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The present work investigated the mechanism of macrovoid initiation in polymeric membranes, with focus on the initiation position of macrovoids. The results show that where macrovoids were initiated strongly depended on the polymer concentration in casting solution and the solvency for polymer of the solvent used. Comparing the results with the rheological properties of casting solutions, we observed a close relationship between the initiation position of macrovoids and the degree of polymer-chain entanglement in the casting solution. High degree of chain entanglement helped to suppress the occurrence of macrovoids. And low quality solvent enhanced chain entanglement and brought about environment not favored for macrovoid initiation. We also obtained results showing that the time needed to induce the phase separation of casting solution played an important role in initiation of macrovoids. A model was developed to take into account the effects of the degree of chain entanglement and the time needed to induce phase separation, by using the chain relaxation time to characterize the effect of chain entanglement and the time constant of the movement of phase separation front to quantify the time needed for phase separation. The model can quantitatively describe the relationship between the initiation position of macrovoids and the polymer concentration in casting solution for the polymeric membranes prepared in the present work. [ABSTRACT FROM AUTHOR]

Highlights: [•] PVDF membrane formation by VIPS using NMP and acetone as solvent and co-solvent, respectively. [•] Control of crystalline domain sizes by tuning solvent composition and nonsolvent activity. [•] Insights on the competition between crystallization and liquid–liquid demixing. [•] Correlation between mechanical properties and crystalline morphologies. [ABSTRACT FROM AUTHOR]

Highlights: [•] Recent advances in the fundamentals of nonsolvent induced phase separation (NIPS). [•] Preparation of membranes containing amphiphilic copolymers or inorganic particles by NIPS. [•] Review on three novel morphology control techniques based on NIPS. [ABSTRACT FROM AUTHOR]

Abstract: The present investigation revealed how the morphology and crystalline forms of a poly(vinylidene fluoride) (PVDF) membrane, prepared by using the vapor-induced phase separation (VIPS) method, were affected by the temperature at which PVDF was dissolved to form the casting solution. The results showed that there existed an important transition dissolution temperature, being referred to as the “critical dissolution temperature”, across which the morphology and crystalline forms of membranes drastically changed. With a dissolution temperature above the critical dissolution temperature, the prepared membranes were composed of nodules and the size of nodules decreased as the dissolution temperature decreased. And, with a dissolution temperature below the critical, membranes with lacy (bi-continuous) structure were obtained. In addition, the α-crystalline form of PVDF grew faster than the β-form when the dissolution temperature was below the critical, and the β-form became faster and dominant as the dissolution temperature increased to be above the critical. The existence of the critical dissolution temperature was observed for all the three solvents used in the present study, N-methyl-2-pyrrolidone (NMP), dimethylacetamide (DMAc), and N,M-dimethylformamide (DMF), indicating that the phenomena are general and not limited to a specific solvent. Also, it was observed that the two PVDF/NMP solutions, prepared with dissolution temperatures above and below the critical temperature, responded in different ways when water was added into the solutions. Though both solutions gelled, the solution with higher dissolution temperature started the gelation with a crystallization-initiation gelling process, while the other with a non-crystallization-initiation gelling. We propose that the competition between the two gelling processes play an important role in determining the PVDF membrane morphology and crystalline polymorphs, and the dissolution temperature can change the competition. And the critical dissolution temperature can be interpreted as a dissolution temperature across which the dominant gelling process switched. [Copyright &y& Elsevier]

Abstract: This work reports the preparation of PEGylated poly(vinylidene fluoride) (PVDF) membranes by vapor-induced phase separation (VIPS). After blending PVDF with poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (Pluronic F108) triblock copolymers, solutions were exposed to water vapors in controlled conditions of relative humidity, temperature and exposure time to vapors. Chemical analysis revealed that the additive was successfully retained onto the surfaces of PVDF membranes. Compared to virgin membranes, PEGylated PVDF membranes presented improved hydrophilic properties, from a 3wt% additive content, both at their surface (low water contact angle) and within their pores (high hydration capability). Bovine–serum–albumin, lysozyme and fibrinogen adsorption tests evidenced that non-specific protein adsorption was associated with the physicochemical nature of the membrane and the chemical nature of the protein. A 35% to 95% decrease of protein adsorption was found and best results were obtained for low molecular weight proteins, presenting less hydrophobic domains, and Pluronic content greater than 3wt%. PEGylated brushes were efficient to prevent attachment of Staphylococcus epidermidis and Escherichia coli, from a 3wt% additive content. Results presented highlighted that PEGylated PVDF membranes obtained by VIPS processing are potentially effective low-biofouling membranes able to resist protein adsorption, bacterial attachment and biofilm formation. [Copyright &y& Elsevier]

Abstract: This work describes the PEGylation of a polysulfone (PSf) membrane incorporated with poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO, Pluronic F108) via two different approaches for PEGylated membrane formation, including liquid-induced phase separation (LIPS) and vapor-induced phase separation (VIPS). Segregated and dispersed PEGylated domains on PSf membrane surfaces were achieved by LIPS and VIPS. The chemical composition and physical microstructure of the various PEGylated PSf membranes were characterized by Fourier transform infrared spectroscopy (FT-IR), contact angle measurements, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) measurements. The results show that the amount of adsorbed proteins and attached bacteria on the modified PSf membranes not only depends on the surface hydrophilicity and hydration capacity but also on the phase separation structures of PEGylated domains distributed on the PSf membrane surface. By controlling the surface uniformity of PEGylated domains, it is possible to highly regulate the PSf membrane to resist the adsorption of proteins and the attachment of bacteria. It was found that the PSf membranes prepared by the VIPS process present a higher polymer chain mobility of PEO–PPO–PEO than those prepared during the LIPS process, resulting in nano-structured phase separation during membrane formation. Consequently, the membranes prepared by VIPS have a better anti-biofouling character with respect to bacterial resistance. This work suggests that membrane formation by controlling PEO–PPO–PEO copolymer structures in nano-scale blending using the VIPS process shows great potential in the molecular design of anti-biofouling membranes for use in ultrafiltration applications. [Copyright &y& Elsevier]

Abstract: The effect of side chain architectures on the properties and proton conductivities of graft copolymer membranes for direct methanol fuel cells (DMFCs) are studied. Poly(vinylidene fluoride)-g-poly(styrene sulfonic acid) (PVDF-g-PSSA) copolymers with either linear or arborescent PSSA side chains are prepared and examined. For the copolymers with similar ion exchange values, both graft copolymers show similar water uptakes and bound water contents. Meanwhile, the arborescent samples exhibit higher proton conductivity, lower methanol permeability, and higher selectivity compared to the linear analogues. Incorporation of highly branched side chains effectively increases the properties of the PVDF-g-PSSA-based PEMs for DMFCs because of formation of agglomerate PSSA domains. The PSSA domains promote proton conduction and depress methanol permeation through the PEMs, consequently significantly increase the selectivities (proton conductivity/methanol permeability) of the PEMs. [Copyright &y& Elsevier]

Highlights: [•] Novel chitosan/pectin/gum Arabic (C/P/A) polyelectrolyte complex (PEC) solutions and membranes were prepared. [•] The appearance of the C/P/A PEC solutions, either turbid or clear, was process-dependent. [•] The C/P/A solutions contained both globe-like and network-like microstructures. [•] The tensile strength and hydrophilicity of the C/P/A (weight ratio: 84/8/8) membranes were enhanced. [•] Using the C/P/A membranes as a drug carrier exhibited steady and complete drug (insulin) release. [ABSTRACT FROM AUTHOR]

Abstract: Novel composite porous membranes made of basic and acidic polysaccharides (i.e. chitosan and pectin) were developed. Three types of chitosan/pectin composite membranes were fabricated: PEC-type (by polyelectrolyte complex (PEC) formation between chitosan and pectin); PEC/Ca-type (by PEC and calcium ion-mediated ionic crosslinking toward pectin); and PEC/covalent-type membranes (by PEC and EDC/NHS-mediated covalent bond). Tensile strengths (about fourfold that of pure chitosan membranes), hydrophilicity and water uptake capability of the composite membranes were significantly greater than membranes composed of chitosan or pectin only. Such composite membranes had faster disintegration rates which could be controlled by the content of pectin. Addition of calcium ions or EDC/NHS further increased the tensile strengths of the composite membranes (by about 1.5-fold) and also changed the profile of disintegration. Such chitosan/pectin composite membranes, due to their enhanced strength, hydrophilicity and controllable disintegration behavior, have potential as edible sponges or disintegrant carrying food or drug. [Copyright &y& Elsevier]

Abstract: In the present work poly (methyl methacrylate) (PMMA) membranes were fabricated by immersing PMMA/1,4-dioxane cast films into the coagulation bath of methanol, ethanol, n-propanol, or n-butanol. Because the alcohols are not strong nonsolvent to PMMA, it was not easy to remove enough solvent from the cast film in the coagulation bath, to avoid the effect of the residual solvent contained in the gelled cast films. Due to the residual solvent, redissolution of the gelled films could occur when they were air dried, and redissolution followed by redemixing was observed when the gelled films were immersed in water bath. The redissolution and redemixing in the water bath was strongly related to the amount of the residual solvent contained in gelled cast film. Also, it was found that, with a fixed time period in the alcohol bath (30min), the amount of residual solvent in the gelled film was strongly related to the carbon number of the alcohol coagulant. The alcohol with higher carbon number resulted in lower solvent removal rate in the coagulation bath and higher amount of residual solvent. With methanol as the coagulant, the residual dioxane contained in the gelled film was too little to induce any redissolution or morphology change of the gelled film in the water bath. With ethanol, only partial redissolution was observed. For the case that n-propanol was the coagulant of the first bath, complete redissolution of the gelled film, followed by immediate redemixing, was observed in the second coagulation bath (water bath). And with n-butanol as the coagulant, the redemixing was further prolonged to allow for delayed redemixing. When redemixing occurred immediately after redissolution, membranes with a three-layer structure were formed, consisting of a lacy-like top porous layer, a dense middle layer, and sponge-like bottom layer. Mechanisms are also proposed to account for the formation of the three-layer structure. [Copyright &y& Elsevier]

Abstract: This work describes the surface grafting control of poly(vinylidene fluoride) (PVDF) membrane with poly(ethylene glycol) methacrylate (PEGMA) via three different modification approaches of surface-initiated radical graft copolymerization, including thermal-induced radical polymerization, surface-initiated atom transfer radical polymerization (ATRP), and low pressure plasma-induced graft-polymerization. Two different surface grafting structures of PEGylated layer, brush-like PEGMA and network-like PEGMA, on PVDF membrane surface were achieved in this study. The chemical composition and microstructure of the various surface-modified PEGylated PVDF membranes were characterized by Fourier transform infrared spectroscopy (FT-IR), contact angle, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) measurements. Antifouling property of the modified PVDF membranes was evaluated according to the amount of protein adsorption and the filtration test for BSA solution in this study. Results show that the amount of adsorbed proteins on the modified PVDF membranes not only depends on the surface hydrophilicity and hydration capacity but also associates with the surface grafting structures of PEGylated layers on PVDF membrane surface. This study not only introduces different practical modification approaches to achieve a hydrophobic PVDF membrane grafting hydrophilic PEGMA, but also provides a fundamental understanding of various PEGylated grafting structures governing the performance of antifouling properties. [Copyright &y& Elsevier]

Abstract: Chitosan (CH) is a biomaterial with antiseptic, bioactive, and biocompatible properties. To further enhance the performance of chitosan membranes, chondroitin sulfate (CS) was utilized to modify the porous chitosan membranes fabricated by the freeze-gelation method. The cross-linking of CS to CH was mediated by ionic interaction and covalent cross-linking using EDC/NHS coupling reagents. The pore structures, mechanical properties, and surface hydrophilicity of the porous chondroitin sulfate-modified chitosan (CH/CS) membranes could be altered by varying the weight ratio of CS to CH. The membranes had interconnected stratiform pore structure with surface pore size ranging from 10 to 40 µm. Among various CH/CS membranes the one with the weight ratio of 90/10 (CH to CS) possessed the highest mechanical strength (18.61 N/g), about 40% increase as compared with the unmodified CH membrane. The addition of CS improved the hydrophilicity of the membranes. Preliminary cell culture experiments revealed that the proliferation of the gingival fibroblasts on the CH/CS surface was slightly better than that on the CH surface. In summary, the CH/CS membranes (especially the 90/10 membrane), due to their higher mechanical strength, hydrophilicity, and better cell compatibility, are promising biomaterials for tissue engineering applications. [Copyright &y& Elsevier]

Abstract: The aim of this paper is to prepare a PVDF (polyvinylidene fluoride) microporous membrane by using alcohol as nonsolvent (coagulant). A porous membrane with high hydrophobic surface (water contact angle ~148°) was facilely created by wet immersion method without modification with special low surface energy compounds and the need of special instruments. In addition, the PVDF casting film was passed through alcohol bath for 2 s and then immersed into water precipitation bath, the porous PVDF membrane with hydrophobic surface (water contact angle ~135°) was also formed. The characterization of the porous PVDF membrane obtained from various coagulation conditions was carried out by a pore size analysis and a direct contact membrane distillation (DCMD). The permeation flux and rejection coefficient of the porous PVDF membrane fabricated via dual-bath coagulated was close to commercial one. [Copyright &y& Elsevier]

Abstract: To improve the pervaporation separation performance, an organic–inorganic hybrid membranes were prepared by adding zeolite 13X into the polyimide (BAPP–BODA), which was synthesized using one-step polycondensation polymerization of 2,2-bis(4-(4-aminophenoxy)phenyl)propane (BAPP) with bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride (BODA). The influence of the zeolite 13X content on the free volume and pervaporation performance of hybrid membranes were systematically analyzed by positron annihilation spectroscopy (PAS). The trend of ortho-positronium (o-Ps) lifetime and free volume holes changes consistently well with the permeation rate variation of aqueous isopropanol mixtures dehydration. The BAPP–BODA/13X hybrid membrane morphology was characterized by AFM and SEM. Compared with the BAPP–BODA membrane, the BAPP–BODA/13X hybrid membrane had good pervaporation performance for aqueous alcohol solution dehydration. The sorption selectivity dominates the behavior of pervaporation while adding zeolite 13X into the polyimide matrix. [Copyright &y& Elsevier]