Gas-phase photoinduced graft polymerization of acrylic acid onto polyacrylonitrile ultrafiltration membranes

Heterogeneous surface modification of polyacrylonitrile (PAN) ultrafiltration (UF) membranes is realized with UV irradiation-initiated graft polymerization of acrylic acid (AA) from the gas phase onto photoinitiator (benzophenone, BP)-coated samples. In the absence of monomer, PAN functionalization by ketyl radicals dominates after UV excitation of sorbed BP. With AA, graft and total polymer yield increase with BP loading and UV irradiation time. Average molecular weight and distribution of PAA homopolymer—formed in parallel during graft polymerization—are analyzed with gel permeation chromatography. Morphology of PAN-gr-AA UF membranes is checked with scanning electron micrographs (SEMs) and atomic force microscopy. Chemical changes are characterized with FTIR-ATR spectroscopy and SEM/EDX analyses, indicating a pronounced surface selctivity of the graft polymer modification (localized in the upper 5-μm membrane thickness). The amount of grafted PAA systematically reduces membrane permeability and increases dextrane retention, as verified in UF experiments. Photo graft polymer modificationof UF membranes will be applied to adjust membrane performance by controlling surface hydrophilicity and permeability using other monomers and/or further graft polymer functionalization. © 1995 John Wiley & Sons, Inc.     

Chemical initiated-grafted nylon 4 membranes

To improve the performance of nylon 4 membranes, this study attempts to utilize chemical initiation which induces different hydrophilicities vinyl monomers grafted onto nylon 4 membranes. Sodium styrene sulfonate, chloromethyl styrene, styrene, and glycidyl methacrylate were used as grafting monomers. The factors that affect the degree of grafting considered were chemical initiators, pH values, kinds and concentrations of monomers, reaction time, and temperatures. The mechanical strength and the transport properties of these chemical-initiated grafted nylon 4 membranes were also investigated. Both the water flux and the salt rejection of sodium styrene sulfonated-grafted membrane were increased significantly, compared to our previous paper,¹ and the casein rejections of all of the four grafted nylon 4 membranes studied exceeded 90%. The quaternized nylon 4-g-poly(chloromethyl styrene) membranes were prepared and possessed high water uptake behavior and high transfer number (0.99) for electrodialysis. The sulfonating process was also applied to improve the hydrophilicity of nylon 4-g-poly(glycidyl methacrylate) membrane so that the water flux and the salt rejection were both increased.     

Ultrafiltration membranes obtained by poly(acrylonitrile) grafted onto poly(vinylchloride)

Acrylonitrile (AN) has been grafted onto Polyvinylchloride (PVC) dissolved in Dimethylformamide (DMF) by means of gamma rays from a Co⁶⁰ source. The grafting yield has been studied as a function of irradiation dose and initial AN/PVC ratio in the solution.

The ratio PAN/PVC has been evaluated both in the grafted copolymer and its fractions by infrared spectroscopy. Ultrafiltration membranes have been prepared from the grafted PVC and its fractions, using the phase inversion technique.

The UF performances have been studied as a function of PAN grafted %.

The results show that both water flux and rejection increase with the grafting. Electron microscopy has also been employed to investigate the membrane structure.     

Surface modification of poly(ether sulfone) ultrafiltration membranes by low‐temperature plasma‐induced graft polymerization

Low‐temperature helium plasma treatment followed by grafting of N‐vinyl‐2‐pyrrolidone (NVP) onto poly(ether sulfone) (PES) ultrafiltration (UF) membranes was used to modify commercial PES membranes. Helium plasma treatment alone and post‐NVP grafting substantially increased the surface hydrophilicity compared with the unmodified virgin PES membranes. The degree of modification was adjusted by plasma treatment time and polymerization conditions (temperature, NVP concentration, and graft density). The NVP‐grafted PES surfaces were characterized by Fourier transform infrared attenuated total reflection spectroscopy and electron spectroscopy for chemical analysis. Plasma treatment roughened the membrane as measured by atomic‐force microscopy. Also, using a filtration protocol to simulate protein fouling and cleaning potential, the surface modified membranes were notably less susceptible to BSA fouling than the virgin PES membrane or a commercial low‐protein binding PES membrane. In addition, the modified membranes were easier to clean and required little caustic to recover permeation flux. The absolute and relative permeation flux values were quite similar for the plasma‐treated and NVP‐grafted membranes and notably higher than the virgin membrane. The main difference being the expected long‐term instability of the plasma treated as compared with the NVP‐grafted membranes. These results provide a foundation for using low‐temperature plasma‐induced grafting on PES with a variety of other molecules, including other hydrophilic monomers besides NVP, charged or hydrophobic molecules, binding domains, and biologically active molecules such as enzymes and ribozymes. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 1699–1711, 1999     

Polyacrylonitrile‐g‐poly(vinyl alcohol) membranes for the pervaporation separation of dimethyl formamide and water mixtures

Poly(vinyl alcohol) as well as its grafted copolymer membranes with polyacrylonitrile (PAN‐g‐PVA) were prepared and used to separate water and dimethyl formamide mixtures by the pervaporation technique. The three following membranes were prepared: (1) pure PVA; (2) 46% grafted PAN‐g‐PVA; and (3) 93% grafted PAN‐g‐PVA. Pervaporation separation experiments were carried out at 25°C for the feed mixture containing 10 to 90% water. By use of the transport data, permeation flux, separation selectivity, swelling index, and diffusion coefficients have been calculated. By increasing the grafting of the membrane, flux decreased, whereas separation selectivity increased slightly over that of pure PVA membrane. Arrhenius activation parameters for transport processes were calculated for 10 mass % water containing feed mixture by using flux and diffusion data obtained at 25, 35, and 45°C. Transport parameters were discussed in terms of sorption‐diffusion principles. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91: 4091–4097, 2004     

Synthesis of well‐defined responsive membranes with fixable solvent responsiveness

A versatile method is described to synthesize a new family of solvent‐responsive membranes whose response states can be not only tunable but also fixable via ultraviolet (UV) irradiation induced crosslinking. The atom transfer radical polymerization (ATRP) initiator 2‐bromoisobutyryl bromide was first immobilized on the poly(ethylene terephthalate) (PET) track‐etched membrane followed by room‐temperature ATRP grafting of poly(2‐hydroxyethyl methacrylate) (PHEMA) and poly(2‐hydroxyethyl methacrylate‐co‐2‐(dimethylamino)ethyl methacrylate) (P(HEMA‐co‐DMAEMA)) respectively. The hydroxyl groups of PHEMA were further reacted with cinnamoyl chloride (a photosensitive monomer) to obtain photo‐crosslinkable PET‐g‐PHEMA/CA membrane and PET‐g‐P(HEMA/CA‐co‐DMAEMA) membrane. The length of grafted polymer chains was controllable by varying the polymerization time. X‐ray photoelectron spectroscopy, Fourier transform infrared spectroscopy in attenuated total reflection and thermogravimetric analysis were employed to characterize the resulting membranes. The various membrane surface morphologies resulting from different states of the grafted chains in water and dimethylformamide were characterized by scanning electron microscopy. It was demonstrated that the grafted P(HEMA/CA‐co‐DMAEMA) chains had more pronounced solvent responsivity than the grafted PHEMA/CA chains. The surface morphologies of the grafted membranes could be adjusted using different solvents and fixed by UV irradiation crosslinking. © 2014 Society of Chemical Industry     

Photo‐induced grafting of poly(ethylene glycol) onto polyamide thin film composite membranes

In this study, the surface grafting of poly(ethylene glycol) (PEG) onto commercial polyamide thin film composite (TFC‐PA) membranes was carried out, using ultraviolet photo‐induced graft polymerization method. The attenuated total reflection Fourier transform infrared spectra verify a successful grafting of PEG onto the TFC‐PA membrane surface. The scanning electron microscope and atomic force microscope analyses demonstrate the changes of the membrane surface morphology due to the formation of the PEG‐grafted layer on the top. The contact angle measurements illustrate the increased hydrophilicity of the TFC‐PA‐g‐PEG membrane surfaces, with a significantly reduced water contact angles compared to the unmodified one. Consequently, the separation performance of the PEG‐grafted membranes is highly improved, with a significant enhancement of flux at a great retention for removal of the different objects in aqueous feed solutions. In addition, the antifouling property of the modified membranes is also clearly improved, with the higher maintained flux ratios and the lower irreversible fouling factors compared to the unmodified membrane. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45454.     

Pore morphology control and hydrophilicity of polyacrylonitrile ultrafiltration membranes

The effects of different solvents (dimethyl formamide: DMF and dimethylsulfoxide: DMSO) on the solubility of polyacrylonitrile (PAN) were investigated by the phase diagrams of H₂O/DMF/PAN and H₂O/DMSO/PAN ternary systems through cloud‐point titration method at low polymer concentration. The influences of polymer concentrations and temperatures on the morphologies of PAN ultrafiltration membranes were elucidated. The morphologies of fabricated UF membranes were characterized by scanning electron microscopy (SEM) and atomic force microscope (AFM), and the basic performance of ultrafiltration including pure water flux and rejection of BSA were explored. At 25°C, the pure water flux of ultrafiltration membranes at the lower PAN content (16 wt % PAN in 84 wt % DMSO) reached 213.8 L/m/bar and the rejection of BSA was 100%. Interestingly, the water flux of UF membranes dramatically decreased to 20.6 L/m/bar (20 wt %) and 2.9 L/m/bar (24 wt %) when increasing PAN concentrations in DMSO. On the other hand, the hydrophilicity of membranes can be enhanced by increasing coagulation temperatures and polymer concentrations which were characterized by static contact angle, fitting well with the variation tendency of roughness. Although there are many works concerning on the effects of phase inversion conditions on the performance of PAN UF membranes, to our best knowledge, there is seldom works focusing on investigating the membrane hydrophilicity trend by adjusting phase inversion conditions. To disclose the reason of the enhanced hydrophilicity, the water and glycol contact angles of various membranes were measured and the surface tensions were presented. The results illustrated that the enhanced hydrophilicity of PAN UF membranes fabricated at higher temperatures or higher polymer concentrations was due to the higher polarity on membrane surface. Since the vast majority of ultrafiltration membranes in labs and in industrial scale have been fabricated by immersion phase inversion method, this work can provide a guidance to obtain hydrophilic PAN UF membranes by adjusting the process of phase inversion. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41991.     

Synthesis of controlled thermoresponsive PET track‐etched membranes by ATRP method

Controlled thermoresponsive PET track‐etched membranes were synthesized by grafting N‐isopropylacrylamide (NIPAAm) onto the membrane surface via atom transfer radical polymerization (ATRP). The initial measurements were made to determine the anchoring of ATRP initiator on PET membrane surface. Thereafter, polymerization was carried out to control the mass of polymer by controlling reaction time grafted from the membrane surface and, ATR‐FTIR, grafting degree measurements, water contact angle measurements, TGA, and SEM were used to characterize changes in the chemical functionality, surface and pore morphology of membranes as a result of modification. Water flux measurements were used to evaluate the thermoresponsive capacity of grafted membranes. The results show the grafted PET track‐etched membranes exhibit rapid and reversible response of permeability to environmental temperature, and its permeability could be controlled by controlling polymerization time using ATRP method. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.     

PAN ultrafiltration membranes grafted with natural amino acids for improving antifouling property

In this report, antifouling polyacrylonitrile (PAN) ultrafiltration membranes were prepared from blends of PAN/polyglycidyl methacrylate (PGMA) via phase inversion method followed by the grafting of natural amino acids through epoxy ring-opening reaction. The grafted PAN membranes possessed highly stable hydrophilic surfaces as a result of the grafting of amino acids, which was adequately demonstrated in attenuated total reflectance–Fourier transform infrared spectroscopy (ATR/FTIR), X-ray photoelectron spectroscopy (XPS), and contact angle measurements. The results of tensile strength and scanning electron microscopy (SEM) images further proved that the surface modification had little effect on their mechanical properties, surface, and cross-sectional morphologies. Meanwhile, remarkable resistance against bovine serum albumin (BSA) and lysozyme (Lyz) fouling was observed for the neutral amino acid-based PAN membranes due to the formation of zwitterionic hydration layer on the membrane surface, while PAN membranes grafted with charged amino acids were able to prohibit the approach of like charged proteins with reduced deposition and provide the driving force for oppositely charged protein adsorption. Furthermore, the ultrafiltration and antifouling performance of PAN membranes were investigated by BSA filtration experiments. Compared with the pristine PAN membrane, all the modified PAN membranes exhibited higher pure water flux, better flux recovery ratio, lower rejection, less total permeation resistance, and preferable stability, having potential applications in protein separation and purification.     

Hydrophilic modification of polyacrylonitrile membranes by oxyfluorination

The surface of polyacrylonitrile (PAN) membranes was modified by oxyfluorination with various conditions to improve its wettability. The membranes were characterized in terms of morphology, structure, hydrophilicity, and membrane performance. The properties and functional groups on the surface of PAN membranes were investigated by contact angle, SEM, ATR-IR and XPS. And permeability of PAN membranes was compared by permeating pure water flux through membrane surface under 100, 150 and 200 kPa pressure. Oxyfluorination introduced oxygen contained functional groups such as the carboxylic acid groups that help increment of wettability on the surface of PAN membrane. Water flux of oxyfluorinated PAN UF membrane increased 20% at pure water permeation pressure 200 kPa compared to that of untreated PAN UF membrane.     

Nano SiO2 and TiO2 embedded polyacrylonitrile/polyvinylidene fluoride ultrafiltration membranes: Improvement in flux and antifouling properties

Polyvinylidene fluoride (PVDF) and polyacrylonitrile (PAN) ultrafiltration (UF) membranes are widely used in drinking water and wastewater applications. These membranes are prone to fouling and membrane efficiency decreases with time under constant operation. Significant improvements/modifications are necessary to apply these polymers as sustainable membrane materials. In this study, PVDF and PAN UF membranes were modified through incorporation of nanoparticles (NPs) namely SiO₂ and TiO₂. PVDF and PAN UF membranes were prepared by phase inversion method from polymer solutions having dispersed SiO₂ and TiO₂ NPs in it. Membrane surface hydrophilicity, charge, roughness, and morphology were studied. Equilibrium water content and molecular weight cut-off of the membranes were also measured. Addition of NPs increased membrane surface hydrophilicity, equilibrium water content, and surface potential. NPs modified membranes exhibited better membrane flux (35–79% higher) and antifouling properties (flux recovery ratio values 28–41% higher) than the virgin membranes.     

Antifouling enhancement of PVDF membrane tethered with polyampholyte hydrogel layers

The preparation and property of antifouling poly(vinylidene fluoride) (PVDF) membrane tethered with polyampholyte hydrogel layers were described in this work. In fabricating these membranes, the [2‐(methacryloyloxy)ethyl] trimethylammonium chloride and 2‐acrylamide‐2‐methyl propane sulfonic acid monomers were grafted onto the alkali‐treated PVDF membrane to yield polyampholyte hydrogel layers via radical copolymerization with N,N′‐methylenebisacrylamide as crosslinking agent. The analyses of fourier transform infrared attenuated total reflection spectroscopy and X‐ray photoelectron spectroscopy confirm the covalent immobilization of polyampholyte hydrogel layer on PVDF membrane surface. The grafting density of polyampholyte hydrogel layer increases with the crosslinking agent growing. Especially for the membrane with a high grafting density, a hydrogel layer can be observed obviously, which results in the complete coverage of membrane pores. Because of the hydrophilic characteristic of grafted layer, the modified membranes show much lower protein adsorption than pristine PVDF membrane. Cycle filtration tests indicate that both the reversible and irreversible membrane fouling is alleviated after the incorporation of polyampholyte hydrogel layer into the PVDF membrane. This work provides an effective pathway of covalently tethering hydrogel onto the hydrophobic membrane surface to achieve fouling resistance. POLYM. ENG. SCI., 55:1367–1373, 2015. © 2015 Society of Plastics Engineers     

Alkali‐responsive membrane prepared by grafting dimethylaminoethyl methacrylate onto ethylene vinyl alcohol copolymer membrane

An alkali‐responsive membrane was prepared by grafting dimethylaminoethyl methacrylate (DMAEMA) onto ethylene vinyl alcohol copolymer (EVAL) membrane using ultraviolet (UV) irradiation graft polymerization. A subtranslucent state of EVAL membrane swelling in the DMAEMA solution was observed, and such a state enabled the passage of UV light through all the pores, inducing graft polymerization inside the pores and on the back. Attenuated total reflectance Fourier‐transform infrared spectrometer (ATR‐FTIR), X‐ray photoelectron spectroscopy (XPS), field‐emission scanning electron microscopy (FESEM), and energy‐dispersive X‐ray spectroscope (EDX) confirmed that the poly(DMAEMA)‐grafted chains existed not only on the top surface, but also inside the pores and on the back. Atomic force microscopy (AFM) and nitrogen adsorption analysis confirmed that the grafted chains collapsed in air, and decreased the surface roughness, surface area, and pore size of the grafted membranes. Alkali‐responsive properties of the poly(DMAEMA)‐grafted EVAL membrane (i.e., contact angle, permeability, and selectivity) were observed in the pH range of 9–10. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41775.     

Formation of multilayer poly(acrylic acid)/poly(vinylidene fluoride) composite membranes for pervaporation

Dual‐ and multilayer composite membranes, consisting of poly(acrylic acid) (PAA) and poly(vinylidene fluoride) (PVDF), were synthesized by the plasma‐induced polymerization technique. The dual‐layer membrane had a dense PAA layer grafted onto a microporous PVDF substrate, whereas in the multilayer membranes, the grafted PAA and the PVDF layers were arranged in an alternating sequence (e.g., PAA/PVDF/PAA and PAA/PVDF/PAA/PVDF/PAA). These membranes were used in a pervaporation process to separate ethanol–water solutions. For the dual‐layer membranes, the results indicated that the separation factor increased and the permeation flux decreased with increasing amounts of grafted PAA. For the case of grafting yield < 0.6 mg/cm², the composite membrane demonstrated poor separation. As the grafting yield reached 0.85 mg/cm², a sharp increase of the separation factor was observed. For the multilayer membranes, the pervaporation performances were very good, with high separation factors (on the order of 100) and reasonable permeation fluxes over a wide ethanol concentration range. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2266–2274, 2004     

Part II. Properties of the grafted and sulfonated membranes

The physical and chemical properties of polystyrene grafted and sulfonated polytetrafluoroethylene (PTFE‐graft‐PSSA) membranes prepared by radiation‐induced grafting of styrene onto commercial PTFE films using simultaneous irradiation technique followed by a sulfonation reaction are evaluated. The investigated properties include water uptake, ion exchange capacity, hydration number and ionic conductivity. All properties are correlated with the amount of grafted polystyrene (degree of grafting). The thermal stability of the membrane evaluated by thermal gravimetric analysis (TGA) is compared with that of original and grafted PTFE films. The membrane surface structural properties are analysed by electron spectroscopy for chemical analysis (ESCA). Membranes having degrees of grafting of 18 % and above show a good combination of physical and chemical properties that allow them to be proposed for use as proton conducting membranes, provided that they have sufficient chemical and mechanical stability. © 2000 Society of Chemical Industry     

Effect of crosslinkers on the preparation and properties of ETFE‐based radiation‐grafted polymer electrolyte membranes

This study concerns a comparative study of three crosslinkers, divinylbenzene (DVB), 1,2‐bis(p,p‐vinylphenyl)ethane (BVPE), and triallyl cyanurate (TAC) crosslinked poly(ethylene‐co‐tetrafluoroethylene) (ETFE)‐based radiation‐grafted membranes, which were prepared by radiation grafting of p‐methylstyrene onto ETFE films and subsequent sulfonation. The effect of the different types and contents of the crosslinkers on the grafting and sulfonation, and the properties such as water uptake, proton conductivity, and thermal/chemical stability of the resulting polymer electrolyte membranes were investigated in detail. Introducing crosslink structure into the radiation‐grafted membranes leads to a decrease in proton conductivity due to the decrease in water uptake. The thermal stability of the crosslinked radiation‐grafted membranes is also somewhat lower than that of the noncrosslinked one. However, the crosslinked radiation‐grafted membranes show significantly higher chemical stability characterized in the 3% H₂O₂ at 50°C. Among the three crosslinkers, the DVB shows a most pronounced efficiency on the crosslinking of the radiation‐grafted membranes, while the TAC has no significant influence; the BVPE is a mild and effective crosslinker, showing the moderate influence between the DVB and TAC crosslinkers. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 4565–4574, 2006     

High‐performance PDMS membranes for pervaporative removal of VOCs from water: The role of alkyl grafting

To improve the pervaporation performance of PDMS membrane, alkyl groups with different chain length were grafted into PDMS matrix. The prepared membranes were characterized by ATR‐IR, DSC, TGA, PALS, and tensile testing. The effects of alkyl grafting on pervaporation performance of PDMS membrane were investigated in separation of ethyl acetate/water mixture. Experimental results show that the separation factor of PDMS membrane is largely improved by alkyl grafting because of the enhanced preferential sorption of ethyl acetate, and this improvement depends on alkyl grafting ratio and alkyl chain length. The total flux of PDMS membrane reduces after alkyl grafting owing to the decreased free volume. When grafting ratio is above 6.9%, membrane grafted with shorter alkyl groups is preferred for pervaporation. The best pervaporation performance is achieved by 9% octyl grafted PDMS membranes with a separation factor of 592 and a total flux of 188 gm^?2 h^?1 in separation of 1% ethyl acetate/water mixture at 40 °C. Moreover, this octyl grafted PDMS membrane also exhibits excellent separation performance in removal of butyl acetate, methyl‐tert‐butyl ether, and n‐butanol from water. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43700.     

Radiation‐induced GMA/DMAA graft copolymerization onto porous PE hollow‐fiber membrane

Radiation‐induced graft copolymerization is a powerful technique to prepare a grafted chain with the desired properties pending onto the trunk material. In this work, a polyethylene hollow‐fiber membrane was modified by this technique. The monomers glycidyl methacrylate (GMA) and N,N‐dimethylacrylamide (DMAA) were cografted onto macroporous polyethylene hollow fiber with a grafting degree in the order of 200%. DMAA/GMA cografted membranes were compared to GMA grafted ones for the introduction of an amino acid as a specific ligand. Grafted membranes with a copolymer composition between 0 and 2 DMAA/GMA were prepared by soaking them in solutions of different mixtures of monomers. Copolymers were characterized by FTIR and their composition was estimated by the analysis of the ratio of carbonyl signals. Copolymers with a higher proportion of DMAA showed improved hydrophilic properties and higher conversion rates of epoxy groups on phenyalanine ligands than those of the GMA grafted ones. However, copolymers with a DMAA/GMA ratio higher than 1 showed a detrimental effect on the pure water flux. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1646–1653, 2003     

Dialysis membranes from polyethylene films grafted with acrylic acid

Low‐density polyethylene‐g‐poly(acrylic acid) membranes were prepared by the direct radiation grafting of aqueous acrylic acid solutions (containing Mohr's salt) onto low‐density polyethylene films and were irradiated at two different irradiation doses (2 and 3 Mrad) at a dose rate of 0.02 Mrad/h. Two series of polyethylene‐g‐poly(acrylic acid) membranes with 100 and 150% grafting were obtained. The free carboxylic acid groups in the grafted films were converted into the corresponding acrylates by reactions with different metal salts. The swelling (water uptake) and dialysis permeability of glucose and urea through the grafted membranes in different metal‐ion forms were investigated. The prepared membranes showed good permeability to both solutes, which increased as the hydrophilicity of the membrane increased. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 10–14, 2004