The structure of cellulose acetate membranes for reverse osmosis. Part 2. Transmission and scanning electron microscopy of membranes cast from an acetone-formamide dope

A study was made of the structure of membranes cast from the Loeb-Manjikian acetone-formamide dope by transmission and scanning electron microscopy, and of the general problem of preparation of specimens and interpretation of electron micrographs of polymer membranes.The formation of the membrane structure during casting of cellulose acetate membranes is discussed in relation to the evaporation time during the casting step. A 3-layer membrane structure is observed similar to that in membranes cast from a dioxan-based dope (1), and it is thought to have similar origins in the casting process. It is concluded that both transmission and scanning microscopy are required for a detailed analysis of all aspects of membrane morphology. Transmission microscopy reveals more information on the “active” surface layer while scanning microscopy is shown to reveal more of the texture of the bulk of the membrane.     

Evaluation of the antifouling and photocatalytic properties of novel poly(vinylidene fluoride) membranes with a reduced graphene oxide–Bi2WO6 active layer

Immobilization of reduced graphene oxide (RGO)–Bi₂WO₆ is an ideal method for obtaining antifouling membranes for membrane distillation (MD) processes. Poly(vinylidene fluoride) membranes modified with RGO–Bi₂WO₆ were successfully obtained with a double‐layer coating method through non‐solvent‐induced phase separation. The water contact angle was improved by about 30° by RGO–Bi₂WO₆; this indicated that the surface modification obviously increased the membrane hydrophobicity. The high desalination rate proved that all of the prepared membranes were appropriate for the MD process. The RGO–Bi₂WO₆‐modified membranes achieve 26.26%–59.95% removal rates in 10 mg/L aqueous ciprofloxacin under visible light for 7.5 h. It was possible to erase strongly bound foulants and recover the prepared membrane's permeation flux by 3 h of visible‐light irradiation. The RGO–Bi₂WO₆‐modified membrane with a high hydrophobicity, fouling mitigation, and photocatalytic capability presents huge potential for the treatment of high‐salt antibiotic wastewater use in the MD process. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45426.     

Facile preparation of ODPA‐ODA type polyetherimide‐based carbon membranes by chemical crosslinking

A chemical crosslinking protocol was developed to prepare carbon membranes from 3,3′,4,4′‐oxydiphthalic dianhydride‐4,4′‐oxydianiline (ODPA–ODA) type polyetherimide on the support of phenolic resin sheets. The effects of support pretreatment, membrane‐coating methods and crosslinking protocols on the resultant carbon membranes were investigated. The microstructure, functional group evolution, thermal stability, mechanics, morphology, and gas separation performance of samples were characterized by XRD, FTIR, TGA, mechanical testing technique, and gas permeation technique, respectively. Results have shown that the chemical crosslinking is more beneficial than the popular thermal crosslinking protocol to fabricate supported carbon membranes for the advantage of simple preparation process. In addition, spin‐coating is superior to drop‐coating in terms of good membrane formation on the support. Under the preferred preparation conditions of crosslinker ethylene glycol usage at 10 wt % and spin‐coating, supported carbon membranes can be obtained with good hydrogen separation performance. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44889.     

PDMS/PVDF microporous membrane with semi‐interpenetrating polymer networks for vacuum membrane distillation

In this article, using the non‐solvent induced phase separation process, a new microporous membrane with the semi‐interpenetrating polymer network (semi‐IPN) structure was produced. For this membrane, polydimethylsiloxane (PDMS) polymer is crosslinking and poly(vinylidene fluoride) (PVDF) polymer is linear, by changing the mass ratio of PDMS/PVDF, the structure and the performance of the prepared membranes were studied. The membranes were also investigated by attenuated total reflection‐Fourier transform infrared (ATR‐FTIR), scanning electron microscopy–energy‐dispersive X‐ray spectroscopy, X‐ray diffraction, thermogravimetric analysis, and water contact angle, etc. ATR‐FTIR spectroscopy confirmed the formation of semi‐IPN; compared with the PDMS/PVDF polymer without semi‐IPNs structure, the viscosity of the semi‐IPNs structured casting solution increased, membrane mechanical property increased but its hydrophobicity decreased. Using the resulting membranes for the vacuum membrane distillation desalt of the NaCl solution (30 g/L), 99.9% salt rejection and reasonable flux were obtained. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45792.     

Morphology and performance of polysulfone hollow fiber membrane

Polysulfone hollow fiber membranes were prepared via the dry-wet spinning process from dope solutions comprised of polysulfone, n-methyl-2-pyrrolidone, polyvinyl-2-pyrrolidone, and dodecylbenzene sulfonic acid, sodium salt. Morphology and performance of the membranes were affected by the compositions of coagulant and dope solution. Pore size and the water flux of the membrane increased by the addition of dodecylbenzene sulfonic acid, sodium salt to water in the coagulation bath, due to the changes of physicochemical properties of the outer coagulant. Addition of dodecylbenzene sulfonic acid, sodium salt to the dope solution also increased the pore size. The absence of polyvinyl-2-pyrrolidone, the pore forming agent, in the dope solution resulted in a remarkable decrease of pore size of the membrane. The distance between the spinneret and coagulation bath affected the membrane structure and performance. The membranes prepared in this study were suitable for hemofiltration in terms of molecular weight cut-off characteristics. © 1993 John Wiley & Sons, Inc.     

Preparation of porous PVC membrane via a phase inversion method from PVC/DMAc/water/additives

Poly(vinyl chloride) membranes were prepared via a phase inversion method, using N,N‐dimethylacetamide (DMAc) as solvent, polyethylene glycol (PEG), polyvinylpyrrolidone (PVP), and sucrose as three typical additives and water as the coagulation medium. The phase diagrams of the PVC/DMAc/additives/water quaternary systems were constructed using cloud‐point experimental data. With the addition of the different additives, the effect of dope solution temperature on the dope solution viscosity and the structure of membranes were investigated. It indicates that the viscosity of the PVC/DMAc dope solution with the additive increase compared with the dope solution without any additive and the addition of the additives into the dope solution alter the morphology and structure of the resultant membranes during the phase‐inversion process. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Surface photochemical graft polymerization of acrylic acid onto polyamide thin film composite membranes

Surface modification is an effective approach to enhance the properties of polymeric membranes. In this work, the UV‐photo‐induced graft polymerization of acrylic acid (AA) onto the surfaces of polyamide thin film composite (TFC‐PA) membranes was carried out using an immersion method performed under ambient conditions. The experimental results indicate that the membrane surface becomes more hydrophilic because of the appearance of new carboxylic groups on the surface after the modification. This reduces the water contact angle and increases the water permeability compared with the unmodified membrane. The membrane surface is relatively compact and smooth due to the formation of the polymeric AA‐grafted layer. The separation performance of the modified membrane is improved with enhancements of the permeate flux and the retention of humic acid from aqueous feed solutions compared with those of the unmodified membrane. The fouling resistance of the membrane is also improved because of the higher maintained flux ratios and the lower irreversible fouling factors for the removal of various organic compounds from feed solutions. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44418.     

Preparation of high‐flux ultrafiltration membranes by blending strongly charged polymer

Three kinds of high‐flux ultrafiltration membranes were fabricated by blending strongly charged polymer [sulfonated poly(phenylene oxide) (SPPO)] with neutral polymer [cellulose acetate (CA), polyethersulfone (PES), or polyvinylidene fluoride (PVDF)]. After blending with SPPO, the pure water flux of CA‐SPPO, PES‐SPPO, and PVDF‐SPPO membrane increase by 3, 76, and 30 times at a transmembrane pressure of 100 kPa. Compared with the unblended membranes, the pore radius of CA‐SPPO, PES‐SPPO, and PVDF‐SPPO membrane increased from 31.9 to 33.2 nm, 26.1 to 28.6 nm, and 19.8 to 25.7 nm, respectively. The addition of strongly charged polymer decreased the thermodynamic stability of casting solutions, promoting the phase inversion process and resulting in highly porous structure. The charged groups and hydrophilicity of the polymer facilitate the formation of an additive concentration gradient (more additive in the active layer), endowing the blend membrane with better hydrophilicity and greater wettability gradient. The high porosity, good hydrophilicity, and larger wettability gradient enable the high permeation of blend membranes. This work shows how the strongly charged polymer affects the formation and performance of blend membrane, which will be useful for designing high‐performance membrane. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44570.     

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.     

Diverse morphologies of PVDF hollow fiber membranes and their performance analysis as gas/liquid contactors

A systematic investigation on the morphology development of polyvinylidene fluoride hollow fiber membrane made using various N‐methyl‐2‐pyrrolidone (NMP) aqueous solutions as an inner coagulant was carried out. The cross‐sectional and inner surface morphology were analyzed with scanning electronic microscopy (SEM). It is found that with increase on NMP concentration, the morphology of the resultant membranes gradually shifted from a double‐skin to a single‐skin structure. When 40.0 ～ 55.0 wt.% NMP solution was used, some unexpected macrovoids near the inner region were observed. This special morphology feature was attributed to the reduced solidification rate of the inner surface as a result of increase on NMP concentration, which sharply weakened the inner skin strength. While the existence of centralized stress formed in the phase inversion process, such as shrinkage stress from syneresis, resulted in fractured points in the nascent skin surface that finally made it difficult to maintain a uniform structure. Investigations on effects of the dope flow rate and the bore fluid velocity on the morphology of PVDF fiber membranes experimentally confirmed the suggestion. Three model membranes with double skins, single skin and single skin with macrovoids structures, respectively, were used to test their permeation performance in a CO₂ membrane contactor system. The experimental results show the membranes without an inner skin present higher permeability and lower mass transfer resistance than the membrane with a double skin structure. © 2010 Wiley Periodicals, Inc. Journal of Applied Polymer Science, 2010     

Microporous polyvinylidene fluoride hollow fiber membrane contactors for CO2 stripping: Effect of PEG-400 in spinning dope

In order to develop the structure of microporous PVDF membranes, PEG-400 was introduced into the polymer dope as a non-solvent additive. The hollow fiber membranes were prepared via a wet phase-inversion process and then used in the membrane contactor modules for CO₂ stripping from water. By addition of different amounts of PEG-400, cloud points of the polymer dope were obtained to examine phase-inversion behavior. From FESEM analysis, the membrane structure changed from a finger-like to an approximately sponge-like morphology with the addition of 4 wt.% of PEG-400. The prepared membranes presented smaller mean pore size (0.13 μm) and significantly higher wetting pressure (550 kPa) compared to the plain membrane. From CO₂ stripping test, at water velocity of 0.4 m/s, the PVDF membranes prepared by 4% PEG-400 demonstrated an approximate CO₂ stripping flux of 4.5 × 10⁻⁵ (mol/m² s) which is 125% higher than the flux of the plain membrane. It could be concluded that structurally developed hydrophobic PVDF hollow fiber membranes can be prepared by a controlled phase-inversion process to enhance the performance of gas–liquid membrane contactor.     

Polyamide–imide membranes with surface immobilized cyclodextrin for butanol isomer separation via pervaporation

A novel cyclodextrin (CD) derivative, m‐xylenediamine‐β‐cyclodextrin (m‐XDA‐β‐CD), has been synthesized and used to graft β‐CD on membrane surface for the pervaporation separation of butanol isomers. The reaction mechanisms for the m‐XDA‐β‐CD synthesis and the membrane surface grafting are confirmed by FTIR and TGA. The as‐fabricated novel CD‐grafted polyamide‐imide (PAI) membranes show homogeneous morphology and significant improved separation performance as compared to the unmodified PAI membranes and PAI/CD mixed matrix membranes made of physical blends. The effects of chemical modification time and dope concentration on the asymmetric membrane have been studied. The optimal separation performance can be found with the CD‐grafted PAI membrane cast from a 22 wt % dope concentration, which exhibits a total butanol flux of 15 g/m²/h and a separation factor of 2.03. This newly developed membrane with surface‐immobilized CD may open new perspective for the development of next‐generation high‐performance pervaporation membranes for liquid separations. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Improvement of antifouling performance of poly(l‐lactic acid) membranes through incorporating polyaniline nanoparticles

Poly(l ‐lactic acid) (PLLA) composite membranes were fabricated by nonsolvent induced phase separation method using polyaniline (PANI) as an additive. Membrane structure was characterized by attenuated total reflectance Fourier transform‐infrared spectroscopy, X‐ray diffraction, scanning electron microscopy, porosity, and pore size analysis. Membrane performance was assessed by goniometer, pure water flux, molecular weight cut‐off, static adsorption and dynamic filtration. The incorporation of PANI significantly improved the hydrophilicity and permeability of PLLA composite membrane, and eventually enhanced the antifouling performance of composite membrane compared with pure PLLA membrane. It was demonstrated that PLLA composite membrane with 1 wt % PANI had better separation and antifouling performance compared with other composite membranes. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44452.     

Mechanically improved superhydrophobic nanofibrous polystyrene/high-impact polystyrene membranes for promising membrane distillation application

Robust membranes for commercial applications of membrane distillation (MD) are nearly the Achilles ankle of the process. Despite from high hydrophobicity requirements of the MD membranes, they must have enough mechanical and thermal stabilities. In this regard, flexible, superhydrophobic, and high-productive nanofibrous membranes were fabricated using mixed dope solutions made of polystyrene (PS) and high-impact PS (HIPS) through the electroblowing process. Although the PS nanofibers can be designed to have hierarchically rough surfaces to show superhydrophobicity, the inherent brittleness of this polymer still remains a big issue for practical application for a longer period of time. Upon adding HIPS into the PS-containing dope solution, the rigid PS membrane turned into a more flexible one with improved elongation at break from 5.83% to 14.89%. Also, excellent direct contact membrane distillation performance was achieved using high saline (up to 150 g/L) and 0.1 mM sodium dodecyl sulfate/35 g/L NaCl feed solutions during 96 and 24 h, respectively. Superhydrophobicity (˃160°) and high LEP value (up to 173.2 kPa) gifted membranes with outstanding wetting resistance. Our proposed procedure can pave the route for the facile fabrication of robust MD membranes using cost-effective materials and a high-throughput fabrication process.     

Phosphonic acid functionalized siloxane crosslinked with 3‐glycidoxyproyltrimethoxysilane grafted polybenzimidazole high temperature proton exchange membranes

Phosphonic acid functionalized siloxane crosslinked with 3‐glycidoxypropyltrimethoxysilane (GPTMS) grafted polybenzimidazole (PBI) membranes are prepared by sol–gel process. The structure of the membranes is characterized by Fourier‐transform infrared spectroscopy and X‐ray diffraction spectroscopy. SEM images of the membranes show that the membranes are homogeneous and compact. The crosslinked membranes exhibit excellent thermal stability, chemical stability and mechanical property. The proton conductivity of the crosslinked membranes increases by an order of magnitude over range of 20 °C to 160 °C under anhydrous condition, which can reach 3.15 × 10^?2 S cm^?1 at 160 °C under anhydrous condition. The activation energy of proton conductivity for membranes decreases with increase of PBI, because the formation of hydrogen bond network between the phosphonic acid and the imidazole ring can enhance the continuity of hydrogen bond in the membrane. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134 , 44818.     

Ranking the key parameters of immersion precipitation process and modeling the resultant membrane structural evolution

Key parameters coupling with the instantaneous nucleation concept (ie, the Big Bang analogy) was used to model immersion precipitation process. The merits of the acquired model were verified via comparing its predictions with experimental results of two well‐prepared and characterized cellulose acetate (CA) and polyacrylonitrile (PAN) membranes. A morphology predictable map, ΔPη^?1 versus ?₁, was constructed, where ΔP, η and ?₁ are osmotic pressure difference between nonsolvent and dope solution, dope viscosity and intruded nonsolvent volume fraction into the dope, respectively. The phase separation map, ΔPη^?1 (proportional with apparent system diffusivity with the unit of time^?1) versus ?₁ showed three regimes which, at least qualitatively, depicted the correct morphological evolution trends of the studied systems. Phase separation in regime one of CA membrane with the longest delayed time or lowest ΔPη^?1, led to bead‐like morphology. CA membrane with the shortest elapsed time or highest ΔPη^?1, separated to finger‐like morphology in regime three. Finally, phase separation in the intermediate regime of CA membrane, ended up to sponge‐like morphology. Phase separation time scales of the PAN membranes versus intruded nonsolvent into the dope solution were located in finger‐like region of the CA membrane, which its downward transition lowered the fingers population. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Effect of Tween 80 on morphology and performance of poly(L‐lactic acid) ultrafiltration membranes

Poly(L‐lactic acid) (PLLA) ultrafiltration (UF) membrane was fabricated by immersion precipitation method using Tween 80 as an additive. Membrane structure was characterized by scanning electron microscopy (SEM), porosity and pore size measurement, and atomic force microscopy (AFM). Membrane performance was evaluated by pure water flux, molecular weight cut‐off, and tensile test. It was found that the addition of Tween 80 into the casting solution significantly increased the permeability and molecular weight cut‐off of membrane. Tensile test confirmed that the as‐prepared PLLA membranes exhibited acceptable mechanical properties for ultrafiltration. Further, the role of Tween 80 in the process of membrane formation was analyzed and proposed. The addition of Tween 80 favored the formation of larger pores by interrupting the polymer chain entanglement and improving the miscibility between solvent and coagulant. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44428.     

Dye removal using hydrophobic polyvinylidene fluoride hollow fibre composite membrane by vacuum membrane distillation

Hydrophobic polyvinylidene fluoride (PVDF) hollow fibre composite membranes were prepared by the dilute solution coating process to build a special surface structure that was similar to the dual micro‐nano structure on the lotus leaf. Poly(vinylidene fluoride‐co‐hexafluoropropene) was chosen as the hydrophobic polymer candidate in dilute solution. Membrane morphology and surface hydrophobicity were evaluated by scanning electron microscopy and dynamic water contact angle measurement. The prepared PVDF hollow fibre membranes were employed to separate dyes (Congo Red and Methylene Blue) from water by vacuum membrane distillation. The effects of operational conditions (feed temperature, vacuum pressure and feed flow rate) on the vacuum membrane distillation performance of different PVDF membranes were investigated. The results indicated that the water contact angle values of PVDF composite membrane surfaces improved from 93.6° to 130.8°, which was mainly attributed to the formation of micro‐nano rods. This structure was similar to the dual micro‐nano structure on the lotus leaf. Under test feed temperature, vacuum pressure and feed flow rate conditions, the dye rejection rate of Congo Red and Methylene Blue by the hydrophobic PVDF hollow fibre membrane remained above 99.5% and 99%, which was higher than that of the pristine PVDF membrane (99% and 98%, respectively). In addition, the hydrophobic PVDF hollow fibre composite membrane showed higher permeation flux under different conditions compared with the pristine PVDF membrane, which was attributed to membrane surface hydrophobicity and the electrostatic interactions between dyes and the PVDF membrane surface.     

Preparation and characterization of PES/CA microporous membranes via reverse thermally induced phase separation process

The blend polyethersulfone (PES)/cellulose acetate (CA) flat‐sheet microporous membranes were prepared by reverse thermally induced phase separation (RTIPS) process. The effects of CA content and coagulation bath temperature on membrane structures and properties were investigated in terms of membrane morphology, water contact angle, permeation performance, and mechanical properties. The cloud point results indicated that the cloud point decreased with the increasing content of CA. When the coagulation bath temperature was lower than the cloud point, the membrane formation process underwent nonsolvent induced phase separation (NIPS) process and dense skin layer and finger‐like structure were formed in membranes. These membranes had lower pure water flux and poor mechanical properties. But when the coagulation bath temperature was higher than the cloud point, the membrane formation process underwent RTIPS process. The porous top surface as well as porous cross‐section of the membranes were formed. Therefore, high pure water flux and good mechanical properties were obtained. The contact angles results indicated that the hydrophilicity of the prepared membranes improved obviously with the addition of CA. When the content of CA was 0.5 wt% and the membrane formation temperature was 323K, the PES/CA microporous membrane which was prepared via the RTIPS process displayed a optimal permeability of the pure water flux of 816 L m^?2 h^?1 and the BSA rejection rate of 49.5%, which showed an increase of 48.9% and 23.6% than that of pure PES membrane, respectively. Moreover, the mechanical strengths of the membranes obtained by RTIPS process were better than those membranes prepared by NIPS process. POLYM. ENG. SCI., 58:180–191, 2018. © 2017 Society of Plastics Engineers     

Development of antifouling poly(vinyl chloride) blend membranes by atom transfer radical polymerization

In this study, antifouling poly(vinyl chloride) (PVC) blend membranes were prepared by blending the PVC based amphiphilic copolymer PVC‐g‐poly(hydroxyethyl methacrylate) (PVC‐g‐PHEMA), synthesized by atom transfer radical polymerization (ATRP), into the hydrophobic PVC matrix via the nonsolvent‐induced phase separation method. The in situ ATRP reaction solutions were also used as the blend additives to improve membrane performance. Attenuated total reflectance–Fourier transform infrared spectroscopy and X‐ray photoelectron spectroscopy indicated that the blend membranes based on the two blend routes exhibited similar surface chemical compositions. The membrane morphology and surface wettability were determined by scanning electronic microscopy and water contact angle measurement, respectively. The blend membranes showed improved water permeability, comparable rejections and enhanced antifouling properties compared with the pure PVC membrane. The PVC blend membranes also had excellent long‐term stability in terms of chemical compositions and fouling resistance. The results demonstrated that ATRP was a promising technique to synthesize amphiphilic copolymer and prepare stable blend antifouling membranes. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45832.