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Biofouling-resistant polymer hydrogel coating for desalination membranes
Applying a hydrophilic coating to the membrane surface is an efficient and promising strategy to control membrane fouling. This work explored the use of a hydrogel coating with a high swelling as a means to reduce the propensity to biofouling.
Bulk poly(ethylene glycol) methyl ether methacrylate (PEGMEMA) hydrogels were prepared in water using macromonomers of three different molecular weights via redox free radical polymerization. To optimize gel composition, the effects of PEGMEMA, initiator and crosslinker concentrations on gel yield and swelling properties were studied. It was found that swelling of polyPEGMEMA gels in water decreases with increasing PEGMEMA concentration and increasing cross-linker content while the effect of these two parameters was opposite corresponding to gel yield. In addition, the chemical structure of the gels was characterized by FTIR and solid state NMR spectroscopy. It showed that the gels contain a small fraction of incompletely reacted cross-linker which increases with the length of the PEG side chain. Rheological measurements are employed to examine gelation time as well as the mechanical strength and mesh size of the gels as a function of synthesis conditions. Mesh sizes were also estimated using swelling data. Sorption experiments using proteins of three different sizes were performed to identify the relationship between the microstructure of the hydrogels and protein adsorption on the surface and its penetration into the hydrogel network. The swelling and rheological behaviors of hydrogels as well as protein partitioning into the gels are discussed in terms of the network mesh size. All results can be interpreted so that PEG side chains with the highest average molecular weight (1000 g/mol) have an additional effect onto physical cross-linking while the networks for the hydrogels with shorter PEG side chain are dominated by chemical cross-linking.
For the preparation of surface-anchored gel, the silanization of glass was used to introduce methacrylate groups and then performed via in situ cross-linking polymerizations in a mold. Contact angle measurement was employed to identify interfacial adaptation between surface anchored hydrogels in water as function of preparation conditions. The bacterial deposition experiments were performed with Pseudomonas fluorescens strain under well-defined laminar flow conditions. The deposition results revealed an interesting non-monotonic dependence on swelling which at the same degree of crosslinking was lower with higher molecular weight of the PEG side chains. The highest efficiency of the gels comprising relatively long PEG chains was attributed to the complex effect of the side chain length. Accelerated biofilm development (biofouling) tests after bacterial attachment in the parallel plate setup also showed that the biofilm growth on polyPEGMEMA gels was strongly inhibited as compared to the hydrophilic glass surface used as reference, which was well correlated with deposition experiments.
The approach of using macroinitiator layer mediated anchored gel layer on membrane was explored to reduce/avoid homopolymerization in bulk solution and prevent delamination of gel layer from membrane surface. The cationic photoreactive macroinitiator based on poly(2-dimethylamino-ethyl methacrylate-co-2-hydroxyethyl methacrylate) comprising photoinitiator side groups was electrostatically adsorbed onto oppositely charged membrane surface. Subsequently hydrogel layers were prepared via surface initiated in situ graft and cross-linking photopolymerization from the membrane surface. Two commercial desalination membranes, NF270 composite membrane with the top layer of polyamide and NTR7450 membrane made from sulfonated polyethersulfone, were used as base membrane to evaluate the effect of surface chemistry of membrane in macroinitiator adsorption and subsequent photopolymerization. The protocols for grafted gel layers were based on procedures developed earlier for bulk gels. The modification degree and its effect on the membrane properties was characterized with respect to membrane chemistry by ATR-IR spectroscopy, surface charge by zeta potential, surface wettability by contact angle, and with respect to pure water permeability and salts rejection measurements as well as propensity to protein fouling. Zeta potential measurements showed an effective reduction in the net surface charge of gel modified membranes relative to pristine membranes. Water permeability decreased and salt rejection increased with degree of grafting. Gel modified membranes demonstrated improved fouling resistance compared to pristine membranes.
In conclusion, this work developed the bulk polyPEGMEMA gels with the low protein sorption and low propensity to bacterial deposition and grafted these gel layers on NF membranes, to yield an efficient antibiofouling surface.