The effect of thermal oxidative degradation of polymers on their viscoelastic response

A model is developed for thermal oxidative degradation of polymer melts and its effect on their viscoelastic response. Based on the theory of temporary heterogeneous networks, stress–strain relations are derived for the time-dependent behavior of a polymer melt at arbitrary three-dimensional deformations with small strains. Within the fragmentation–annihilation concept, kinetic equation is proposed for the distribution of strands with various lengths in a network, and its explicit solution is obtained. Material parameters in the constitutive equations are found by fitting experimental data in shear oscillation tests on a polypropylene melt annealed at elevated temperatures for various times. Good agreement is demonstrated between the observations and the results of numerical simulation.