Membrane Pore Structure Research Articles

Polysulfone Membranes Embedded with Halloysites Nanotubes: Preparation and Properties.

In the present study, nanocomposite ultrafiltration membranes were prepared by incorporating nanotubes clay halloysite (HNTs) into polysulfone (PSF) and PSF/polyvinylpyrrolidone (PVP) dope solutions followed by membrane casting using phase inversion method. Characterization of HNTs were conducted using scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and thermogravimetric (TGA) analysis. The pore structure, morphology, hydrophilicity and mechanical properties of the composite membranes were characterized by using SEM, water contact angle (WCA) measurements, and dynamic mechanical analysis. It was shown that the incorporation of HNTs enhanced hydrophilicity and mechanical properties of the prepared PSF membranes. Compared to the pristine PSF membrane, results show that the total porosity and pore size of PSF/HNTs composite membranes increased when HNTs loadings were more than 0.5 wt % and 1.0 wt %, respectively. These findings correlate well with changes in water flux of the prepared membranes. It was observed that HNTs were homogenously dispersed within the PSF membrane matrix at HNTs content of 0.1 to 0.5 wt % and the PSF/HNTs membranes prepared by incorporating 0.2 wt % HNTs loading possess the optimal mechanical properties in terms of elastic modulus and yield stress. In the case of the PSF/PVP matrix, the optimal mechanical properties were obtained with 0.3 wt % of HNTs because PVP enhances the HNTs distribution. Results of bovine serum albumin (BSA) filtration tests indicated that PSF/0.2 wt % HNTs membrane exhibited high BSA rejection and notable anti-fouling properties.

Preparation and nanofiltration properties of thin supported meso-porous ceria membranes

Thin film meso-porous ceria membranes were prepared by colloidal deposition of ceria precursor nanoparticles on ~2 mm thick macro-porous α-Al2O3 supports, followed by rapid thermal processing at 600 °C or conventional thermal processing at 500 °C. Stable ceria precursor nanoparticles with a diameter of 4–6 nm were made by ultrasound-assisted precipitation and stabilization with bicine. The ceria precursor concentration, poly-vinyl alcohol concentration in the deposition dispersion, and the coating times were varied to obtain the optimum membrane deposition. The effect was investigated of deposition conditions on membrane thickness, water permeation and salt rejection at ambient circumstances. Meso-porous ceria membranes made on monolithic macro-porous α-Al2O3 supports with a bulk pore size of 100 nm and a porosity of 30% exhibited high salt rejection for aqueous solutions with low salt concentrations. For instance at a pH of 4 and a trans-membrane pressure of 0.68 MPa, a ceria membrane had stable maximum rejections of 83% and >95% for 1 mol/m3 NaCl and CaCl2, respectively. At a concentration of 1 mol/m3, >81% Na2SO4 was rejected at a pH of 10. These salt-rejections are caused by surface charging of the porous ceria membrane structure by chemi-sorption of H+, Ca2+ and SO42−. Ceria membrane structures with just monolithic α-Al2O3 supports had a volumetric water permeance of 5.9 L/(m2 h bar). To increase the overall permeance, a ceria membrane was deposited onto a ~10 μm thick macroporous α-Al2O3 layer, in turn supported by a 2 mm thick and more permeable macroporous α-Al2O3 support. With this three-layer structure a salt rejection of 73% was obtained for 1 mol/m3 NaCl at pH = 4 and 25 °C with an overall volumetric water permeance of 43 L/(m2 h bar). During the experiments no obvious changes in transport properties occurred while the membranes were also fully stable during 6 weeks exposure to ultrapure water at 60 °C. This indicates a complete stability in aqueous solutions at room temperature and moderate pH.

Distinct Behaviors between Gypsum and Silica Scaling in Membrane Distillation.

Mineral scaling constrains membrane distillation (MD) and limits its application in treating hypersaline wastewater. Addressing this challenge requires enhanced fundamental understanding of the scaling phenomenon. However, MD scaling with different types of scalants may have distinctive mechanisms and consequences which have not been systematically investigated in the literature. In this work, we compared gypsum and silica scaling in MD and demonstrated that gypsum scaling caused earlier water flux decline and induced membrane wetting that was not observed in silica scaling. Microscopic imaging and elemental mapping revealed contrasting scale morphology and distribution for gypsum and silica, respectively. Notably, while gypsum crystals grew both on the membrane surface and deep in the membrane matrix, silica only formed on the membrane surface in the form of a relatively thin film composed of connected submicrometer silica particles. We attribute the intrusion of gypsum into membrane pores to the crystallization pressure as a result of rapid, oriented crystal growth, which leads to pore deformation and the subsequent membrane wetting. In contrast, the silica scale layer was formed via polymerization of silicic acid and gelation of silica particles, which were less intrusive and had a milder effect on membrane pore structure. This hypothesis was supported by the result of tensile testing, which showed that the MD membrane was significantly weakened by gypsum scaling. The fact that different scaling mechanisms could yield different consequences on membrane performance provides valuable insights for the future development of cost-effective strategies for scaling control.

Stimuli-Responsive Membranes through Sustainable Aqueous Phase Separation.

Polymeric membranes are used on huge scales for kidney dialysis, wastewater treatment, and drinking water production. However, almost all polymeric membranes are fabricated by a process reliant on the use of unsustainable, expensive, and reprotoxic dipolar aprotic solvents. In this work, we propose an aqueous phase separation approach for preparing porous membrane films. Poly(4-vinylpyridine) (P4VP), a pH-responsive polymer, is first dissolved at low pH where the polymer is charged and subsequently cast as a thin film. Switching to a high pH where the polymer is uncharged and insoluble results in controlled phase separation and solidification of the polymer into porous membrane structures. This approach gives a large degree of control over membrane structure, leading to symmetric porous microfiltration membranes and asymmetric dense nanofiltration membranes. Moreover, the use of a pH-responsive polymer leads directly to a pH-responsive membrane, where the degree of responsive behavior can be tuned by the degree of cross-linking. Such responsive behavior allows effective cleaning of the membrane, without the use of harsh chemicals. This work outlines an approach toward preparing membranes in a more sustainable fashion—an approach that allows control over the membrane structure and one that naturally leads to advanced membranes with responsive properties.

Impact of three-dimensional tortuous pore structure on polyethersulfone membrane morphology and mass transfer properties from a manufacturing perspective.

We examined typical commercial poly(ethersulfone) (PESf) hemodialysis and hemoconcentration membranes successfully used in manufacturing, and employed scanning probe microscope (SPM) to achieve a structural observation of the pores in the inner membrane surfaces, as well as measure the pore diameters and their distribution, verifying the relationship between the typical mass transfer properties. We focused on the differences between the PESf membranes which were expected to further improve the advanced pore structure control and functional design for various medical uses. The three-dimensional tortuous capillary pores on the inner surface of hollow fiber hemodialysis and hemoconcentrator membranes were investigated using dynamic force microscopy (DFM), and the pore diameter and distribution were measured through a line analysis. Compared with PUREMA-A, PES-Sα hemodialysis membranes have smaller three-dimensional tortuous capillary pore diameters and pore areas, as well as a smaller pore diameter distribution and pore area distribution, which make the accurate measurements of the pore diameter using FE-SEM impossible. These PESf membranes are almost the same in pure water permeability, but greatly differ in pore diameter and pore diameter distribution. By comparing and verifying as above, we may gain insight into the flexibility, versatility, and superior structural and functional controllability of PESf membrane pore structures, which could advance the development of pore structure control. Pending issues include the fact that, using a line analysis software of SPM devices, it is very difficult to measure hundred pores which clearly reflects the poor quality of pore size distributions obtained in this study, measurement accuracy must be improved further.

Effect of pore structure in polymer membrane from various preparation techniques on cyclic stability of 4.9 V LiNi0.5Mn1.5O4 at elevated temperature

Pore structure of the membrane which was determined by the preparation techniques played a key role in its thermal stability and electrochemical properties. Two of the most common fabrication techniques, phase inversion and electrospinning, were used to obtain the porous polymer membrane employing poly (methyl methacrylate-butyl acrylate-acrylonitrile-styrene) (P(MMA-BA-AN-St)) as the polymer matrix. Scanning electron microscopy suggested that the morphology of polymer membranes was different: a loose and porous structure was obtained by phase inversion method, while a compact and interconnected structure with stacked and interlaced fiber filaments was prepared through electrospinning technique. Two different membranes exhibited a little discrepancy in the physical property and electrochemical performance under room temperature. However, under the elevated temperature, the pore structure of membrane prepared by phase inversion method was unstable and the mechanical strength was sharply decreased, while the electrospinning membrane maintained the original pore structure and better mechanical strength. Subsequently, the discharge capacity of the Li/LiNi0.5Mn1.5O4 cell using the electrospinning membrane kept 82% of its initial capacity after 80 cycles at 60 °C, while the phase inversion membrane only retained 10% of capacity under the same condition. Thus, the matched polymer membrane from electrospinning technique was suggested to effectively improve the cyclic stability of 4.9 V LiNi0.5Mn1.5O4 cathode under elevated temperature. This fundamental research revealed that the pore structure of the polymer membrane played a decisive role in the cycle performance of the high-energy density Li-ion battery, suggesting that the more attention should be paid particularly to the preparation technique of the membrane, in addition to designing novel polymer materials.

Filamentous virus-based membrane prepared by chemical cross-linking at liquid/liquid interface for a tailored molecular separation system

Membranes with selective separation capability are desirable for use in energy-efficient processes. However, precise control of target selectivity for separation is still challenging due to difficulty in controlling porous membrane structures and interactions between membrane components and target molecules. In this study, we report a filamentous virus (M13 phage)-based membrane prepared by chemical cross-linking at liquid/liquid interfaces for selective molecular separation. The M13 phage membrane composed of oriented M13 phage assemblies showed a molecularly porous structure, enabling the size-selective separation of small organic compounds. Furthermore, the pore structure was controlled by the initial concentration of M13 phage solution, enabling broad size selectivity. The rejection rates of positively charged molecules tend to be greater than those of negatively charged molecules with similar molecular weights, possibly due to the negatively charged M13 phage membrane, indicating certain electrostatic interactions. Membrane preparation using M13 phage modified by peptides with a specific affinity for neodymium ions (Nd3+) was performed to realize the selective separation of small Nd3+. The membrane preferentially adsorbed Nd3+ over iron ions, demonstrating that the molecular recognition capability of the peptide clearly remained even after the chemical cross-linking reaction. This broad-spectrum separation membrane has potential applicability in various fields, such as wastewater treatment, the food industry, and the life sciences.

Pore Structure and Properties of PEEK Hollow Fiber Membranes: Influence of the Phase Structure Evolution of PEEK/PEI Composite.

Poly(ether ether ketone) (PEEK) hollow fiber membranes were successfully prepared from miscible blends of PEEK and polyetherimide (PEI) via thermally-induced phase separation (TIPS) with subsequent extraction of the PEI diluent. The phase structure evolution, extraction kinetics, membrane morphology, pore size distribution and permeability for the hollow fiber membrane were studied in detail. Extraction experiments, differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMA) studies showed that the heat treatment had a significant influence on the two-phase structure of PEEK/PEI, and that it was controlled by the crystallization kinetic of PEEK and the diffusion kinetic of PEI. As the annealing temperature increased, the controlling factor of the phase separation changed from PEEK crystallization to PEI diffusion, and the main distribution of the amorphous PEI chains were changed from the interlamellar region to the interfibrillar or interspherulitic regions of PEEK crystallization. When the annealing temperature increased from 240 °C to 280 °C, the extracted amount of PEI increased from 85.19 to 96.24 wt %, and the pore diameter of PEEK membrane increased from 10.59 to 37.85 nm, while the surface area of the PEEK membrane decreased from 111.9 to 83.69 m2/g. Moreover, the water flux of the PEEK hollow fiber membranes increased from 1.91 × 10−2 to 1.65 × 10−1 L h−1 m−2 bar−1 as the annealing temperature increased from 240 °C to 270 °C. The structure and properties of the PEEK hollow fiber membrane can be effectively controlled by regulating heat treatment conditions.

Flux-enhanced polysulfone membranes blended with ABPBI as a novel additive

Enhanced-flux polysulfone membranes were prepared with poly (2,5-benzimidazole) additive. Structural and morphological characterizations of prepared membranes were performed by using Scanning electron microscope and Infrared spectroscopy techniques. Hydrophilicity, porosity and pore structure of the composite membranes were investigated with wet-dry membrane weight and contact angle measurements. Flux, rejection and fouling resistance tests were performed by using cross-flow filtration studies. Scanning electron microscope images showed that a low ratio of poly (2,5-benzimidazole) additive increased the top and bottom skin layer thickness and membrane structure turned from finger-like to sponge-like but increasing the poly (2,5-benzimidazole) ratio changed the pore structure to finger-like again. Infrared spectra showed that good interactions were established between poly (2,5-benzimidazole) and polysulfone. Contact angles of the membranes decreased from 60° to 38° with the addition of poly (2,5-benzimidazole) which proved the excellent hydrophilicity of the membranes. Also increasing the hydrophilicity was supported by Infrared spectra. At 200 kPa, pure water fluxes of the membranes increased from 228 to 455 L/m2.h. Bovine serum albumin rejection also increased with the addition of poly (2,5-benzimidazole), especially with low additive ratios. Flux recovery ratios of the membranes increased about 20% after the addition of poly (2,5-benzimidazole) and this showed that poly (2,5-benzimidazole) blended composite membranes have higher fouling resistance than pristine polysulfone membrane. Also, poly (2,5-benzimidazole) blended polysulfone membranes compared with poly [2,2-(m-phenylene)-5,5-bibenzimidazole] blended membrane which was our previous study. The comparison showed that that poly (2,5-benzimidazole) more effective and functional additive than poly [2,2-(m-phenylene)-5,5-bibenzimidazole] with its higher flux performance, fouling resistance and hydrophilicity characteristics. And also, poly (2,5-benzimidazole) additive more useful than poly [2,2-(m-phenylene)-5,5-bibenzimidazole] for using additive in membranes because of its so easy synthesis in one step, inexpensive monomer and low-cost advantages.

An Experimental Investigation of Humidity Effect on fabrication of Green Ceramic Membrane Prepared Via Dry Phase Inversion

Ceramic membranes are gaining a market demand nowadays due to its characterization and performance. However, in the preparation of ceramic membrane for water and wastewater separation, the processes involved are taking a higher cost. In this work, ceramic membranes were prepared via phase inversion within controlled humidity by using different composition of low cost and green ceramic material, kaolin. The preparation of ceramic membranes was conducted via phase inversion in high humidity; i) 3°C and, ii) room temperature. In this paper, we report on the high humidity affected the viscosity of membrane suspension. Thus, the viscosity of ceramic suspension was giving integration towards membrane characterization and performance. A dense membrane structure was observed when the ceramic membranes were prepared in high humidity and kaolin composition. The morphology of overall, porous and dense structure of ceramic membranes was characterized by field emission scanning electron microscope (FESEM) at x50, x500 and x5000 magnification respectively.

PVDF/palygorskite composite ultrafiltration membranes: Effects of nano-clay particles on membrane structure and properties

Palygorskite (PGS) was incorporated into a polyvinylidene fluoride (PVDF) matrix and PVDF/PGS nanocomposite UF membranes were prepared successfully via a combination of non-solvent-induced phase separation (NIPS) and thermally-induced phase separation (TIPS) processes. The influence of the size of the PGS particles (PGS-L, 450 nm and PGS-S, 302 nm) on the membrane pore structure, morphology, mechanical strength, filtration performance and antifouling property were investigated. The results showed that the PVDF/PGS-S composite membranes had higher porosity and lower overall membrane thickness and skin layer thickness. This directly resulted in better PWF, BSA flux, BSA rejection and FRR for the membranes loaded with PGS-S, when compared with the characteristics of PVDF/PGS-L membranes. Thus, the physical structures are dominant factors in determining the separation performance of PVDF/PGS composite membranes. However, the surface roughness and thermal stability of PVDF/PGS-L membranes were greater than those of PVDF/PGS-S membranes and their surface was also more hydrophilic. Moreover, they exhibited higher mechanical strength owing to stronger binding forces and lower porosity. Thus, the composite properties of PVDF/PGS UF membranes could be adjusted by controlling the scale of the inorganic filler.

Structure–Property Relationships of Pure Cellulose and GO/CEL Membranes Regenerated from Ionic Liquid Solutions

Two types of cellulose membranes were produced by a classical wet phase inversion method from a solution of the polymer in 1-ethyl-3-methylimidazolium acetate (EMIMAc) by coagulation in water and selected primary alcohols. The first type were membranes made from pure cellulose (CEL). The second type were membranes obtained by adding nanosized graphene oxide (GO) to the cellulose solution. The process of precipitation and selection of the coagulant affected the structure of the membranes, which in turn affected their usability and applicability. The results of the presented studies show that the physicochemical properties of the coagulant used (e.g., molecular mass and dipole moment) play important roles in this process. It was found that both the content and dimensions of the pores depended on the molecular mass of the coagulant used. It was also found that the dipole moment of coagulant molecules had a large influence on the volume content of the pores (e.g., the 1-octanol (Oc) membrane had a dipole moment of 1.71 D; Φ = 1.82%). We investigated the effect of the type of coagulant on the porous structure of CEL membranes and how this affected the transport properties of the membranes (e.g., for the distilled water (W) membrane, Jv = 5.24 ± 0.39 L/m2 h; for the Oc membrane, Jv = 92.19 ± 1.51 L/m2 h). The paper presents the results of adding GO nanoparticles in terms of the structure, morphology, and transport properties of GO/CEL membranes (e.g., for composite membrane F (containing 20% GO), Jv = 40.20 ± 2.33 L/m2 h). In particular, it describes their extremely high ability to remove heavy metal ions.

P (VDF‐co‐CTFE)‐g‐P2VP amphiphilic graft copolymers: Synthesis, structure, and permeation properties

A series of amphiphilic graft copolymers of poly (vinylidene fluoride‐co‐chlorotrifluoroethylene)‐g‐poly(2‐vinyl pyridine), P (VDF‐co‐CTFE)‐g‐P2VP, with different degrees of P2VP grafting (from 26.3 to 45.6 wt%) was synthesized via one‐pot atom transfer radical polymerization (ATRP). The amphiphilic properties of P (VDF‐co‐CTFE)‐g‐P2VP graft copolymers allowed itself to self‐assemble into nanoscale structures. P (VDF‐co‐CTFE)‐g‐P2VP graft copolymers were introduced into neat P (VDF‐co‐CTFE) as additives to form blending membranes. When two different solvents, N‐methyl‐2‐pyrrolidone (NMP) and dimethylformamide (DMF), were used, specific organized crystalline structures were observed only in the NMP systems. P (VDF‐co‐CTFE)‐g‐P2VP played a pivotal role in controlling the morphology and pore structure of membranes. The water flux of the membranes increased from 57.2 to 310.1 L m−2 h−1 bar−1 with an increase in the PVDF‐co‐CTFE‐g‐P2VP loading (from 0 to 30 wt%) due to increased porosity and hydrophilicity. The flux recovery ratio (FRR) increased from 67.03% to 87.18%, and the irreversible fouling (Rir) decreased from 32.97% to 12.82%. Moreover, the pure gas permeance of the membranes with respect to N2 was as high as 6.2 × 104 GPU (1 GPU = 10–6 cm3[STP]/[s cm2 cmHg]), indicating their possible use as a porous polymer support for gas separation applications.

Tailoring hydrophilic and porous nature of polysiloxane derived ceramer and ceramic membranes for enhanced bioelectricity generation in microbial fuel cell

Selection of proton conducting membrane is currently a key factor that decides the performance of microbial fuel cell (MFC). Uniaxial pressed polysiloxane-derived ceramer and ceramic membrane with proton conducting fillers like montmorillonite and H3PMo12O40/SiO2 were applied for the first time as separator in MFC. Here, we present a series of polymer-derived ceramic membranes tailored based on pyrolysis temperature and filler addition, in which ion exchange capacity, cation transport number, and oxygen permeability are influenced through the hydrophilic and porous structural property. The maximum power density of MFC with polysiloxane-derived ceramer membrane modified with 20 wt% montmorillonite and 10 wt% H3PMo12O40/SiO2 reached a value of 5.66 W m−3, which was four times higher than that with non-modified polysiloxane-derived ceramer membrane. Furthermore, the specific power recovery per unit cost of the membrane was found to be 2-fold higher than MFC using polymeric Nafion membrane. In contrast, MFC with polysiloxane-derived ceramic membrane modified with 20 wt% montmorillonite delivers 1.2 times lower power density (4.20 W m−3) than that with non-modified macroporous polysiloxane-derived ceramic membrane. Hence, the findings demonstrated that tailoring the hydrophilic and porous structure of the ceramic membrane is a new and promising approach to enhance the performance of MFC.

Tailoring waterproof and breathable properties of environmentally friendly electrospun fibrous membranes by optimizing porous structure and surface wettability

Developing a feasible and efficient waterproof and breathable (W&B) membrane with environmentally friendliness for high-end protective garments is of great significance but very challenging. Herein, an environmentally friendly and high-performing W&B membrane via a facile strategy of one-step electrospinning is created. It has been confirmed that the porous structure and surface wettability greatly affected W&B properties of the electrospun membranes, and excessive utilization of fluorinated segments with low surface energy would not further enhance the hydrophobicity . Consequently, the prepared composite membranes exhibit an exceptional hydrostatic pressure of 88.2 kPa, high water vapor transmission (WVT) rate of 11.8 kg m −2 d −1 , together with comparable mechanical strength of 11.4 MPa, which are expected to reach the requirements of upscale market for high-performing protective garments. • Porous structure and surface chemical properties of the fibrous membranes were subtly regulated. • The effects of pore size and surface wettability on waterproof breathable properties were thoroughly investigated. • The resultant environmentally friendly fibrous membranes exhibited good comprehensive properties.

How rheological behaviors of concentrated starch affect graft copolymerization of acrylamide and resultant hydrogel

Corn starches with different amylose/amylopectin ratios were used to explore the effect of rheological behaviors of concentrated system on the graft copolymerization of acrylamide and resultant hydrogels, which sheds a light on their reactive extrusion process. The viscoelastic moduli of starch melts increased with increasing amylose content (AC), leading to a decreased extent of micro-mixing detected by a reduced rheokinetic rate. With increasing AC, the graft efficiency was decreased but with almost similar monomer conversion (about 87.5%) and nearly equivalent graft content. XRD and SAXS spectra revealed that the extent of retrogradation of the starches were increased and two-phase separation was enhanced for hydrogels with increasing AC. Interestingly, microscopic analysis showed the superabsorbent hydrogel from the starch with AC of 50% exhibited a gridding membrane porous structure, resulting in a higher water absorbent capacity of 550 g/g. This was attributed to the moderate crosslinking and the slightly greater graft content.

Collagen-alginate-nano-silica microspheres improved the osteogenic potential of human osteoblast-like MG-63 cells.

Modular bone tissue engineering is touted as an alternative approach to replace the damaged bone tissue. Hydrogel microcapsules could promote therapeutic properties by providing 3D condition and an increased cell-to-cell interaction. We investigated the osteogenic properties of alginate-nano-silica hydrogels enriched with collagen and gelatin on human osteoblast-like MG-63 cells. For encapsulation, cells were divided into three groups; control (alginate+ nano-silica), collagen (alginate + collagen + nano-silica), and gelatin (alginate + gelatin + nano-silica) and expanded for 28 days. Cell survival was determined by trypan blue staining and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay. To confirm the osteogenic potential, we measured the alkaline phosphatase activity. Alizarin red S staining was used to reveal the existence of hydroxyapatite and transcription BMP-2, osteocalcin and osteonectin evaluated by the real-time polymerase chain reaction. Collagen substrate caused a reduced swelling ratio compared with the control and gelatin groups (P < 0.05). Compared with other groups, collagen had potential to improve mechanical strength and generate porous membrane structure. The addition of collagen (4-fold) and gelatin (1.5-fold) increased cell proliferation rate compared with the control (P < 0.05). Biochemical analysis and Alizarin red S staining showed that collagen-induced osteogenesis by induction of alkaline phosphatase and matrix mineralization. The expression of osteocalcin and BMP-2 was increased in cells from the collagen group. As a result, the combination of natural polymers collagen and gelatin with alginate + nano-silica can increase the osteogenic potential of human osteoblasts.

Modification of surface charge characteristics for unsupported nanostructured titania–zirconia UF/NF membrane top layers with calcination temperature

Ceramic membranes are more advantageous alternatives especially for harsh working conditions when compared with the polymeric membranes. The porous multilayer structure of the ceramic membranes (composed of support, intermediate, and top layers) can be prepared via different oxides. Titania and zirconia, having superior properties, are mainly preferred for the top layer formation. The separation properties of the membrane are both dependent on pore morphology and surface charge of the oxide(s) forming the top layer. The effect of surface charge in separation may be very significant in case of filtration of charged species with relatively lower mass as in the ultrafiltration (UF) and nanofiltration (NF). In this study, unsupported membrane top layers were prepared with varying titania/zirconia ratios by sol-gel technique. Their surface charges at different pH conditions after calcination at varying temperatures (400°, 500°, and 600 °C) were determined. The surface charge of the pure titania (full Ti) top layer was decreasing with the increasing calcination temperature. The highest magnitudes of zeta potential for both acidic and basic conditions were measured via Zr rich top layer (TiZr2575) at calcination temperatures ≥ 500 °C, which was composed of anatase, rutile (titania), and tetragonal (zirconia) phases after calcination. The tailor-made top layer can be prepared with modifications during membrane preparation.

Oxidative crosslinking of copolyimides at sub-Tg temperatures to enhance resistance against CO2-induced plasticization

Polymer crosslinking via thermal oxidation is a well-established technique that enhances the gas selectivity of a polymer membrane, but usually at the expense of lower gas permeability. As thermal oxidation is typically performed at temperatures higher than the glass transition temperature (Tg) of polymers, large energy footprints are incurred while pore structures in asymmetric membranes – the preferred physical configuration of polymer membranes are collapsed. To overcome these drawbacks, here we report a strategy to simultaneously enhance both gas permeability and selectivity via thermal oxidation at sub-Tg temperatures. This was achieved with a copolymer consisting a component that enhanced Tg, and another component with functional groups that decomposed and induced crosslinking at lower temperatures. We used a series of complementary characterization techniques to pinpoint the actual mechanism underpinning our sub-Tg thermal oxidation approach. Crosslinking slightly reduced the d-space between polymer chains but increased polymer fractional free volume and most importantly, inhibited CO2-induced plasticization. The pure CO2 permeability of the crosslinked polymer reached 88.5 Barrer with a CO2/CH4 ideal selectivity of 38.8, while no plasticization behavior was detected at a CO2 pressure up to 30 atm at 35 °C. When separating CO2:CH4 (1:1) mixed gas at total pressures between 10 and 60 atm, the CO2 permeability and CO2/CH4 selectivity of this crosslinked polymer were reduced to 55 Barrer and 29 due to competitive permeation.

Graphene oxide quantum dots embedded polysulfone membranes with enhanced hydrophilicity, permeability and antifouling performance

Graphene oxide (GO) has been demonstrated to be an effective hydrophilic nanofiller to modify the polymeric membranes when forming a mixed matrix structure. GO quantum dots (QDs) are promising candidates to fully exert the rich oxygen containing functional groups due to their unique size induced edge effects. In this work, GO QDs modified polysulfone (PSF) ultrafiltration (UF) membranes were prepared by phase inversion method with various GO QDs loadings (0.1–0.5 wt.%). A proper amount of GO QDs addition led to a more porous and hydrophilic membrane structure. With 0.3 wt.% GO QDs, the membranes showed a 60% increase in permeability (130.54 vs . 82.52 LMH bar–1 ). The pristine PSF membranes had a complete cutoff of bovine serum albumin molecules and it was well maintained with GO QDs incorporated. The membranes with 0.5 wt.% GO QDs exhibited the highest flux recovery ratio of 89.7% and the lowest irreversible fouling of 10.3% (54.5% and 33.3% for the pristine PSF membranes). Our results proved that GO QDs can function as effective nanofillers to enhance the hydrophilicity, permeability and antifouling performance of PSF UF membranes.