Water diffusion in poly(vinyl alcohol) membranes: A rigorous analysis of the pervaporation process

In the design of crosslinked polymer elements for separation and fuel cell technology, controlled drug delivery devices or micro-scale sensor-actuator systems, profound knowledge of mass transport in the polymer network is of substantial interest for a systematic ab initio evaluation of separation efficiency, drug release rates and response characteristics. The study of solvent transport in polymeric membranes is frequently based on the pervaporation process, analysing the flux of one or more permeants under well-defined boundary conditions. Yet, for quantitative analysis, assumptions such as concentration-independent diffusion coefficients and neglect of gas-phase boundary layer resistances and local mass transfer effects are common, but remain largely unverified as information on the solvent distribution in the membrane is not usually available. The present study therefore introduces an advanced experimental methodology and combines the routine permeation flux measurement with the determination of local concentration profiles in the membrane, allowing diffusion coefficients to be specified directly as a function of solvent content. For physically crosslinked poly(vinyl alcohol) membranes, it could thus be demonstrated that water transport is successfully described by pure Fickian diffusion with an exponential expression accounting for the concentration-dependent solvent diffusion coefficient.