Differential constitutive equations for entangled polymer systems are evaluated experimentally. Applicability of three constitutive models proposed by Leonov, Giesekus and Larson, respectively, is investigated based on stress relaxation data for polystyrene melts. Predictions of these three models on strain-dependent relaxation modulus G (t, γ) are comparcd with experimental data obtaincd for samples with narrow and bimodal molecular weight distributions. It is found from the comparison that

more »

... e of these three models can describe the following two experimental results: (1) In entangled polymer systems, there appears a nonlinear relaxation process at short times characterized by a relaxation time τeq, which is shorter than the maximum reiaxation time τ1. (2) Even when linear relaxation spectra of two samples with narrow and bimodal molecular weight distributions have the same shape at long and intermediate times, shape of G (t, γ) curves of the two samples is different at short and intermediate times . These three models cannot give a quantitative description of G (t, γ) at short times , because they neglect the fast relaxation process.