The Effects of Small Molecules on the Thermal Degradations of Some Vinyl Polymers

In the introductory chapter, the development of research into thermal degradation is traced from study of homopolymers, through copolymers and binary polymer blends, to the present study of polymer/small molecule blends. Following a chapter describing the experimental techniques employed, Chapter Three consists of results, abstracted into tabular form, of a survey of the effects of various small molecules on the thermal degradation of a range of polymers. On the basis of this survey, three blends were chosen, for reasons given in Chapters Four, Five, and Six, to be studied in detail. Zinc bromide was found to exert a massive effect on the thermal degradation of poly(methyl methacrylate) by combining with pendant ester groups to form a co-ordination complex, which subsequently allows cyclisation reactions at low temperature, with liberation of methyl bromide. A complete reaction mechanism for blend degradation is proposed. The effect of zinc bromide on poly(vinyl acetate) degradation, again thought to proceed through complex formation, is of a catalytic nature, resulting in liberation, of acetic acid at temperatures much lower than those required for release of acetic acid from poly(vinyl acetate) alone. Chapter Six deals with the effect, on poly(methyl methacrylate) degradation, of zinc oxide, a widely used commercial additive. In this case, the effect is less marked, with zinc oxide promoting some decomposition of methyl methacrylate units at high temperature, accompanied by some reduction to zinc in the process. In overall conclusion, it can be seen that, whereas some small molecules do not affect polymer degradation, a large number exert a significant influence. The type of effect depends on particular polymer/additive selection, but on the basis of Chapters Four and Five, one possible generalisation is that the likelihood of low temperature degradation is increased if polymer-additive complex formation is possible