The effect of blending on polymer stability: kinetics and mechanisms

A polarising microscope has been used to study the gross homogeneity of a number of polymer blends: PMMA/P alpha MeSt, PMMA/HDPE, and PMMA/PVC. This technique has shown that homogeneity is evident in the PMMA/PaMeSt and the PMMA/PSt blends, but that there is some inhomogeneity in the PMMA/ HDPE and PMMMA/PVC blended systems for samples melted at 200 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 these were compared with those for the homopolymers pyrolysed under identical 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 molecule migration across these phase boundaries. For the systems studied this cross-product formation is very much a secondary effect, because the observed rate of formation of MMA monomer from both heterogeneous blends was not found to be significantly different from that shown by the homopolymer. Pyrolysis of homogeneous blends has been shown to involve interacting mechanisms, which may or may not lead to cross-product formation. However, Hates of formation of major pyrolysis product peaks are found to alter significantly, in some cases by orders of magnitude, and these results have been interpreted in terms of degradation mechanisms. In particular, it has been suggested that the following effects are involved in the degradation mechanisms of homogeneous blends. (a) Cross-termination of depropagating chains. If this is strongly preferred to like-like termination, the effect is to stabilise both components in the blend. (b) INTERmolecular transfer of a hydrogen atom from the first component to a depropagating chain of the second component. The effect of this is to activate the degradation of the first component and to stabilise the second component. (c) Diffusion restriction of INTRAmolecular transfer. If the first component is present as an 'inert diluent' in the vicinity of an end-bite (INTRAmolecular transfer), the facility with which the latter can occur is reduced. This reduces 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 component dilutes the bimolecular termination process of the other, causing some enhancement of the overall rate. (C) 1998 Elsevier Science Limited. All rights reserved.