Variable permeability membranes: Network structure of poly (methacrylic acid) and its relation to diffusive transport

This paper focusses on the crosslink density and solvent-polymer interaction parameter of uncharged PMAA membranes. Crosslink densities and the solvent-polymer interaction parameter were obtained for a series of PMAA membranes which differed only in the ratio of crosslinker to monomer at the time of polymerization and crosslinking. Tensile force versus elongation, as well as polymer volume fraction in both the relaxed state (i.e. at the time of polymerization and crosslinking) and the swollen state (but not ionized - e.g. pH 3, 0.1M KCl) were measured. The crosslink density and interaction parameter for each of the membranes was calculated from the tensile force and polymer volume fraction data, using the method of Meissner and Janacek. The results were used to relate crosslink density to membrane permeability, which had previously been measured as a function of membrane stochiometry. For swollen state polymer volume fractions v_s between 0.251 and 0.377 at 23.3°C, we found that the interaction parameter χ₁ could be approximated by a quadratic dependence on υ_s. The χ₁ values obtained in this study are consistent with literature values for PMAA which had been polymerized and crosslinked in solutions; this χ₁, can be used in the Flow swelling equation to calculate the crosslink density of PMAA gels whose equilibrium polymer volume fractions are known to be within the range spanned by the tensile measurements. However, the extreme sensitivity of the calculated crosslink density to small changes in χ₁ makes it difficult to use the swelling equation to calculate crosslink density without tensile measurements that cover the polymer volume fraction range of interest.