Degradation on freezing dilute polystyrene solutions in p‐xylene

Abstract

AbstractChain scission was observed during the crystallization of p‐xylene in dilute polystyrene solutions. Degradation yields were determined by gel permeation chromatography, as a function of the number of freeze‐and‐thaw cycles, polymer concentration, and initial polymer molecular weight (M). The rate constant for chain scission Kc increases with the polymer chain length, from 0.021%/cycle at M = 110·103 to 4.7%/cycle at M = 8.5·106. Over the two decades range of investigated molecular weights, Kc follows an empirical scaling law of the form Kc ∼ (M − Mlim)1.17578, where Mlim is a limiting molecular weight ≅ 29,000 g. mol−1 below which no degradation could be induced. Some propensity for midchain scission was detected, although this tendency was much weaker in comparison to flow‐induced degradation.A chain scission model based on crack propagation failed to reproduce the experimental results. To explain the observed dependence of Kc with the square of the radius of gyration, an interfacial stress transmission mechanism between the crystallization fronts and the polymer coil has been proposed. © 1994 John Wiley & Sons, Inc.