Liquid-Glassy Polymer Diffusion: Rate-Controlling Step and Diffusion Mechanism

Abstract

We investigate the diffusion mechanism at a liquid-glassy polymer interphase, produced in this case between poly(vinyl methyl ether) (PVME) as the liquid polymer and polystyrene (PS) as the glassy matrix. The evolution of the interphase was directly measured by using confocal Raman microspectroscopy in the depth-profiling mode. Diffusion experiments were performed in the range 85125°C, with the specific purpose of encompassing the glass transition temperature (Tg) of the glassy matrix (PS, 100°C). In this way, direct evidence about the effect of the physical state of the (glassy or liquid) PS matrix on the diffusion modes was obtained. We found that the diffusion experiments performed at temperatures below the matrix Tg (liquid-glassy polymer diffusion) are controlled by the same parameters and show the same features as those performed at temperatures above the matrix T g (liquid-liquid polymer diffusion). Furthermore, a Fickean diffusion model developed for liquid-liquid polymer diffusion correlates precisely with the whole set of data, including liquid-glassy polymer diffusion experiments, without invoking case II diffusion theory. It is concluded that the diffusion-controlling step of the process is placed at the liquid PVME-PS interphase. These observations are in marked contrast with interpretations from other authors that used the context of case II to explain the mechanisms that control the evolution of these interphases, an idea often proposed to interpret experimental results for this polymer pair. The origin of the discrepancy is discussed.