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Thermal degradation of poly(dimethyl-p-silphenylene) and poly(tetramethyl-p-silphenylene-siloxane)
O014-3057 81 (150503-06S02.{X1 0 Cop~,right ~. 198l Pergamon Press Lid
Eurolwan Polt'mer Journal \ ol. 17, pp. 503 to 508. 1981 Printed in Greal Britain. All rights reserved
THERMAL DEGRADATION OF POLY(DIMETHYLp-SILPHENYLENE) AND POLY(TETRAMETHYLp-SILPHENYLENE-SILOXANE) B. ZELE|, M. BLAZSOand S. DOBOS Research Laboratory for Inorganic Chemistry, Hungarian Academy of Sciences, Budapest 1112, BudaSrsi ut 45, Hungary
(Receired 8 September 1980) Abstract--ix. Spectra of poly(dimethyl-p-silphenylene), poly(tetramethyl-p-silphenylene-siloxane) and their pyrolysis residue were investigated and gas chromatographic analyses of the volatile pyrolysis products were performed. Scission of the polymer chain results in radicals, leading to some volatile products and also to cross-linking. It represents the main thermal degradation reaction in both polymers. A rearrangement of the siloxane bonds, altering the polydispersity and producing a small amount of some cyclic volatile products, may also take place in poly(tetramethyl-p-silphenylene-siloxane).
INTRODUCTION
The thermal decompositions of dimethylsilane and dimethylsiloxane polymers and copolymers containing short aliphatic segments in the chain have been discussed [1-7]. Poly(dimethylsilane) splits into radicals and cyclic pentamer and hexamer are the main volatile products of the thermal degradation at 300-400 ~ in an inert atmosphere. Besides these products, there is a smaller amount of dimethylsilane formed by hydrogen abstraction from methyl groups, leading to cross-linking [1]. The thermal degradation of poly(dimethylsiloxane) takes place through siloxane decomposition first suggested by Thomas and Kendrick [3]. Cyclotrisiloxane, cyclotetrasiloxane and higher cyclic oligomers are formed at 300-500'. In polymer chains terminated by OH groups, oligomer formation is promoted at the chain ends [7, 8]. A free radical decomposition characteristic of hydrocarbons becomes predominant in the thermal degradation, provided short aliphatic segments are built into the silane or siloxane chain [2, 8]. The thermal stability of these copolymers diminished in consequence of the change in their thermal degradation mechanism. In the present paper we deal with two siliconorganic polymers containing p-phenylene groups in the main chain. No weak bonds are inserted into the silicon-organic chain by the phenylene groups but the chain flexibility decreases[-9], thus the secondary structure formation favourable for thermal decomposition is prevented. RESULTS
AND
sions about the mechanism of the thermal degradation. Poly(dimethyl-p-silphenylene)
DISCUSSION
We have studied the chain structure of the macromolecules and the interactions of their segments by vibrational spectroscopy. Alterations in the sample composition due to thermal degradation have been investigated by comparison of the i.r. spectra of the original samples and those of the heat treated material. The nature of the volatile decomposition products found by gas chromatography led to conclu503
CIH3 -
SI I
CH3
- ( O ) n
The i.r. spectrum of an oriented film of this sample cast from benzene solution, registered by polarized attenuated total reflexion method, can be explained by a zig-zag chain structure with nearly perpendicular planes of the two p-phenylene groups attached to the same silicon atom. A propeller-like librational movement of the p-phenylene groups is possible around the Si---C bonds. Only the B~u ring modes having the transition moment in the direction of the S i . . . Si axis split into two components in the high resolution spectra. We presume that this splitting is caused by the inand the out-of-phase coupling of these modes across the silicon atoms in the polymer chain which is rather stiff in this sample. No splitting of the vibrational modes of the p-phenylene groups due to interchain forces was observed. The i.r. active Si---Cphe,y~e,~ stretching mode is shifted towards higher frequencies compared with that observed in the monomer or in poly(tetramethyl-p-silphenylene-siloxane) and the monomer of the latter [10]. A gradual development of this shift can be observed by building up the polymer chain by monomer units r l l ] . Accordingly a weak n-electron delocalization can be supposed along the polymer chain. The yellowish colour of this polymer is probably due to these mobile electrons. The i.r. spectrum of poly(dimethyl-p-silphenylene) in KI pellet and that of a sample heat treated at 400 for 3 min are shown in Fig. 1. These spectra are similar but the bands are broader for the pyrolysed sample indicating either an increased polydispersity or/and a decreased crystallinity. New bands on h e a t treating are assigned to the phenyl group (1600, 1420, 990 and 400cm -1) and the the silanolic hydroxyl
504
B. ZELEI, M. BLAZS~ and S. Doaos
."
o
E E
t-
Pyrolysis residue I
I
3500
30(X)
I 1400
I 1200
I 10(30
I 800
I 600
I 400
200
cm"l
Wovenumber,
Fig. 1. i.r. Spectra of poly(dimethyl-p-silphenylene) and its residue after pyrolysis at 4 0 0 for 3 rain.
~oo -
4
7
50-
E
(a)
'll213
o
8
0
IO
[
2o
i
~o
3
Joo
50
Ii
J
..
(b)
I,
I
t6 2'o Retention time, rain
3b
Fig. 2. Bar graph of the pyrolysis products of poly(dimethyl-p-silphenylene) at 400 ° in argon (a) and in air (b). 1. methane; 3. benzene; 2.
6.
CIH3 H-Si -H I CH
CIH3
4.
CH3 H-Si - ~
CH3 5. H - S i ~ (!H~
~H3 - -
/c. 3
__/c%
\
H
c~
\CH3
/2
C IH3 Si-H Ch~
8.
Si
\ - H
Thermal degradation group (3700-3600 c m - t ) and a broad shoulder to the corresponding Si---O group (900cm-~). Since the pyrolysis was carried out in an oxygen-free atmosphere, silanoles must be formed after pyrolysis, during manipulation from the reactive Si--H groups. The intense doublet at 250-200cm-~ in the i.r. spectra, cannot be assigned with certainty. Although it appears in the range of the Si------Cphenylene in-plane bending mode, it is too intense to be caused by this deformational mode. Figure 2(a) shows the volatile pyrolysis products at 400 ° in an inert atmosphere, analysed by gas chromatography. The nature of these products remains unchanged in the pyrolysis temperature range from 350 to 550". Pyrolysis in air (Fig. 2b) leads to a large quantity of methane. The same is observed for poly(dimethylsiloxane). The GC peaks were assigned by the injection of standards available as commercial products. We have applied the generally accepted rule that the retention index of a compound on a given GC phase is closely connected with the molecular weight as well as with the composition and shape of the molecule[12, 13]. The GC peak assignments given in the captions are in agreement with the observation that the product molecules with two Si--H bonds practically disappear and the quantity of components with one Si--H bond decreases considerably in air. To explain this finding, we suppose that silanole formation takes place in air but cannot be analysed with gas chromatography under the conditions described. The i.r. spectrum of the pyrolysis residue and the nature of the volatile products therefore indicate that the thermal degradation occurs by radical bond scission of the macromolecular chain. The radicals are
505
terminated by hydrogen abstracted probably from methyl groups, as in the case of poly(dimethylsilane) and poly(methyl-phenylsiloxane)[1, 14]. The combination of two macromolecular chains through methylene groups formed in this way results in cross-linking [14]. The bonds along the chain are stronger in poly(dimethyl-p-silphenylene)than in poly(dimethylsilane), and no depolymerization chain-reaction can be initiated by the polymer chain scission as in the case of certain organic polymers with flexible chains. Poly(tetramethyl-p-silphenylene-siloxane)
CH 5
CH 3
Based on the vibrational spectra, the arrangement
of the siloxane and p-phenylene groups in the unit cell of the oriented sample was investigated [10]. The repeating unit of the helical chain consists of four monomer units (P4t chain symmetry) in which the p-phenylene groups are symmetry equivalent and possess D2h pseudo-symmetry. The quasi-free libration of the p-phenylene groups around the Si---Cphooyz¢,e bonds also requires the p-phenylene groups be equivalent. Consequently, there is no interchain vibrational coupling between the phenylene groups. The tetramethyi-disiloxy groups show high flexibility and/or conformational disorder in the crystalline sample since the asymmetric Si--O---Si stretching mode yields a very broad and intense i.r. band without dichroic properties (ordered conformation). The symmetric Si---O--Si stretching band is split into two components in the Raman spectrum indicating the
c g c o
ol
l
CH 3
CH 3
"
_ si,_o_i,_ CH 3
CH 3
=_ 1001 E ,c,,, m c:
2 I-
0
i 1600
1400
I 1200
I I000 Wovenumber,
Pyrolysis
residue
I 600
I 400
I 800
200
cm 1
Fig. 3. i.r. Spectra of poly(tetramethyl-p-silphenylene-siloxane) and its residue after pyrolysis at 500° for 10rain (dashed line) and for 30rain (solid line). E,PJ. 17 5 --D
506
B. ZELEL M . BLAZSO a n d S. D o B o s ioe
P,h
c
/I
"
E I-
i
¢ iI
,, ~
Pyrolysis residue
o
I
,~oo ,~oo
,~o
,o'oo
~;o
~,o
~o
~oo
Wovenurnber~ cm- I
Fig. 4. i.r. Spectra of polytphenyl-silsesquioxane)and its residue after pyrolysis at 300 for 30 min (dashed line) and at 450: for 30 min (solid line).
vibrational interaction of this mode either inside the unit cell or of the neighbouring polymer chains. The i.r. spectra of poly(tetramethyl-p-silphenylenesiloxane) and of samples heat treated at 500~ for 10 (dashed line) and 30min (solid line) are shown in Fig. 3; the first was measured in KI pellet, the others in Nujol mull. When the sample was heat treated at a low temperature of 280 :, the bands became somewhat broader but their wavelengths and relative intensities did not change. After pyrolysis at 500~. the vibrational bands of the Si-O-Si groups became predominant, and new bands assigned to the phenyl group (1600, 800-700 cm- 1) appeared. The intensities of the latter bands decreased for extensive pyrolysis (30 min. solid line in Fig. 3). The i.r. spectrum of the residue of the extensive pyrolysis is surprisingly similar to that of poly(phenyl-silsesquioxane)shown in Fig. 4.
• -0
-Si
-0-
o
s,
o-s,,
-o
© © The result of pyrolysis gas chromatographic analysis of poly(tetramethyl-p-silphenylene-siloxane) is
shown in Fig. 5(a). The volatile products of a pyrolysis carried out at 500 in an inert atmosphere are indicated. The same products are formed in the pyrolysis temperature range between 350 and 550 :. When the pyrolysis is carried out in air (Fig. 5b), the same changes occur as in the case of poly(dimethyl-p-silphenylene), viz. methane formation and disappearance of Si--H bond containing products. The formation of hexamethyl-cyclotrisiloxane (assigned by standard injection) and some cyclic dimer and trimer pyrolysis products indicates that, in addition to the radical scission of the macromolecular chain, siloxane type degradation may also take place. The macromolecules split at Si---C bonds; the scission leads to phenyl groups in the pyrolysed residue and volatile products containing Si--H bonds. The siloxane cycle formation leading to cyclic dimer and trimer is rather negligible. We may conclude that the splitting off of oligomeric cycles through rearrangement of siloxane bonds from poly(tetramethyl-p-silphenylene-siloxane) is hindered since its chain is less flexible than the poly(dimethylsiloxane) chain. Further. radical scission of the p-silphenylene-siloxane polymer chain at Si~ph,,,~e,e bonds occurs at a higher temperature. The formation of hexamethyl-cyclotrisiloxane may be attributed to an intermolecular rearrangement of siloxane bonds (most probably not in a single step) during which three phenylene-Si(CH3)2-phenylenesegments and one hexamethyl-cyclotrisiloxane molecule are formed. Although the amount of this product is very small compared to the main volatile products, its presence is important. In order to explain the formation of this
Thermal degradation
507
I00
9
'~
i Iii
4
3
J ,81
E o
i
2O
0
(a)
12 I 3(3
3
Joo o
g~ (b)
5O
5 I
J
.
12
I,
I
I0
0
,
I
3O
20
Retention time, min 5. Bar graph of pyrolysis products of poly(tetramethylp-silphenylene-siloxane) at 500 ~ in argon (a) and in air (b) separated on OV-1 gas chromatographic phase. I. methane; 3. benzene; Fig.
CHI3
CH5
/ CH 3 \
H-S,-O-S, -H ch~ ~H 3
2.
4.
( - Si-O-)
CH 3 C,H3 --
5.
H-Si-O-Si-(O >
&~ ~ % - -
\CH3/3
H_~H_30_!H~
CiH3 CH3
CH
CH 3
CH 3 CH 3
CH 3 CH3 ~
_s,_o_si_,
6. CH 3 CH 3
CH3 CIH3/~ \
8.
I
9,10. H ( - S i - O - S i - ~ ~
CH3
CH3-
\cH 3 ~H3~
/2
/CH 3 CH3
\
\ CH3 CH3 ~
"2 CH3 CH3
c.~
fH3 CH3
,
12.
CH3 CH3
/3
product, we have to assume a rearrangement of the S i - - O and Si'---Cph~,rlene bonds as follows:
/ Sf /\
/
\
/
residues of poly(tetramethyl-p-silphenylene-siloxane) and poly(phenyl-silsesquioxane) (Figs 3b and 4b) indicates that both residues are networks comprising siloxane chains crosslinked either by oxygen or by phenylene groups. As a matter of fact, no volatile siloxane forms from poly(phenyl-silsesquioxane) under pyrolysis at 500, although benzene is produced.
\
0
The similarity between the spectra of the pyrolysis
EXPERIMENTAL
The polymer samples were synthesized in the authors" laboratory [15. 16]. The number-average molecular weight
B. ZELEI, M. BLAZS~and S. Doaos
508
(determined by Vapor Pressure Osmometer in benzene, at 37) of poly(dimethyl-p-silphenylene) was 2.5 x 103 and that of poly(tetramethyl-p-silphenylene-siloxane) 4 x 104 (by extrapolation). i.r. Spectra were registered on a Perkin-Elmer 225 type grating spectrophotometer equipped with AgBr grid polarizer and attenuated total reflexion unit in the spectral range 4000-200 cm- 1. Raman spectra were measured on a Cary 81 type Ar ÷ laser excited intrument using the exciting lines of 488 and 514.3 rim. Pyrolyses of the samples were carried out in an argon flow of 1 cmsec -1 rate. For analysis of the volatile products, a micro-scale pyrolyser [81 coupled to a PerkinElmer 900 gas chromatograph with flame ionization detector was applied. The sample weights were about 0.1 mg; the pyrolysis time was 30sec. Analysis of the volatile products was performed on a 3 m, 2 mm i.d. gas chromatographic column packed with 80-100 mesh Chromosorb G coated with 5°00V-I silicon phase. A temperature program from 4 0 to 300 ~ at a rate of 10:/min was applied. In addition, two other columns with OV-17 phenyl-methyl silicon phase and with F-50 methyl-cyanopropyl silicon phase were used for qualitative identification. REFERENCES
1. T. Sz6kely, O. M. Nefedov, G. Garz6, V. 1. Shiryayev and D. Fritz, Acta Chim. Acad. Sci. Hung. 54, 341 (1967). 2. M. Blazs6 and E. Jakab, Preprints Int. Syrup. Macromolecules (Edited by I. Ltiderwald and R. Weis), Vol. l, p. 625. Mainz, 17-21 September (1979).
3. T. H. Thomas and T. C. Kendrick, J. Polym. Sci., Part A-2 7, 537 (1969). 4. T. H. Thomas and T. C. Kendrick, J. Polym. Sci., Part A-2 8, 1823 (1970). 5. K. A. Andrianov, G. Garzo, T. Sz6kely, M. Blazso. L. M. Volkova, N. V. Delazari and J. Tam~.s, Vysokomolek. Soedin. B13, 593 (1971). 6. M. Blazs6, G. Garzo and T. Sz~kely, J. Organomet. Chem. 54, 105 (1973). 7. N. Grassie and I. G. Macfarlane, Eur. Polym. J. 14, 875 (1978). 8. M. Blazs6, G. Garz6 and T. Sz6kely, Chromatographia 5, 485 (1972). 9. W. R. Dunnavant, Inorg. Macromolec. Rev. 1, 165 (1971). 10. S. Dobos and B. Zelei, J. Polym. Sei., Polym. Chem. Ed. 17, 2679 (1979). 11. The Sadtler Standard Spectra. Organometallic Grating Spectra N 89K-93K. Sadtler Research Laboratory Philadelphia (1966). 12. E. Kovats, In Advances in Chromatography, Vol I. (Edited by J. C. Giddings and R. A. Keller), pp. 229-247. Marcel Dekker, New York (1965). 13. G. Schomburg, in Advances in Chromatography VoL 6 (Edited by J. C. Giddings and R. A. Keller), pp. 211-245. Marcel Dekker, New York (1968). 14. N. Grassie, 1. G. Macfarlane and K. F. Francey, Eur. Polym. J. 15, 415 (1979). 15. J. G. Noltes and G. J. M. van der Kerk. Rec. Tray. Chim. 81,565 (1962). 16. R. L. Merker and M. J. Scott, J. Polym. Sci. Part A, 2, 15 (1964).
Eurolwan Polt'mer Journal \ ol. 17, pp. 503 to 508. 1981 Printed in Greal Britain. All rights reserved
THERMAL DEGRADATION OF POLY(DIMETHYLp-SILPHENYLENE) AND POLY(TETRAMETHYLp-SILPHENYLENE-SILOXANE) B. ZELE|, M. BLAZSOand S. DOBOS Research Laboratory for Inorganic Chemistry, Hungarian Academy of Sciences, Budapest 1112, BudaSrsi ut 45, Hungary
(Receired 8 September 1980) Abstract--ix. Spectra of poly(dimethyl-p-silphenylene), poly(tetramethyl-p-silphenylene-siloxane) and their pyrolysis residue were investigated and gas chromatographic analyses of the volatile pyrolysis products were performed. Scission of the polymer chain results in radicals, leading to some volatile products and also to cross-linking. It represents the main thermal degradation reaction in both polymers. A rearrangement of the siloxane bonds, altering the polydispersity and producing a small amount of some cyclic volatile products, may also take place in poly(tetramethyl-p-silphenylene-siloxane).
INTRODUCTION
The thermal decompositions of dimethylsilane and dimethylsiloxane polymers and copolymers containing short aliphatic segments in the chain have been discussed [1-7]. Poly(dimethylsilane) splits into radicals and cyclic pentamer and hexamer are the main volatile products of the thermal degradation at 300-400 ~ in an inert atmosphere. Besides these products, there is a smaller amount of dimethylsilane formed by hydrogen abstraction from methyl groups, leading to cross-linking [1]. The thermal degradation of poly(dimethylsiloxane) takes place through siloxane decomposition first suggested by Thomas and Kendrick [3]. Cyclotrisiloxane, cyclotetrasiloxane and higher cyclic oligomers are formed at 300-500'. In polymer chains terminated by OH groups, oligomer formation is promoted at the chain ends [7, 8]. A free radical decomposition characteristic of hydrocarbons becomes predominant in the thermal degradation, provided short aliphatic segments are built into the silane or siloxane chain [2, 8]. The thermal stability of these copolymers diminished in consequence of the change in their thermal degradation mechanism. In the present paper we deal with two siliconorganic polymers containing p-phenylene groups in the main chain. No weak bonds are inserted into the silicon-organic chain by the phenylene groups but the chain flexibility decreases[-9], thus the secondary structure formation favourable for thermal decomposition is prevented. RESULTS
AND
sions about the mechanism of the thermal degradation. Poly(dimethyl-p-silphenylene)
DISCUSSION
We have studied the chain structure of the macromolecules and the interactions of their segments by vibrational spectroscopy. Alterations in the sample composition due to thermal degradation have been investigated by comparison of the i.r. spectra of the original samples and those of the heat treated material. The nature of the volatile decomposition products found by gas chromatography led to conclu503
CIH3 -
SI I
CH3
- ( O ) n
The i.r. spectrum of an oriented film of this sample cast from benzene solution, registered by polarized attenuated total reflexion method, can be explained by a zig-zag chain structure with nearly perpendicular planes of the two p-phenylene groups attached to the same silicon atom. A propeller-like librational movement of the p-phenylene groups is possible around the Si---C bonds. Only the B~u ring modes having the transition moment in the direction of the S i . . . Si axis split into two components in the high resolution spectra. We presume that this splitting is caused by the inand the out-of-phase coupling of these modes across the silicon atoms in the polymer chain which is rather stiff in this sample. No splitting of the vibrational modes of the p-phenylene groups due to interchain forces was observed. The i.r. active Si---Cphe,y~e,~ stretching mode is shifted towards higher frequencies compared with that observed in the monomer or in poly(tetramethyl-p-silphenylene-siloxane) and the monomer of the latter [10]. A gradual development of this shift can be observed by building up the polymer chain by monomer units r l l ] . Accordingly a weak n-electron delocalization can be supposed along the polymer chain. The yellowish colour of this polymer is probably due to these mobile electrons. The i.r. spectrum of poly(dimethyl-p-silphenylene) in KI pellet and that of a sample heat treated at 400 for 3 min are shown in Fig. 1. These spectra are similar but the bands are broader for the pyrolysed sample indicating either an increased polydispersity or/and a decreased crystallinity. New bands on h e a t treating are assigned to the phenyl group (1600, 1420, 990 and 400cm -1) and the the silanolic hydroxyl
504
B. ZELEI, M. BLAZS~ and S. Doaos
."
o
E E
t-
Pyrolysis residue I
I
3500
30(X)
I 1400
I 1200
I 10(30
I 800
I 600
I 400
200
cm"l
Wovenumber,
Fig. 1. i.r. Spectra of poly(dimethyl-p-silphenylene) and its residue after pyrolysis at 4 0 0 for 3 rain.
~oo -
4
7
50-
E
(a)
'll213
o
8
0
IO
[
2o
i
~o
3
Joo
50
Ii
J
..
(b)
I,
I
t6 2'o Retention time, rain
3b
Fig. 2. Bar graph of the pyrolysis products of poly(dimethyl-p-silphenylene) at 400 ° in argon (a) and in air (b). 1. methane; 3. benzene; 2.
6.
CIH3 H-Si -H I CH
CIH3
4.
CH3 H-Si - ~
CH3 5. H - S i ~ (!H~
~H3 - -
/c. 3
__/c%
\
H
c~
\CH3
/2
C IH3 Si-H Ch~
8.
Si
\ - H
Thermal degradation group (3700-3600 c m - t ) and a broad shoulder to the corresponding Si---O group (900cm-~). Since the pyrolysis was carried out in an oxygen-free atmosphere, silanoles must be formed after pyrolysis, during manipulation from the reactive Si--H groups. The intense doublet at 250-200cm-~ in the i.r. spectra, cannot be assigned with certainty. Although it appears in the range of the Si------Cphenylene in-plane bending mode, it is too intense to be caused by this deformational mode. Figure 2(a) shows the volatile pyrolysis products at 400 ° in an inert atmosphere, analysed by gas chromatography. The nature of these products remains unchanged in the pyrolysis temperature range from 350 to 550". Pyrolysis in air (Fig. 2b) leads to a large quantity of methane. The same is observed for poly(dimethylsiloxane). The GC peaks were assigned by the injection of standards available as commercial products. We have applied the generally accepted rule that the retention index of a compound on a given GC phase is closely connected with the molecular weight as well as with the composition and shape of the molecule[12, 13]. The GC peak assignments given in the captions are in agreement with the observation that the product molecules with two Si--H bonds practically disappear and the quantity of components with one Si--H bond decreases considerably in air. To explain this finding, we suppose that silanole formation takes place in air but cannot be analysed with gas chromatography under the conditions described. The i.r. spectrum of the pyrolysis residue and the nature of the volatile products therefore indicate that the thermal degradation occurs by radical bond scission of the macromolecular chain. The radicals are
505
terminated by hydrogen abstracted probably from methyl groups, as in the case of poly(dimethylsilane) and poly(methyl-phenylsiloxane)[1, 14]. The combination of two macromolecular chains through methylene groups formed in this way results in cross-linking [14]. The bonds along the chain are stronger in poly(dimethyl-p-silphenylene)than in poly(dimethylsilane), and no depolymerization chain-reaction can be initiated by the polymer chain scission as in the case of certain organic polymers with flexible chains. Poly(tetramethyl-p-silphenylene-siloxane)
CH 5
CH 3
Based on the vibrational spectra, the arrangement
of the siloxane and p-phenylene groups in the unit cell of the oriented sample was investigated [10]. The repeating unit of the helical chain consists of four monomer units (P4t chain symmetry) in which the p-phenylene groups are symmetry equivalent and possess D2h pseudo-symmetry. The quasi-free libration of the p-phenylene groups around the Si---Cphooyz¢,e bonds also requires the p-phenylene groups be equivalent. Consequently, there is no interchain vibrational coupling between the phenylene groups. The tetramethyi-disiloxy groups show high flexibility and/or conformational disorder in the crystalline sample since the asymmetric Si--O---Si stretching mode yields a very broad and intense i.r. band without dichroic properties (ordered conformation). The symmetric Si---O--Si stretching band is split into two components in the Raman spectrum indicating the
c g c o
ol
l
CH 3
CH 3
"
_ si,_o_i,_ CH 3
CH 3
=_ 1001 E ,c,,, m c:
2 I-
0
i 1600
1400
I 1200
I I000 Wovenumber,
Pyrolysis
residue
I 600
I 400
I 800
200
cm 1
Fig. 3. i.r. Spectra of poly(tetramethyl-p-silphenylene-siloxane) and its residue after pyrolysis at 500° for 10rain (dashed line) and for 30rain (solid line). E,PJ. 17 5 --D
506
B. ZELEL M . BLAZSO a n d S. D o B o s ioe
P,h
c
/I
"
E I-
i
¢ iI
,, ~
Pyrolysis residue
o
I
,~oo ,~oo
,~o
,o'oo
~;o
~,o
~o
~oo
Wovenurnber~ cm- I
Fig. 4. i.r. Spectra of polytphenyl-silsesquioxane)and its residue after pyrolysis at 300 for 30 min (dashed line) and at 450: for 30 min (solid line).
vibrational interaction of this mode either inside the unit cell or of the neighbouring polymer chains. The i.r. spectra of poly(tetramethyl-p-silphenylenesiloxane) and of samples heat treated at 500~ for 10 (dashed line) and 30min (solid line) are shown in Fig. 3; the first was measured in KI pellet, the others in Nujol mull. When the sample was heat treated at a low temperature of 280 :, the bands became somewhat broader but their wavelengths and relative intensities did not change. After pyrolysis at 500~. the vibrational bands of the Si-O-Si groups became predominant, and new bands assigned to the phenyl group (1600, 800-700 cm- 1) appeared. The intensities of the latter bands decreased for extensive pyrolysis (30 min. solid line in Fig. 3). The i.r. spectrum of the residue of the extensive pyrolysis is surprisingly similar to that of poly(phenyl-silsesquioxane)shown in Fig. 4.
• -0
-Si
-0-
o
s,
o-s,,
-o
© © The result of pyrolysis gas chromatographic analysis of poly(tetramethyl-p-silphenylene-siloxane) is
shown in Fig. 5(a). The volatile products of a pyrolysis carried out at 500 in an inert atmosphere are indicated. The same products are formed in the pyrolysis temperature range between 350 and 550 :. When the pyrolysis is carried out in air (Fig. 5b), the same changes occur as in the case of poly(dimethyl-p-silphenylene), viz. methane formation and disappearance of Si--H bond containing products. The formation of hexamethyl-cyclotrisiloxane (assigned by standard injection) and some cyclic dimer and trimer pyrolysis products indicates that, in addition to the radical scission of the macromolecular chain, siloxane type degradation may also take place. The macromolecules split at Si---C bonds; the scission leads to phenyl groups in the pyrolysed residue and volatile products containing Si--H bonds. The siloxane cycle formation leading to cyclic dimer and trimer is rather negligible. We may conclude that the splitting off of oligomeric cycles through rearrangement of siloxane bonds from poly(tetramethyl-p-silphenylene-siloxane) is hindered since its chain is less flexible than the poly(dimethylsiloxane) chain. Further. radical scission of the p-silphenylene-siloxane polymer chain at Si~ph,,,~e,e bonds occurs at a higher temperature. The formation of hexamethyl-cyclotrisiloxane may be attributed to an intermolecular rearrangement of siloxane bonds (most probably not in a single step) during which three phenylene-Si(CH3)2-phenylenesegments and one hexamethyl-cyclotrisiloxane molecule are formed. Although the amount of this product is very small compared to the main volatile products, its presence is important. In order to explain the formation of this
Thermal degradation
507
I00
9
'~
i Iii
4
3
J ,81
E o
i
2O
0
(a)
12 I 3(3
3
Joo o
g~ (b)
5O
5 I
J
.
12
I,
I
I0
0
,
I
3O
20
Retention time, min 5. Bar graph of pyrolysis products of poly(tetramethylp-silphenylene-siloxane) at 500 ~ in argon (a) and in air (b) separated on OV-1 gas chromatographic phase. I. methane; 3. benzene; Fig.
CHI3
CH5
/ CH 3 \
H-S,-O-S, -H ch~ ~H 3
2.
4.
( - Si-O-)
CH 3 C,H3 --
5.
H-Si-O-Si-(O >
&~ ~ % - -
\CH3/3
H_~H_30_!H~
CiH3 CH3
CH
CH 3
CH 3 CH 3
CH 3 CH3 ~
_s,_o_si_,
6. CH 3 CH 3
CH3 CIH3/~ \
8.
I
9,10. H ( - S i - O - S i - ~ ~
CH3
CH3-
\cH 3 ~H3~
/2
/CH 3 CH3
\
\ CH3 CH3 ~
"2 CH3 CH3
c.~
fH3 CH3
,
12.
CH3 CH3
/3
product, we have to assume a rearrangement of the S i - - O and Si'---Cph~,rlene bonds as follows:
/ Sf /\
/
\
/
residues of poly(tetramethyl-p-silphenylene-siloxane) and poly(phenyl-silsesquioxane) (Figs 3b and 4b) indicates that both residues are networks comprising siloxane chains crosslinked either by oxygen or by phenylene groups. As a matter of fact, no volatile siloxane forms from poly(phenyl-silsesquioxane) under pyrolysis at 500, although benzene is produced.
\
0
The similarity between the spectra of the pyrolysis
EXPERIMENTAL
The polymer samples were synthesized in the authors" laboratory [15. 16]. The number-average molecular weight
B. ZELEI, M. BLAZS~and S. Doaos
508
(determined by Vapor Pressure Osmometer in benzene, at 37) of poly(dimethyl-p-silphenylene) was 2.5 x 103 and that of poly(tetramethyl-p-silphenylene-siloxane) 4 x 104 (by extrapolation). i.r. Spectra were registered on a Perkin-Elmer 225 type grating spectrophotometer equipped with AgBr grid polarizer and attenuated total reflexion unit in the spectral range 4000-200 cm- 1. Raman spectra were measured on a Cary 81 type Ar ÷ laser excited intrument using the exciting lines of 488 and 514.3 rim. Pyrolyses of the samples were carried out in an argon flow of 1 cmsec -1 rate. For analysis of the volatile products, a micro-scale pyrolyser [81 coupled to a PerkinElmer 900 gas chromatograph with flame ionization detector was applied. The sample weights were about 0.1 mg; the pyrolysis time was 30sec. Analysis of the volatile products was performed on a 3 m, 2 mm i.d. gas chromatographic column packed with 80-100 mesh Chromosorb G coated with 5°00V-I silicon phase. A temperature program from 4 0 to 300 ~ at a rate of 10:/min was applied. In addition, two other columns with OV-17 phenyl-methyl silicon phase and with F-50 methyl-cyanopropyl silicon phase were used for qualitative identification. REFERENCES
1. T. Sz6kely, O. M. Nefedov, G. Garz6, V. 1. Shiryayev and D. Fritz, Acta Chim. Acad. Sci. Hung. 54, 341 (1967). 2. M. Blazs6 and E. Jakab, Preprints Int. Syrup. Macromolecules (Edited by I. Ltiderwald and R. Weis), Vol. l, p. 625. Mainz, 17-21 September (1979).
3. T. H. Thomas and T. C. Kendrick, J. Polym. Sci., Part A-2 7, 537 (1969). 4. T. H. Thomas and T. C. Kendrick, J. Polym. Sci., Part A-2 8, 1823 (1970). 5. K. A. Andrianov, G. Garzo, T. Sz6kely, M. Blazso. L. M. Volkova, N. V. Delazari and J. Tam~.s, Vysokomolek. Soedin. B13, 593 (1971). 6. M. Blazs6, G. Garzo and T. Sz~kely, J. Organomet. Chem. 54, 105 (1973). 7. N. Grassie and I. G. Macfarlane, Eur. Polym. J. 14, 875 (1978). 8. M. Blazs6, G. Garz6 and T. Sz6kely, Chromatographia 5, 485 (1972). 9. W. R. Dunnavant, Inorg. Macromolec. Rev. 1, 165 (1971). 10. S. Dobos and B. Zelei, J. Polym. Sei., Polym. Chem. Ed. 17, 2679 (1979). 11. The Sadtler Standard Spectra. Organometallic Grating Spectra N 89K-93K. Sadtler Research Laboratory Philadelphia (1966). 12. E. Kovats, In Advances in Chromatography, Vol I. (Edited by J. C. Giddings and R. A. Keller), pp. 229-247. Marcel Dekker, New York (1965). 13. G. Schomburg, in Advances in Chromatography VoL 6 (Edited by J. C. Giddings and R. A. Keller), pp. 211-245. Marcel Dekker, New York (1968). 14. N. Grassie, 1. G. Macfarlane and K. F. Francey, Eur. Polym. J. 15, 415 (1979). 15. J. G. Noltes and G. J. M. van der Kerk. Rec. Tray. Chim. 81,565 (1962). 16. R. L. Merker and M. J. Scott, J. Polym. Sci. Part A, 2, 15 (1964).