A Study of Case II Transport by Laser Interferometry

Abstract

Laser interferometry is found to be a very convenient technique for studying gradient diffusion in the poly(methyl methacrylate)/methanol (PMMA/MeOH) system. We studied the process in initially dry, supported PMMA films suddenly immersed in liquid MeOH and examined the effects of temperature and sample thermal history. Over the range of conditions studied, the diffusion process was entirely Case II. A simple model was developed to predict the time dependence of the intensity of light reflected from a supported film during Case II. An FFT algorithm made it possible to accurately match the reflected intensity data and the model predictions to determine the key parameters defining the Case II process: The volume fraction of methanol behind the Case II front (ϕe), the front velocity (v), and the front reflectivity (rf2). Free-volume theory describes the temperature dependence of v very well and enables calculation of the individual apparent activation energies for methanol diffusion in and viscous flow of the dry glassy polymer. The polymer sample's thermal history has a large effect on v: Samples quenched through Tg2 faster show higher v with a lower apparent activation energy; free-volume theory can describe this thermal history effect fairly well. Over the whole range of conditions examined, we observed rf2 at least an order of magnitude smaller than that calculated from the Fresnel formula for a true discontinuity in fluid concentration, proving that the sharp Case II front has a finite width on the order of 100 nm. However, the current theory for Case II cannot fully explain the trends found in rf2 with temperature or with the sample's thermal history. © 1995, American Chemical Society. All rights reserved.