Inverse method for determining general molecular weight distribution from polymer rheology

Abstract

Determination of polymer molecular weight distribution (MWD) from rheological measurements is desirable due to the ease and low cost of rheometry compared to other methods such as gel permeation chromatography. However, relating MWD to rheology requires the inversion of rheological models, for which there is no analytic solution. Prior approaches assume a functional form for the MWD (such as a lognormal or generalized exponential distribution), minimizing the error with respect to the functional form's degrees of freedom. While this is a powerful and robust technique for determining general polymer properties, such as average MWD or polydispersity, it requires former knowledge of the shape of the MWD. This work presents a generalized approach to solving the inverse problem directly, with no former knowledge of the MWD or assumptions regarding its functional form. To close the inverse problem and establish uniqueness, Lagrange multipliers constraints on the MWD are included. The method is applied with reptation-based models to a variety of polycarbonate, polyethylene and polystyrene polymers. For samples whose rheology are well-described by reptation, the predicted MWD is shown to match experimental measurements very well. For samples that are not well-described by reptation, the predicted MWD naturally differs from experiment. Nevertheless, the results still offer insight into how reptation-described polymers differ from their counterparts. This establishes the proposed inverse method as a viable practical tool for rheology-based characterization.