ANALYSIS OF THE KINETICS OF VAPOR ABSORPTION DESORPTION IN/FROM SILICONE-RUBBER AND CELLULOSE-ACETATE MEMBRANES IN THE PRESENCE OF STAGNANTBOUNDARY-LAYERS/

The application of an interferometric technique (optical thickness met er, OTM) to the measurement of vapor sorption kinetics in both rubbery and glassy polymers is presented. In this technique, the membrane is formed by casting on a suitable glass surface and interferometry is ap plied in situ. The use of a carrier gas loaded with penetrant vapor in troduced a stagnant boundary layer (SBL) effect which had to be correc ted, in order to determine true sorption kinetics. The said SBL effect was estimated, on the basis of existing theory for the silicone rubbe r-methylene chloride (SR/MC) system and found to be more pronounced in the case of desorption. Upon correction for this effect, Fickian sorp tion curves were obtained; which yielded nearly constant values of the diffusion coefficient, not materially different for absorption and de sorption, in line with theoretical expectation. Cellulose acetate-meth ylene chloride (CA/MC) was then studied as an example of a glassy poly mer-vapor system, where the SBL effect distorts the absorption kinetic curve in the same way as the non-Fickian mechanism of sorption inhere nt in this kind of polymer-penetrant system. Here, the vapor sorption data were corrected using the results obtained from the Fickian SR/MC system. The corrected results were checked by comparison with independ ent data reflecting the true kinetic behavior of CA/MC, obtained with a vacuum balance apparatus (VBA), which is free of SBL effects. It is shown that this novel method of applying the SBL correction was reason ably successful in favorable circumstances, while a criterion is provi ded to identify cases where reasonably reliable correction is not poss ible.