POLYMER DEGRADATION MODIFIED BY BLENDING WITH POLYMERS CHOSEN ON THE BASIS OF THEIR PHI-FACTORS

It is difficult to modify the rate of pyrolysis of a polymer by the addition of conventional free radical initiators, inhibitors or retarders, because these would generally be decomposed or volatilised below the elevated temperatures of interest in degradation work. The objective in the present work is to modify the thermal degradation rate of a polymer by blending it with another polymer to either enhance or reduce the degradation rate. With respect to the latter, it was thought possible that the selection of the blend polymer could be based on the assumption that high PHI-factors for monomer pairs in copolymerisation would imply that the long chain radicals in a degrading blend of the corresponding polymers would preferentially interact, causing mutual termination. To test this idea in the present work, poly(styrene) was used as the host polymer, and poly(methyl acrylate) or poly(butyl acrylate) were chosen on the above basis as the blend polymers. All degradation rates were monitored by pyrolysis-gas chromatography. The results for these systems show that the degradation rates of the added blend polymers are reduced by a factor of about eight, supporting the PHI-factor proposal, and indicating homogeneous kinetics at the pyrolysis temperature (430-degrees-C). However, the results also show that the rate of degradation of the host poly(styrene) is enhanced, so cross-termination is not the principal factor influencing the rate in this case. Hence, the PHI-factor proposal alone is too simplistic. It is proposed that chain transfer processes also have an effect, and play a more important role in determining the degradation rate of the polystyrene in the blends. Further experiments performed with poly(alpha-methyl styrene) as the host polymer provided supporting evidence for this proposal. Consideration is also given to the effect of other factors, such as heterogeneity, on the pyrolysis rates of the blends.