Nanofiltration using pore-filled membranes: effect of polyelectrolyte composition on performance

Several series or membranes composed of microporous poly(propylene) substra tes filled with polyelectrolyte gels of different chemical structure, polym er concentration and charge densities have been prepared in order to examin e the effect of polyelelctrolyte composition on the performance of these me mbranes in nanofiltration. The membranes were made by two different routes involving either in situ chemical cross-linking of poly(4-vinylpyridine) or poly(vinylbenzyl chloride), or by in situ polymerization of acrylic acid w ith tetra(ethylene glycol) diacrylate or N,N-methylenebisacrylamide as cros s-linking agents in the pores of the substrates. The resulting pore-filled membranes Were characterized by the concentration (volume fraction) of the polyelectrolyte in the pores, ion exchange capacity (charge density), water content, and thickness. The different series of membranes were tested unde r pressure to determine their hydrodynamic permeabilities and salt separati on properties (NaCl). It was found that there was a good correlation betwee n hydrodynamic (Darcy) permeability and gel polymer concentration that hold s irrespective of the gel polymer chemistry in the pore-filling gels. Membr anes with different pore-filling gels, whether positively or negatively cha rged, followed the same relationship. The separation properties of the memb ranes are very good and the salt rejection was found to be practically cons tant over a wide range of permeabilities (gel concentrations). It Was also round that the increase in the nominal charge density above approximately 0 .2 mmol/cm(3) of the swollen gel had a negligible effect on the separation properties of the gel-filled membranes. The results of this study provide a basis for the further design and optimization of polyelectrolyte filled me mbranes for nanofiltration applications. (C) 2001 Elsevier Science B.V. All rights reserved.