Sorption and diffusion data were obtained for water vapor in four different polymers: poly (methylmethacrylate) (PMMA), poly (2hydroxyethylmethacrylate) (PHEMA), poly (N-vinyl-2-pyrrolidone) (PVP) and poly (acrylonitrile) (PAN) at 35 8C using a gravimetric sorption method. Highest sorption was for PVP, followed by PHEMA. PMMA and PAN sorbed very little water. All the polymers exhibit a BET type III sorption isotherm; the large upturn at high activity for PVP and PHEMA is probably due to plasticization of the polymers by water vapor. Sorption data were interpreted using Flory -Huggins theory and the Zimm and Lundberg cluster integral.
Fickian diffusion is observed for PHEMA. For PVP, the fractional uptake M t =M 1 is linear with the square root of the time up to M t =M 1 ¼ 0:6 2 0:8 for all water activities a w ; but it shows a clear water sorption overshoot at a w ¼ 55:3% and a w ¼ 72:1%; probably due to macromolecular relaxation. PMMA sorption kinetics is also characterized by a maximum in the water uptake. The diffusion coefficient increases significantly with water concentration for PVP and PHEMA, weakly for PMMA, but it is independent of concentration for PAN.