THE EFFECT OF BLENDING ON POLYMER STABILITY - KINETICS AND MECHANISMS

A polarising microscope has been used to study the gross homogeneity o f a number of polymer blends: PMMA/P alpha MeSt, PMMA/HDPE, and PMMA/P VC. This technique has shown that homogeneity is evident in the PMMA/P aMeSt and the PMMA/PSt blends, but that there is some inhomogeneity in the PMMA/ HDPE and PMMMA/PVC blended systems for samples melted at 20 0 degrees C and then cooled to room temperature. The blends contained 1:1 (w:w) parts of each component. Pyrolysis-gas chromatography (py-g. c.) and pyrolysis-gas chromatography-mass spectrometry (py-g.c.-m.s.) have been used to measure the rate constants for the evolution of the principal pyrolysis products from these blends at 500 degrees C, and t hese were compared with those for the homopolymers pyrolysed under ide ntical conditions. The results show that heterogeneous blends degrade predominantly within their phase-separated regions, but may also give rise to some cross-products, which may form by small radical or molecu le migration across these phase boundaries. For the systems studied th is cross-product formation is very much a secondary effect, because th e observed rate of formation of MMA monomer from both heterogeneous bl ends was not found to be significantly different from that shown by th e homopolymer. Pyrolysis of homogeneous blends has been shown to invol ve interacting mechanisms, which may or may not lead to cross-product formation. However, Hates of formation of major pyrolysis product peak s are found to alter significantly, in some cases by orders of magnitu de, and these results have been interpreted in terms of degradation me chanisms. In particular, it has been suggested that the following effe cts are involved in the degradation mechanisms of homogeneous blends. (a) Cross-termination of depropagating chains. If this is strongly pre ferred to like-like termination, the effect is to stabilise both compo nents in the blend. (b) INTERmolecular transfer of a hydrogen atom fro m the first component to a depropagating chain of the second component . The effect of this is to activate the degradation of the first compo nent and to stabilise the second component. (c) Diffusion restriction of INTRAmolecular transfer. If the first component is present as an 'i nert diluent' in the vicinity of an end-bite (INTRAmolecular transfer) , the facility with which the latter can occur is reduced. This reduce s the possibility of oligomer formation. (d) Diffusion restriction of termination. Hf the two components in a homogeneous blend depropagate independently and cross-termination is not favoured, then each compone nt dilutes the bimolecular termination process of the other, causing s ome enhancement of the overall rate. (C) 1998 Elsevier Science Limited . All rights reserved.