Abstract

Diffusion through polymers impacts a wide range of existing applications, and could create new applications for polymers. Diffusion in polymer melts has gained considerable interest, and its industrial importance has triggered the need for faster measurement techniques. Rheological measurements characterize the behavior of polymeric materials, and are an effective tool to study various aspects of diffusion and interdiffusion in molten polymers. The rheological behavior in the molten state of high density polyethylene exposed to carbon dioxide has been probed under small amplitude oscillatory shear, but relatively few papers have been published on this topic. Results show that SAOS accelerates diffusion, but only a limited number of cases have been reported. This observation provides evidence that SAOS accelerates diffusion, and defines new research directions. This study aims at developing a robust experimental technique and accurate analytical and numerical methods to probe diffusion and interdiffusion in molten polymers. Both approaches can be applied to parallel disk rheometry data, to determine fundamental properties governing the diffusion process in polymer melts. Diffusion of small solvent molecules through molten polystyrene, as well as interdiffusion across a binary polystyrene interface, are studied here. We found that applying SAOS accelerates diffusion, even thought there is no large scale net convection of fluid. At constant temperature, the diffusion coefficient is independent of the oscillation frequency, and at temperatures closer to the glass transition temperature, applying SAOS further accelerates the diffusion.