Flow-enhanced nucleation of poly(1-butene): Model application to short-term and continuous shear and extensional flow

Abstract

A modeling framework for flow-enhanced nucleation of polymers is applied to a broad set of data from literature. Creation of flow-induced pointlike nuclei is coupled to chain stretch of the high-molecular weight tail of the material, calculated with a rheological constitutive model. As the flow-induced nuclei grow, the crystalline volume fraction increases and with it the viscosity of the material. This is accounted for by describing the material as a suspension of spheres in a viscoelastic matrix. Calculations are compared with a broad set of experimental data from literature on three grades of poly(1-butene). First, a parameter set is determined by fitting model results to flow-induced nucleation densities from short-term shear experiments. Next, this parameter set is used to validate the framework in continuous flow experiments in which viscosity is monitored during a constant flow rate. In this way, we demonstrate the approach is applicable to not only short-term shear but also continuous flow. It was observed in experiments that for continuous extensional flow, the viscosity shows an upturn at a constant strain, the value of which is independent of strain rate. We hypothesize that this upturn is related to long chains entering the chain stretch regime, as a result of the extension rate exceeding the inverse of the Rouse time of the longest chains.