Low protein fouling synthetic membranes by UV-assisted surface grafting modification: varying monomer type

The sensitivity of photo-induced grafting and polymerization and the filtration performance of six different grafted monomers [2 neutral (N-2-vinyl pyrolidinone (NVP), 2-hydroxyethyl methacrylate (HEMA)), 2 weak (carboxylic) acids (acrylic acid (AA), 2-acrylamidoglycolic acid (AAG)), and 2 strong (sulfonic) acids (3-sulfopropyl methacrylate (SPMA), 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS))] on poly(ether sulfone) (PES) membranes were measured. The approach was to (i) follow the degree of grafting on commercial PES ultrafiltration membranes with varying pore size (10-300 kDa) as a function of monomer concentration multiplied by irradiation time (the initial linear slopes are termed "sensitivity"), (ii) compare the wettabilities of various grafted membranes (PES-gr-monomer) as a measure of the hydrophilicity of the grafted surfaces, and (iii) contrast their the filtration performances (protein retention, hydrodynamic resistance, and cleanability) with 1 g/l protein (bovine serum albumin, BSA) in phosphate buffer solution (PBS). Although all the grafted and polymerized monomers increased the surface wettability of the PES-grafted membranes over that for the unmodified PES membranes, their effect on filtration performance was different. Using the 50 kDa PES membranes for grafting, membranes with superior performance (high protein retention, high protein solution flux, and low irreversible fouling) were obtained with the NVP, AMPS and AA monomers. Neither commercial regenerated cellulose or PES membranes were as competitive. For larger pore-size membranes (70 and 100 kDa), however, PES-gr-AMPS and PES-gr-AA membranes exhibited reasonably high BSA rejection and protein solution fluxes with excellent cleaning capability (with projected high long-term performance) as compared with the control membranes. Grafting of NVP and HEMA resulted in an initial substantial decrease in BSA rejection due to their tendency to dissolve PES, but with further grafting rejection was recovered. These two monomers also exhibited reduced grafting sensitivity and reduced total hydraulic resistance with increases in wettability (or degree of grafting, DG) as compared with the as-received commercial PES membrane. Grafting sensitivity was inversely proportional to monomer size [(molar volume) (1/3)] for the charged monomers. Also, universal plots of DG versus a composite parameter of monomer concentration multiplied by time of irradiation were obtained and can be used for prediction. We also provide a detailed experimental protocol of how to choose vinyl monomers for preparing high performance ultrafiltration membranes using graft-assisted photo-polymerization and a new figure-of-merit to help compare modifications. (C) 2003 Elsevier B.V. All rights reserved.