Study of molten isotactic polystyrene. II. The kinetics of thermal degradation from melt viscosity data

Abstract

Abstract Starting from the established relation: log η = A + 3.4 log M̄ v , the kinetics of thermal degradation of isotactic polystyrene has been investigated in terms of melt viscosities measured as a function of melting time at constant temperatures between 265 and 320°C. Random chain scission predominates over radical chain depolymerization and can be described by a zero‐order reaction with an activation energy E of about 39 kcal./mole. At the same time, the kinetic plots permit evaluation of the temperature dependence of melt viscosity without interference by degradation. This leads to an activation energy for viscous flow E′ of 20 ± 2 kcal./mole. As the same value holds for atactic polystyrene; viscous flow seems not to be influenced by the stereoregularity of polystyrene. In addition to the above activation energies only a single run of melt viscometry with 1–2 g. of a new sample of isotactic polystyrene is needed for a quantitative prediction of its melt viscosity at any practical melting time and temperature.