Quantitative predictions of polymer melt dynamics.

Abstract

Quantitative prediction of the rheological properties of polymers will become an important tool that connects molecular properties of polymers to their processibility and mechanical properties, thus providing a strong contribution in choosing or tailoring polymers based on the desired criteria. It can also be used to optimize existing processing conditions, i.e. molecular weight, molecular weight distribution, degree of branching, and operating temperature. Our research seeks to provide quantitative predictions of polymers in both linear and nonlinear regimes by extending molecular theories based on the tube theory of Doi and Edwards. For the linear regime, the dual constraint model recently developed by Mead, Van Dyke et al. is extended by inclusion of a new crossover function from the early-time to late-time contour-length fluctuations, constraint-release Rouse relaxation, and fragmented high-frequency Rouse modes, and then evaluated by comparing its predictions with literature data for over 60 different linear and star polymers. In most cases, the predictions of the model are quantitative for monodisperse, bidisperse, and polydisperse linear and star polymers, except at low molecular weights. This capability is particularly useful in allowing us to distinguish between the effects of polydispersity and branching even for highly polydisperse polymers like polyethylenes. For the nonlinear regime, the most recent molecular model was developed by Mead, Larson, and Doi (1998). In our research, we conducted a quantitative comparison of the prediction of the MLD model with that of the Doi-Edwards-Marrucci-Grizzuti (DEMG) models and with the rheological data in start up of steady shearing and uniaxial extensional flows. Our studies confirmed that the convective constraint release included in the orientation equation of the MLD model remedies the extreme shear thinning predicted by the DEMG model. The reptative constraint release of the MLD model also extends the good predictions found for monodisperse polymers to bidisperse systems, especially at steady state. Nevertheless, the drawbacks of the MLD model are that it predicts lower magnitude of stretching in both shear and extensional flows.