Studies of the chemical degradation of polysiloxanes by hydrofluoric acid: (2). Poly(tetramethyl- p-silphenylene siloxane-dimethyl siloxane) block copolymers

Abstract

Block copolymers of tetramethyl- p-silphenylene siloxane (TMPS) and dimethyl siloxane (DMS) have been selectively degraded with hydrofluoric acid solution. The HF preferentially attacks the SiO bonds, particularly those of the non-crystalline DMS component. The copolymers which were precipitated by the self-seeding technique were etched with 48% HF at 30°C (established as suitable conditions for our work). The crystallinity change on etching these is largest for the sample of lowest TMPS content; it varies from about 20% at zero etching time to 98% after a period of 40 h approximately. The molecular weight, small-angle X-ray period, melting temperature, heat of fusion and degree of crystallinity were all determined as a function of etching time. The correlated results were found to be consistent with a two-phase model in which the DMS component was essentially excluded from the TMPS crystalline core. Some predictions, based on copolymer theories, were found to be consistent with the analysis of the experimental observations.