EFFECT OF PROCESSING VARIABLES ON THE PROPERTIES OF POLYSILANE CERAMIC PRECURSORS

The Wurtz synthesis of polysilanes is a useful route in the production of polymeric precursors for silicon carbide ceramics and especially f ibres. Copolymers have been synthesized by the dechlorination of methy lphenyldichlorosilane and phenyltrichlorosilane in a 1 : 1 molar ratio using sodium metal dispersed in xylene. Changes have been made to the work-up procedure, leading to the separation of the various reaction products, in an attempt to improve the processing of ceramic fibres vi a polymeric precursors. It has been found that variation in the solven t used to separate the reaction products modified their yields and cha racteristic properties. Gel permeation chromatography (g.p.c.) showed that the molecular weight distribution of the soluble solid fraction, of importance as a potential precursor for ceramic fibres, had changed , which altered the softening temperature range observed. Thermomechan ical analysis demonstrated that these changes could be as much as 70 d egrees C, which has important consequences for the processing of ceram ic fibres from these precursors. Although some small variations in the ceramic yield were observed by thermogravimetry, these did not correl ate with the g.p.c. data, suggesting that their chemical structure is at least as important as their molecular weight distribution in determ ining this yield. Copyright (C) 1996 Elsevier Science Ltd.