Thermal degradation of polymer blends: polystyrene/PMMA blend behaviour related to the copolymerization Φ-factor for this system

Abstract

It has been found that blending polystyrene (PSt) with poly(methyl methacrylate) (PMMA) modifies the thermal degradation behaviour of each of the corresponding polymers. Thus, during pyrolysis at 500 °C, rate constants for the evolution of MMA and St monomers from the blend were each found to be smaller by a factor of about 10 than the corresponding rate constants measured using the individual polymers. The explanation proposed for this is that the ‘unlike’ long chain radicals in the blend preferentially interact, causing mutual termination of the depropagating chains. This preference for cross-termination is consistent with the high Φ-factor reported from the kinetic studies of the copolymerization of this monomer pair at much lower temperatures. This pyrolysis behaviour of the PSt/PMMA blend systems differs from that of poly(alkyl acrylate)/polystyrene blend systems, for which it is reported that chain transfer plays a more important role than cross-termination in the degradation mechanism. For the latter systems, the evidence quoted for the transfer mechanism was that no kinetic effects were observed when the poly(alkyl acrylate) was blended with poly(α-methyl styrene), in which there is no tertiary hydrogen atom available for transfer. Blends of poly(α-methyl styrene) with PMMA have also been studied in the present work, and again only a small effect has been observed; the degradation rates are comparable with those measured for the individual polymers. The implications of this is that the poly (α-methyl styrene) radical cannot cross-terminate with the same facility as the polystyryl radical, presumably because of additional steric hindrance from the methyl group. Actually there is a few percent enhancement of the degradation rates in the poly(α-methyl styrene) blends, and this is attributed to the blended polymer diluting the bimolecular termination reactions. Conversely there are orders of magnitude reductions of the rates of styrene oligomer formation in the PSt/PMMA blends. These are associated with the PMMA ‘solvent’ in the melt causing steric hindrance of those intramolecular (backbiting) transfer reactions of the polystyryl radical which lead to oligomer formation. All rate measurements in this work have been made by the pyrolysis-g.c. technique.