Photoinduced grafting of ultrafiltration membranes: comparison of poly(ether sulfone) and poly(sulfone)

Abstract

Using UV-assisted graft polymerization of three hydrophilic monomers, N-vinyl-2-pyrrolidinone (NVP), 2-acrylamidoglycolic acid monohydrate (AAG) and 2-acrylamido-2-methyl-1-propanesulfonic acid (AAP), both poly(ether sulfone) (PES) and poly(sulfone) (PSf) 50 kDa ultrafiltration (UF) membranes were modified using the dip method with 300 nm wavelength lamps. The concentration of monomer and irradiation energy supplied to the membranes was varied. The modified membranes were characterized physicochemically by their degree of grafting using attenuated total reflection-Fourier transform infrared spectra (ATR-FTIR) and by their wettability using a captive air bubble technique. They were also characterized by their filtration performance with a 0.1 wt.% bovine serum albumin (BSA) solution in phosphate buffered saline (PBS) at 20±2°C and at pH 7.4. The filtration protocol gave several measures of performance including the initial water permeability, the protein solution permeability and the water permeability after flushing with DI water. Four different modification conditions were found that gave modified UF membranes with superior filtration performance than the base unmodified PES, the base unmodified PSf, or a regenerated cellulose (RC) control membrane. Slightly compromised protein solution permeabilities were compensated for with low fouling modified membranes that exhibited excellent cleaning characteristics. All the best cases were at the highest monomer concentrations (5 wt.%) and lowest irradiation energy (<65 mJ/cm 2 for PES and <130 mJ/cm 2 for PSf). This work also suggests that low degrees of grafting (DG<0.53) and intermediate wettabilities ( 0.74< cos θ<0.82 ) were sufficient to obtain attractive non-fouling membranes. Since BSA is strongly negatively charged at pH 7.4, it is not surprising that AAP was the monomer that exhibited best performance for two of the four best cases. An important finding was that PES is far more sensitive to UV-assisted graft polymerization and, thus, requires far less energy to attain a desired degree of grafting than PSf.