THERMAL-DEGRADATION OF POLYMER BLENDS - POLYSTYRENE PMMA BLEND BEHAVIOR RELATED TO THE COPOLYMERIZATION PHI-FACTOR FOR THIS SYSTEM/

It has been found that blending polystyrene (PSt) with poly(methyl met hacrylate) (PMMA) modifies the thermal degradation behaviour of each o f the corresponding polymers. Thus, during pyrolysis at 500 degrees C, rate constants for the evolution of MMA and St monomers from the blen d were each found to be smaller by a factor of about 10 than the corre sponding rate constants measured using the individual polymers. The ex planation proposed for this is that the 'unlike' long chain radicals i n the blend preferentially interact, causing mutual termination of the depropagating chains. This preference for cross-termination is consis tent with the high phi-factor reported from the kinetic studies of the copolymerization of this monomer pair at much lower temperatures. Thi s pyrolysis behaviour of the PSt/PMMA blend systems differs from that of poly(alkyl acrylate)/polystyrene blend systems, for which it is rep orted that chain transfer plays a more important role than cross-termi nation in the degradation mechanism. For the latter systems, the evide nce quoted for the transfer mechanism was that no kinetic effects were observed when the poly(alkyl acrylate) was blended with poly(a-methyl styrene), in which there is no tertiary hydrogen atom available for t ransfer. Blends of poly(cr-methyl styrene) with PMMA have also been st udied in the present work, and again only a small effect has been obse rved; the degradation rates are comparable with those measured for the individual polymers. The implications of this is that the poly(a-meth yl styrene) radical cannot cross-terminate with the same facility as t he polystyryl radical, presumably because of additional steric hindran ce from the methyl group. Actually there is a few percent enhancement of the degradation rates in the poly(cr-methyl styrene) blends, and th is is attributed to the blended polymer diluting the bimolecular termi nation reactions. Conversely there are orders of magnitude reductions of the rates of styrene oligomer formation in the PSt/PMMA blends. The se are associated with the PMMA 'solvent' in the melt causing steric h indrance of those intramolecular (backbiting) transfer reactions of th e polystyryl radical which lead to oligomer formation. All rate measur ements in this work have been made by the pyrolysis-g.c. technique. (C ) 1997 Elsevier Science Limited.