Predicting the linear viscoelastic properties of monodisperse and polydisperse polystyrenes and polyethylenes

Abstract

 For linear homopolymers the linear viscoelastic predictions of the double reptation model are compared to those of a recent, more detailed model, the “dual constraint model” and to experimental data for monodisperse, bidisperse, and polydisperse polystyrene melts from several laboratories. A mapping procedure is developed that links the empirical parameter K of the double reptation model to the molecular parameter τ_e of the dual constraint model, thereby allowing the parameter K to be related to molecular characteristics such as the monomeric friction coefficient ζ. Once K (or τ_e) are determined from data for monodisperse polymers, the double reptation model predicts that for fixed weight-average molecular weight M_w, the zero-shear viscosity η₀ increases slightly with increasing polydispersity M_w/M_n for log normal distributions, while for the dual constraint model η₀ is almost independent of M_w/M_n. Experimental data for polystyrenes show no increase (or even a slight decrease) in η₀ with increasing M_w/M_n at fixed M_w, indicating a deficiency in the double reptation model. The dual constraint theory is also applied to hydrogenated polybutadienes and commercial high-density polyethylenes, where we believe it can be used to indicate the presence of long side branches, which are difficult to detect by other analytic methods.