The effect of phenyl content on the degradation of poly(dimethyl diphenyl)siloxane copolymers

Inert atmosphere thermal degradation of polysiloxanes having different side groups (-CH3 and -Ph) was studied at 364 degreesC. The process was charact erized using gas chromatography-mass spectrometry, Si-29 NMR and elemental analysis. The polysiloxanes studied included polydimethylsiloxane (PDMS), p oly(3.5%diphenyl-96.5%dimethyl)siloxane (DP-3.5) and poly(25%diphenyl-75%di methyl)siloxane (DP-25). The degradation products were primarily cyclic oli gomers ranging from hexamethylcyclotrisiloxane and octamethylcyclotetrasilo xane for PDMS with benzene as an additional product for PDPDMS. No cyclic o ligomers with phenyl substituents were observed. Various mechanisms for deg radation of PDPDMS were reviewed and discussed in relation to the Si-29-NMR results and elemental analysis. The Si-29 NMR analysis shows the mechanism for generation of benzene to be thermally induced random free-radical reac tion. (C) 2001 Elsevier Science Ltd. All rights reserved.