Low-biofouling membranes prepared by liquid-induced phase separation of the PVDF/polystyrene-b-poly (ethylene glycol) methacrylate blend

Tailoring the Wettability and Substrate Adherence of Thin Polymer Films with Surface-Segregating Bottlebrush Copolymer Additives

We developed "reactive" bottlebrush polymers based on styrene (S) and t-butyl acrylate (tBA) as additives for polystyrene (PS) coatings. The bottlebrush polymers spontaneously bloom to both the air and substrate interfaces during solution casting. While neat PS films are hydrophobic and poorly adhere to the native oxide on clean silicon wafers, the hydrophilicity and substrate adherence of bottlebrush-incorporating PS films can be tailored through the thermally activated deprotection of tBA to produce acrylic acid (AA) and acrylic anhydride (AH). A critical design parameter is the manner by which tBA is incorporated into the bottlebrush: When the bottlebrush side chains are copolymers of S and tBA, the extent of deprotection is extremely low, even after prolonged thermal annealing at elevated temperature. However, when the bottlebrush contains a mixture of poly(t-butyl acrylate) (PtBA) and PS side chains, nearly all tBA is converted to AA and AH. Consequently, using the "mixed-chain" bottlebrush design with thermal processing and appropriate conditioning, the water contact angle is reduced from over 90° on unmodified PS down to 75° on bottlebrush-incorporating PS films, and the substrate adherence is improved in proportion to the extent of tBA deprotection.

Recent Advances on the Fabrication of Antifouling Phase-Inversion Membranes by Physical Blending Modification Method

Membrane technology is an essential tool for water treatment and biomedical applications. Despite their extensive use in these fields, polymeric-based membranes still face several challenges, including instability, low mechanical strength, and propensity to fouling. The latter point has attracted the attention of numerous teams worldwide developing antifouling materials for membranes and interfaces. A convenient method to prepare antifouling membranes is via physical blending (or simply blending), which is a one-step method that consists of mixing the main matrix polymer and the antifouling material prior to casting and film formation by a phase inversion process. This review focuses on the recent development (past 10 years) of antifouling membranes via this method and uses different phase-inversion processes including liquid-induced phase separation, vapor induced phase separation, and thermally induced phase separation. Antifouling materials used in these recent studies including polymers, metals, ceramics, and carbon-based and porous nanomaterials are also surveyed. Furthermore, the assessment of antifouling properties and performances are extensively summarized. Finally, we conclude this review with a list of technical and scientific challenges that still need to be overcome to improve the functional properties and widen the range of applications of antifouling membranes prepared by blending modification.

Antifouling improvement of a polyacrylonitrile membrane blended with an amphiphilic copolymer

The amphiphilic copolymer polyacrylonitrile-co-poly(hydroxyethyl methacrylate) (PAN-co-PHEMA) was readily blended with polyacrylonitrile (PAN) to fabricate a flat-sheet blending membrane through non-solvent induced phase separation (NIPS). In the membrane-forming process, the hydrophilic PHEMA chains are uniformly distributed on the surface, as revealed by the energy-dispersive X-ray tests. The sponge-like sub-layer embedded with droplet-shaped structures is formed at the cross-sections of membranes, because of the high viscosity of the casting solution. With the increase of copolymer concentration, the mean pore size of the blending membranes increases from 26.9 to 99.8 nm, leading to the increase of membrane flux from 93.6 to 205.4 l/(m2h). The incorporation of PAN-co-PHEMA copolymer endows the blending membrane with a rough surface microstructure and enhanced hydrophilicity. The rejection ratio of membranes for emulsified pump oil reaches 99.9%, indicating a prominent separation performance. In the cycle permeation experiments, the flux recovery ratio of the blending membranes is as high as 99.6%, which is much higher than those of PAN membrane. The irreversible fouling of blending membranes induced by oil adsorption is alleviated, and converted into reversible fouling, owing to the reduction of the adhesion force between foulant and membrane surface. These results suggest that the anti-fouling property of PAN membranes has been dramatically strengthened via the addition of PAN-co-PHEMA copolymer.

A Biofouling Resistant Zwitterionic Polysulfone Membrane Prepared by a Dual-Bath Procedure

This study introduces a zwitterionic material to modify polysulfone (PSf) membranes formed by a dual bath procedure, in view of reducing their fouling propensity. The zwitterionic copolymer, derived from a random polymer of styrene and 4-vinylpyrridine and referred to as zP(S-r-4VP), was incorporated to the PSf solution without any supplementary pore-forming additive to study the effect of the sole copolymer on membrane-structuring, chemical, and arising properties. XPS and mapping FT-IR provided evidence of the modification. Macrovoids appeared and then disappeared as the copolymer content increased in the range 1-4 wt%. The copolymer has hydrophilic units and its addition increases the casting solution viscosity. Both effects play an opposite role on transfers, and so on the growth of macrovoids. Biofouling tests demonstrated the efficiency of the copolymer to mitigate biofouling with a reduction in bacterial and blood cell attachment by more than 85%. Filtration tests revealed that the permeability increased by a twofold factor, the flux recovery ratio was augmented from 40% to 63% after water/BSA cycles, and irreversible fouling was reduced by 1/3. Although improvements are needed, these zwitterionic PSf membranes could be used in biomedical applications where resistance to biofouling by cells is a requirement.

Highly Effective Anti-Organic Fouling Performance of a Modified PVDF Membrane Using a Triple-Component Copolymer of P(Stx-co-MAAy)-g-fPEGz as the Additive

In this study, a triple-component copolymer of P(St_x-co-MAA_y)-g-fPEG_z containing hydrophobic (styrene, St), hydrophilic (methacrylic acid, MAA), and oleophobic (perfluoroalkyl polyethylene glycol, fPEG) segments was synthesized and used as an additive polymer to prepare modified PVDF membrane for enhanced anti-fouling performance. Two compositions of St:MAA at 4:1 and 1:1 for the additive and two blending ratios of the additive:PVDF at 1:9 and 3:7 for the modified membranes were specifically examined. The results showed that the presence of the copolymer additive greatly affected the morphology and performance of the modified PVDF membranes. Especially, in a lower ratio of St to MAA (e.g., St:MAA at 1:1 versus 4:1), the additive polymer and therefore the modified PVDF membrane exhibited both better hydrophilic as well as oleophobic surface property. The prepared membrane can achieve a water contact angle at as low as 48.80° and display an underwater oil contact angle at as high as 160°. Adsorption experiments showed that BSA adsorption (in the concentration range of 0.8 to 2 g/L) on the modified PVDF membrane can be reduced by as much as 93%. From the filtration of BSA solution, HA solution, and oil/water emulsion, it was confirmed that the obtained membrane showed excellent resistance to these organic foulants that are often considered challenging in membrane water treatment. The performance displayed slow flux decay during filtration and high flux recovery after simple water cleaning. The developed membrane can therefore have a good potential to be used in such applications as water and wastewater treatment where protein and other organic pollutants (including oils) may cause severe fouling problems to conventional polymeric membranes.

Cyclic on-chip bacteria separation and preconcentration

Nanoparticles and biological molecules high throughput robust separation is of significant interest in many healthcare and nanoscience industrial applications. In this work, we report an on-chip automatic efficient separation and preconcentration method of dissimilar sized particles within a microfluidic platform using integrated membrane valves controlled microfiltration. Micro-sized E. coli bacteria are sorted from nanoparticles and preconcentrated on a microfluidic chip with six integrated pneumatic valves (sub-100 nL dead volume) using hydrophilic PVDF filter with 0.45 μm pore diameter. The proposed on-chip automatic sorting sequence includes a sample filtration, dead volume washout and retentate backflush in reverse flow. We showed that pulse backflush mode and volume control can dramatically increase microparticles sorting and preconcentration efficiency. We demonstrate that at the optimal pulse backflush regime a separation efficiency of E. coli cells up to 81.33% at a separation throughput of 120.45 μL/min can be achieved. A trimmed mode when the backflush volume is twice smaller than the initial sample results in a preconcentration efficiency of E. coli cells up to 121.96% at a throughput of 80.93 μL/min. Finally, we propose a cyclic on-chip preconcentration method which demonstrates E. coli cells preconcentration efficiency of 536% at a throughput of 1.98 μL/min and 294% preconcentration efficiency at a 10.9 μL/min throughput.

A Review on Membrane Technology and Chemical Surface Modification for the Oily Wastewater Treatment

Cleaning of wastewater for the environment is an emerging issue for the living organism. The separation of oily wastewater, especially emulsified mixtures, is quite challenged due to a large amount of wastewater produced in daily life. In this review, the membrane technology for oily wastewater treatment is presented. In the first part, the global membrane market, the oil spill accidents and their results are discussed. In the second and third parts, the source of oily wastewater and conventional treatment methods are represented. Among all methods, membrane technology is considered the most efficient method in terms of high separation performance and easy to operation process. In the fourth part, we provide an overview of membrane technology, fouling problem, and how to improve the self-cleaning surface using functional groups for effectively treating oily wastewater. The recent development of surface-modified membranes for oily wastewater separation is investigated. It is believed that this review will promote understanding of membrane technology and the development of surface modification strategies for anti-fouling membranes.

Amphiphilic Block Copolymer of Poly(dimethylsiloxane) and Methoxypolyethylene Glycols for High-Permeable Polysulfone Membrane Preparation

Poly(dimethylsiloxane)-block-methoxypolyethylene glycols (PDMS-b-mPEG) were synthesized by Steglich esterification. The high-permeable membrane (PSf/PDMS-b-mPEG) was prepared by using PDMS-b-mPEG as additives. The successful synthesis of PDMS-b-mPEG was confirmed by nuclear magnetic resonance. Field emission scanning electron microscopy images show that the distribution of finger-like macroporous and sponge-like macroporous can be modulated by controlling the ratio of the hydrophilic/hydrophobic components of additives. The distribution of additives and membrane wettability are validated with X-ray photoelectron spectroscopy and water contact angle test. The permeability of the blended membrane, especially for the membrane PSf/PDMS-b-mPEG1900 (M3), was remarkably improved. The water permeability of M3 (239.4 L/m²·h·bar) was 6.6 times that of the unblended membrane M0 (42.5 L/m²·h·bar). The findings of protein BSA filtration show that the flux recovery ratio of M3 is 89.2% at a BSA retention rate of about 80%, which demonstrates that the polysulfone membranes blended with PDMS-b-mPEG have excellent antifouling performance and extraordinary permeability.

Enhanced Fouling Resistance and Antibacterial Properties of Novel Graphene Oxide-Arabic Gum Polyethersulfone Membranes

Membrane biofouling has proved to be a major obstacle when it comes to membrane efficiency in membrane-based water treatment. Solutions to this problem remain elusive. This study presents novel polyethersulfone (PES) membranes that are fabricated using the phase inversion method at different loadings of graphene oxide (GO) and 1 wt. % arabic gum (AG) as nanofiller and pore forming agents. Synthesized GO was examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) for morphological studies and energy-dispersive X-ray spectroscopy (EDX) for elemental analysis. The prepared GO flakes showed a high content of oxygen-containing groups (~31%). The fabricated membranes were extensively characterized, including water contact angle analysis for hydrophilicity, zeta potential measurements for surface charge, SEM, total porosity and pore size measurements. The prepared membranes underwent fouling tests using bovine serum albumin (BSA) solutions and biofouling tests using model Gram-positive (Bacillus subtilis) and Gram-negative (Escherichia coli) bacterial suspensions as well as real treated sewage effluent (TSE). The results showed that the novel PES/GO membranes possessed strong hydrophilicity and negative surface charge with an increase in porosity, pore size and water flux. The PES/GO membranes exhibited superior antibacterial action against both Gram-positive and Gram-negative bacterial species, implicating PES membranes which incorporate GO and AG as novel membranes that are capable of high antibiofouling properties with high flux.

Molecular Dynamics Study on Wettability of Poly(vinylidene fluoride) Crystalline and Amorphous Surfaces

The present study investigates the effect of microscopic structure on the wettability of poly(vinylidene fluoride) (PVDF) surfaces using all-atom molecular dynamics simulations of water droplets brought into contact with both crystal and amorphous PVDF surfaces. For each case, computations were performed using five different droplet diameters, and the corresponding water droplet contact angles θ were obtained. Using the fact that the cosine of these contact angles for both surfaces are inversely proportional to the radius of the droplet contact surface ( r_dr( Z₀)), the contact angle θ_∞ of the macroscopic water droplet was obtained by extrapolating cos θ to 1/ r_dr( Z₀) = 0. The estimated values of θ_∞ on the crystal and amorphous surfaces were 96° and 86°, respectively, showing that the amorphous surface is less hydrophobic than the crystal surface. The contact angle of the crystalline/amorphous mixed surface was estimated using the Cassie equation to be 91°. This value agrees well with experimental measurement of the water contact angle on the PVDF film. Furthermore, the interaction energy, interface structure, and electrostatic potential were analyzed to clarify the reason for the lower hydrophobicity of the amorphous surface. This surface interacts more favorably with water than the crystal surface. Such an interaction reduces the excess free energy (interfacial tension) at the PVDF and water interface and makes the amorphous surface less hydrophobic. The amorphous interfacial region contains more water molecules than the crystal one, and water molecules are oriented toward the PVDF. This interface structure makes water strongly interact with the PVDF.

Hydrophilic modification and anti-fouling properties of PVDF membrane via in situ nano-particle blending

Two hydrophilic poly-vinylidene fluoride (PVDF) ultrafiltration membranes were prepared via in situ embedment of nanoparticles (NP), i.e., TiO₂ and Al₂O₃, respectively, and their anti-organic-fouling and anti-biofouling were comprehensively investigated. Characterization of modified PVDF-NP membranes by XRD and FTIR exhibited that nanoparticles were embedded successfully. Series of fast filtration tests demonstrated that in contrary to virgin PVDF membrane, PVDF-NP membranes have high permeability and anti-organic-fouling ability by decreasing the possibility of organic matters deposition and accumulation. Co-existed Ca²⁺ in feed solution deteriorated the organic fouling in virgin PVDF and PVDF-NP membranes, which was mainly caused by gelation of macromolecular foulants. PVDF-NP membranes were used to form MBR modules for domestic wastewater treatment, and the long-term monitoring evidenced that hydrophilic modified membranes achieved stably high COD and [Formula: see text] rejection efficiencies, and better organic rejection capability than mAO process. PVDF-NP membranes possessed consistently high anti-biofouling ability to maintain stable membrane permeability.

New insight into the adsorption behaviour of effluent organic matter on organic-inorganic ultrafiltration membranes: a combined QCM-D and AFM study

Adsorption of organic matter on membranes plays a major role in determining the fouling behaviour of membranes. This study investigated effluent organic matter (EfOM) adsorption behaviour onto poly(vinylidene fluoride) (PVDF) membrane blended with SiO₂ nanoparticles using quartz crystal microbalance with dissipation monitoring (QCM-D) and atomic force microscopy (AFM). The QCM-D results suggested that low adsorption of EfOM and an EfOM layer with a non-rigid and open structure was formed on SiO₂-terminated membrane surfaces. Conformational assessment showed that EfOM undergoes adsorption via two steps: (i) in the initial stage, a rapid adsorption of EfOM accumulated onto the membrane; (ii) the change in dissipation was still occurring when the adsorption frequency reached balance, and the layer tended towards a more rearranged or organized secondary structure upon adsorption onto the more hydrophilic surface. For the AFM force test, when a self-made EfOM-coated probe approached the membrane, a 'jump-in' was observed for the hydrophobic membrane after repulsion at a small distance, while only repulsive forces were observed for PVDF/SiO₂ membranes. This study demonstrated that the PVDF/SiO₂ membrane changed the entire filtration process, forming a 'soft' open conformation in the foulant layer.