Extensional properties of monodisperse and bidisperse polystyrene solutions

Abstract

A filament stretching rheometer is used to study a series of monodisperse and bidisperse polystyrene solutions and to test predictions of the simplified Mead–Larson–Doi (MLD) “toy” model. This model incorporates convective constraint release, diffusive constraint release, and chain stretching into the reptation-based theory of Doi and Edwards but ignores contour length fluctuations (CLFs). The model parameters are the plateau modulus and the two time constants, the Rouse time and the reptation time for each monodisperse component. In order to artificially incorporate CLFs into the toy model, we relate the constants of the toy model to parameters of the Milner–McLeish model obtained using dynamic data. The predictions of this toy MLD model are in good agreement with experimental data for monodisperse solutions in steady shear, startup of shear, and steady state and transient extensional flow. At high strain rates in extensional flow, good agreement is only obtained if the extensibility of the chains is arbitrarily reduced by a factor of 2 below the molecularly derived value, perhaps reflecting the limitations of a single mode model. Without any additional parameters, the model also yields reasonable agreement with experimental data for bidisperse solutions. The study illustrates the advantages of extensional data in justifying parameter estimates and also the sensitivity of extensional measurements to the high molecular weight component of blends.