Photografted thin polymer hydrogel layers on PES ultrafiltration membranes: characterization, stability, and influence on separation performance

UV-photo graft functionalization of polyethersulfone membrane with strong polyelectrolyte hydrogel and its application for nanofiltration

A strong polyelectrolyte hydrogel was graft copolymerized on a polyethersulfone (PES) ultrafiltration (UF) membrane using vinyl sulfonic acid (VSA) as the functional monomer, and N,N'-methylenbisacrylamide (MBAA) as the cross-linker monomer. This was carried out in one simple step using the UV photoirradiation method. The effect of the polymerization conditions on the degree of grafting (DG) was investigated using the gravimetric method which measures the total hydrogel grafted on the membrane, and with ATR-FTIR spectroscopy which indicates the functional monomer fraction in the hydrogel layer. The VSA could not graft polymerize without the cross-linker as comonomer. An increase in the cross-linker fraction from 0.25 to 2.5 mol % (relative to the functional monomer VSA) resulted in a higher DG. Although the surface morphology changed upon modification, the resulting surface roughness as measured by AFM was very low. From the monitoring of DG with UV time (4.5-30 min) at constant conditions, it was deduced that during the early stages of the polymerization mainly the cross-linker was grafted, thus inducing the graft copolymerization of the functional monomer. Polymerization using a higher monomer concentration (12.5-40% VSA) at constant monomer/cross-linker ratio resulted in a higher VSA fraction in the grafted hydrogel, although the gravimetric DG was similar. Ion exchange capacity and X-ray photoelectron spectroscopy measured after modification under the different conditions supported these findings. The new membranes were tested under nanofiltration (NF) conditions. A NF membrane could be obtained when the MBAA fraction was above 0.25%. The Na2SO4 rejection was 90-99% and the permeability 10-1 L m(-2) h(-1) bar(-1) when the MBAA fraction increased from 0.75 to 2.5%. The order of rejection of single salts solution was Na2SO4 > MgSO4 ≈ NaCl > CaCl2, as expected on the basis of Donnan exclusion for negatively charged NF membranes. An increase in the salts rejection with increasing degree of cross-linking and VSA fraction was attributed to an increase in the membrane charge density and to steric exclusion that also resulted in an increase of rejection for uncharged solutes such as sucrose or glucose. The new membrane presented a high, essentially unchanged Na2SO4 rejection (>97%) in the range of salt concentrations up to 4 g/L, and only slightly reduced rejection (>92%) at a concentration of 8 g/L; this can be related to its high barrier layer charge density measured by ion exchange capacity. In addition, because poly(vinyl sulfonic acid) (PVSA) is a strong polyelectrolyte the membrane separation performance was stable in the range of pH 1.5 to pH 10.

Biocompatibility of modified polyethersulfone membranes by blending an amphiphilic triblock co-polymer of poly(vinyl pyrrolidone)-b-poly(methyl methacrylate)-b-poly(vinyl pyrrolidone)

An amphiphilic triblock co-polymer of poly(vinyl pyrrolidone)-b-poly(methyl methacrylate)-b-poly(vinyl pyrrolidone) (PVP-b-PMMA-b-PVP) was synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization. The block co-polymer can be directly blended with polyethersulfone (PES) using dimethylacetamide (DMAC) as the solvent to prepare flat sheet and hollow fiber membranes using a liquid-liquid phase separation technique. The PVP block formed a brush on the surface of the blended membrane, while the PMMA block mingled with the PES macromolecules, which endowed the membrane with permanent hydrophilicity. After adding the as-prepared block co-polymer the modified membranes showed lower protein (bovine serum albumin) adsorption, suppressed platelet adhesion, and a prolonged blood coagulation time, and thereby the blood compatibility was improved. Furthermore, the modified PES membranes showed good cytocompatibility, ultrafiltration and protein anti-fouling properties. These results suggest that surface modification of PES membranes by blending with the amphiphilic triblock co-polymer PVP-b-PMMA-b-PVP allows practical application of these membranes with good biocompatibility in the field of blood purification, such as hemodialysis and bioartificial liver support.

Bacterial attachment to RO membranes surface-modified by concentration-polarization-enhanced graft polymerization

Concentration polarization-enhanced radical graft polymerization, a facile surface modification technique, was examined as an approach to reduce bacterial deposition onto RO membranes and thus contribute to mitigation of biofouling. For this purpose an RO membrane ESPA-1 was surface-grafted with a zwitterionic and negatively and positively charged monomers. The low monomer concentrations and low degrees of grafting employed in modifications moderately reduced flux (by 20-40%) and did not affect salt rejection, yet produced substantial changes in surface chemistry, charge and hydrophilicity. The propensity to bacterial attachment of original and modified membranes was assessed using bacterial deposition tests carried out in a parallel plate flow setup using a fluorescent strain of Pseudomonas fluorescens. Compared to unmodified ESPA-1 the deposition (mass transfer) coefficient was significantly increased for modification with the positively charged monomer. On the other hand, a substantial reduction in bacterial deposition rates was observed for membranes modified with zwitterionic monomer and, still more, with very hydrophilic negatively charged monomers. This trend is well explained by the effects of surface charge (as measured by ζ-potential) and hydrophilicity (contact angle). It also well correlated with force distance measurements by AFM using surrogate spherical probes with a negative surface charge mimicking the bacterial surface. The positively charged surface showed a strong hysteresis with a large adhesion force, which was weaker for unmodified ESPA-1 and still weaker for zwitterionic surface, while negatively charged surface showed a long-range repulsion and negligible hysteresis. These results demonstrate the potential of using the proposed surface- modification approach for varying surface characteristics, charge and hydrophilicity, and thus minimizing bacterial deposition and potentially reducing propensity biofouling.

Magnetically activated micromixers for separation membranes

Presented here is a radically novel approach to reduce concentration polarization and, potentially, also fouling by colloids present in aqueous feeds: magnetically responsive micromixing membranes. Hydrophilic polymer chains, poly(2-hydroxyethyl methacrylate) (PHEMA), were grafted via controlled surface-initiated atom transfer radical polymerization (SI-ATRP) on the surface of polyamide composite nanofiltration (NF) membranes and then end-capped with superparamagnetic iron oxide magnetite (Fe(3)O(4)) nanoparticles. The results of all functionalization steps, that is, bromide ATRP initiator immobilization, SI-ATRP, conversion of PHEMA end groups from bromide to amine, and carboxyl-functional Fe(3)O(4) nanoparticle immobilization via peptide coupling, have been confirmed by X-ray photoelectron spectroscopy (XPS) and field emission scanning electron microscopy (FESEM). These nanoparticles experience a magnetic force as well as a torque under an oscillating external magnetic field. It has been shown, using particle image velocimetry (PIV), that the resulting movement of the polymer brushes at certain magnetic field frequencies induces mixing directly above the membrane surface. Furthermore, it was demonstrated that with such membranes the NF performance could significantly be improved (increase of flux and salt rejection) by an oscillating magnetic field, which can be explained by a reduced concentration polarization in the boundary layer. However, the proof-of-concept presented here for the active alteration of macroscopic flow via surface-anchored micromixers based on polymer-nanoparticle conjugates has much broader implications.

Surface modification of dense membranes using radical graft polymerization enhanced by monomer filtration

Surface graft polymerization is a promising way to modify membranes for improved performance. Redox-initiated graft polymerization of vinyl monomers is a facile and inexpensive method carried out at room temperature in aqueous media; however, its use is often limited by slow kinetics, low surface specificity, and excessive consumption of chemicals on undesired homopolymerization. It is shown that in the case of RO or NF membranes these drawbacks may be eliminated by utilizing the selectivity of the membranes toward monomers and carrying out the polymerization while applying pressure, i.e., under filtration conditions. Concentration polarization that ensues raises the concentration of reagents near the membrane surface and thereby drastically increases the rate of reaction and preferentially directs it towards surface grafting. Grafting experiments using 2-hydroxyethyl methacrylate and other monomers and characterization of modified membranes using permeability measurements, ATR-FTIR, AFM, XPS, and contact angle demonstrate that the required monomer concentrations can be drastically reduced, particularly when a small fraction of a cross-linker is added. As an additional benefit, this approach enables broadening the spectrum of utilizable monomers to sparingly soluble hydrophobic, charged, and macro-monomers, as was demonstrated using sparingly soluble ethyl methacrylate and 2-ethoxyethyl methacrylate and other monomers. Even though the kinetics of the process is substantially complicated by evolution and concentration polarization of oligomeric and polymeric species, especially in the presence of a cross-linker, it is well offset by the benefits of higher rate, specificity, and reduced monomer consumption.

Anti-nonspecific protein adsorption properties of biomimetic glycocalyx-like glycopolymer layers: effects of glycopolymer chain density and protein size

In many cases, biomaterials surfaces are desired to be resistant to protein adsorption. A system fulfilling this task in nature is the so-called glycocalyx. The glycocalyx is an outer layer on the cell membrane with bound glycoproteins and glycolipids, exposing a pattern of carbohydrate groups. There is a growing interest to mimic this glycocalyx layer to have a tool to overcome the problems with uncontrolled protein adsorption on biomaterials. In this work a glycocalyx-like layer is artificially imitated by surface-initiated atom transfer radical polymerization (ATRP) of a glycomonomer, D-gluconamidoethyl methacrylate (GAMA), from a mixed self-assembled monolayer (SAM) of an ATRP initiator-immobilized hydroxyl-terminated thiol and a methyl-terminated thiol as diluent. Fourier transform infrared spectroscopy (FT/IR-ATR), contact angle, and ellipsometry measurements were employed to confirm the grafting of the glycopolymer. The anti-nonspecific protein binding properties of this glycopolymer layer were then investigated with surface plasmon resonance (SPR). Three proteins with different size, lysozyme, bovine serum albumin (BSA), and fibrinogen were used as model solutes to investigate the influence of protein size on the protein resistance behavior. The glycopolymer chain density was controlled during surface-initiated ATRP by varying the ratio of the components in the mixed SAM, and the chain length was adjusted by ATRP time. The effect of chain density in combination with the protein size was also evaluated. The most important results are that poly(GAMA) layers of higher grafting density show resistance to adsorption of the model proteins used in this work and that the amount of adsorbed protein depends on the length and density of the glycopolymer chains and also on the size of the proteins.

Biofilm growth of individual and dual strains of Klebsiella oxytoca from the dairy industry on ultrafiltration membranes

Formation of biofilms in dairy membrane plants causes membrane pore blocking, product contamination and subsequent economic loss. To investigate the biofilm growth, two Klebsiella oxytoca strains, K. B006 and TR002, previously isolated from New Zealand dairy membrane plants, were grown both individually and combined on three types of ultrafiltration (UF) membranes in different concentrations of whey medium in biofilm reactors (CBR 90, BioSurface Technologies, Bozeman, USA). Biofilms of both the individual and combined strains grew on the membrane surfaces to levels of 4.9-7.99 log colony-forming units (CFU) cm(-2) measured by standard plate counting after removing the cells by sonication. More biofilm grew on used polyethersulfone (PES) membranes than on new PES and polyvinylidene fluoride (PVDF) membranes. Both strains formed good biofilms, although K. B006 formed a denser biofilm than TR002. This corresponded to our previous study on the attachment of these organisms, where K. B006 attached in greater numbers than K. TR002. The dual strains produced a higher biofilm density than single strains on the new membranes. Biofilm density tended to increase with increased whey concentration. The saturated biofilm was approximately 10(8) CFU cm(-2). PES membranes appeared to support biofilm growth less readily than did PVDF membranes and therefore may be the preferred material for UF membranes to reduce problems with microbial colonisation. Used membranes were more readily colonised with biofilm than were new membranes. Therefore, selecting a membrane type and monitoring membrane age will help manage biofilm development during UF.