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Crosslinking of polydimethylvinylsiloxane rubber by oligoorganosilane with functional groups at the silicon atom
1192
A. A. ZHDANOV e$ a~.
5. Kh. S. BAGDASARYAN, Teoriya radikal'noi polhnerizatsii (Theory of Radical Polymerization), 2nd ed. suppl, a n d ammended, l~auka, Moscow, 1966 6. R. (L NORRISH and J. P. S1MONS, Proc. Roy. Soe. A 251: 4, 1264, 1959 7. V. M. GRANCHAK, V. P. SHERSTYUK and I. I. DILUNG, Dokl. Adad. N a u k SSSR 235: 3, 611, 1977 8. S. S. WANSER and G. S. HAMMOND, J. Amer. Chem. Soc. 92: 6362, 1970
Polymer Science U.S.S.R. Vol. 25, No. 5, pp. 1192-1198, 1983 Printed in Poland
0032-3950/83 $10.00 +.00 © 1984 Pergamon Press Ltd.
CROSSHNKING OF POLYDIMETHYLVINYLSILOXANE RUBBER BY OLIGOORGANOSILANE WITH FUNCTIONAL GROUPS AT THE SILICON ATOM* A. A. ZHDANOV,V. Yu. LEVIN, G. L. SLONIMSKII, Yr. P. KVACHEV, O. T. GRITSE~KO, N. V. DELAZARI, N. A. CHARNYAVSKAYA,V. E. MmHLI~ and Yr.. S. OBOLONKOVA Nesmeyanov Hetero-organic Compounds Institute, U.S.S.R. A c a d e m y of Sciences
(Received 26 December 1981) The feasibility of crosslinking polydimethylvinylsiloxane rubber b y oligoorganosilane with functional groups at the Si a t o m has been investigated. I t is shown t h a t the addition of oligoorganosilane to an organosilicon rubber is accompanied b y a m a r k e d increase in tensile strength and in Young's modulus. This is a t t r i b u t e d to chemical erosslinking of the rubber, a n d to the formation of a two-phase system.
ORGAI~OSILICONrubbers maintain their serviceability over a wide range of temperature, but poor strength properties are among their main disadvantages. I t is now urgently necessary to find means of enhancing the strength properties of these rubbers. This paper relates to our s~udy of the feasibility of erossllnldng of the organosilicon rubber SKTV-1 (polydimethylsiloxane rubber containing 0.5 mole % methylvinylsiloxane units) by means of "Ethylan" grade oligoorganosilane (OS). We took as s t u d y objects films based on mixtures of the rubber and OS in an ahuninium foil cuvette, casting from a 20-25% benzene solution. After elimination of benzene the film remaining in the cuvette was k e p t at 473 K for 6 hr. F o r comparison a s t u d y was made of the systems r u b b e r - c u m y l peroxide and r u b b e r - O S - p e r o x i d e crosslinked under identical conditions. The OS concentration was varied within the limits of 1 to 25%, a n d the peroxide * Vysokomol. soyed. A25: No. 5, 1030-1034, 1983.
Crosslinking of polydimethylvinylsiloxane rubber
11-93
concentration from" 0.5 to 2.0~o. Statio mechanical properties of the erosslinl~ed rubber were investigated by a different method using a Polyani type dynamometer [1] at room temperature, the stretching rate being 6.5 × 10-= mm/sec. Dynamic mechanical measurements were carried out under a regime of freely extinguishing torsional vibrations at a frequency of the order of 1 Hz [2]. Thermomechanical experiments were based on the method outlined in [3]. In both cases the heating of abruptly cooled samples was carried out at a rate of the order of 1 deg/min. The morphology of the crosslinked systems was investigated by meazls of carbon-platinum replicas using an EMV-100L electron microscope. Replicas were applied ~o the surface of a section of a sample or to the fracture surface, and were subsequently removed in a saturated alcoholic solution of an alkali. According t o t h e results o f t h e r m o m e c h a n i c a l tests, the d e f o r m a t i o n response t o t h e action of a force field a n d ~emperature field in t h e case of r u b b e r - O S syst e m s is similar, irrespective of the OS concentration, t o the d e f o r m a t i o n changes occurring in r u b b e r - p e r o x i d e systems, and m a y be described b y a t h e r m o m e c h a n ieal curve of t h e t y p e associated w i t h erosslinked organosilicon rubbers [4].
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100
300
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FIG. 2
Fro. 1. Thermomechanieal curves of samples of the rubber crosslinked with 9% of OS (1), by 1~o of cumyl peroxide (2), curves of the initial OS (3) and the OS l~ept at 473 K for 11 hr (4). Pressure on sample = 7-2 × 10-= MPa. FIG. 2. Tensile curves of the rubber eroaslinl~ed with 1 (1), 4 (2), 9 (3), 16 (4) and 25~o (5) OS, and for the rubber erosslinked with 0.5 (6), 1 (7) and 2% (8) cumyl peroxide. This is exemplified b y curves in Fig. 1 where we h a v e t h e r u b b e r containing 9~/o of OS (curve 1) a n d t h e r u b b e r crosslinlred by t h e peroxide (curve 2). I n b o t h cases a rise in ~emperature is a c c o m p a n i e d b y similar d e f o r m a t i o n changes: in the region of 150 K increased d e f o r m a t i o n corresponds t o devi%rification o f t h e a m o r p h o u s phase of t h e rubber; n e x t , u p ~o t e m p e r a t u r e s o f t h e order o f 220 K , t h e degree of d e f o r m a t i o n is r e d u c e d slightly; t h e n it increases again a t a r o u n d 220 K in view o f t h e melting o f crystalline phase, a f t e r which t h e r e is a slight r e d u c t i o n t y p i c a l for t h e region of t h e high elastic state. T h u s w i t h i n t h e limits of a c c u r a c y o f t h e t h e r m o m e c h a n i c a l m e t h o d we m a y say t h a t a n addition o f u p t o 2 5 % of OS t o t h e r u b b e r has scarcely a n y effect on m a j o r transitions in t h e
A. A. ZmD~ov a ~.
1194
latchet, viz. on the temperature intervals of glass,transition and melting. O n t h e basis of considerations cited in [5] we surmise t h a t the additi6n of 0S leads t 0 a kinetic change in the crystallization of the rubber. !
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~-'IO. 3. Breaking stress o-B vs. OS concentration for systems containing 0 (1), 0"5 (2) and 1% (3) cumyl peroxide. An estimate of the static mechanical properties of the systems was based on an analysis of the tensile curves. I n Fig. 2 we have as an example the tensile curves for the systems rubber-0S and rubber-peroxide. It can be seen t h a t an increase in the 0S concentration in the system is accompanied by a reduction in the breaking deformation ~B over the entire range of 0S concentration, and by an increase in tensile strength aB Up tO an 0S concentration of 16~. Figure 3 shows the eul'ves of {TB VS. the 0S concentration in the rubbers containing 0, 0.5 and 1 ~o o f cumyl peroxide. I t is seen that, in all cases, an increase in the 0S concentration is accompanied by a rise in aB. A maximum value of aB is obtained for the ternary system rubber-OS-peroxide containing 16% of OS, and amounts to ~0.9 MPa. The {TB values for the rubber crosslinked with 0.5 and 1 and 2 ~ of the peroxide are 0.1, 0.15 and 0.2 MPa respectively (Fig. 2, curves 6-8). I t appears t h a t crosslinking of the 0S makes it possible, over the entire interval of 0S concentrations, to obtain crosslinked systems whose strength is 4-5 times that of the rubbers crosslinkcd by cumy! peroxide, which at present is widely used as a crosslinking agent. An increase in the peroxide concentration in the systems containing OS is accompanied b y a rise in aB in the interval of 0S concentrations from 9 to 25% (Fig. 3). At 0S concentrations of 1 and 40//oa difference in the peroxide concentration has scarcely any effect on aB. In Fig. 4 we have plots of Young's modulus E vs, the OS concentration for the s y s ~ m s containing 0.5 and 1 °/o of the pero.vide. I r i s seen t h a t an increase in the 0 8 concentration is accompanied by a rise in E, irrespective of the peroxide concenVration. After comparing the value of E for the rubber crosslln_ked by the peroxide (-~0-25 MPa at a 1 ~ peroxide concentra#ion) with t h a t for the systems containing 0S we conclude that ~he modulus is ~ 4 times higher for the latter. ; The increase in,~ and ~he reduction in 8B as t h e OS Concentration increa,ses a m
Crosslinkingof polydlmethylvinylsiloxanerubber
1195,
similar to what is observed when network densities in crosslinlredhigh-elastic systems are inoreased. In this connection it is helpful to compare the increase in E with the increased density of the three-dimensional network that occurs w h e n the O S concentration is increased. In a concrete case we are not concerned with the absolute value of the crosslink concentration, and so we will limit ourselves to a study of changes in the equilibrium degree of swelling Q in benzene for the rubber-OS-peroxide systems as the OS concentration is increased (Fig. 5). It can be seen t h a t the introduction of 1°/o of OS brings a marked reduction in Q. An increase in the OS concentration to 4% is accompanied by a smaller change in Q, and there is scarcely any change in Q if the OS concentration is further increased. The swelling Q is proportional to the MW of a chain segment between erosslinks Me, and so we surmise t h a t a marked change in Me will occur only if we introduce up to 40/o of the OS, and t h a t a further change in the OS concenti~ttion will be accompanied by scarcely any reduction in Me. Comparing plots of Q and E vs. the OS concentration it is seen t h a t these curves are not similar in slope (symbate), since a significant change in E, in contradistinction to Q, takes place over the entire OS concentration interval. Lu view of this an increase in E at OS concentrations exceeding 4% cannot be attributed to increased network density. The question t h a t now arises concerns the distribution of OS when introduced into an organosilicon rubber. This quest i o n ! s made all the more appropriate by the fact t h a t the latter could hardly be compatible with the OS, particularly if the OS concentration is high. To investigate the distribution of OS in the rubber we made an electron microscope study of samples containing differing amounts of OS. As an e x a m p l e we have in Fig. 6 the photomicrograph of the rubber-OS system containing 9 % of 0S. I t is seen that spherical and oval formations appear against a background of the structureless matrix. The number of these formations increases with the OS concentration. I t looks as if the OS is present (at any rate in the case of high concentrations) in the form of a separate phase in the rubber. To determine features of the two-phase n a t u r e of the systems containing OS we carried out measurements of dynamic mechanical properties. Figure 7 shows the temperature dependences of the logarithmic decrement of extinction and of the real part of the complex shear modulus for the rubber-OS systems containing l, 9 and 25% of OS. In all cases there are clear-cut transitions in the vicinity of 150 and 220 K corresponding to molecular mobility upon devitrification and melting respectively. As the OS concentration increases, losses in the temperature region situated between the temperatures of glass-transition and melting increase, and appear in the form of a diffuse peak at an OS concentration of 25 o/o. According to the thermomechanical data the softening point of the initial OS is ~180 K, or for the OS hardened in air at 473 C, ~ 200 K (Fig. 1, curves 3 and 4). iY[easurement of mechanical losses in the temperature interval lying between temperatures of glass transition and melting are, in our view, due preferentially to molecular mobility on thawing OS separated out as a separate phase.
1196
A.A.
ZHDA~OV et al.
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/5
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:FIG. 4. Young's modulus E vs. OS concentration for systems containing 0.5 (1) a n d 1 ~o {2) of cumyl peroxide. The points show the experimental values of E. The lines were drawn in accordance with the results of least squares approximations. FzG. 5. Degree of equilibrium swelling Q vs. OS concentration for systems containing 0.5 (1) a n d 1~/o (2) of cumyl peroxide.
:FIG. 6. Photomicrograph of the rubber crosslinked with 9 % of the OS ( × 25,000).
Crosslinking of polydimethylvinylsiloxanorubber
1197
On the basis of an analysis of the experimental results we surmise that the increase in aB and in E observed for the organosilicon rubber on introducing the OS is accounted for by two factors: these are crossiinking of the rubber and the emergence of a two-phase system in which one phase is formed by the rubber erosslinked by the OS and the peroxide (or by OS only), while the other phase consists of cros~ilnl~ed OS. We surmise that the hardened OS forming a separate phase is partly llnt:ed to the rubber matrix by chemical bonds, and that accordingly the structure of the rubber-OS-peroxide systems will be closer to that of two-phase nonstereospecific block copolymers than to that of incompatible blends. The observed increase in aB and reduction in 8~ when the concentration is increased are similar to changes in the tensile strength and deformation of block eopolymers accomlaany~mg an increase in the relative amount of rigid component in a eopolymer [6].
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FIG. 7. Temperature dependences of the real part of the complex shear modulus 6/' (1-3) and of the logarithmic decrement of damping A (1"-3') for the rubber crosslinked with 1 (1, 1'), 9 (9, 2') and 25% of OS (3, 3'). Thus the use of OS acting as a crosslJni~ng agent for an organosilicon rubber and, at sufficiently high concentrations forming a separate phase, provides a means of enhacing the strength properties of organosilicon rubbers. It is highly probable that this means of preparing crosslinked organosilicon rubbers may be equally effective in the case of filled systems. Trar~sZa~d by R . J . A . HENDRY
1198
~r. V. PCHELINTS~V and Y~.. T. D E , SOy
REFERENCES 1. A. Ya. W.~LKIN, A. A. ASKADSKII and V. V. KOVRIGA, Metody izmereniya mekha. nicheskikh svoistv polimerov (Methods for Measuring the Mechanical Properties of Polymers). p. 33, Khimiya, Moscow, 1978 2. V. Yu. LEVIN, A. A. ZHDANOV, G. L. SLONIMSKII, V. A. MARTIROSOV, Yu. P. KVACHEV, G. Ye. GOLUBKOV and O. T. GRITSENK0, Vysokomol. soyed. A24: 2115, 1982 (Translated in Polymer Sci. U.S.S.R. 24: 10, 2422, 1982) 3. B. L. TSETLIN, V. I. GAVRIL0V, N. A. VELIKOVSKAYA and V. V. KOCHKIN, Zavodsk. lab. 22: 3, 352, 1956 4. K. A. ANDRIANOV, G. L. SLONIMSKII, A. A. ZDANOV, V. Yu. LEVIN, Yu. K. GODOVSKH and V. A. MOSKALENKO, J, Polymer Sci. A-1 10: 1, 23, 1972 5. V. Yu. LEVIN, Avteref. dis. na soiskanie ueh. st. dokt. khim. nauk (Patents Discussion at Chem. Doctorate Degree Contest). p. 15, INEOS Akad. Nauk SSSR, Moscow, 1977 6. A. NOSHEI and D. A. WAK-GRAT, Blok-sopolimery (Block Copolymers). p. 416, Miro Moscow, 1980
Pol.~mer Science U.S.S.R. ~ol. 25, No. 5, !01~.1198-1206, 1988 Printed in Poland
0032-3950/83 $10.00 -i-.00 © 1984 Pergamon Press Ltd.
KINETICS OF THE THERMOOXIDATIVE DEGRADATION OF cis-I,4-POLYISOPRENE IN BULK IN THE PRESENCE OF PHE.NOL ANTIOXIDANTS* V. V. PCl~V.LINTSEVand Y~,. T. Dv.~ISOV S. V. Lebedev All.Union Synthetic Rubber Research Institute Department of the Chemical Physics Institute, ~T.S.S.R. Academy of Sciences
(Received 27 December
1981)
The kinetics of oxidation and of C--C bond seission have been investigated for c/~-l,4-polyisoprene in the presence of sterically hindered a]kyl phenols and azobisisobutyronitrile (ABN). Relations between rates of oxidation and degradation, and the inhibitor concentration and rate of initiation have been determined. Degradation of the polymer is thought to take place through the ~-peroxyperoxide radical reacting with the phenoxyl radical via breakdown of the unstable quinolide peroxide.
IN AN earlier paper [1] we studied the initiated oxidation of cis-1,4-polyisoprene (PI) and showed that degradation of the polymer was the result of rapid decay of unstable alkoxyperoxide radicals emerging during the disproportionation of * Vysokomol. soyed. A25: No. 5, 1035-1040, 1983.
A. A. ZHDANOV e$ a~.
5. Kh. S. BAGDASARYAN, Teoriya radikal'noi polhnerizatsii (Theory of Radical Polymerization), 2nd ed. suppl, a n d ammended, l~auka, Moscow, 1966 6. R. (L NORRISH and J. P. S1MONS, Proc. Roy. Soe. A 251: 4, 1264, 1959 7. V. M. GRANCHAK, V. P. SHERSTYUK and I. I. DILUNG, Dokl. Adad. N a u k SSSR 235: 3, 611, 1977 8. S. S. WANSER and G. S. HAMMOND, J. Amer. Chem. Soc. 92: 6362, 1970
Polymer Science U.S.S.R. Vol. 25, No. 5, pp. 1192-1198, 1983 Printed in Poland
0032-3950/83 $10.00 +.00 © 1984 Pergamon Press Ltd.
CROSSHNKING OF POLYDIMETHYLVINYLSILOXANE RUBBER BY OLIGOORGANOSILANE WITH FUNCTIONAL GROUPS AT THE SILICON ATOM* A. A. ZHDANOV,V. Yu. LEVIN, G. L. SLONIMSKII, Yr. P. KVACHEV, O. T. GRITSE~KO, N. V. DELAZARI, N. A. CHARNYAVSKAYA,V. E. MmHLI~ and Yr.. S. OBOLONKOVA Nesmeyanov Hetero-organic Compounds Institute, U.S.S.R. A c a d e m y of Sciences
(Received 26 December 1981) The feasibility of crosslinking polydimethylvinylsiloxane rubber b y oligoorganosilane with functional groups at the Si a t o m has been investigated. I t is shown t h a t the addition of oligoorganosilane to an organosilicon rubber is accompanied b y a m a r k e d increase in tensile strength and in Young's modulus. This is a t t r i b u t e d to chemical erosslinking of the rubber, a n d to the formation of a two-phase system.
ORGAI~OSILICONrubbers maintain their serviceability over a wide range of temperature, but poor strength properties are among their main disadvantages. I t is now urgently necessary to find means of enhancing the strength properties of these rubbers. This paper relates to our s~udy of the feasibility of erossllnldng of the organosilicon rubber SKTV-1 (polydimethylsiloxane rubber containing 0.5 mole % methylvinylsiloxane units) by means of "Ethylan" grade oligoorganosilane (OS). We took as s t u d y objects films based on mixtures of the rubber and OS in an ahuninium foil cuvette, casting from a 20-25% benzene solution. After elimination of benzene the film remaining in the cuvette was k e p t at 473 K for 6 hr. F o r comparison a s t u d y was made of the systems r u b b e r - c u m y l peroxide and r u b b e r - O S - p e r o x i d e crosslinked under identical conditions. The OS concentration was varied within the limits of 1 to 25%, a n d the peroxide * Vysokomol. soyed. A25: No. 5, 1030-1034, 1983.
Crosslinking of polydimethylvinylsiloxane rubber
11-93
concentration from" 0.5 to 2.0~o. Statio mechanical properties of the erosslinl~ed rubber were investigated by a different method using a Polyani type dynamometer [1] at room temperature, the stretching rate being 6.5 × 10-= mm/sec. Dynamic mechanical measurements were carried out under a regime of freely extinguishing torsional vibrations at a frequency of the order of 1 Hz [2]. Thermomechanical experiments were based on the method outlined in [3]. In both cases the heating of abruptly cooled samples was carried out at a rate of the order of 1 deg/min. The morphology of the crosslinked systems was investigated by meazls of carbon-platinum replicas using an EMV-100L electron microscope. Replicas were applied ~o the surface of a section of a sample or to the fracture surface, and were subsequently removed in a saturated alcoholic solution of an alkali. According t o t h e results o f t h e r m o m e c h a n i c a l tests, the d e f o r m a t i o n response t o t h e action of a force field a n d ~emperature field in t h e case of r u b b e r - O S syst e m s is similar, irrespective of the OS concentration, t o the d e f o r m a t i o n changes occurring in r u b b e r - p e r o x i d e systems, and m a y be described b y a t h e r m o m e c h a n ieal curve of t h e t y p e associated w i t h erosslinked organosilicon rubbers [4].
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300
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FIG. 2
Fro. 1. Thermomechanieal curves of samples of the rubber crosslinked with 9% of OS (1), by 1~o of cumyl peroxide (2), curves of the initial OS (3) and the OS l~ept at 473 K for 11 hr (4). Pressure on sample = 7-2 × 10-= MPa. FIG. 2. Tensile curves of the rubber eroaslinl~ed with 1 (1), 4 (2), 9 (3), 16 (4) and 25~o (5) OS, and for the rubber erosslinked with 0.5 (6), 1 (7) and 2% (8) cumyl peroxide. This is exemplified b y curves in Fig. 1 where we h a v e t h e r u b b e r containing 9~/o of OS (curve 1) a n d t h e r u b b e r crosslinlred by t h e peroxide (curve 2). I n b o t h cases a rise in ~emperature is a c c o m p a n i e d b y similar d e f o r m a t i o n changes: in the region of 150 K increased d e f o r m a t i o n corresponds t o devi%rification o f t h e a m o r p h o u s phase of t h e rubber; n e x t , u p ~o t e m p e r a t u r e s o f t h e order o f 220 K , t h e degree of d e f o r m a t i o n is r e d u c e d slightly; t h e n it increases again a t a r o u n d 220 K in view o f t h e melting o f crystalline phase, a f t e r which t h e r e is a slight r e d u c t i o n t y p i c a l for t h e region of t h e high elastic state. T h u s w i t h i n t h e limits of a c c u r a c y o f t h e t h e r m o m e c h a n i c a l m e t h o d we m a y say t h a t a n addition o f u p t o 2 5 % of OS t o t h e r u b b e r has scarcely a n y effect on m a j o r transitions in t h e
A. A. ZmD~ov a ~.
1194
latchet, viz. on the temperature intervals of glass,transition and melting. O n t h e basis of considerations cited in [5] we surmise t h a t the additi6n of 0S leads t 0 a kinetic change in the crystallization of the rubber. !
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~-'IO. 3. Breaking stress o-B vs. OS concentration for systems containing 0 (1), 0"5 (2) and 1% (3) cumyl peroxide. An estimate of the static mechanical properties of the systems was based on an analysis of the tensile curves. I n Fig. 2 we have as an example the tensile curves for the systems rubber-0S and rubber-peroxide. It can be seen t h a t an increase in the 0S concentration in the system is accompanied by a reduction in the breaking deformation ~B over the entire range of 0S concentration, and by an increase in tensile strength aB Up tO an 0S concentration of 16~. Figure 3 shows the eul'ves of {TB VS. the 0S concentration in the rubbers containing 0, 0.5 and 1 ~o o f cumyl peroxide. I t is seen that, in all cases, an increase in the 0S concentration is accompanied by a rise in aB. A maximum value of aB is obtained for the ternary system rubber-OS-peroxide containing 16% of OS, and amounts to ~0.9 MPa. The {TB values for the rubber crosslinked with 0.5 and 1 and 2 ~ of the peroxide are 0.1, 0.15 and 0.2 MPa respectively (Fig. 2, curves 6-8). I t appears t h a t crosslinking of the 0S makes it possible, over the entire interval of 0S concentrations, to obtain crosslinked systems whose strength is 4-5 times that of the rubbers crosslinkcd by cumy! peroxide, which at present is widely used as a crosslinking agent. An increase in the peroxide concentration in the systems containing OS is accompanied b y a rise in aB in the interval of 0S concentrations from 9 to 25% (Fig. 3). At 0S concentrations of 1 and 40//oa difference in the peroxide concentration has scarcely any effect on aB. In Fig. 4 we have plots of Young's modulus E vs, the OS concentration for the s y s ~ m s containing 0.5 and 1 °/o of the pero.vide. I r i s seen t h a t an increase in the 0 8 concentration is accompanied by a rise in E, irrespective of the peroxide concenVration. After comparing the value of E for the rubber crosslln_ked by the peroxide (-~0-25 MPa at a 1 ~ peroxide concentra#ion) with t h a t for the systems containing 0S we conclude that ~he modulus is ~ 4 times higher for the latter. ; The increase in,~ and ~he reduction in 8B as t h e OS Concentration increa,ses a m
Crosslinkingof polydlmethylvinylsiloxanerubber
1195,
similar to what is observed when network densities in crosslinlredhigh-elastic systems are inoreased. In this connection it is helpful to compare the increase in E with the increased density of the three-dimensional network that occurs w h e n the O S concentration is increased. In a concrete case we are not concerned with the absolute value of the crosslink concentration, and so we will limit ourselves to a study of changes in the equilibrium degree of swelling Q in benzene for the rubber-OS-peroxide systems as the OS concentration is increased (Fig. 5). It can be seen t h a t the introduction of 1°/o of OS brings a marked reduction in Q. An increase in the OS concentration to 4% is accompanied by a smaller change in Q, and there is scarcely any change in Q if the OS concentration is further increased. The swelling Q is proportional to the MW of a chain segment between erosslinks Me, and so we surmise t h a t a marked change in Me will occur only if we introduce up to 40/o of the OS, and t h a t a further change in the OS concenti~ttion will be accompanied by scarcely any reduction in Me. Comparing plots of Q and E vs. the OS concentration it is seen t h a t these curves are not similar in slope (symbate), since a significant change in E, in contradistinction to Q, takes place over the entire OS concentration interval. Lu view of this an increase in E at OS concentrations exceeding 4% cannot be attributed to increased network density. The question t h a t now arises concerns the distribution of OS when introduced into an organosilicon rubber. This quest i o n ! s made all the more appropriate by the fact t h a t the latter could hardly be compatible with the OS, particularly if the OS concentration is high. To investigate the distribution of OS in the rubber we made an electron microscope study of samples containing differing amounts of OS. As an e x a m p l e we have in Fig. 6 the photomicrograph of the rubber-OS system containing 9 % of 0S. I t is seen that spherical and oval formations appear against a background of the structureless matrix. The number of these formations increases with the OS concentration. I t looks as if the OS is present (at any rate in the case of high concentrations) in the form of a separate phase in the rubber. To determine features of the two-phase n a t u r e of the systems containing OS we carried out measurements of dynamic mechanical properties. Figure 7 shows the temperature dependences of the logarithmic decrement of extinction and of the real part of the complex shear modulus for the rubber-OS systems containing l, 9 and 25% of OS. In all cases there are clear-cut transitions in the vicinity of 150 and 220 K corresponding to molecular mobility upon devitrification and melting respectively. As the OS concentration increases, losses in the temperature region situated between the temperatures of glass-transition and melting increase, and appear in the form of a diffuse peak at an OS concentration of 25 o/o. According to the thermomechanical data the softening point of the initial OS is ~180 K, or for the OS hardened in air at 473 C, ~ 200 K (Fig. 1, curves 3 and 4). iY[easurement of mechanical losses in the temperature interval lying between temperatures of glass transition and melting are, in our view, due preferentially to molecular mobility on thawing OS separated out as a separate phase.
1196
A.A.
ZHDA~OV et al.
E MP~
I.L
CL~~, IGO0' 0.6
IZO0
0"2 i
5
I
I
/5 F~G. 4
I
08,%
1
25
qO0~-
5
/5
OS, %
25
FIG. 5
:FIG. 4. Young's modulus E vs. OS concentration for systems containing 0.5 (1) a n d 1 ~o {2) of cumyl peroxide. The points show the experimental values of E. The lines were drawn in accordance with the results of least squares approximations. FzG. 5. Degree of equilibrium swelling Q vs. OS concentration for systems containing 0.5 (1) a n d 1~/o (2) of cumyl peroxide.
:FIG. 6. Photomicrograph of the rubber crosslinked with 9 % of the OS ( × 25,000).
Crosslinking of polydimethylvinylsiloxanorubber
1197
On the basis of an analysis of the experimental results we surmise that the increase in aB and in E observed for the organosilicon rubber on introducing the OS is accounted for by two factors: these are crossiinking of the rubber and the emergence of a two-phase system in which one phase is formed by the rubber erosslinked by the OS and the peroxide (or by OS only), while the other phase consists of cros~ilnl~ed OS. We surmise that the hardened OS forming a separate phase is partly llnt:ed to the rubber matrix by chemical bonds, and that accordingly the structure of the rubber-OS-peroxide systems will be closer to that of two-phase nonstereospecific block copolymers than to that of incompatible blends. The observed increase in aB and reduction in 8~ when the concentration is increased are similar to changes in the tensile strength and deformation of block eopolymers accomlaany~mg an increase in the relative amount of rigid component in a eopolymer [6].
Io9 G'L,,~7
,4
O'q
0"2
,-a'
,, ~
°
.
_
,tD
I
100
I
I
I
22O T,K
FIG. 7. Temperature dependences of the real part of the complex shear modulus 6/' (1-3) and of the logarithmic decrement of damping A (1"-3') for the rubber crosslinked with 1 (1, 1'), 9 (9, 2') and 25% of OS (3, 3'). Thus the use of OS acting as a crosslJni~ng agent for an organosilicon rubber and, at sufficiently high concentrations forming a separate phase, provides a means of enhacing the strength properties of organosilicon rubbers. It is highly probable that this means of preparing crosslinked organosilicon rubbers may be equally effective in the case of filled systems. Trar~sZa~d by R . J . A . HENDRY
1198
~r. V. PCHELINTS~V and Y~.. T. D E , SOy
REFERENCES 1. A. Ya. W.~LKIN, A. A. ASKADSKII and V. V. KOVRIGA, Metody izmereniya mekha. nicheskikh svoistv polimerov (Methods for Measuring the Mechanical Properties of Polymers). p. 33, Khimiya, Moscow, 1978 2. V. Yu. LEVIN, A. A. ZHDANOV, G. L. SLONIMSKII, V. A. MARTIROSOV, Yu. P. KVACHEV, G. Ye. GOLUBKOV and O. T. GRITSENK0, Vysokomol. soyed. A24: 2115, 1982 (Translated in Polymer Sci. U.S.S.R. 24: 10, 2422, 1982) 3. B. L. TSETLIN, V. I. GAVRIL0V, N. A. VELIKOVSKAYA and V. V. KOCHKIN, Zavodsk. lab. 22: 3, 352, 1956 4. K. A. ANDRIANOV, G. L. SLONIMSKII, A. A. ZDANOV, V. Yu. LEVIN, Yu. K. GODOVSKH and V. A. MOSKALENKO, J, Polymer Sci. A-1 10: 1, 23, 1972 5. V. Yu. LEVIN, Avteref. dis. na soiskanie ueh. st. dokt. khim. nauk (Patents Discussion at Chem. Doctorate Degree Contest). p. 15, INEOS Akad. Nauk SSSR, Moscow, 1977 6. A. NOSHEI and D. A. WAK-GRAT, Blok-sopolimery (Block Copolymers). p. 416, Miro Moscow, 1980
Pol.~mer Science U.S.S.R. ~ol. 25, No. 5, !01~.1198-1206, 1988 Printed in Poland
0032-3950/83 $10.00 -i-.00 © 1984 Pergamon Press Ltd.
KINETICS OF THE THERMOOXIDATIVE DEGRADATION OF cis-I,4-POLYISOPRENE IN BULK IN THE PRESENCE OF PHE.NOL ANTIOXIDANTS* V. V. PCl~V.LINTSEVand Y~,. T. Dv.~ISOV S. V. Lebedev All.Union Synthetic Rubber Research Institute Department of the Chemical Physics Institute, ~T.S.S.R. Academy of Sciences
(Received 27 December
1981)
The kinetics of oxidation and of C--C bond seission have been investigated for c/~-l,4-polyisoprene in the presence of sterically hindered a]kyl phenols and azobisisobutyronitrile (ABN). Relations between rates of oxidation and degradation, and the inhibitor concentration and rate of initiation have been determined. Degradation of the polymer is thought to take place through the ~-peroxyperoxide radical reacting with the phenoxyl radical via breakdown of the unstable quinolide peroxide.
IN AN earlier paper [1] we studied the initiated oxidation of cis-1,4-polyisoprene (PI) and showed that degradation of the polymer was the result of rapid decay of unstable alkoxyperoxide radicals emerging during the disproportionation of * Vysokomol. soyed. A25: No. 5, 1035-1040, 1983.