POLYSTYRENE PYROLYSIS MECHANISMS .1. AS DEDUCED FROM THE DEPENDENCE OF PRODUCT YIELDS ON FILM THICKNESS

Previous work on secondary reactions occurring during polymer pyrolysis is reviewed, and on this basis it was decided that an investigation of the dependence of product yields on sample thickness could provide most information about such secondary reactions. The principles underlying this thickness-dependence approach and its interpretation are discussed in detail. The experimental work consists of a pyrolysis-gas-chromatographic study of polystyrene samples with sizes in the microgram range. This has been performed after depositing the samples from solution on to a thermocouple-filament pyrolyser. The sample masses ranged from 1 to 15 mug, which corresponds to nominal thicknesses of 0.3-4.5 mum. The pyrolysis temperature was 500 +/- 1-degrees in all cases. Eleven products from ethyl benzene to trimer were monitored, and the dependence of the yields of all of these products on sample thickness was investigated. The results show that whilst all products may be formed directly as primary products (parallel reactions), many consecutive processes also occur. These include (i) monomer recombination reactions to produce dimers, (ii) dimer isomerization reactions, collisionally-induced by monomer, and (iii) collisionally-induced trimer decomposition reactions, in which the principal products are believed to be dimeric. On this basis, a general mechanistic scheme for polystyrene pyrolysis can be proposed as an array of parallel and consecutive processes. Mechanisms for the formation of the minor products are speculative at the present stage; the proposals will be modified or confirmed in the light of structural investigations now proceeding.