Surface modification of polysulfone ultrafiltration membranes and fouling by BSA solutions

Five different chemically modified versions of polysulfone were prepared via two different homogeneous chemical reaction pathways. They, together with the base polymer, were cast as membranes by a phase-inversion process. The surface energies of these membranes, as measured by contact angles, were used to characterize the different membranes. Streaming-potential measurements were obtained to probe the surface charge of the membranes. The surface roughness of each membrane was also determined by atomic-force microscopy. Each membrane was then exposed to deionized water, 0.08 g/1 bovine serum albumin solution and deionized water using a standard filtration procedure to simulate protein fouling and cleaning potential. Both the chemistry and the size of the grafted molecules were correlated with respect to volumetric flux during ultrafiltration of protein solutions. Surface roughness seemed to be important for filtering pure water. Hysteresis between advancing and receding contact angles increased with hydrophilicity of the membrane surfaces. One possible explanation could be that surface reorientation was more likely with hydrophilic than with hydrophobic membranes. The membrane modified by direct sulfonation had the lowest surface energy and the shortest grafted chain length and exhibited the highest volumetric flux with BSA solution. It was also the easiest to clean and exhibited the highest initial flux recovery by stirring (91%) and backflush (99%) methods with deionized water. In most cases, backflushing rather than stirring was more effective in recovering the water flux.