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The effect of slow physical relaxation processes on the properties of elastomers over a wide temperature range
570
L . A . AKOPYAN et aL
11. W. A. P R Y O R and I. N . ' C O C O , Macromolecules 3: 500, 1970 12. V. P. KARTA~CYKH, Ye. N. BARANTISE~¢ICI-I, V. A. I,AVROV and S. S. I V A N C H E V , Vysokomol. soyed. A22: 1203, 1980 (Translated in Polymer Sci. U.S.S.R. 22: 6, 1319, 1980) 13. S. I. KUCI-IANO¥, NIetody kineticheskikh raschetov v khimii polimerov (Methods o f Kinetic Calculation in Polymer Chemistry). p. 221, Khimiya, Moscow, 1978; Y. A. K H O K H L O V and S. G. LYUBETSKII, V kn.: Polimerizatsionnyie protsessy. Apparaturnoye oformleniye i matematicheskoye modelirovaniye (In book: Polymerization Processes: Instrumental Formation and Mathematical Modelling). p. 83, Leningrad, 1976 14. J. B R A N D R U P and Ye. N. I N N E R G U T , Polymer Handbook, Intersci. Publ., N.Y,, 1975 15. V. I. VALUYEV, T. S. DMITRIEYA, N . N . T R I Z N A and R. A. SHLYAKHTER, Vysokomol. soyed. B19: 172, 1977 (Not translated in Polymer Sci. U.S.S.R.) 16. T. A. TIME, A. Ye. KALAUS, Ye. L. MACHEVSKAYA, G. S. SOLODO'VNIKOVA and A. B. KORENNAYA, V kn.: Sintez i svoistva zhidkikl-t uglevodorodnykh kauchukov i elastomerov na ikh osnove (In book: Synthesis and Properties of Liquid Hydrocarbon Rubbers ant[ Elastomers based on Them). p. 22, TSNIITENeftekhim, Moscow, 1979
Polymer ScienceU.S.S.R. Vol. 26, No. 3, pp. 570-576, 1984 Printed in Poland
0032-3950/84 $I0.00+ .00' © 1985 Pergamon Press Ltd.
THE EFFECT OF SLOW PHYSICAL RELAXATION PROCESSES ON THE PROPERTIES OF ELASTOMERS OVER A WIDE TEMPERATURE RANGE* L. A . AKOPYAN, N . A . OVRUTSKAYA, E. V. GRONSKAYA a n d G. M . BARTENEV Leningrad Branch, Rubber Industry Research Institute Institute of Physical Chemistry, U.S.S.R. Academy of Sciences (Received 14 June 1982)
The relaxation properties of crosslinked elastomers and of rubbers based on SKMS-10, S K N - 1 8 + S K N - 1 6 and SKN-40, modified with surface-active agents were investigated by relaxation spectrometry. Not only the rapid, but also the slow physical relaxation processes, related to fluctuational decomposition of the microblocks of supersegmental and supermolecular structures (2-processes) and to the thermal mobility of the active filler particles (~-processes) had a significant effect on such elastomer properties as capability for molecular orientation and durability to multiple strain at temperature of T> T~, on maintaining high-elasticity properties at low temperatures, etc. Surface active agents are efficacious modifiers, affecting the time and energy of activation of 2- and ~p-relaxation processes, the effect of the surfactants being increased with a temperature decrease. A scheme for predicting these processes in a low temperature region was developed and the temperature boundaries for reliable prediction of relaxation processes were determined. * Vysokomol. soyed. A26: No. 3, 512-517, 1984.
571
Properties of elastomers over a wide temperature range
RELAXATION spectrometry is a new perspective in polymer physics being a specific means of studying polymer structures in the non-crystalline state and particularly of elastomers and rubbers [1]. The main advantage of relaxation spectrometry is the fact that in the first place, the formal mathematical description of a relaxation process by a continuous spectrum is defined by the physical-structural constants of the material: the discrete relaxation time z~, the activation energy Ui, the dimension Bi and the contribution Ei of each ith subsystem of the assembly of structural sub-systems, in the relaxation process. A higher glass transition temperature, Tg, is exhibited in elastomers by the relaxation spectrometry method and a group of slow relaxation processes are present, due to the mobility of the ordered microblocks of supersegmental and supermolecular structure. Microblocks may have various internal structures: globular, micellar or folded. It is important to note that they have a fluctuational nature i.e. they are pseudo discrete particles. The ¢ process of filler particle relaxation was discovered earlier and is related to reorganization and disintegration of chemical bonds in crosslinked elastomers. High elasticity, the most important property of elastomers, is usually related to ~-relaxation processes. However, the role of slow physical relaxation processes at the molecular level, in causing the stress and tensile properties of elastomers, remains unclear. Besides, the possibility of regulating the relaxation processes by adding surfacrants, over a wide temperature range, has been insufficiently studied. Elastomers and filled rubbers (see Table) were studied. The relaxation properties were changed by varying the fillers and by using various surfactant concentrations. The following anionic surfactants were studied, being according to [2], universal modifiers for the relaxation properties of polymers: primary alkaryl sulphonates R - \ /=/ /--' % SO 3 Na with branched (tetrapropylene benzene sulphonate, brand NP-I) and linear (dodecylbenzene sulphonate, brand KB) alkyl radicals and fillers of various types. The parameters of the discrete relaxation time spectra were calculated b>, the
COMPOSITION OF RUBBERS STUDIED
Vulcanization Rubber
SKMS-10 SKN-40M
I l )
Vulcanization group
i Thiuram + A l t a x + s u l p h u r + ZnO Thiuram + dithiomor! pholine + ZnO
SKN-18 + SK N-26 I Sulphur + ZnO + dibutyl' sebacate + thiuram
Filler and surfactant*
PM-15 PM-15+KB DG-100 PM-15 BS-100 BS-120 KB, NP-I, KB+NP-1; PM-15 + DG-100; PM-15 + DG-100 + N P - I , KB, NP-1 + K B
* PM-15, DG-100, BS-100 and BS-120 are fillers, KB and NP-I are surfactants.
time, min
! T'
30
50
40
1:t.3
30
572
L . A . AKOPYANet aL
method of [I1 on an EVM from the stress relaxation isotherm with 20 70 strain at uniaxial compression and temperatures of 20-130 °. Relaxation processes and their regulation. For different relaxational transitions, their
activation energy Us and coefficient B, were ~lefined by the equation z i = B i exp ( U J k T )
(1)
The coefficient B, is related Ill to the volume v, of a kinetic unit Bi = v~/6( 6 k T /pi)- 1/2,
(2)
where p~ is the density of the substance of which the kinetic unit is composed. The physical representation of the coefficient Bt in formula (1), having a time dimension, is the period of thermal oscillation of the kinetic units around the equilibrium position. For comparative estimates, the sizes of the relaxing structures may be taken as B ~ v . Above 20 °, a group was noticed, consisting of 3 2-relaxation processes, related to the ordering of the microblocks of supersegmental and supermolecular structure. In filled elastomers, there are ~0-relaxation processes for the discrete filler particles. Two chemical processes were disclosed earlier on: the first, the Ol-process is related to the formation of bulky aggregates of colloidal dimensions [3]; the second, the ~2-process with the oscillation of sulphur crosslinks. The chemical processes were almost independent of the presence of fillers or surfactants, only a small acceleration of the Ol-process being noted when they were introduced. Fillers do not change the activation energy of 2-processes, which have the same value for 3 ).-processes, since U~ is determined by the mobility of the same structural e l e m e n t - a segment. But at the same time, the relaxation time ~ and the size of each of the microblocks, characterized by coefficient B~, are changed. The introduction of a surfactant changes all the characteristics of 2-processes, including also Ua, and also the ~0-process characteristics. Finally we will consider the effect of a surfactant on :~ processes, using one as an e x a m p l e - t h e 23-process, as the longest of this group and also the effect of a surfactant on the ~0-process in filled elastomers. Figure 1 depicts the relation of the logarithmic time of the 23 and ~0relaxation processes at 20 ° and also the activation energy of these processes, to surfactant concentration in a rubber based on SKSM-10. As can be seen f r o m the Figure, surfactants change not only the time but the activation energy of slow physical relaxation processes. The 2-process rates are most appreciably changed (7-8 times) and the ~0-process rates less (3-5 times). It is interesting to note that tear resistance clearly correlates precisely with the change in the )~-relaxation processes of microblocks of supermoleeular structure: with an increase in 2-process rate (i.e. decrease in va), tear resistance increases. These results agree with those of [4] in which it was shown that lamination resistance of styrene rubber cloth samples increases with acceleration of the slow physical relaxation processes. Tear resistance is a particular case of the adhesion test by a lamination method. Analogous results on the effect of surfactants on 2- and q~-processes were found for crosslinked elastomers and filled rubbers, based on S K N - 1 8 + S K N - 2 6 (see Fig. 2).
573
Properties of elastomers over a wide temperature range
in these systems, with o p t i m u m Sulphonol K B concentrations, the value o f Ux is dereased by 110-12 and that o f U~,, by 5 kJ/mole. A t the same time, the size o f the relaxing element B~, calculated f r o m the f o r m u l a (1), is increased by 100 times in elastomers (from B ~ = 4-9 x 10- 5 to 5.0 x 10- 3 see) and in a filled rubber by 31 times (from Ba~=8"3 x 10 - s to 2"6x 10-a).
109~,[seel U;9, kJ/mole 80 ~
Ui, AJ/mole 70 ~...
./._.
Y"
1
1
. ....
A,kN/m 30~
I
D 2
a
u~ , /~2/mo/e
C
I _ 7 FIG. 1
70i~
h 2 c,%
G
o
c
a
l
I
2 3
C,%
Fro. 2
Fro. 1. Effect of surfactant (K.B) concentration in an SKMS-10 based rubber on the logarithmic time of 23 (1) and q~(2) relaxation processes at 20° (a), on activation energy of these processes (b) and on tear resistance at 20 ° (c). FIG. 2. Effect of surfactants NP-I (l) and KB (2) concentrations on activation energy of );~ (a, b) and ~0-relaxation processes (c), in an SKN-18+SKN-26 based rubber, without (a) and with filler (b, c). Thus surfactants are effective agents for regulating the structure o f microblocks o f supersegmental and supermolecular structures and consequently the 2-processes. Moreover, the characteristics o f f-processes o f active filler relaxation are changed. In its turn, the introduction o f the filler modifies 2-structures, whilst being less effective m comparison with the surfactant. It is also interesting to note that an increase in the value of B~ and a decrease in Ua in a non-crystallizing elastomer under the action o f a surfactant causes the morphological type o f the microblocks o f the supermolecular structure to a p p r o a c h that o f microbloeks which are capable o f crystallizing. The latter are characterized [5] by comparatively large values o f B~ and low ones o f Ux. Molecular orientation and 2, ~o-relaxation processes. Molecular orientation is developed by straining crosslinked elastomers a n d determines a series o f application prop-
574
L.A.
AKOPYAN et aL
erties of manufactured articles. Nevertheless, it remained unclear whether it was related to rapid or slow physical processes. The capability for molecular orientation under strain was estimated by the method of [6], based on the anisotropic effect of wetting uniaxially strained elastomers [7, 8]. The essence of the method consists in measuring the marginal angles of wetting, 0, for a homologous series of saturated hydrocarbons in the direction of the stretching. We determined the critical surface tension ? of the elastomers by extrapolation of the ratio of cos 0 to the surface tension of the liquids, to cos 0 = 1. The ratio of ?, to stress, tr, with stretched elastomers was linear, which permitted introduction of the orientation parameter, b, = dT/dcr, characterizing the molecular orientation capacity under stress.
Uo~,kJ/mole
3o-' lO
N~ lO-a
2
ao
8 b
f
E;'-,, II o z 41oq:~-~
~"
/f l/
11
I
....
gr# B
1 I
;
~.~
3.'7~-r
qo
5
I
,"
logrilsec]
I
2 FIG. 3
I
-8
4 b~-'lO FIG. 4
FIG. 3. Relation of bo to logarithm of time of 23 (1) a n d ~-relaxation (2) processes in a rubber based o n S K N - 4 0 M (a) a n d also the effect of b~ o n durability N (b) of rubbers based on SKN-40M (3) a n d on SKN-18 + SKN-26, modified with KB + NP-1 (4). FIG. 4. Relations of log r~ (1, 2) and log z, (3) to liT(a) a n d of U~ (4) to 1/T(b) for rubbers based o n S K M S - 1 0 without surfactant (1, 3, 4) and with 1 ~ of surfactant K B (2).
Figure 3 shows the relation of the parameter b~ to logarithmic time for 23- and 9-relaxation processes for SKN-40M based rubber, containing various fillers. It is seen from the Figure that with a decrease in zx and z~, the size of b, grows i.e. slow physical relaxation processes take a direct part in molecular orientation under strain or stress. This is confirmed by the dependance of durability, N, estimated by the number of cycles to rupture, under multiple stretching of the spade shaped samples (according to GOST 261-79), on the parameter b~ (see Fig. 3b). In fact, increases in b, for an SKN-40M-based rubber with various fillers (see curve 3) and for an SKN-18 + SKN-26based rubber containing surfactant (see curve 4), causes an increase in the durability N. Earlier it had been shown by Gul' and co-workers [9-11] that conditions facilitating molecular orientation in place of expansion of the rupture, favour strengthening of polymers in the highly elastic state. At the same time, the extent of supplementary orientation in place of rupture was estimated from the ratio of stretching strain at the top of a growing crack to the strain in the non-rupturing part of the sample. Therefore it is quite natural that an increase in the parameter which characterizes the molecular
Properties of elastomers over a wide temperature range
575
orientation capability under stress or strain leads (as in [121) to an increase in s a m p l e durability with multiple loading. Predicti~r,9 2-relaxation processes at low temperatures. ).-Processes are usually observed o na high-elasticity plateau, above Tg and as shown above, affect a series of elastomer properties. To elucidate their role at low temperatures, rubber based on SKSM-10 alone and also on one with a surfactant, differing from the former in ,~-process behav iour, were studied. According to equation (1), at high temperatures, U / k T ~ O and relaxation time ~ is determined by the coefficient Bz. For a rubber based on SKMS-10 without a surfaetant with T ~ co, B~ = 2.5 x 10- s see and for one containing 1 ~ KB surfa ctant, Ba,= 1.6x 10 -3 sec i.e. the difference is 64 times (see Fig. 4a). To forecast in t h e low temperature range at the limit U / k T ~ o o the value of Uz is determined. As Fig. 4a shows, a reduction in Uz due to a surfactant, is caused by the fact that if the relaxation time at 293°K for a ).s-process in a resin containing surfactant accelerates 7-8 t i m e s (see Fig. la), then at the glass transition point of SKMS-10, the difference in v~ reaches 2-5 orders. However, the problem arises of finding the low temperature boundary for reliable prediction of the times of 2-processes.
-80
O
0.8
8
o.~
oq
-88
0
T°
FiG. 5. Temperature relation of recoverability coefficient of rubbers based on SKN-18 +SKN-26 (a) and on SKMS-10 (b) without surfactant (l) and with surfactant KB (2). The kinetic units, determining the mobility of the microblocks o f supermolecular ~supersegrnental) structures are the free and bound segments. Under normal conditions, the value of Uz is somewhat greater than the activation energy of an u-relaxation process, determining the mobility of the free segments. With a reduction in temperature, the segments' mobility is decreased. The relaxation times for different temperatures may be calculated f r o m equation (1), taking account of the fact that Bi = Bo = 5 × 1 0 - t2 see, the vibration period of a segment around the equilibrium time position and Uz = U~ is calculated from Williams-Landel-Ferry equation
U~=Uoo(1-- T o / T ) - ' ,
(3)
where T o = T , - 5 0 , L~= Uoo at T ~ c o . For SKMS-10, Tg=201°K. To=151°K, U ~ = I 2 k J/mole. [t is seen from Fig. 4b, that at high and moderate temperatures, the relation o f U~ to I / T does not change, but it increases suddenly at low temperatures and with T:= To, U , ~ ~ . The ratio of log v, to 1/T(see Fig. 4a, curve 3) is similar, having a rounded
576
L . A . AKoPYANetaL
type of curve. The Figure shows that at T = Tg, the ~-process time becomes approximately equal to the extrapolation value o f that of 2-processes. This temperature also determines the physical basis of the limit of predicting 2-processes in the low temperatere region. Figure 5 shows the temperature dependance o f the rubber recoverability coefficient with a different "permissibility" of 2-relaxation process, owing to their modification with a surfactant. The reductions in U~ and ~ mean that these rubbers possess greater elasticity at low temperatures than those without a surfactant. This difference disappears as ~r-, 7~. Thus not only rapid but also slow physical relaxation processes determine a series o f important properties of elastomers: the capability for molecular orientation and durability towards multiple straining, the preservation o f high elasticity at low temperatures and tear-resistance. Surfactants effectively regulate the times and activation ~ energies o f these processes, their effect being increased as temperature is reduced. A scheme for reliable prediction o f relaxation properties at low temperatures w a s worked out, based on relaxation spectrometry. Translated by C. W. CAPP
REFERENCES 1. G. M. BARTENEV, Struktura i relaksatsionnyie svoistva elastomerov (Structure and Relaxation Properties of Polymers). p. 288, Khimiya, Moscow, 1979 2. L. A. AKOPYAN, L. M. BARTENEV, E. V. GRONSKAYA and N. A. OVRUTSKAYA~ Khim. prom-st', 8, 610, 1976 3. A. A. D O - - V , Protsessy strukturovaniya elastomerov (Structuring Processes in Rubbers). p. 288, Khimiya, 1978 4. L. A. AKOPYAN, E. V. GROWSKAYA and G. M. BARTENEV, Adhesives 6: 177, 1974 5. L. A. AKOPYAN, M. V. ZABINA and G. M. BARTENEV, Vysokomol. soyed. A24: 58, 1982 (Translated in Polymer Sci. U.S.S.R. 24: 1, 67, 1982) 6. L. A. AKOPYAN, N. A. OVRUTSKAYA and V. P. NIKIFOROV, Authors' cert. No. 657314 (U.S.S.R.); Publ. in Bull. Inventions, No. 14, 174, 1979 7. G. M. BARTENEV and L. A. AKOPYAN, Vysokomol. soyed. B12: 359, 1970 (Not translated in Polymer Sci. U.S.S.R.); Plaste und Kautschuk 16: 655, 1969 8. A. I. RUSANOV, Kolloid zh. 39: 704, 1977 9. V. Ye. GUL', G. P. KRUTETSKAYA and V. V. KOVRIGA, Kauchuk i resina, 12, l, 1957 10. B. A. DOGADKIN, D. L. FEDYUKIN and V. Ye. GUL', Kolloid z.h. 19: 292, 1957 11. V. Ye. GUL', Struktura i prochnost' polimerov (Structure and Durability of Polymers). p, 106, Khimiya, Moscow, 1978 12. ]L. A. AKOPYAN, N. A. OVRUTSKAYA and G. M. BARTENEV, Vysokomol. soyed. A24:: 1705, 1982 (Translated in Polymer Sci. U.S.S.R. 24: 8, 1944, 1982)
L . A . AKOPYAN et aL
11. W. A. P R Y O R and I. N . ' C O C O , Macromolecules 3: 500, 1970 12. V. P. KARTA~CYKH, Ye. N. BARANTISE~¢ICI-I, V. A. I,AVROV and S. S. I V A N C H E V , Vysokomol. soyed. A22: 1203, 1980 (Translated in Polymer Sci. U.S.S.R. 22: 6, 1319, 1980) 13. S. I. KUCI-IANO¥, NIetody kineticheskikh raschetov v khimii polimerov (Methods o f Kinetic Calculation in Polymer Chemistry). p. 221, Khimiya, Moscow, 1978; Y. A. K H O K H L O V and S. G. LYUBETSKII, V kn.: Polimerizatsionnyie protsessy. Apparaturnoye oformleniye i matematicheskoye modelirovaniye (In book: Polymerization Processes: Instrumental Formation and Mathematical Modelling). p. 83, Leningrad, 1976 14. J. B R A N D R U P and Ye. N. I N N E R G U T , Polymer Handbook, Intersci. Publ., N.Y,, 1975 15. V. I. VALUYEV, T. S. DMITRIEYA, N . N . T R I Z N A and R. A. SHLYAKHTER, Vysokomol. soyed. B19: 172, 1977 (Not translated in Polymer Sci. U.S.S.R.) 16. T. A. TIME, A. Ye. KALAUS, Ye. L. MACHEVSKAYA, G. S. SOLODO'VNIKOVA and A. B. KORENNAYA, V kn.: Sintez i svoistva zhidkikl-t uglevodorodnykh kauchukov i elastomerov na ikh osnove (In book: Synthesis and Properties of Liquid Hydrocarbon Rubbers ant[ Elastomers based on Them). p. 22, TSNIITENeftekhim, Moscow, 1979
Polymer ScienceU.S.S.R. Vol. 26, No. 3, pp. 570-576, 1984 Printed in Poland
0032-3950/84 $I0.00+ .00' © 1985 Pergamon Press Ltd.
THE EFFECT OF SLOW PHYSICAL RELAXATION PROCESSES ON THE PROPERTIES OF ELASTOMERS OVER A WIDE TEMPERATURE RANGE* L. A . AKOPYAN, N . A . OVRUTSKAYA, E. V. GRONSKAYA a n d G. M . BARTENEV Leningrad Branch, Rubber Industry Research Institute Institute of Physical Chemistry, U.S.S.R. Academy of Sciences (Received 14 June 1982)
The relaxation properties of crosslinked elastomers and of rubbers based on SKMS-10, S K N - 1 8 + S K N - 1 6 and SKN-40, modified with surface-active agents were investigated by relaxation spectrometry. Not only the rapid, but also the slow physical relaxation processes, related to fluctuational decomposition of the microblocks of supersegmental and supermolecular structures (2-processes) and to the thermal mobility of the active filler particles (~-processes) had a significant effect on such elastomer properties as capability for molecular orientation and durability to multiple strain at temperature of T> T~, on maintaining high-elasticity properties at low temperatures, etc. Surface active agents are efficacious modifiers, affecting the time and energy of activation of 2- and ~p-relaxation processes, the effect of the surfactants being increased with a temperature decrease. A scheme for predicting these processes in a low temperature region was developed and the temperature boundaries for reliable prediction of relaxation processes were determined. * Vysokomol. soyed. A26: No. 3, 512-517, 1984.
571
Properties of elastomers over a wide temperature range
RELAXATION spectrometry is a new perspective in polymer physics being a specific means of studying polymer structures in the non-crystalline state and particularly of elastomers and rubbers [1]. The main advantage of relaxation spectrometry is the fact that in the first place, the formal mathematical description of a relaxation process by a continuous spectrum is defined by the physical-structural constants of the material: the discrete relaxation time z~, the activation energy Ui, the dimension Bi and the contribution Ei of each ith subsystem of the assembly of structural sub-systems, in the relaxation process. A higher glass transition temperature, Tg, is exhibited in elastomers by the relaxation spectrometry method and a group of slow relaxation processes are present, due to the mobility of the ordered microblocks of supersegmental and supermolecular structure. Microblocks may have various internal structures: globular, micellar or folded. It is important to note that they have a fluctuational nature i.e. they are pseudo discrete particles. The ¢ process of filler particle relaxation was discovered earlier and is related to reorganization and disintegration of chemical bonds in crosslinked elastomers. High elasticity, the most important property of elastomers, is usually related to ~-relaxation processes. However, the role of slow physical relaxation processes at the molecular level, in causing the stress and tensile properties of elastomers, remains unclear. Besides, the possibility of regulating the relaxation processes by adding surfacrants, over a wide temperature range, has been insufficiently studied. Elastomers and filled rubbers (see Table) were studied. The relaxation properties were changed by varying the fillers and by using various surfactant concentrations. The following anionic surfactants were studied, being according to [2], universal modifiers for the relaxation properties of polymers: primary alkaryl sulphonates R - \ /=/ /--' % SO 3 Na with branched (tetrapropylene benzene sulphonate, brand NP-I) and linear (dodecylbenzene sulphonate, brand KB) alkyl radicals and fillers of various types. The parameters of the discrete relaxation time spectra were calculated b>, the
COMPOSITION OF RUBBERS STUDIED
Vulcanization Rubber
SKMS-10 SKN-40M
I l )
Vulcanization group
i Thiuram + A l t a x + s u l p h u r + ZnO Thiuram + dithiomor! pholine + ZnO
SKN-18 + SK N-26 I Sulphur + ZnO + dibutyl' sebacate + thiuram
Filler and surfactant*
PM-15 PM-15+KB DG-100 PM-15 BS-100 BS-120 KB, NP-I, KB+NP-1; PM-15 + DG-100; PM-15 + DG-100 + N P - I , KB, NP-1 + K B
* PM-15, DG-100, BS-100 and BS-120 are fillers, KB and NP-I are surfactants.
time, min
! T'
30
50
40
1:t.3
30
572
L . A . AKOPYANet aL
method of [I1 on an EVM from the stress relaxation isotherm with 20 70 strain at uniaxial compression and temperatures of 20-130 °. Relaxation processes and their regulation. For different relaxational transitions, their
activation energy Us and coefficient B, were ~lefined by the equation z i = B i exp ( U J k T )
(1)
The coefficient B, is related Ill to the volume v, of a kinetic unit Bi = v~/6( 6 k T /pi)- 1/2,
(2)
where p~ is the density of the substance of which the kinetic unit is composed. The physical representation of the coefficient Bt in formula (1), having a time dimension, is the period of thermal oscillation of the kinetic units around the equilibrium position. For comparative estimates, the sizes of the relaxing structures may be taken as B ~ v . Above 20 °, a group was noticed, consisting of 3 2-relaxation processes, related to the ordering of the microblocks of supersegmental and supermolecular structure. In filled elastomers, there are ~0-relaxation processes for the discrete filler particles. Two chemical processes were disclosed earlier on: the first, the Ol-process is related to the formation of bulky aggregates of colloidal dimensions [3]; the second, the ~2-process with the oscillation of sulphur crosslinks. The chemical processes were almost independent of the presence of fillers or surfactants, only a small acceleration of the Ol-process being noted when they were introduced. Fillers do not change the activation energy of 2-processes, which have the same value for 3 ).-processes, since U~ is determined by the mobility of the same structural e l e m e n t - a segment. But at the same time, the relaxation time ~ and the size of each of the microblocks, characterized by coefficient B~, are changed. The introduction of a surfactant changes all the characteristics of 2-processes, including also Ua, and also the ~0-process characteristics. Finally we will consider the effect of a surfactant on :~ processes, using one as an e x a m p l e - t h e 23-process, as the longest of this group and also the effect of a surfactant on the ~0-process in filled elastomers. Figure 1 depicts the relation of the logarithmic time of the 23 and ~0relaxation processes at 20 ° and also the activation energy of these processes, to surfactant concentration in a rubber based on SKSM-10. As can be seen f r o m the Figure, surfactants change not only the time but the activation energy of slow physical relaxation processes. The 2-process rates are most appreciably changed (7-8 times) and the ~0-process rates less (3-5 times). It is interesting to note that tear resistance clearly correlates precisely with the change in the )~-relaxation processes of microblocks of supermoleeular structure: with an increase in 2-process rate (i.e. decrease in va), tear resistance increases. These results agree with those of [4] in which it was shown that lamination resistance of styrene rubber cloth samples increases with acceleration of the slow physical relaxation processes. Tear resistance is a particular case of the adhesion test by a lamination method. Analogous results on the effect of surfactants on 2- and q~-processes were found for crosslinked elastomers and filled rubbers, based on S K N - 1 8 + S K N - 2 6 (see Fig. 2).
573
Properties of elastomers over a wide temperature range
in these systems, with o p t i m u m Sulphonol K B concentrations, the value o f Ux is dereased by 110-12 and that o f U~,, by 5 kJ/mole. A t the same time, the size o f the relaxing element B~, calculated f r o m the f o r m u l a (1), is increased by 100 times in elastomers (from B ~ = 4-9 x 10- 5 to 5.0 x 10- 3 see) and in a filled rubber by 31 times (from Ba~=8"3 x 10 - s to 2"6x 10-a).
109~,[seel U;9, kJ/mole 80 ~
Ui, AJ/mole 70 ~...
./._.
Y"
1
1
. ....
A,kN/m 30~
I
D 2
a
u~ , /~2/mo/e
C
I _ 7 FIG. 1
70i~
h 2 c,%
G
o
c
a
l
I
2 3
C,%
Fro. 2
Fro. 1. Effect of surfactant (K.B) concentration in an SKMS-10 based rubber on the logarithmic time of 23 (1) and q~(2) relaxation processes at 20° (a), on activation energy of these processes (b) and on tear resistance at 20 ° (c). FIG. 2. Effect of surfactants NP-I (l) and KB (2) concentrations on activation energy of );~ (a, b) and ~0-relaxation processes (c), in an SKN-18+SKN-26 based rubber, without (a) and with filler (b, c). Thus surfactants are effective agents for regulating the structure o f microblocks o f supersegmental and supermolecular structures and consequently the 2-processes. Moreover, the characteristics o f f-processes o f active filler relaxation are changed. In its turn, the introduction o f the filler modifies 2-structures, whilst being less effective m comparison with the surfactant. It is also interesting to note that an increase in the value of B~ and a decrease in Ua in a non-crystallizing elastomer under the action o f a surfactant causes the morphological type o f the microblocks o f the supermolecular structure to a p p r o a c h that o f microbloeks which are capable o f crystallizing. The latter are characterized [5] by comparatively large values o f B~ and low ones o f Ux. Molecular orientation and 2, ~o-relaxation processes. Molecular orientation is developed by straining crosslinked elastomers a n d determines a series o f application prop-
574
L.A.
AKOPYAN et aL
erties of manufactured articles. Nevertheless, it remained unclear whether it was related to rapid or slow physical processes. The capability for molecular orientation under strain was estimated by the method of [6], based on the anisotropic effect of wetting uniaxially strained elastomers [7, 8]. The essence of the method consists in measuring the marginal angles of wetting, 0, for a homologous series of saturated hydrocarbons in the direction of the stretching. We determined the critical surface tension ? of the elastomers by extrapolation of the ratio of cos 0 to the surface tension of the liquids, to cos 0 = 1. The ratio of ?, to stress, tr, with stretched elastomers was linear, which permitted introduction of the orientation parameter, b, = dT/dcr, characterizing the molecular orientation capacity under stress.
Uo~,kJ/mole
3o-' lO
N~ lO-a
2
ao
8 b
f
E;'-,, II o z 41oq:~-~
~"
/f l/
11
I
....
gr# B
1 I
;
~.~
3.'7~-r
qo
5
I
,"
logrilsec]
I
2 FIG. 3
I
-8
4 b~-'lO FIG. 4
FIG. 3. Relation of bo to logarithm of time of 23 (1) a n d ~-relaxation (2) processes in a rubber based o n S K N - 4 0 M (a) a n d also the effect of b~ o n durability N (b) of rubbers based on SKN-40M (3) a n d on SKN-18 + SKN-26, modified with KB + NP-1 (4). FIG. 4. Relations of log r~ (1, 2) and log z, (3) to liT(a) a n d of U~ (4) to 1/T(b) for rubbers based o n S K M S - 1 0 without surfactant (1, 3, 4) and with 1 ~ of surfactant K B (2).
Figure 3 shows the relation of the parameter b~ to logarithmic time for 23- and 9-relaxation processes for SKN-40M based rubber, containing various fillers. It is seen from the Figure that with a decrease in zx and z~, the size of b, grows i.e. slow physical relaxation processes take a direct part in molecular orientation under strain or stress. This is confirmed by the dependance of durability, N, estimated by the number of cycles to rupture, under multiple stretching of the spade shaped samples (according to GOST 261-79), on the parameter b~ (see Fig. 3b). In fact, increases in b, for an SKN-40M-based rubber with various fillers (see curve 3) and for an SKN-18 + SKN-26based rubber containing surfactant (see curve 4), causes an increase in the durability N. Earlier it had been shown by Gul' and co-workers [9-11] that conditions facilitating molecular orientation in place of expansion of the rupture, favour strengthening of polymers in the highly elastic state. At the same time, the extent of supplementary orientation in place of rupture was estimated from the ratio of stretching strain at the top of a growing crack to the strain in the non-rupturing part of the sample. Therefore it is quite natural that an increase in the parameter which characterizes the molecular
Properties of elastomers over a wide temperature range
575
orientation capability under stress or strain leads (as in [121) to an increase in s a m p l e durability with multiple loading. Predicti~r,9 2-relaxation processes at low temperatures. ).-Processes are usually observed o na high-elasticity plateau, above Tg and as shown above, affect a series of elastomer properties. To elucidate their role at low temperatures, rubber based on SKSM-10 alone and also on one with a surfactant, differing from the former in ,~-process behav iour, were studied. According to equation (1), at high temperatures, U / k T ~ O and relaxation time ~ is determined by the coefficient Bz. For a rubber based on SKMS-10 without a surfaetant with T ~ co, B~ = 2.5 x 10- s see and for one containing 1 ~ KB surfa ctant, Ba,= 1.6x 10 -3 sec i.e. the difference is 64 times (see Fig. 4a). To forecast in t h e low temperature range at the limit U / k T ~ o o the value of Uz is determined. As Fig. 4a shows, a reduction in Uz due to a surfactant, is caused by the fact that if the relaxation time at 293°K for a ).s-process in a resin containing surfactant accelerates 7-8 t i m e s (see Fig. la), then at the glass transition point of SKMS-10, the difference in v~ reaches 2-5 orders. However, the problem arises of finding the low temperature boundary for reliable prediction of the times of 2-processes.
-80
O
0.8
8
o.~
oq
-88
0
T°
FiG. 5. Temperature relation of recoverability coefficient of rubbers based on SKN-18 +SKN-26 (a) and on SKMS-10 (b) without surfactant (l) and with surfactant KB (2). The kinetic units, determining the mobility of the microblocks o f supermolecular ~supersegrnental) structures are the free and bound segments. Under normal conditions, the value of Uz is somewhat greater than the activation energy of an u-relaxation process, determining the mobility of the free segments. With a reduction in temperature, the segments' mobility is decreased. The relaxation times for different temperatures may be calculated f r o m equation (1), taking account of the fact that Bi = Bo = 5 × 1 0 - t2 see, the vibration period of a segment around the equilibrium time position and Uz = U~ is calculated from Williams-Landel-Ferry equation
U~=Uoo(1-- T o / T ) - ' ,
(3)
where T o = T , - 5 0 , L~= Uoo at T ~ c o . For SKMS-10, Tg=201°K. To=151°K, U ~ = I 2 k J/mole. [t is seen from Fig. 4b, that at high and moderate temperatures, the relation o f U~ to I / T does not change, but it increases suddenly at low temperatures and with T:= To, U , ~ ~ . The ratio of log v, to 1/T(see Fig. 4a, curve 3) is similar, having a rounded
576
L . A . AKoPYANetaL
type of curve. The Figure shows that at T = Tg, the ~-process time becomes approximately equal to the extrapolation value o f that of 2-processes. This temperature also determines the physical basis of the limit of predicting 2-processes in the low temperatere region. Figure 5 shows the temperature dependance o f the rubber recoverability coefficient with a different "permissibility" of 2-relaxation process, owing to their modification with a surfactant. The reductions in U~ and ~ mean that these rubbers possess greater elasticity at low temperatures than those without a surfactant. This difference disappears as ~r-, 7~. Thus not only rapid but also slow physical relaxation processes determine a series o f important properties of elastomers: the capability for molecular orientation and durability towards multiple straining, the preservation o f high elasticity at low temperatures and tear-resistance. Surfactants effectively regulate the times and activation ~ energies o f these processes, their effect being increased as temperature is reduced. A scheme for reliable prediction o f relaxation properties at low temperatures w a s worked out, based on relaxation spectrometry. Translated by C. W. CAPP
REFERENCES 1. G. M. BARTENEV, Struktura i relaksatsionnyie svoistva elastomerov (Structure and Relaxation Properties of Polymers). p. 288, Khimiya, Moscow, 1979 2. L. A. AKOPYAN, L. M. BARTENEV, E. V. GRONSKAYA and N. A. OVRUTSKAYA~ Khim. prom-st', 8, 610, 1976 3. A. A. D O - - V , Protsessy strukturovaniya elastomerov (Structuring Processes in Rubbers). p. 288, Khimiya, 1978 4. L. A. AKOPYAN, E. V. GROWSKAYA and G. M. BARTENEV, Adhesives 6: 177, 1974 5. L. A. AKOPYAN, M. V. ZABINA and G. M. BARTENEV, Vysokomol. soyed. A24: 58, 1982 (Translated in Polymer Sci. U.S.S.R. 24: 1, 67, 1982) 6. L. A. AKOPYAN, N. A. OVRUTSKAYA and V. P. NIKIFOROV, Authors' cert. No. 657314 (U.S.S.R.); Publ. in Bull. Inventions, No. 14, 174, 1979 7. G. M. BARTENEV and L. A. AKOPYAN, Vysokomol. soyed. B12: 359, 1970 (Not translated in Polymer Sci. U.S.S.R.); Plaste und Kautschuk 16: 655, 1969 8. A. I. RUSANOV, Kolloid zh. 39: 704, 1977 9. V. Ye. GUL', G. P. KRUTETSKAYA and V. V. KOVRIGA, Kauchuk i resina, 12, l, 1957 10. B. A. DOGADKIN, D. L. FEDYUKIN and V. Ye. GUL', Kolloid z.h. 19: 292, 1957 11. V. Ye. GUL', Struktura i prochnost' polimerov (Structure and Durability of Polymers). p, 106, Khimiya, Moscow, 1978 12. ]L. A. AKOPYAN, N. A. OVRUTSKAYA and G. M. BARTENEV, Vysokomol. soyed. A24:: 1705, 1982 (Translated in Polymer Sci. U.S.S.R. 24: 8, 1944, 1982)