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Some regularities of thermal oxidation of polyorganosiloxane rubbers
Thermal oxidation of polyorganosiloxane rubbers
1589
10. Yu. v. BRESTKIN, B. M. GINZBUR(~, P. A. IL'CHENKO, K. B. KURBANOV, M. A. MARTYNOV, Sh. TUICHIEV and S. Ya. FRENKEL', Vysokomol. soyed. A15: 621, 1973 (Translated in Polymer Sci. U.S.S.R. 15: 1973) 11. B. M. GINZBURG, Dissertatsiya (Dissertation) 1967 12. R. J. SAMUELS, J. Polymer Sci. 6, A-2: 1101, 1968 13. A. I. SLUTSKER, Dissertation 1968; A. Ye. GROMOV and A. I. SLUTSKER, Sb. Karbotsepnye vysokomolekulyarnye soyedineniya (Carbon-Chain High-Molecular ~Vcight Compounds)• Izd. AN SSSR, 1963 14. Ye. L. GAL'PERII~, V. F. M:INDRUL and V. K. SMIRNOV, Vysokomol. soyed, t l 2 : 1949, 1970 (Translated in Polymer Sci. U.S•S.R• 12: 9, 2207, 1970) 15. L. G. KAZARYAN, Dissertation, 1966 16. D. Ya. TSVANKIN, Dissertation, 1970. 17. B. K. VAINSHTEIN, Diffraktsiya rentgenovykh luchei na tsepnykh molekulakh (X-ray Diffraction on Chain Molecules). Izd. AN SSSR, 1963 18. Z. MENCHIK, Khimiya i tekhnol, polimerov, No. 6, 3, 1961 19. W. O. STATTON, J. Polymer Sci. 22: 385, 1965
SOME
REGULARITIES
OF
THERMAL
OXIDATION
OF
POLYORGANOSILOXANE RUBBERS* S. M. MEZHIKOVSKII, 1~. M. ASEYEVA a n d A. A. BERLIN Institute of Chemical Physics, U.S•S•R. Academy of Sciences
(Received 23 September 1971) T ~ direction a n d t y p e of b r e a k d o w n processes t a k i n g place a t increased t e m p e r a t u r e s in linear p o l y o r g a n o s i l o x a n e s (POS) h a v e b e e n o u t l i n e d in r e c e n t years• K i n e t i c s a n d m e c h a n i s m of t h e r m a l b r e a k d o w n of P O S h a v e b e e n s t u d i e d a n d discussed [1-3] a n d t h e general relations which g o v e r n t h e r m a l o x i d a t i o n o f P O S e x a m i n e d in m a n y p a p e r s , e.g. [4]. K i n e t i c s o f d e c o m p o s i t i o n o f P O S in t h e presdnce o f oxygen, h o w e v e r , h a v e b e e n m u c h less s t u d i e d t h a n in v a c u u m or i n e r t media. K i n e t i c i n v e s t i g a t i o n s o f t h e r m a l oxidizing b r e a k d o w n of P O S h a v e b e e n q u a l i t a t i v e in m o s t cases a n d c o n c e r n e d w i t h e a r l y stages of t h e process; t h e y w e r e carried out w i t h i n a n a r r o w t e m p e r a t u r e range. I t is of i n t e r e s t to e x a m i n e kinetics of t h e r m a l - o x i d a t i v e b r e a k d o w n o f P O S r u b b e r s o v e r a w i d e r r a n g e o f t e m p e r a t u r e a n d e x p l a i n t h e effect o f s o m e factors on effective k m e t m p a r a m e t e r s of i n d i v i d u a l stages of t h e process. T h i s p a p e r r e p o r t s results d e r i v e d b y T G A a n d D T A of s o m e P O S r u b b e r s while h e a t i n g f r o m 25 to 600 ° . •
.
.
I
.
,
°
* Vysokomol. soyed, hi5: No. 6, 1416-1420, 1973.
1590
S. M. M E z n ~ o v s ~ I e$a/. OBJECTS AND METHODS OF EXPERIMENT
The f o l l o w i n g POS rubbers were used*.
SKTV--an industrial sample of polydimethylsiloxane rubber containing a small prop. ortion of vinyl groups. I t was prepared with an acid catalyst. SKTVa~--a rubber, which differs from the previous rubber in that it was synthesized with an alkaline catalyst. SKTVbl--samo rubber as the previous one, but with blocked (trimethylsilyl) end groups. SKTFV-803--an experimental-industrial rubber containing a small proportion of vinyl groups, as well as methyl and phenyl groups in the organic structure. Some properties of the rubbers used in the study are as follows: I
Rubber SKTV Solubility in benzene, Yo 100 Reaction of the aqueous extract Amount ofvolatilos at 150°, ~ 5"25 Molecular weight, M × 10 -5 5.2
SKTVal 100 6.5 --
SKTVbl 100 neutral 1.3 4.5
SKTFV-803 100 2.77 6.9
Thermal oxidizing breakdown of POS in air was studied using a derivatograph and thermal breakdown in a vacuum of 10 -4 mm was carried out in ¥TV-1 automatic scales (developed by the Institute of Chemical Physics, U.S.S.R. Academy of Sciences). The rate of heating was 3q-0.1 dog/rain in all experiments; the samples weighted 100 rag. Alumina calcined up to a temperature of 1200° was used as standard for DTA. Kinetic parameters of breakdown were calculated by a method [5] based on determining Che rate of gravimetric loss at the point of inflexion 'of the TG curve. The method of calculation is described in detail [6]. Comparison by sampling of results obtained with data calculated by the FreemanCarroll [7] method proved to be satisfactory.
RESULTS A c o m p a r i s o n of t h e r m a l a n d t h e r m a l - o x i d i z i n g stability of samples e x a m i n e d (Fig. 1) indicates t h a t SKTVbl is t h e m o s t resistant to t h e effect of h e a t w i t h o u t o x y g e n : t h e initial t e m p e r a t u r e of intensive decomposition is displaced to t h e h i g h - t e m p e r a t u r e region b y ~ 100 °, c o m p a r e d w i t h S K T V , which confirms t h e conclusions p r e v i o u s l y d r a w n [2]. I n a n oxidizing a t m o s p h e r e judging b y t h e initial t e m p e r a t u r e of intensive w e i g h t loss (Fig. la) t h e differences in stability of S K T V a n d SKTVbl r u b b e r s were n o t so significant. This is, a p p a r e n t l y , because oxidation of organic hydrocarbon radicals, w h i c h p r o m o t e s the a c c u m u l a t i o n of a c t i v e centres of depolym e r i z a t i o n of t h e siloxane chain a n d acceleration of t h e :liberation of volatiles, t a k e s place a t r a n d o m or at t h e w e a k bonds. F i g u r e 2a shows a t y p i c a l d e r i v a t o g r a m of ageing S K T V in air. I n first a p p r o x i m a t i o n t h e T G integral curve m a y be s e p a r a t e d into t h r e e parts: ~ 3 1 0 , 3 1 0 - 4 3 0 a n d 430-510 ° . H o w e v e r , s t u d y of t h e differential curve of weight loss e n a b l e s a more detailed analysis to be carried o u t a n d a c o m p a r i s o n of b o t h * The authors are grateful to S. Ya. Yakusina and M. P. Grinblat for providing the sample.
1591
T h e r m a l o x i d a t i o n of polyorganosiloxane r u b b e r s
curves with the thermogram indicates that, in the temperature ranges given, several competing reactions take place. A comparison of derivatograms of three types of polydimethylvinyl siloxane rubbers shows that the type of the DTG and DTA curves is practically the same, but some peaks are displaced in relation to each other, according to temperature. •
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~
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b
.
.,~'///f /
6o
/;/ ///
/,/~-3
'
/ /
I
/,;7 /
/
J,/ /
20
./
.J// f00
800
500
fog
300
5"00
c,oc 7CL7
FIo. l. T h e r m o g r a v i m e t r i c curves of the b r e a k d o w n of S K T V (1), SKTVat (2), SKTVbl(3) a n d S K T F V - 8 0 3 (4) in air (a) a n d in v a c u u m (b). The rate of heating: 3 deg/min.
Weight losses observed in the low-temperature region are basically due to volatilization of cyclosiloxane present in equilibrium concentration in the polymer. Somewhat lower weight losses on heating to 3]0 ° observed for SKTVbl may be due to partial removal of low molecular weight compounds during the preparation of a given sample. Active interaction of oxygen with POS begins at a temperature higher than 300 °, which is proved by the appearance on thermograms of an exothermie ~ 80 \ \
.~o
\\ 7"G \ --
\rG \
\\ ]
0
I -T--~-~
I
\ '% r - . . DTA i
/'
o.8
4'
500
300
~a
~1"2,
f00
T,oc
o.~i ÷ 500
300
100
T,oc
FIa. 2. D e r i v a t o g r a m s of S K T V (a) a n d S K T F V - 8 0 3 (b).
1592
S.M.
MEZHIKOVSKII et al.
peak in this region. In an inert medium this peak has not been observed. A comparison of molecular weights of products of heat treatment of POS rubber samples studied prepared by heating in" air to the initial temperature of the exothermic peak indicates that in the low temperature range, in addition to the simple evaporation of low molecular weight volatile components, the main chain may break and depolymerization and structure formation may take place. The rate of these reactions depends on structural properties of rubbers, particularly the type of radicals and blocking of end groups. The presence of oxygen in the system has a marked effect on these reactions. K I N E T I C PARA~vIETERS OF T H E R M A L O X I D A T I O N OF R U B B E R S
Rubber
T °, C
Eett, keal/
Eeft
-~eft,
n*
T, °C
/mole
keal/
n
T, °C
/mole
kcal/ /mole
n
I SKTV SKTVbl SKTVal SKTV-803
310-365 335-385 300-375 350-420
67.5 63.5
7O 80.5
1-8 2 2 1.5
375-410 380-450 380-440
81 91 60
2 I 430-515 1"8 460-510 "1 450-515 470-500
115 113 68 120
2
2 2 0.7
* n is tim order of the reaction.
Effective kinetic parameters of thermal oxidizing breakdown of POS rubbers were calculated separately for each of the temperature ranges, selected from an analysis of derivatograms. Kinetic characteristics of the low temperature stage are tabulated since the accuracy of calculation related to this part of the TG curve is poor. Since the t y p e of process in the low temperature range should affect the direction of reactions of subsequent stages, the problem of the mechanism of the early stage of ageing POS rubbers requires special examination. I t is interesting to note that the effective activation energy Eeff of the second stage of breakdown of polydimethylvinyl siloxane rubbers, due to oxidation of lateral radicals, depends little on the type of catalyst and macromolecular end groups: Eerr is 63"5-70 kcal/mole. This value considerably exceeds Eelf normally observed during oxidation of low molecular weight or polymer hydrocarbons. The result obtained proves that purely thermal processes have a considerable effect on the overall activation energy of the second stage of weight losses of POS rubbers. Oxygen only accelerates structure formation in P0S and forms active centres of depolymerization but has no direct effect on the main chain of the macromolecule. As oxidation proceeds, stronger Si-O-Si crosslinks accumulate in the polymer. At the same time with an increase in the number of crosslinks in the polymer, diffusion conditions of oxygen penetration and the removal of low molec-
Thermal oxidation of polyorganosilox~me rubbers
1593
ular weight products formed deteriorate. It is no coincidence that experimentM x~alues of Eelf of the breakdown of SKTV rubber gradually increase as the process goes on and reach 113-115 kcM/mole at the last stage. I t is significant that advanced stages of the thermal oxidizing process of SKTV~1 are characterized b y lower Eelf values, compared with SKTV and SKTV u (Table). This is, apparently, due to the specific t y p e of competing rca.ctions of breakdown of the main chain and structure formation at high temperature. The presence of alkali metal cations in an SKTV,~I sample (initially ,ontained as silanolate end groups) accelerates depolymerization. The rate of structure formation of the polymer decreases accordingly. This results in ;~ reduction of E~ff at advanced stages of the process. It. has been indicated [8] that with an increase in the density of arrangement of bridge-type bonds in the tlu'ee-dimensional network of polydimethylsiloxanes Em. values of breakdown in vacuum increase. It should be noted that Eelf values for the second stage of breakdown of three-dimensionM polymers in vacuum are close to the E,ff values of the breakdown of .POS derived by the authors in the presence of oxygen. Of the two competing directions, the reaction of substituents and macromolecular chain rupture, the latter has the main effect on the energetics of tile process of weight loss. The SKTFV-803 sample containing phenyl radicals in the side chain has no n()ticcahle advantages (luring breakdown in vacuum over other rubbers (Fig. la). A small proportion of phenyl groups in the rubber examined (less than 10%) does n()t. apparently, ensure significant disruption of molecular conformation and v~u'iation in spiral structure; it is insufficient to suppress depolarization. The rat¢' of weight losses in SKTFV-802 is much higher than for SKTV type rubber. This is due to the fact that the weight of volatile fragments formed during the hreakd°wn of S~KTI~\~ is higher on average than for polydimethylvinyl siloxanes. Its the presence of oxygen the pattern of breakdown of SKTFV-803 is different; the beginning of intensive decomposition is displaced in the direction of high temperature; the same tendency is observed tbr the exothermic peak, which is r(,~l)onsible for oxidation of hydrocarbon radicals (Fig. 2b); Eerf of weight losses in('rcases at the second stage to 80.5 compared with 67.5 kcM/mole for SKTV (Table). However, as oxidation proceeds, the number of phenyl nuclei capable of ao(.epting free radicals formed during the interaction of substituents with oxygen, al~t)¢,ars to be insufficient for preventing structure formation in rubber. As a result Eerf at the last stage of breakdown of SKTFV-803 markedly increases an~l a,pproximates to Eeff found for SKTV. CONCLUSIONS
(1) It was found that thermal oxidizing breakdown of polyorganosiloxane rubbers at temperatures of 25-600 ° takes place ill several stages with different effective activation energies.
1594
I.I.
PETROVA et al.
(2) Blocking of macromolecular end groups has little effect on the thermaloxidizing process; the addition of phenyl nuclei in the organic component of the siloxane chain stabilizes the system during thermal oxidation. Translated by E. S E ~ R E
REFERENCES 1. K. A. ANDRIANOV, Syrup. Starenie i stabilizatsiya polimerov (Ageing and Stabilization of Polymers), Izd. " N a u k a " , 1964 2. K. A. ANDRIANOV, V. S. PAPKOV, G. L. SLONIMSKII, A. A. ZHDANOV and S. Ya. YAKUSHKINA, Vysokomol. soyed. A l l : 2030, 1969 (Translated in Polymer Sci. U.S.S.R. 11: 9, 2313, 1969) 3. Yu. A. ALEKSANDROVA, T. S. NIKITINA and A. N. PRAVEI)NIKOV, Vysokomol. soyed. A10: 1078, 1968 (Translated in Polymer Sci. U.S.S.R. 10: 5, 1250, 1968) 4. A. S. KUZ'MINSKII, K a u c h u k i rezina, No. 11, 3, 1969 5. V. S. PAPKOV and G. L. SLONIMSKII, Vysokomol. soyed. 8: 80, 1966 (Translated in Polymer Sci. U.S.S.R. 8: 1, 84, 1966) 6. S. M. MEZHIKOVSKII, A. A. GUROV, N. I. IVIYAGCHILOVA, B. I. LIGON'KII and A. A. BERLIN, Vysokomol. soyed. A15: 3, 1973 (Translated in Polymer Sei. U.S.S.R. 15: 1, 1973) R. B. VHtNIK, S. M. MEZH1KOVSKII, R. M. ASEEVA, A. A. BERLIN and Yu. A. YERSHOV, Vysokomol. soyed. A13: 1125, 1971 (Translated in Polymer Sci. U.S.S.R. 13: 5, 1265, 1971) 7. E. S. FREEMAN and B. CARROLL, J. Phys. Chem. 62: 394, 1958 8. K. A. ANDRLkNOV, A. A. ZHI)ANOV, N. A. KURASHEVA, G. L. SLONIMSKII and V. S. PAPKOV, T r u d y soveshchaniya pc kremniiorganicheskim soyedineniyam (Proceedings of a Conference on Organo-Silicon Compounds). IV, p. 33, 1966
METHODS OF INVESTIGATION TEMPERATURE CONDITIONS OF ETCHING POLYMERS IN THE HI,GH-FREQUENCY OXYGEN DISCHARGE PLASMA* I. I. PETROVA, A. YE. CHALYXH, A. A:CGANOVand V. M. LIJK'YAI~OVlCH I n s t i t u t e of Physical Chemistry, U.S.S.R. A c a d e m y of Sciences (Received 26 August 1971) IT was shown recently [1-4] t h a t etching polymers in the plasma of a high-frequency (HF) olectrodeless oxygen discharge t has several advantages over other methods [5-7] of preparing high-molecular weight specimens for studying supermolecular organization in an electron microscope. I t is assumed in most studies t h a t decomposition of polymers in discharge is * Vysokomol. soyed. AI5: No. 6, 1421-1425, 1973. t This method of etching is also known as "etching with active oxygen" [4].
1589
10. Yu. v. BRESTKIN, B. M. GINZBUR(~, P. A. IL'CHENKO, K. B. KURBANOV, M. A. MARTYNOV, Sh. TUICHIEV and S. Ya. FRENKEL', Vysokomol. soyed. A15: 621, 1973 (Translated in Polymer Sci. U.S.S.R. 15: 1973) 11. B. M. GINZBURG, Dissertatsiya (Dissertation) 1967 12. R. J. SAMUELS, J. Polymer Sci. 6, A-2: 1101, 1968 13. A. I. SLUTSKER, Dissertation 1968; A. Ye. GROMOV and A. I. SLUTSKER, Sb. Karbotsepnye vysokomolekulyarnye soyedineniya (Carbon-Chain High-Molecular ~Vcight Compounds)• Izd. AN SSSR, 1963 14. Ye. L. GAL'PERII~, V. F. M:INDRUL and V. K. SMIRNOV, Vysokomol. soyed, t l 2 : 1949, 1970 (Translated in Polymer Sci. U.S•S.R• 12: 9, 2207, 1970) 15. L. G. KAZARYAN, Dissertation, 1966 16. D. Ya. TSVANKIN, Dissertation, 1970. 17. B. K. VAINSHTEIN, Diffraktsiya rentgenovykh luchei na tsepnykh molekulakh (X-ray Diffraction on Chain Molecules). Izd. AN SSSR, 1963 18. Z. MENCHIK, Khimiya i tekhnol, polimerov, No. 6, 3, 1961 19. W. O. STATTON, J. Polymer Sci. 22: 385, 1965
SOME
REGULARITIES
OF
THERMAL
OXIDATION
OF
POLYORGANOSILOXANE RUBBERS* S. M. MEZHIKOVSKII, 1~. M. ASEYEVA a n d A. A. BERLIN Institute of Chemical Physics, U.S•S•R. Academy of Sciences
(Received 23 September 1971) T ~ direction a n d t y p e of b r e a k d o w n processes t a k i n g place a t increased t e m p e r a t u r e s in linear p o l y o r g a n o s i l o x a n e s (POS) h a v e b e e n o u t l i n e d in r e c e n t years• K i n e t i c s a n d m e c h a n i s m of t h e r m a l b r e a k d o w n of P O S h a v e b e e n s t u d i e d a n d discussed [1-3] a n d t h e general relations which g o v e r n t h e r m a l o x i d a t i o n o f P O S e x a m i n e d in m a n y p a p e r s , e.g. [4]. K i n e t i c s o f d e c o m p o s i t i o n o f P O S in t h e presdnce o f oxygen, h o w e v e r , h a v e b e e n m u c h less s t u d i e d t h a n in v a c u u m or i n e r t media. K i n e t i c i n v e s t i g a t i o n s o f t h e r m a l oxidizing b r e a k d o w n of P O S h a v e b e e n q u a l i t a t i v e in m o s t cases a n d c o n c e r n e d w i t h e a r l y stages of t h e process; t h e y w e r e carried out w i t h i n a n a r r o w t e m p e r a t u r e range. I t is of i n t e r e s t to e x a m i n e kinetics of t h e r m a l - o x i d a t i v e b r e a k d o w n o f P O S r u b b e r s o v e r a w i d e r r a n g e o f t e m p e r a t u r e a n d e x p l a i n t h e effect o f s o m e factors on effective k m e t m p a r a m e t e r s of i n d i v i d u a l stages of t h e process. T h i s p a p e r r e p o r t s results d e r i v e d b y T G A a n d D T A of s o m e P O S r u b b e r s while h e a t i n g f r o m 25 to 600 ° . •
.
.
I
.
,
°
* Vysokomol. soyed, hi5: No. 6, 1416-1420, 1973.
1590
S. M. M E z n ~ o v s ~ I e$a/. OBJECTS AND METHODS OF EXPERIMENT
The f o l l o w i n g POS rubbers were used*.
SKTV--an industrial sample of polydimethylsiloxane rubber containing a small prop. ortion of vinyl groups. I t was prepared with an acid catalyst. SKTVa~--a rubber, which differs from the previous rubber in that it was synthesized with an alkaline catalyst. SKTVbl--samo rubber as the previous one, but with blocked (trimethylsilyl) end groups. SKTFV-803--an experimental-industrial rubber containing a small proportion of vinyl groups, as well as methyl and phenyl groups in the organic structure. Some properties of the rubbers used in the study are as follows: I
Rubber SKTV Solubility in benzene, Yo 100 Reaction of the aqueous extract Amount ofvolatilos at 150°, ~ 5"25 Molecular weight, M × 10 -5 5.2
SKTVal 100 6.5 --
SKTVbl 100 neutral 1.3 4.5
SKTFV-803 100 2.77 6.9
Thermal oxidizing breakdown of POS in air was studied using a derivatograph and thermal breakdown in a vacuum of 10 -4 mm was carried out in ¥TV-1 automatic scales (developed by the Institute of Chemical Physics, U.S.S.R. Academy of Sciences). The rate of heating was 3q-0.1 dog/rain in all experiments; the samples weighted 100 rag. Alumina calcined up to a temperature of 1200° was used as standard for DTA. Kinetic parameters of breakdown were calculated by a method [5] based on determining Che rate of gravimetric loss at the point of inflexion 'of the TG curve. The method of calculation is described in detail [6]. Comparison by sampling of results obtained with data calculated by the FreemanCarroll [7] method proved to be satisfactory.
RESULTS A c o m p a r i s o n of t h e r m a l a n d t h e r m a l - o x i d i z i n g stability of samples e x a m i n e d (Fig. 1) indicates t h a t SKTVbl is t h e m o s t resistant to t h e effect of h e a t w i t h o u t o x y g e n : t h e initial t e m p e r a t u r e of intensive decomposition is displaced to t h e h i g h - t e m p e r a t u r e region b y ~ 100 °, c o m p a r e d w i t h S K T V , which confirms t h e conclusions p r e v i o u s l y d r a w n [2]. I n a n oxidizing a t m o s p h e r e judging b y t h e initial t e m p e r a t u r e of intensive w e i g h t loss (Fig. la) t h e differences in stability of S K T V a n d SKTVbl r u b b e r s were n o t so significant. This is, a p p a r e n t l y , because oxidation of organic hydrocarbon radicals, w h i c h p r o m o t e s the a c c u m u l a t i o n of a c t i v e centres of depolym e r i z a t i o n of t h e siloxane chain a n d acceleration of t h e :liberation of volatiles, t a k e s place a t r a n d o m or at t h e w e a k bonds. F i g u r e 2a shows a t y p i c a l d e r i v a t o g r a m of ageing S K T V in air. I n first a p p r o x i m a t i o n t h e T G integral curve m a y be s e p a r a t e d into t h r e e parts: ~ 3 1 0 , 3 1 0 - 4 3 0 a n d 430-510 ° . H o w e v e r , s t u d y of t h e differential curve of weight loss e n a b l e s a more detailed analysis to be carried o u t a n d a c o m p a r i s o n of b o t h * The authors are grateful to S. Ya. Yakusina and M. P. Grinblat for providing the sample.
1591
T h e r m a l o x i d a t i o n of polyorganosiloxane r u b b e r s
curves with the thermogram indicates that, in the temperature ranges given, several competing reactions take place. A comparison of derivatograms of three types of polydimethylvinyl siloxane rubbers shows that the type of the DTG and DTA curves is practically the same, but some peaks are displaced in relation to each other, according to temperature. •
/00
&
2.
~
,f/~l>iliO
b
.
.,~'///f /
6o
/;/ ///
/,/~-3
'
/ /
I
/,;7 /
/
J,/ /
20
./
.J// f00
800
500
fog
300
5"00
c,oc 7CL7
FIo. l. T h e r m o g r a v i m e t r i c curves of the b r e a k d o w n of S K T V (1), SKTVat (2), SKTVbl(3) a n d S K T F V - 8 0 3 (4) in air (a) a n d in v a c u u m (b). The rate of heating: 3 deg/min.
Weight losses observed in the low-temperature region are basically due to volatilization of cyclosiloxane present in equilibrium concentration in the polymer. Somewhat lower weight losses on heating to 3]0 ° observed for SKTVbl may be due to partial removal of low molecular weight compounds during the preparation of a given sample. Active interaction of oxygen with POS begins at a temperature higher than 300 °, which is proved by the appearance on thermograms of an exothermie ~ 80 \ \
.~o
\\ 7"G \ --
\rG \
\\ ]
0
I -T--~-~
I
\ '% r - . . DTA i
/'
o.8
4'
500
300
~a
~1"2,
f00
T,oc
o.~i ÷ 500
300
100
T,oc
FIa. 2. D e r i v a t o g r a m s of S K T V (a) a n d S K T F V - 8 0 3 (b).
1592
S.M.
MEZHIKOVSKII et al.
peak in this region. In an inert medium this peak has not been observed. A comparison of molecular weights of products of heat treatment of POS rubber samples studied prepared by heating in" air to the initial temperature of the exothermic peak indicates that in the low temperature range, in addition to the simple evaporation of low molecular weight volatile components, the main chain may break and depolymerization and structure formation may take place. The rate of these reactions depends on structural properties of rubbers, particularly the type of radicals and blocking of end groups. The presence of oxygen in the system has a marked effect on these reactions. K I N E T I C PARA~vIETERS OF T H E R M A L O X I D A T I O N OF R U B B E R S
Rubber
T °, C
Eett, keal/
Eeft
-~eft,
n*
T, °C
/mole
keal/
n
T, °C
/mole
kcal/ /mole
n
I SKTV SKTVbl SKTVal SKTV-803
310-365 335-385 300-375 350-420
67.5 63.5
7O 80.5
1-8 2 2 1.5
375-410 380-450 380-440
81 91 60
2 I 430-515 1"8 460-510 "1 450-515 470-500
115 113 68 120
2
2 2 0.7
* n is tim order of the reaction.
Effective kinetic parameters of thermal oxidizing breakdown of POS rubbers were calculated separately for each of the temperature ranges, selected from an analysis of derivatograms. Kinetic characteristics of the low temperature stage are tabulated since the accuracy of calculation related to this part of the TG curve is poor. Since the t y p e of process in the low temperature range should affect the direction of reactions of subsequent stages, the problem of the mechanism of the early stage of ageing POS rubbers requires special examination. I t is interesting to note that the effective activation energy Eeff of the second stage of breakdown of polydimethylvinyl siloxane rubbers, due to oxidation of lateral radicals, depends little on the type of catalyst and macromolecular end groups: Eerr is 63"5-70 kcal/mole. This value considerably exceeds Eelf normally observed during oxidation of low molecular weight or polymer hydrocarbons. The result obtained proves that purely thermal processes have a considerable effect on the overall activation energy of the second stage of weight losses of POS rubbers. Oxygen only accelerates structure formation in P0S and forms active centres of depolymerization but has no direct effect on the main chain of the macromolecule. As oxidation proceeds, stronger Si-O-Si crosslinks accumulate in the polymer. At the same time with an increase in the number of crosslinks in the polymer, diffusion conditions of oxygen penetration and the removal of low molec-
Thermal oxidation of polyorganosilox~me rubbers
1593
ular weight products formed deteriorate. It is no coincidence that experimentM x~alues of Eelf of the breakdown of SKTV rubber gradually increase as the process goes on and reach 113-115 kcM/mole at the last stage. I t is significant that advanced stages of the thermal oxidizing process of SKTV~1 are characterized b y lower Eelf values, compared with SKTV and SKTV u (Table). This is, apparently, due to the specific t y p e of competing rca.ctions of breakdown of the main chain and structure formation at high temperature. The presence of alkali metal cations in an SKTV,~I sample (initially ,ontained as silanolate end groups) accelerates depolymerization. The rate of structure formation of the polymer decreases accordingly. This results in ;~ reduction of E~ff at advanced stages of the process. It. has been indicated [8] that with an increase in the density of arrangement of bridge-type bonds in the tlu'ee-dimensional network of polydimethylsiloxanes Em. values of breakdown in vacuum increase. It should be noted that Eelf values for the second stage of breakdown of three-dimensionM polymers in vacuum are close to the E,ff values of the breakdown of .POS derived by the authors in the presence of oxygen. Of the two competing directions, the reaction of substituents and macromolecular chain rupture, the latter has the main effect on the energetics of tile process of weight loss. The SKTFV-803 sample containing phenyl radicals in the side chain has no n()ticcahle advantages (luring breakdown in vacuum over other rubbers (Fig. la). A small proportion of phenyl groups in the rubber examined (less than 10%) does n()t. apparently, ensure significant disruption of molecular conformation and v~u'iation in spiral structure; it is insufficient to suppress depolarization. The rat¢' of weight losses in SKTFV-802 is much higher than for SKTV type rubber. This is due to the fact that the weight of volatile fragments formed during the hreakd°wn of S~KTI~\~ is higher on average than for polydimethylvinyl siloxanes. Its the presence of oxygen the pattern of breakdown of SKTFV-803 is different; the beginning of intensive decomposition is displaced in the direction of high temperature; the same tendency is observed tbr the exothermic peak, which is r(,~l)onsible for oxidation of hydrocarbon radicals (Fig. 2b); Eerf of weight losses in('rcases at the second stage to 80.5 compared with 67.5 kcM/mole for SKTV (Table). However, as oxidation proceeds, the number of phenyl nuclei capable of ao(.epting free radicals formed during the interaction of substituents with oxygen, al~t)¢,ars to be insufficient for preventing structure formation in rubber. As a result Eerf at the last stage of breakdown of SKTFV-803 markedly increases an~l a,pproximates to Eeff found for SKTV. CONCLUSIONS
(1) It was found that thermal oxidizing breakdown of polyorganosiloxane rubbers at temperatures of 25-600 ° takes place ill several stages with different effective activation energies.
1594
I.I.
PETROVA et al.
(2) Blocking of macromolecular end groups has little effect on the thermaloxidizing process; the addition of phenyl nuclei in the organic component of the siloxane chain stabilizes the system during thermal oxidation. Translated by E. S E ~ R E
REFERENCES 1. K. A. ANDRIANOV, Syrup. Starenie i stabilizatsiya polimerov (Ageing and Stabilization of Polymers), Izd. " N a u k a " , 1964 2. K. A. ANDRIANOV, V. S. PAPKOV, G. L. SLONIMSKII, A. A. ZHDANOV and S. Ya. YAKUSHKINA, Vysokomol. soyed. A l l : 2030, 1969 (Translated in Polymer Sci. U.S.S.R. 11: 9, 2313, 1969) 3. Yu. A. ALEKSANDROVA, T. S. NIKITINA and A. N. PRAVEI)NIKOV, Vysokomol. soyed. A10: 1078, 1968 (Translated in Polymer Sci. U.S.S.R. 10: 5, 1250, 1968) 4. A. S. KUZ'MINSKII, K a u c h u k i rezina, No. 11, 3, 1969 5. V. S. PAPKOV and G. L. SLONIMSKII, Vysokomol. soyed. 8: 80, 1966 (Translated in Polymer Sci. U.S.S.R. 8: 1, 84, 1966) 6. S. M. MEZHIKOVSKII, A. A. GUROV, N. I. IVIYAGCHILOVA, B. I. LIGON'KII and A. A. BERLIN, Vysokomol. soyed. A15: 3, 1973 (Translated in Polymer Sei. U.S.S.R. 15: 1, 1973) R. B. VHtNIK, S. M. MEZH1KOVSKII, R. M. ASEEVA, A. A. BERLIN and Yu. A. YERSHOV, Vysokomol. soyed. A13: 1125, 1971 (Translated in Polymer Sci. U.S.S.R. 13: 5, 1265, 1971) 7. E. S. FREEMAN and B. CARROLL, J. Phys. Chem. 62: 394, 1958 8. K. A. ANDRLkNOV, A. A. ZHI)ANOV, N. A. KURASHEVA, G. L. SLONIMSKII and V. S. PAPKOV, T r u d y soveshchaniya pc kremniiorganicheskim soyedineniyam (Proceedings of a Conference on Organo-Silicon Compounds). IV, p. 33, 1966
METHODS OF INVESTIGATION TEMPERATURE CONDITIONS OF ETCHING POLYMERS IN THE HI,GH-FREQUENCY OXYGEN DISCHARGE PLASMA* I. I. PETROVA, A. YE. CHALYXH, A. A:CGANOVand V. M. LIJK'YAI~OVlCH I n s t i t u t e of Physical Chemistry, U.S.S.R. A c a d e m y of Sciences (Received 26 August 1971) IT was shown recently [1-4] t h a t etching polymers in the plasma of a high-frequency (HF) olectrodeless oxygen discharge t has several advantages over other methods [5-7] of preparing high-molecular weight specimens for studying supermolecular organization in an electron microscope. I t is assumed in most studies t h a t decomposition of polymers in discharge is * Vysokomol. soyed. AI5: No. 6, 1421-1425, 1973. t This method of etching is also known as "etching with active oxygen" [4].