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 (51-52 wt%) undergo macrophase separation from PVDF, and the product membrane shows poor performance. On the other hand, with the appropriate copolymers that contain 18-19 wt% 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 5 wt% in PVDF can lead to membranes with doubled water flux (up to 99 L/m 2 .h.bar) and complete irreversible fouling resistance against oil suspensions and protein solutions. Only 10 wt% additive can yield membranes with even higher flux (up to 165 L/m 2 .h.bar), and complete resistance to irreversible fouling by an oil suspension in 24-hour 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.

Electrospun fiber membranes from blends of poly(vinylidene fluoride) with fouling‐resistant zwitterionic copolymers

“…The spectra of pure copolymers, C‐SB2VP and C‐SBMA, are shown as the topmost spectra, and display almost identical features. Two characteristic peaks appear for both copolymers at around 1146 and 1724 cm −1 , corresponding to SO asymmetric stretch and OCO ester carbonyl stretch, respectively. In agreement with the literature, as‐spun fibers that contain PVDF display strong characteristic crystal peaks of β ‐phase (1275 cm −1 ), with weak α ‐phase (614 and 763 cm −1 ) and γ ‐phase (811 and 1234 cm −1 ).…”

“…Oil Red O was purchased from Sigma Aldrich (St Louis, Missouri). Poly(MMA‐ random ‐SBMA) and poly(MMA‐ random ‐SB2VP) were synthesized using free radical polymerization following protocols found in previous studies . These two copolymers will be referred to by C‐SBMA and C‐SB2VP, which stand for “copolymer poly(MMA‐ ran ‐SBMA)” and “copolymer poly(MMA‐ ran ‐SB2VP)”, respectively.…”

“…For PVDF blends with C‐SB2VP, solutions were obtained with 100, 95, 90, 85, 80 and 0 wt% PVDF of the total polymer blend content. For PVDF blends with C‐SBMA, only 100, 95, 90, 85 and 0 wt% PVDF formed well‐dissolved solutions, due to relatively lower miscibility between PVDF and SBMA‐containing copolymers . For all polymer blend solutions, first the PVDF pellets were dissolved in DMAC, using magnetic stirring for several hours at 90 °C.…”

“…This method enables the formation of fouling‐resistant membranes without any added manufacturing steps. Various groups have applied this approach using zwitterionic groups as the hydrophilic, fouling‐resistant repeat units in the copolymer . Of these, only one focused on membranes prepared from PVDF, the membrane material of choice for many applications due to its chemical resistance and mechanical properties.…”

A Comprehensive Review on Polymeric Nano-Composite Membranes for Water Treatment

“…Metal cations used for membrane preparation used were non-toxic, environment friendly and cheap [188][189][190][191][192][193][194]. Their results revealed that when metal cations are incorporated within GO, resulted membranes were flexible with metal cations having no effect on GO membranes mechanical stability.…”