Thermal degradation of poly(methyl methacrylate) in the range 300–500°C. Changes in mechanism confirmed by trends in molecular weight with conversion

A heterodisperse sample of poly(methylmethacrylate), and several fractions derived from this sample, have been pyrolysed in vacuo at temperatures in the region of 300 and 500°C. The samples were deposited as thin films on both sides of a 260 cm ribbon filament, which was heated resistively, and also boosted to ensure a rapid initial rise in temperature. The initial samples, and the residues after pyrolysis, were analysed by Gel Permeation Chromatography to obtain the trends in molecular weight with conversion. From previous kinetic studies, a change in mechanism is expected over the above temperature range, and on this basis it was predicted that during degradation at 300°C the molecular weight of an unfractionated sample would remain invariant, but at 500°C the molecular weight of such a sample would decrease; on the other hand the molecular weight of fractionated samples is expected to decrease at 300°C and to remain invariant at 500°C. The results are in general agreement with these predictions except that at 500°C only low molecular weight fractions behave in the expected manner. This work therefore confirms the mechanisms postulated from kinetic studies, but in the case of high molecular weight samples at high temperature some qualification of the mechanism has been necessary.