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Effect of the chemical structure on the visco-elastic behaviour of elastomers
Polymer ScienceU.S.S.R.Vol.30, No. 5, pp. 947-950, 1988 Printed in Poland
0032-3950/88 $10.00+.00 0 1989 Pergamon Press plc
EFFECT OF THE CHEMICAL STRUCTURE ON THE VISCO-ELASTIC BEHAVIOUR OF ELASTOMERS* I. I. PEREPECHKO and V. P. KUZ'MIN Moscow Automechanical Institute (Recdved 19 October 1987)
Elastomers based on a series of rubbers have been investigated by the method of torsional vibrations in the temperature interval 77-323 K. Multiplicity of the vitrification process was observed in all the elastomers studied. With fall in temperature the difference in the dynamic shear modulus of all the elastomers studied diminishes tending to a common limit. ~ 2 GPa. UNTIL now study of the properties of elastomers has focussed o n t h e region of the highly elastic state and the transitional zone [I]. The region of the glassy state remains little studied. This p a p e r reports the results o f investigation of the effect of chemical structure on the visco-elastic behaviour of crosslinked SKI-3, butadiene-ct-methylstyrene rubber S K M S - 3 0 A R K and the butadiene-nitrile rubber SKN-26M in the interval 77-323 K. This temperature range covers the regions both of the highly elastic and glassy states and the transitional zone. The investigations were by the method of free torsional vibrations [2] at the frequencies 0.2 and 2-5 Hz. In the course of the experiment we determined the effective part G' of the complex shear modulus and the tangent of the angle of mechanical losses tan ,~ and calculated the low frequency speed of the shear waves ct = x / G ~ (p is the density of the polymer). The error in determining these magnitudes was 4, 7-8 and 2 % respectively. The rubbers SKN-26M and S K M S - 3 0 A R K are statistical copolymers, the first of butadiene and acrylonitrile and the second of butadiene and ~-methylstyrene. The elastomer based on SKI-3 has the most flexible chain. Because of this and also because it possesses a sparse spatial network the dynamic modulus in the highly elastic state is lower than in the other elastomers (Figure). The high value for SKN-26M may be explained by the interaction of the polar nitrile groups and for S K M S - 3 0 A R K by the presence of a rigid c o m p o n e n t (~-methylstyrene). In the glassy state SKI-3 has the highest value of G' which is explained by the tightest packing of the macromolecules of this rubber despite the fact that it has the most flexible chains and sparsest spatial network 2.4x 10 -4 mole/cm 3 (3.68x 10-* mole/cm 3 for S K N - 2 6 M and 3.72 x 10 -4 mole/cm 3 for SKMS-30ARK. The lowest G' value in the glassy state is observed in S K M S - 3 0 A R K since it has an irregular structure and, in addition, this polymer also contains phenyl groups C6H5 impeding tight packing of the chains. The higher G' value in SKN-26M of irregular * Vysokomol. soyed. A30: No. 5, 930-933, 1988. 947
948
I. I. PE~PECnKO and V. P. Kuz'~I[N
chemical structure may be explained by the strong intermolecular interaction of the polar g r o u p s - C N . The strong intermolecular interaction due to the presence of the acrylonitrile units is also confirmed by the fact that with increase in their content the glass transition point of the elastomer rises [3].
c,~ , f O-J, rn /sec
toe G'EMPaJ
i
2 1
IG
1"0
Z3
b
O.5
1
0123
223
~K
323
128
223
7",K
323
la9 "tan5" !
2
0-I -2 I
123
I
I
223
I
7-,K
I
323
Temperatur© dependences of G' (a), c, (b) and tan ~ (c) of elastomcrs based on SKI-3 (1); SKN-26M
(2); and SKMS-30ARK (3)
In the graph ct=f(T) (Figure) in the region of vitrification two temperature transitions are observed detected from change in the temperature coefficient of the speed of the shear waves. From acoustic measurements it was earlier established that in linear amorphous polymers in the region of vitrification two temperature transitions are also present [4]. The lower temperature transition Tz corresponds to unfreezing of segmental
Visco-elastlc behaviour of elastomers
949
mobility in the unordered matrix and the higher one Tt = TBto unfreezing of segmental mobility in the more ordered or more tightly packed regions [4], in SKI-3 to the transitions at T2= 195 K and/'1=206 K. The results of reference [4] indicate two levels of supramolecular organization in glassy amorphous polymers. The presence of two temperature transitions also supports this. In SKI-3 the presence of two regions with different mobility of the nuclei was also established by the PMR method [5]. The high temperature transition always corresponds to the glass transition point of polymers [2]. Consequently, Tg of vulcanizates of the rubbers SKI-3, SKN-26M and SKMS-30ARK are respectively 206, 240 and 219 K. The activation energy E corresponding to the vitrification process of these elastomers is 273, 373 and 319 kJ/mole. Thus, the highest T~ and E values are characteristic of the polar elastomer SKN-26M with strong intermolecular interaction. The lowest Tg and E are noted in the most flexible chain rubber SKI-3. The value of the temperature coefficient of the speed of the shear waves at T> Ts indicates the degree of cooperativeness of the vitrification process: the higher this parameter the higher the degree of cooperativeness of the vitrification process [6]. This value is maximum in SKI-3 (110 m - s e c - l . K -1) which again confirms the greatest density or orderliness of the packing of the polymer chains. The molectilar mobility was judged from the maxima of the mechanical losses. The most flexible chain and most tightly packed SKI-3 in the Ts region has the highest peak (tan 6max=4"3). On the temperature scale the 0t peak of this rubber (T,= 217 K) comes below the ~ peaks for the other rubbers. The high value of the magnitude tan 6 confirms the high degree of cooperativeness of the vitrification process and the low value of the temperature of the ct maximum indicates the high kinetic flexibility of the polymer chains. In the region T< Tg in SKI-3 there is only rise in tan ~ which may be due to unfreezing of the mobility of the methyl, methylene and terminal groups [5, 7]. The low value of the secondary losses here is explained by the high packing density of the macromolecules of the rubber and the resulting insufficiency of this free volume for the unfreezing of rapid mobility both of the side groups and portions of the main chain of the polymer. The highest value (tan 6=0.036) of the secondary processes is observed in SKMS30ARK of irregular chemical structure with a rigidity of the polymer chain high as compared with that of SKI-3. This leads to looser packing of the macromolecules in the glassy state and retention of sufficient free volume for the unfreezing of the mobility of the phenyl and methyl groups and also possibly fragments of the main chain. In SKN-26M in the low temperature region we observe a wide maximum of tan 6 the level of which is, however, lower than in SKMS-30ARK. The lower tan 6 level in SKN-26M as compared with SKMS-30ARK may be explained by the presence in SKN-26M of a strong intermolecular interaction of the polar nitrile groups and the resulting damping of mobility of a local type. With change in frequency by an order the temperature position and the form of the fl peak of this rubber change. From the Figure it may be established that the G' value of all the elastomers studied with fall in temperature tends to a certain common limit. This value of G' was evaluated from the magnitude G~ ___2GPa through linear extrapolation of the functions G' = f ( T )
950
I . I . PEREPECHKOand V. P. Kuz'Mn~
to the low temperature region. The temperature at which this value is reached from calculation is N51 K. The expression for the dynamic modulus may be represented in the form
G'=Co+
i H (T) (.D2T2
dT,
(1)
0 where Go is the equilibrium modulus; H(z) is the density o f the relaxation time spectrum z; o~ is frequency. Then a t o 9 ~ - ~ the magnitude Go as compared with G® may be disregarded and the expression (1) may be reduced to the form
oo G~o= S g (z) d'r
O
Using the limiting value of Goo obtained we get
oo 0
H (~) d~ = 2 (GPa)
Convergence of G' for all the elastomers studied towards a common limiting value and the independence o f G.o from the chemical structure, polarity, etc. are possibly due to the fact that all flexible chain polymers in these conditions possess roughly the same relaxation time spectrum. Translated by A. CRozY REFERENCES
1. G. M. BARTENEV, Struktura i relaksatsionnye svoistva elastomerov (Structure and Relaxational Properties of Elastomers). 288 pp., Moscow, 1979 2. I. I. PEREPECHKO, Akusticheskiye metody issledovaniya polimerov (Acoustic Methods for Investigating Polymers), 296 pp., Moscov, 1973 3. B. A. DOGADKIN, A. A. DONTSOV and V. A. SHERSHNEV, Khimiya elastomerov (The Chemistry of Elastomers). 376 pp., Moscow, 1981 4. I. L PEREPCHKO and O. V. STARTSEV, Vysokomol. soyed, BIS: 321, 1973 (Not translated in Polymer Sci. U.S.S.R.) 5. S. K. FROLOVA, Z. L SAVEL'EVA, O. V. NIKITINA and N. N. LEZHNEV, Ibid. A19: 208, 1977 (Translated in Polymer Sci. U.S.S.R. 19: 1,243, 1977) 6. O.V. STARTSEV, Dissert. Cand. Phys.-Math. Sci (in Russian). 166 pp., MOPI im N. K. Krupskoi 166 pp., Moscow, 1975 7. Yu. V. ZELENEV, Releksatsionndye yavleniya v blochnykh polimerakh (Relaxational Phenomena in Block Polymers). p. 25, Leningrad, 1972
0032-3950/88 $10.00+.00 0 1989 Pergamon Press plc
EFFECT OF THE CHEMICAL STRUCTURE ON THE VISCO-ELASTIC BEHAVIOUR OF ELASTOMERS* I. I. PEREPECHKO and V. P. KUZ'MIN Moscow Automechanical Institute (Recdved 19 October 1987)
Elastomers based on a series of rubbers have been investigated by the method of torsional vibrations in the temperature interval 77-323 K. Multiplicity of the vitrification process was observed in all the elastomers studied. With fall in temperature the difference in the dynamic shear modulus of all the elastomers studied diminishes tending to a common limit. ~ 2 GPa. UNTIL now study of the properties of elastomers has focussed o n t h e region of the highly elastic state and the transitional zone [I]. The region of the glassy state remains little studied. This p a p e r reports the results o f investigation of the effect of chemical structure on the visco-elastic behaviour of crosslinked SKI-3, butadiene-ct-methylstyrene rubber S K M S - 3 0 A R K and the butadiene-nitrile rubber SKN-26M in the interval 77-323 K. This temperature range covers the regions both of the highly elastic and glassy states and the transitional zone. The investigations were by the method of free torsional vibrations [2] at the frequencies 0.2 and 2-5 Hz. In the course of the experiment we determined the effective part G' of the complex shear modulus and the tangent of the angle of mechanical losses tan ,~ and calculated the low frequency speed of the shear waves ct = x / G ~ (p is the density of the polymer). The error in determining these magnitudes was 4, 7-8 and 2 % respectively. The rubbers SKN-26M and S K M S - 3 0 A R K are statistical copolymers, the first of butadiene and acrylonitrile and the second of butadiene and ~-methylstyrene. The elastomer based on SKI-3 has the most flexible chain. Because of this and also because it possesses a sparse spatial network the dynamic modulus in the highly elastic state is lower than in the other elastomers (Figure). The high value for SKN-26M may be explained by the interaction of the polar nitrile groups and for S K M S - 3 0 A R K by the presence of a rigid c o m p o n e n t (~-methylstyrene). In the glassy state SKI-3 has the highest value of G' which is explained by the tightest packing of the macromolecules of this rubber despite the fact that it has the most flexible chains and sparsest spatial network 2.4x 10 -4 mole/cm 3 (3.68x 10-* mole/cm 3 for S K N - 2 6 M and 3.72 x 10 -4 mole/cm 3 for SKMS-30ARK. The lowest G' value in the glassy state is observed in S K M S - 3 0 A R K since it has an irregular structure and, in addition, this polymer also contains phenyl groups C6H5 impeding tight packing of the chains. The higher G' value in SKN-26M of irregular * Vysokomol. soyed. A30: No. 5, 930-933, 1988. 947
948
I. I. PE~PECnKO and V. P. Kuz'~I[N
chemical structure may be explained by the strong intermolecular interaction of the polar g r o u p s - C N . The strong intermolecular interaction due to the presence of the acrylonitrile units is also confirmed by the fact that with increase in their content the glass transition point of the elastomer rises [3].
c,~ , f O-J, rn /sec
toe G'EMPaJ
i
2 1
IG
1"0
Z3
b
O.5
1
0123
223
~K
323
128
223
7",K
323
la9 "tan5" !
2
0-I -2 I
123
I
I
223
I
7-,K
I
323
Temperatur© dependences of G' (a), c, (b) and tan ~ (c) of elastomcrs based on SKI-3 (1); SKN-26M
(2); and SKMS-30ARK (3)
In the graph ct=f(T) (Figure) in the region of vitrification two temperature transitions are observed detected from change in the temperature coefficient of the speed of the shear waves. From acoustic measurements it was earlier established that in linear amorphous polymers in the region of vitrification two temperature transitions are also present [4]. The lower temperature transition Tz corresponds to unfreezing of segmental
Visco-elastlc behaviour of elastomers
949
mobility in the unordered matrix and the higher one Tt = TBto unfreezing of segmental mobility in the more ordered or more tightly packed regions [4], in SKI-3 to the transitions at T2= 195 K and/'1=206 K. The results of reference [4] indicate two levels of supramolecular organization in glassy amorphous polymers. The presence of two temperature transitions also supports this. In SKI-3 the presence of two regions with different mobility of the nuclei was also established by the PMR method [5]. The high temperature transition always corresponds to the glass transition point of polymers [2]. Consequently, Tg of vulcanizates of the rubbers SKI-3, SKN-26M and SKMS-30ARK are respectively 206, 240 and 219 K. The activation energy E corresponding to the vitrification process of these elastomers is 273, 373 and 319 kJ/mole. Thus, the highest T~ and E values are characteristic of the polar elastomer SKN-26M with strong intermolecular interaction. The lowest Tg and E are noted in the most flexible chain rubber SKI-3. The value of the temperature coefficient of the speed of the shear waves at T> Ts indicates the degree of cooperativeness of the vitrification process: the higher this parameter the higher the degree of cooperativeness of the vitrification process [6]. This value is maximum in SKI-3 (110 m - s e c - l . K -1) which again confirms the greatest density or orderliness of the packing of the polymer chains. The molectilar mobility was judged from the maxima of the mechanical losses. The most flexible chain and most tightly packed SKI-3 in the Ts region has the highest peak (tan 6max=4"3). On the temperature scale the 0t peak of this rubber (T,= 217 K) comes below the ~ peaks for the other rubbers. The high value of the magnitude tan 6 confirms the high degree of cooperativeness of the vitrification process and the low value of the temperature of the ct maximum indicates the high kinetic flexibility of the polymer chains. In the region T< Tg in SKI-3 there is only rise in tan ~ which may be due to unfreezing of the mobility of the methyl, methylene and terminal groups [5, 7]. The low value of the secondary losses here is explained by the high packing density of the macromolecules of the rubber and the resulting insufficiency of this free volume for the unfreezing of rapid mobility both of the side groups and portions of the main chain of the polymer. The highest value (tan 6=0.036) of the secondary processes is observed in SKMS30ARK of irregular chemical structure with a rigidity of the polymer chain high as compared with that of SKI-3. This leads to looser packing of the macromolecules in the glassy state and retention of sufficient free volume for the unfreezing of the mobility of the phenyl and methyl groups and also possibly fragments of the main chain. In SKN-26M in the low temperature region we observe a wide maximum of tan 6 the level of which is, however, lower than in SKMS-30ARK. The lower tan 6 level in SKN-26M as compared with SKMS-30ARK may be explained by the presence in SKN-26M of a strong intermolecular interaction of the polar nitrile groups and the resulting damping of mobility of a local type. With change in frequency by an order the temperature position and the form of the fl peak of this rubber change. From the Figure it may be established that the G' value of all the elastomers studied with fall in temperature tends to a certain common limit. This value of G' was evaluated from the magnitude G~ ___2GPa through linear extrapolation of the functions G' = f ( T )
950
I . I . PEREPECHKOand V. P. Kuz'Mn~
to the low temperature region. The temperature at which this value is reached from calculation is N51 K. The expression for the dynamic modulus may be represented in the form
G'=Co+
i H (T) (.D2T2
dT,
(1)
0 where Go is the equilibrium modulus; H(z) is the density o f the relaxation time spectrum z; o~ is frequency. Then a t o 9 ~ - ~ the magnitude Go as compared with G® may be disregarded and the expression (1) may be reduced to the form
oo G~o= S g (z) d'r
O
Using the limiting value of Goo obtained we get
oo 0
H (~) d~ = 2 (GPa)
Convergence of G' for all the elastomers studied towards a common limiting value and the independence o f G.o from the chemical structure, polarity, etc. are possibly due to the fact that all flexible chain polymers in these conditions possess roughly the same relaxation time spectrum. Translated by A. CRozY REFERENCES
1. G. M. BARTENEV, Struktura i relaksatsionnye svoistva elastomerov (Structure and Relaxational Properties of Elastomers). 288 pp., Moscow, 1979 2. I. I. PEREPECHKO, Akusticheskiye metody issledovaniya polimerov (Acoustic Methods for Investigating Polymers), 296 pp., Moscov, 1973 3. B. A. DOGADKIN, A. A. DONTSOV and V. A. SHERSHNEV, Khimiya elastomerov (The Chemistry of Elastomers). 376 pp., Moscow, 1981 4. I. L PEREPCHKO and O. V. STARTSEV, Vysokomol. soyed, BIS: 321, 1973 (Not translated in Polymer Sci. U.S.S.R.) 5. S. K. FROLOVA, Z. L SAVEL'EVA, O. V. NIKITINA and N. N. LEZHNEV, Ibid. A19: 208, 1977 (Translated in Polymer Sci. U.S.S.R. 19: 1,243, 1977) 6. O.V. STARTSEV, Dissert. Cand. Phys.-Math. Sci (in Russian). 166 pp., MOPI im N. K. Krupskoi 166 pp., Moscow, 1975 7. Yu. V. ZELENEV, Releksatsionndye yavleniya v blochnykh polimerakh (Relaxational Phenomena in Block Polymers). p. 25, Leningrad, 1972