m i x t u r e o f identical a m o u n t s of c o m p a t i b l e r u b b e r s in a s u l p h u r vulcanizate, a h i g h - t e m p e r a t u r e m a x i m u m is n o t d e t e c t e d in practice.

1. 2. 3.

4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

Translated by E. SE.~IERE

REFERENCES V. I. ALEKSEYENKO and I. U. MASHUSTIN, Vysokomol. soyed. 1 : 1953, 1959 G. V. STRUMINSKII and G. L. SLONIMSKII, Zh. fiz. khimii 30: 1941, 1956 G. L. SLONIMSKII and G. V. STRUMINSKII, Zh. fiz. khimii 30: 2143, 1956 P. P. ZUBOV and M. P. ZVEREV, Kolloidn. zh. I8: 679, 1956 G. L. SLONIMSKII and N. F. KOMSKAYA, Zh. fiz. khimii 30: 1529, 1746, 1956 L. Ye. KALININA, V. I. ALEKSEYENKO and S. S. VOYUTSKII, Kolloidn. zh. 18: 180, 1956 R. A. REZNIKOVA, S. S. VOYUTSKII and A. D. ZAIONCHKOVSKII, Kolloidn. zh. 16: 204, 1954; 17: 108, 1955 G. M. BARTENEV and G. S. KONGAROV, Vysokomol. soyed. 2: 1692, 1960 A. I. MAREI, Kauchuk i rezina, No. 2, 1, 1960 V. D. ZAITSEVA and G. M. BARTENEV, Vysokomol. soyed. 2: 1301, 1960 V. A. KARGIN and Yu. M. MALINSKII, Dokl. Akad. Nauk SSSR, 73: 967, 1950 S. N. ZHURKOV, Dokl. Akad. Nauk SSSR, 47: 7, 1945 J. D. FERRY, Viscoelastic Properties of Polymers, New York-London, 1961 G. P. MIKIIAILOV, Uspekhi khimii 24: 875, 1955 G. M. BARTENEV and Yu. V. ZELENEV, Vysokomol. soyed. 4: 66, 1962 Yu. V. ZELENEV, G. M. BARTENEV and G. K. DEMISHEV, Zavodsk. lab. No. 7, 1963 Yu. V. ZELENEV, Vysokomo]. soyed. 4: 1486, 1962 G. M. BARTENEV and Yu. V. ZELENEV, Dokl. Akad. Nauk SSSR 154: 661, 1964 Yu. V. ZELENEV and G. M. BARTENEV, Vysokomol. soyed. 6: 918, 1964 J. C. POWLES and J. A. E. KAIL, Trans. Faraday Soc. 55: 1996, 1960