UV-assisted graft polymerization of N-vinyl-2-pyrrolidinone onto poly(ether sulfone) ultrafiltration membranes:: Comparison of dip versus immersion modification techniques

Two different techniques were used to photochemically modify 50 kDa poly(ether sulfone) (PES) membranes with the monomer N-vinyl-2-pyrrolidinone (NVP) to increase surface wettability and decrease adsorptive fouling during the constant volume diafiltration of 0.1 wt % bovine serum albumin (BSA). The filtration performance of the modified membranes was compared to that of a commercially available PES membrane and a regenerated cellulose membrane. Both the dip and immersion modification techniques produced membranes with essentially the same wettability as regenerated cellulose, a wettability increase of 30% over the base PES membrane. There was a substantial decrease in the irreversible adsorptive fouling of the base membrane as measured by the permanent flux drop after water cleaning with respect; to the initial buffer flux using either modification (from 0.42 to 0-0.09). The immersion-modified membrane with the best performance exhibited no adsorptive fouling, similar permeability, and higher rejection than the regenerated cellulose membrane. Both modification techniques sharply decreased membrane permeability at high monomer concentrations due to pore blockage by grafted polymer chains. The dip-modified membranes exhibited simultaneous loss of BSA rejection and permeability, which suggested that although radiation cleaved PES bonds and enlarged the pores, the high degree of grafted polymer chains on the surface blocked the pores and decreased the permeability. The immersion-modified membranes retained their rejection because the monomer NVP solution was found to absorb up to 88% of the emitted energy, depending on its concentration, thereby protecting the pore structure from intense irradiation. Thus, the dip and immersion techniques are useful for applications where high protein transmission and retention are desired, respectively.