Water transport study in crosslinked poly(ethylene oxide) hydrogels as fouling-resistant membrane coating materials
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The major objective of this research is a systematic experimental exploration of hydrophilic materials that can be applied as coating materials for conventional ultrafiltration (UF) membranes to improve their fouling resistance against organic components. This objective is achieved by developing new, fouling-reducing membrane coatings and applying these coatings to conventional UF membranes, which can provide unprecedented reduction in membrane fouling and marked improvements in membrane lifetime. Novel polymeric materials are synthesized via free-radical photopolymerization of mixtures containing poly(ethylene glycol) diacrylate (PEGDA), photoinitiator, and water. PEGDA chain length (n=10-45, where n is the average number of ethylene oxide units in the PEGDA molecule) and water content in the prepolymerization mixture (0-80 wt.%) were varied. Crosslinked PEGDA (XLPEGDA) exhibited high water permeability and good fouling resistance to oil/water mixtures. Water permeability increased strongly with increasing the water content in the prepolymerization mixture. Specifically, for XLPEGDA prepared with PEGDA (n=13), water permeability increased from 0.6 to 150 L um/(m2 h bar) as prepolymerization water content increased from 0 to 80 wt.%. Water permeability also increased with increasing PEGDA chain length. Moreover, water permeability exhibits a strong correlation with equilibrium water uptake. However, solute rejection, probed using poly(ethylene glycol)s of well defined molar mass, decreased with increasing prepolymerization water content and increasing PEGDA chain length. That is, there is a tradeoff between water permeability and separation properties: Materials with high water permeability typically exhibit low solute rejections, and vice versa. The fouling resistance of XLPEGDA materials was characterized via contact angle measurements and static protein adhesion experiments. From these results, XLPEGDA surfaces are more hydrophilic in samples prepared at higher prepolymerization water content or with longer PEGDA chains, and the more hydrophilic surfaces generally exhibit less BSA accumulation. These materials were applied to polysulfone (PSF) UF membranes to form coatings on the surface of the PSF membranes. Oil/water crossflow filtration experiments showed that the coated PSF membranes had water flux values 400% higher than that of an uncoated PSF membrane after 24 h of operation, and the coated membranes had higher organic rejection than the uncoated membranes.