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 characterized using gas chromatography-mass spectrometry, Si-29 NMR and elemental analysis. The polysiloxanes studied included polydimethylsiloxane (PDMS), poly(3.5%diphenyl-96.5%dimethyl)siloxane (DP-3.5) and poly(25%diphenyl-75%dimethyl)siloxane (DP-25). The degradation products were primarily cyclic oligomers ranging from hexamethylcyclotrisiloxane and octamethylcyclotetrasiloxane for PDMS with benzene as an additional product for PDPDMS. No cyclic oligomers with phenyl substituents were observed. Various mechanisms for degradation 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 reaction. (C) 2001 Elsevier Science Ltd. All rights reserved.