The Thermal Decomposition of Some Polysiloxanes

There is an ever increasing demand for polymers and copolymers having high thermal and oxidative stability. Since the silicone industry began in 1931, silicone polymers have provided for a wide range of industrial needs as thermally stable oils, rubbers and resins. In recent years research has been carried out by a number of workers into the mechanism of breakdown processes in these materials in an attempt to bridge the gap between empirical knowledge and scientific understanding. The present work is aimed at supplementing information in the literature on the thermal breakdown of some linear polysiloxanes containing methyl or methyl and phenyl side groups. Brief notes on the history of silicones, on common nomenclature and on some aspects of the physical chemistry of silicon and its compounds are given in Chapter 1 and these are followed by a comprehensive review of the literature dealing with work related to the title of this thesis. Chapter 2 includes a list of sources for the chemicals used in this work, gives details of polymerisation techniques and summarises the apparatus and experimental techniques employed in this study. The first part deals with the thermal analysis techniques and the second part describes the techniques used to identify the degradation products both qualitatively and quantitatively. The thermal degradation of poly(dimethylsiloxane) is discussed in Chapter 3. The main products are cyclic siloxanes containing up to eighteen or more dimethylsiloxane units. The cyclic trimer, hexamethylcyclotrisiloxane is the major product. Methane is a product from samples degraded in the presence of base. In Chapter 4 the preparation and thermal properties of poly(methylphenylsiloxane) are discussed. The two isomeric forms of the cyclic siloxane trimer and the four forms of the cyclic tetramer have been shown to be the major products of degradation. Benzene is also formed. Several attempts to prepare high molecular weight samples of poly(diphenylsiloxane) are described in Chapter 5. These attempts were not successful. In Chapter 6 some preliminary studies on copolymers of poly(dimethyldiphenylsiloxane) are described. Although benzene is produced there is evidence that some diphenyl units remain intact and are incorporated into siloxane ring compounds which appear as degradation products