Impact of gel morphology on pore-filled membranes

Abstract

<p>Pore-filled cation exchange membranes were prepared by the co-polymerization of acrylic acid and N,N' -methylenebisacrylamide within the pores of a microporous polypropylene substrate. The poly(acrylic acid) gels in the membranes exhibited heterogeneity when synthesized with 1,4-dioxane, a poor solvent for poly(acrylic acid), or with increase in degree of cross-linking. Corresponding bulk gels were prepared and revealed heterogeneity as the volume percent of 1,4-dioxane was increased in the DMF mixture, or as the cross linking ratio was increased. Confocal laser scanning microscopy revealed a globular morphology of both the bulk gel and the pore-filling gel in the membrane when formed in a poor solvent. However, no difference in morphology was detected with the change in degree of cross-linking. The degree of swelling of the membranes was consistent with a change in gel morphology associated with change in reaction solvent or degree of cross-linking. The hydrodynamic (Darcy) permeabilities of the membranes were determined by measuring the fluxes at various pressures. The pure water-fluxes were found to be dependent on the mass-loading and morphology of the gel within the membranes. It was found that the permeability of the membranes decreased with increasing polymer volume fraction. The membranes were evaluated in terms of their nanofiltration capabilities with 5 mM solutions of NaCl. The extended Nemst-Planck equation, which takes account of the effective charge densities, was used to model rejection of single electrolyte solutions. The separation properties of the PAA membranes are good and the salt rejection was found to be practically constant, at constant pressure, over a wide range of gel concentrations. However, the performance was affected by the morphology of the gel. Asymmetric membranes were synthesized by photopolymerization of acrylic acid within a porous substrate. The monomer solution contained 2,2 ' -dihydroxy-4,4' -dimethoxybenzophenone (DDB), a UV absorber, which controlled the penetration depth of UV radiation. (Abstract shortened by UMI.)</p>