Zwitterion-containing polymer additives for fouling resistant ultrafiltration membranes

Abstract

Fouling is likely the most important obstacle to the use of membranes in many applications, especially in those that the feed contains high concentrations of organics such as oil and biomacromolecules. Zwitterions, defined as molecules with equal numbers of positively and negatively charged functional groups, show excellent fouling resistance and hydrophilicity. These features can be incorporated into ultrafiltration (UF) membranes during their manufacture by blending a commodity polymer like polyvinylidene fluoride (PVDF) with a copolymer containing zwitterionic groups. This approach can be used directly in existing membrane production systems, with no need for post-processing. Research to date, however, does not provide any guidelines for designing or selecting a zwitterion-containing polymer for this purpose to achieve the best possible performance. In this work, we synthesized copolymers of methyl methacrylate (MMA), whose homopolymer is compatible with PVDF, with two different zwitterionic copolymers, sulfobetaine methacrylate (SBMA) and sulfobetaine-2-vinylpyridine (SB2VP). These copolymers were not previously investigated as surface segregating additives in membrane manufacture. We investigate the impact of different copolymer properties such as zwitterion chemistry, copolymer composition (i.e. zwitterionic/hydrophobic monomer ratio), and blend composition on the performance of membranes manufactured from their blends with PVDF. We report how changing these variables affect the morphology, selectivity, permeance and fouling resistance of membranes, and associate this data with design rules for selecting favorable copolymers. Our study showed that, in contrast to previous literature, increasing the hydrophilic/zwitterionic monomer amount in the additive copolymer does not always result in improved membrane performance. Instead, during membrane formation by non-solvent induced phase separation (NIPS), copolymer additives with high zwitterion content (51–52wt%) undergo macrophase separation from PVDF, and the membrane produced shows poor performance. On the other hand, with the appropriate copolymers that contain 18–19wt% zwitterionic monomer, membranes with significantly higher permeance and remarkable fouling resistance can be attained even with very small amounts of additive copolymer. Zwitterionic additive contents as low as 5wt% in PVDF can lead to membranes with doubled water flux (up to 99L/m2hbar) and complete irreversible fouling resistance against oil suspensions and protein solutions. Only 10wt% additive can yield membranes with even higher flux (up to 165L/m2hbar), and complete resistance to irreversible fouling by an oil suspension in 24-h dead-end fouling experiments. This degree of fouling resistance have not previously been reported for PVDF-based membranes, to our knowledge, and indicates the promise of this membrane modification approach for a wide range of applications.