Sorption and transport of water vapor in thin polymer films at 35°C

Using a gravimetric technique, experimental data at 35degreesC are reported for isothermal sorption equilibria and sorption kinetics of water vapor in poly(N,N-dimethyl methacrylamide) (PDMAA), poly(2-dimethyl aminoethyl methacrylate) (PDMAEMA), poly(acrylic acid) (PAA), and in a typical membrane of a commercial soft-contact lens made of poly(2-hydroxyethyl methacrylate) (PHEMA). The highest sorption of water vapor is in PDMAA. The Flory-Huggins model and the Zimm-Lundberg clustering theory are used to interpret the equilibrium data. Over the entire range of water activity, the four materials show clustering functions larger than -1 indicating that water molecules cluster together in the polymers. The least hydrophilic polymer, PDMAEMA, and the most hydrophilic one, PDMAA, show respectively, the highest and the lowest tendency of water molecules to form clusters. Water diffusion into the polymer matrix is faster in PDMAEMA (diffusion coefficient, D = 10-20 x 10(-8) cm(2) s(-1)) and in PDMAA (D = 4.9-8.7 x 10(-8) cm(2) s(-1)) than in PHEMA-lens material (D = 0.55-3.4 x 10(-8) cm(2) s(-1)) and PAA (D = 0.98-3.5 x 10(-8) cm(2) s(-1)). The measured diffusion coefficients increase with water activity in the PHEMA-lens, decrease with activity in PDMAEMA, and show a more complex concentration dependence in PDMAA and PAA. By writing the diffusion coefficient as a product of an intrinsic mobility, D, and a non-ideality thermodynamic factor, Gamma, the concentration dependence of D is explained by the interplay of two factors: rising plasticization with water activity, which causes an increase in D, and the effect of composition on Gamma, decreasing with water activity.