Permeation of water through polar and nonpolar polymers and copolymers: Determination of the concentration-dependent diffusion coefficient

The diffusion and permeation properties of liquid water through different p olar and nonpolar polymers and copolymers were studied with a highly sensit ive permeameter. The transient permeation fluxes through the polar polymer films could be fitted well only with an exponential equation for the diffus ivity concentration dependence; this empirical exponential equation represe nted the diffusion plasticization effect of water on the materials. For the hydrophobic polyolefins, this exponential equation was no longer valid, an d another form of the equation was empirically found to account for the red uction of the water diffusivity with the extent of the permeation. Such a n egative plasticization effect might be attributed to the formation of water clusters in the polyolefins. The values of the diffusion coefficient of wa ter in the dry polar polymers were smaller than those in dry polyolefins, b ut the opposite behavior was found for the permeability because it was much more favorable for water sorption in the polar polymers than in the hydrop hobic polyolefins. For the ethylene-vinylacetate copolymers, the plasticiza tion effect of water on its own diffusion was negative for the sample with a low vinyl acetate (VA) content; it became nil at 19 wt % VA and positive at higher VA contents. This increase in the extent of the water sorption wi th the increase in the VA content led to a steady increase in the water per meability in the poly(ethylene-co-vinylacetate) copolymers. (C) 2000 John W iley & Sons, Inc.