Stress relaxation in polymer melts following equibiaxial step strain

The nonlinear equibiaxial relaxation modulus for two polymer melts, one with linear and one with branched structure, were measured using the technique of lubricated squeezing flow. The experimental technique was validated for Hencky strains up to ∼1.6 by demonstrating that results are independent of the lubricant viscosity and thickness. Time-strain factorability of the relaxation modulus was observed for both materials. The measured damping functions were compared with tube model (Doi-Edwards) and pom-pom model (McLeish-Larson) predictions. Damping function data for the linear polymer showed reasonably good agreement with the tube model predictions. The relaxation modulus and the damping function of the branched polymer showed significant differences from the pom-pom model predictions.