Superoleophobic micro-nanostructure surface formation of PVDF membranes by tannin and a condensed silane coupling agent

A tannin-based hybrid coating was coated on the PVDF membrane surface through a simple one-step co-deposition of tannin and KH550. A micro/nano hierarchical structure was formed on the PVDF membrane surface through hydrolysis/condensation of KH550 and Michael addition reaction between oxidized tannin and an amino group revealed by the field-emission scanning electron microscopy, atomic force microscopy and Fourier transform infrared spectroscopy, which established a harsh surface. Abundant hydrophilic groups and high surface roughness endowed the modified membranes with high hydrophilicity and underwater superoleophobicity. The modified PVDF membranes possess excellent oil/water separation and antifouling performance due to the underwater superoleophobicity. Moreover, the modified membrane exhibited outstanding stability.

A tannin-based hybrid coating was coated on the PVDF membrane surface through a simple one-step co-deposition of tannin and KH550.

The oil/water separation process, especially for separating emulsified oil-in-water emulsions, has become an increasingly important part of water treatment as more and more oily wastewater has been produced from industrial processing and oil spills.^1–5 Among many conventional methods, polymer-dominated filtration technology has been widely applied to water treatment for its simple operation, low cost and easy control of pores.^6–10 Polyvinylidene fluoride (PVDF) is a promising membrane material owing to its excellent mechanical and chemical properties. However, the intrinsic hydrophobicity makes PVDF easily fouled during treatment of aqueous solutions containing organic matters, decreasing its service life, which has limited its application in the field of oil/water separation.^11–14 Thus, it is of great importance to improve the hydrophilicity of PVDF porous membranes for highly efficient and eco-friendly separation of oily wastewater.The concept of bio-adhesion stems from mussel adhesion protein (MAP) with broad adhesion potency. MAP rich in catechol, amino acids and 3,4-dihydroxyphenylalanine (DOPA) allows mussels to adhere to a variety of materials. Dopamine containing amine and catechol groups exhibits a molecular structure similar to that of DOPA, and has become a focus of attention as a novel bioadhesive coating.^15–19 Low-cost tannin, which has analogous polyphenols units with dopamine, has also been reported to have excellent interfacial properties and could be coated on a variety of substrates including hydrophobic polymer materials.^20–24 In our previous work,²⁵ we demonstrated that the plant tannin layer can strongly construct on the surface of the PVDF membrane and effectively improve the hydrophilicity and filtration performance of PVDF membrane. In this work, in order to further enhance the underwater superoleophobicity of PVDF membrane for oil-in-water emulsion separation, we constructed a tannin-based hybrid coating on the PVDF membrane surface through a simple one step co-deposition of tannin and (3-aminopropyl) triethoxy-silane (KH550) which is a silane coupling agent containing amino group. On the one hand, hybrid coating can coat on the PVDF membrane surface because of the existence of polyphenols tannin, on the other hand, tannin and KH550 can bond together by means of Michael addition reaction between oxidized tannin and amino of KH550, at the same time, hydrolysis and condensation of KH550 can form crosslinking agents, which can greatly enhance the adhesive ability of coating layer.The modified membranes were fabricated through a simple one step dip-coating method in plant tannin and KH550 hybrid solution in the context of a weak alkaline solution (pH 7.8) and contacting with atmosphere. Fig. 1 shows the co-deposition pathway. The phenolic group of tannin is oxidized to benzoquinone under weakly alkaline conditions and undergoes a Michael addition reaction with the amino group in KH550.^26–28 The alkoxy group of the silane coupling agent has a hydrogen bond with the phenolic hydroxyl group of the tannin after hydrolysis, and will condense with itself to form a silicon-containing oligomer. Tannin and KH550 form a strong coating on the membrane surface through this complex cross-linking structure. The silicon-containing oligomer produced by KH550 changed the surface morphology and roughness of the membrane surface, and the wettability of the membrane surface was significantly improved.Open in a separate windowFig. 1The co-deposition pathway of tannin and KH550 for membrane modification.The surface morphology of the pristine membrane (TK-0) and modified membranes with different adding contents of KH550 were observed via FESEM and AFM, which can be seen in the Fig. 2a and b. As indicated by the FESEM images in Fig. 2a, some spherical particles have formed on the all modified membranes through co-deposition process, but the size and number of particles are greatly different among various membranes. With increasing the content of KH550, the particle size and number sharply increase. Moreover, it can be seen that the top surface of TKN-0.6 is almost completely coated by the particles, which shows a micro/nano hierarchical structure. These phenomena can be rationalized by the formation of hybrid oligomer. The more the content of KH550 in the co-deposition process, the more and bigger hybrid oligomer will be formed through hydrolysis and condensation reaction of KH550. AFM images are shown in Fig. 2b, the roughness obviously increased with extending the content of KH550. This result corresponds with the FESEM and indicates the formation of micro/nano structure. Interestingly, the roughness of TK-0 are higher than TKN-0.15, it can be explained that the main part of the coating layer on the TKN-0.15 are tannin due to the less adding content of KH550, which can form a homogeneous coating with a few particles. In contrast to polydopamine, our modified membranes show a slight change in color (Fig. S1^†), which indicates that this novel process has more widely application.Open in a separate windowFig. 2Characterizations of morphology and chemical composition of the different PVDF membranes. (a) FESEM images of the membrane top surface. (b) AFM images of the membrane top surface. (c) XPS spectra and atomic ratio of the membrane surface.The deposition ratio (DR) can directly reflect the amount of coating layer which deposited on the membranes. As increasing of the content of KH550, the DR of the modified membranes are increasing linearly, which demonstrates that more hybrid oligomer was formed and anchored on the membrane surface (Fig. S2^†). The surface chemical elements and groups are critical for wettability of materials. The XPS spectrum of the membranes were shown in Fig. 2c and Table S1,^† and the elemental compositions of different modified membranes were determined. The presence of oxygen, nitrogen, and silicon elements can be confirmed in the XPS spectrum, wherein the oxygen element is derived from tannin and KH550, and the nitrogen and silicon elements are derived only from PVP. Therefore, the Si/O, Si/C and O/C ratio can illustrate the content of tannin and KH550 applied to the surface of the membrane to a certain extent. The atomic ratio about Si/O, Si/C and O/C are all raising with the increase of KH550, this shows that more silicon-containing oligomers are fixed on the membrane surface. These indicate that tannin and KH550 have co-deposited on membrane surface successfully. This result is also confirmed by ATR-FTIR measurement (Fig. S3^†). Four notably absorption peaks at 1608 cm⁻¹, 1499 cm⁻¹, 1323 cm⁻¹, and 1098 cm⁻¹ were detected, which assigned to skeletal vibration of aromatic rings, N–H bending vibrations, C–N stretching vibration and Si–O–Si stretching vibration, respectively. These imply that a lot of hydrophilic groups have coated on the hydrophobic PVDF membrane surface, which can significantly improve its hydrophilicity.As an interfacial issue, membrane surface energy and surface roughness can simultaneously affect the wettability.^29–32 According to Wenzel''s model, roughness can increase the actual contact area between droplet and substrate, thus improving the hydrophilicity and total surface energy of hydrophilic substrate.^33,34 In this work, the hybrid coating layer contains abundant hydrophilic groups, thus, the hydrophilicity of modified PVDF membranes have improved greatly. As shown in Fig. 3a, the water contact angle (WCA) of modified membranes sharply decreased comparing with TK-0, and the minimum WCA value was occurred to TKN-0.3 rather than TKN-0.6, which indicates that this modified process exists a threshold WCA with the increase of KH550. The water droplet permeation of modified membrane can be observed in Fig. 3b, the WCA rapidly decrease to 0° within 5 s, which exhibit much high hydrophilicity. Fig. 3b shows the dynamic underwater oil-adhesion property of TKN-0.6. An extremely low oil-adhesion performance was detected, it can be explained that a water barrier has formed on the modified PVDF membrane surface under water due to the high hydrophilicity, which can greatly reject the oil droplets and effectively decrease fouling. The underwater oil contact angle (OCA) directly revealed the excellent oleophobicity of our modified membranes as shown in Fig. 3c and d. The underwater chloroform contact angle of different membranes are raising with the increase of KH550. The value of TK-0 is 127°, but the modified membranes are all higher than 150°, the value of TKN-0.6 is 163° which show a outstanding oleophobicity. Therefore, underwater different oils (dichloromethane, toluene, petroleum ether and diesel) contact angle of TKN-0.6 were measured to further characterize the oil repelling performance. It can be seen that the OCA of four oils are 161°, 162°, 163° and 156° respectively, indicating a underwater superoleophobic property which is critical to oil/water separation. This underwater superoleophobicity is caused by the cooperation of high hydrophilicity and roughness of tannin and KH550 hybrid coating.Open in a separate windowFig. 3Characterizations of hydrophilicity and oleophobicity of the different PVDF membranes. (a) Water contact angle of membrane surface. (b) Water droplet permeation and dynamic underwater oil-adhesion measurements. (c) Underwater chloroform contact angle of various membranes. (d) Different oils underwater contact angle of TKN-0.6.The pure water flux, emulsion filtration flux and flux recovery ratio (FRR) were measured by a vacuum driven filtration system (Fig. S4^†) to evaluate filtration performance, as shown in Fig. 4. The pristine membrane and modified membranes were all wetted before measurement to avoid the compaction effect. The pure water flux was shown in Fig. 4a, the flux value of pristine membrane is 14 133 L m⁻² h⁻¹, however, the modified membranes are 18 982 L m⁻² h⁻¹, 19 863 L m⁻² h⁻¹ and 18 480 L m⁻² h⁻¹ respectively, which are much higher than pristine membrane. This result is triggered by the excellent hydrophilicity of coating layer on modified membranes. Meanwhile, the slightly different flux values among the modified membranes can be detected, which result in the blocking of micro/nano particles. The emulsion filtration performance is shown in Fig. 4b, the prepared emulsion had an average particle diameter of 487 nm as measured by a Malvern particle size analyzer. The flux value of pristine membrane is 782 L m⁻² h⁻¹, however, all the modified membranes exhibit high emulsion filtration flux, as the flux of 2512 L m⁻² h⁻¹, 4178 L m⁻² h⁻¹ and 4632 L m⁻² h⁻¹ for TKN-0.15, TKN-0.3 and TKN-0.6 respectively were obtained. This indicates the great emulsion separation efficiency of modified membranes. After filtration, the toluene-in-water emulsion is transformed into transparent exhibited by the inset images. The underwater superoleophobic property of modified membranes is the main reason for this outstanding filtration performance. The antifouling property was characterized by flux recovery ratio (FRR) shown in Fig. 4c. It can be seen that the FRR value of modified membranes are all higher than the pristine membrane (82%) and all more than 98%, even reaches 100% for TKN-0.3 and TKN-0.6. This excellent antifouling property should give the credit to the superoleophobic property which is aroused by the high hydrophilicity and roughness of tannin and KH550 hybrid coating. To measure the stability of modified membranes, a water rinsing experiment was taken and underwater chloroform contact angle was measured to evaluate the stability of underwater superoleophobic property of modified membrane (Fig. 5a). It is found that the underwater chloroform contact angle of TKN-0.3 stabilized at 157–160° during seven days rinsing, which disclosed the outstanding stability of hybrid coating coated PVDF membrane. As shown in Fig. 5b, the emulsion flux declines gradually with the increase of time due to the fouling of the membrane. The emulsion flux was almost completely recovered by washing with pure water after 1 hour of emulsion filtration, indicating the stability of the TKN-0.3 membrane.Open in a separate windowFig. 4Filtration performance of the different membranes. (a) Pure water flux of the wetted PVDF membranes. (b) Flux of toluene-in-water emulsion, inset images is the emulsion before (left) and after (right) filtration. (c) Flux recovery ratio after the emulsion separation.Open in a separate windowFig. 5(a) The underwater chloroform contact angle of TKN-0.3 membrane during the pure water rinsing test for 7 days. (b) Emulsion flux and flux recovery over three cycles.In summary, a hydrophilic and underwater superoleophobic PVDF membrane was fabricated through a simple one step co-deposition method, the low cost tannin and commercial KH550 hybrid coating layer was coated on PVDF membrane surface successfully. A micro/nano hierarchical structure, which can be adjusted by the adding content of KH550, was observed on the modified membrane surface. It can increase the roughness of membrane surface and obtain an excellent hydrophilic and underwater superoleophobic property. The underwater superoleophobicity endows the membrane with a superior antifouling property (underwater oil contact angle reaches 160°, FRR reaches 100%) and high oil/water separation efficiency (the emulsion flux reaches 4600 L m⁻² h⁻¹). The great performance indicates that this strategy is promising for practical applications in the field of treating oily wastewater.     

Improved filtration performance and antifouling properties of polyethersulfone ultrafiltration membranes by blending with carboxylic acid functionalized polysulfone

To improve the filtration performance and antifouling properties of ultrafiltration (UF) membranes, novel polymer blend UF membranes were fabricated in this study. Carboxylic acid functionalized polysulfone (PSFNA) was synthesized by modifying polysulfone (PSF) with 6-hydroxy-2-naphthoic acid (HNA). A series of polymer blend UF membranes were fabricated by adding different amounts of PSFNA into polyethersulfone (PES) to form a homogeneous casting solution. The influences of PSFNA on the morphology, thermal stability, hydrophilicity, filtration performance and antifouling properties of the blend membranes were investigated. The results indicated that by adding PSFNA into PES membranes, the finger-like pores in the membranes became larger, and the porosity and surface hydrophilicity of the membranes were improved. Compared with the pristine PES membrane, PES/PSFNA membranes demonstrated improved filtration performance, resulting in both increased water flux and higher bovine serum albumin (BSA) rejection. At a feed pressure of 0.1 MPa, the PES/PSFNA membrane with 4.0 wt% PSFNA had a pure water flux of 478 L m⁻² h⁻¹, which was 1.7 times higher compared with the PES membrane (287 L m⁻² h⁻¹). In addition, the antifouling properties of PES membranes were also enhanced with the addition of PSFNA. The PES/PSFNA membranes with 3.0 wt% PSFNA had a total fouling ratio (TFR) of 49.6%, as compared with 62.5% for PES membranes.

Ultrafiltration membranes with improved filtration performance and antifouling properties have been synthesized through blending polyethersulfone with carboxylic acid functionalized polysulfone.     

Fabrication of photo-Fenton self-cleaning PVDF composite membrane for highly efficient oil-in-water emulsion separation

The anti-fouling performance of membranes is an important performance in the separation of oil/water. However, the membrane with anti-fouling performance will also have surface scaling phenomenon when it runs for a long time. Therefore, there is still a great demand for stain-resistant membranes with good self-cleaning ability and high flux recovery rate. Based on this, this paper firstly prepared a hydrophilic membrane with carboxyl group and carboxyl ion by blending poly(ethylene-alt-maleic anhydride) (PEMA) and polyvinylidene fluoride (PVDF), and then prepared a self-cleaning composite membrane by in situ mineralization of β-FeOOH particles on the surface of the membrane for efficient oil-in-water emulsion separation. A large number of –COOH/COO⁻ and β-FeOOH particles on the membrane surface make the composite membrane have strong hydrophilic properties (WCA = 20.34°) and underwater superoleophobicity (UOCA = 155.10°). These composite membranes have high separation efficiency (98.8%) and high flux (694.56 L m⁻² h⁻¹ bar⁻¹) for soybean oil-in-water emulsion. Importantly, the as-prepared membrane shows excellent flux recovery rate (over 99.93%) attributed to the robust photo-Fenton catalytic activity of β-FeOOH, and the β-FeOOH is chemically bonded to the as-prepared membrane, which makes the as-prepared membrane have good reusability. This work provides hope for the application of self-cleaning membranes in the construction of anti-fouling membranes for wastewater remediation.

The anti-fouling performance of membranes is an important performance in the separation of oil/water.     

Fabrication and characterization of polyamide thin-film composite membrane via interfacial polycondensation for pervaporation separation of salt and arsenic from water

Pervaporation, mainly utilized to separate azeotropic mixtures, has been paid much attention for desalination in recent years due to its numerous advantages. The membranes based on thin-film composite structure have gained great interest in pervaporation due to their thin thickness, controllable hydrophilicity, and crosslinking density which affects the permeation flux and selectivity of the membranes. In this study, a polyamide thin-film composite (PA-TFC) membrane was fabricated through interfacial polymerization between amine monomers and trimesoyl chloride (TMC) on a polysulfone porous substrate (PSf). Four different diamine monomers, including ethylenediamine (EDA), triethylenetetramine (TETA), m-phenylenediamine (MPD), and piperazine (PIP) were used to investigate the effect of the monomers on the pervaporation performance of the resulting membrane for separation of sodium chloride (NaCl) and arsenate (As(v)) aqueous solution. The physicochemical properties of the membrane were characterized using attenuated total reflection Fourier transform infrared (ATR-FTIR), scanning electron microscopy (SEM), atomic force microscopy (AFM), and pure water contact angle measurement. Furthermore, the performance of the fabricated membranes was studied by pervaporation separation of 0.15 mg L⁻¹ As(v) and 5 g L⁻¹ NaCl aqueous solution at 40 °C, respectively. The results show that the rejections of the membrane are insignificantly affected by the chemical structures of the amines, and both the As(v) rejection and NaCl rejection are higher than 99.9%. However, the permeation flux decreases in the order of PIP-TMC membrane > TETA-TMC membrane ∼ EDA-TMC membrane > MPD-TMC membrane. Furthermore, the operating conditions are found to affect the separation performance of the PIP-TMC membrane significantly. In particular, the elevating operation temperature profoundly increases the permeation flux, while the increase in high salt concentration leads to a slight decrease in rejection but a significant decline in permeation flux. The derived membrane shows a reasonable permeation flux of 16.1 kg m⁻² h⁻¹ and ca. 99.9% rejection for 1.5 mg L⁻¹ As(v) removal, as well as 13 kg m⁻² h⁻¹ and 99.3% rejection for 30 g L⁻¹ NaCl separation at 60 °C. The sufficient permeation flux and good rejection of As(v) and NaCl of the membrane suggested the promising application of PA-TFC membrane for pervaporation removal of toxic arsenic from water and desalination of seawater.

Pervaporation, mainly utilized to separate azeotropic mixtures, has been paid much attention for desalination in recent years due to its numerous advantages.     

High-performance membranes with full pH-stability

Following current strong demands from, among others, paper, food and mining industries, a novel type of nanofiltration membrane was developed, which displays excellent performance in terms of selectivity/flux with a unique combination of chemical stability over the full (0–14) pH-range and thermal stability up to 120 °C. The membrane consists of polyvinylidene fluoride grafted with polystyrene sulfonic acid. The optimum membrane showed water permeances of 2.4 L h⁻¹ m⁻² bar⁻¹ while retaining NaCl, MgSO₄ and Rhodamine B (479 Da) for respectively ≈60%, ≈80% and >96%.

Grafted PVDF membranes were synthesized with improved nanofiltration properties (R_NaCl ≈ 60%) and stability over the full pH range.     

Anhydrous proton conductivity of electrospun phosphoric acid-doped PVP-PVDF nanofibers and composite membranes containing MOF fillers

A high-temperature proton exchange membrane was fabricated based on polyvinylidene fluoride (PVDF) and polyvinylpyrrolidone (PVP) blend polymer nanofibers. Using electrospinning method, abundant small ionic clusters can be formed and agglomerated on membrane surface, which would facilitate the proton conductivity. To further enhance the conductivity, phosphoric acid (PA) retention as well as mechanical strength, sulfamic acid (SA)-doped metal–organic framework MIL-101 was incorporated into PVP-PVDF blend nanofiber membranes. As a result, the anhydrous proton conductivity of the composite SA/MIL101@PVP-PVDF membrane reached 0.237 S cm⁻¹ at 160 °C at a moderate acid doping level (ADL) of 12.7. The construction of long-range conducting network by electrospinning method combined with hot-pressing and the synergistic effect between PVP-PVDF, SA/MIL-101 and PA all contribute to the proton conducting behaviors of this composite membrane.

A composite SA/MIL101@PVP-PVDF membrane was fabricated via electrospinning and reached a conductivity of 0.237 S cm⁻¹ at 160 °C with a moderate acid doping level (12.7).     

Fabrication of an antifouling PES ultrafiltration membrane via blending SPSF

Sulfonated polysulfone (SPSF) with different sulfonation degrees (10%, 30%, and 50%) was added to polyethersulfone (PES) to improve the separation and antifouling performance of polyethersulfone ultrafiltration membranes. The PES/SPSF blend ultrafiltration membrane was prepared by the non-solvent induced phase inversion method (NIPS), and the effect of sulfonation degree on the ultrafiltration performance was studied. The compatibility of SPSF and PES was calculated by the group contribution method, and confirmed by differential scanning calorimetry (DSC). The morphology and surface roughness of the membrane were characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM), the chemical composition of the membrane was analyzed by X-ray photoelectron spectroscopy (XPS) and infrared spectroscopy (FTIR), and the permeability and anti-fouling performance of the blend membrane were studied through filtration experiments. The research shows that the flux and anti-fouling performance of the blend membrane have been improved after adding SPSF. When the sulfonation degree of the SPSF is 30%, the pure water flux of the blend membrane can reach 530 L m⁻² h⁻¹, the rejection rate of humic acid (HA) is 93%, the flux recovery rate of HA increases from 69.23% to 79.17%, and the flux recovery rate of BSA increases from 72.56% to 83%.

The chemical structures of (a) PES and (b) SPSF.     

Enhanced desalination using a three-layer OTMS based superhydrophobic membrane for a membrane distillation process

Superhydrophobic membranes are essential for improved seawater desalination. This study presents the successful casting of a three-layered membrane composed of a top superhydrophobic coating onto a polypropylene (PP) mat through simple sol–gel processing of octadecyltrimethoxysilane (OTMS), and the bottom layer was casted with hydrophilic poly(vinyl alcohol) (PVA) by using a knife casting technique; this membrane represents a novel class of improved-performance membranes consisting of a top superhydrophobic coating onto a hydrophobic PP mat and a hydrophilic layer (PVA) at the bottom. OTMSs are well known low-surface-energy materials that enhance superhydrophobicity, and they were observed to be the ideal chemical group for increasing the hydrophobicity of the PP mat. The PVA layer acted as base layer absorbing the condensed vapor and thus enhancing the vapor flux across the membrane. The hybrid three-layered membrane exhibited superhydrophobicity, with an average contact angle of more than 160°, and demonstrated high performance in terms of rejection and water flux. This study also examined the pore size distribution, surface roughness, surface area, tensile strength, water flux, and salt rejection of the fabricated membrane. The salt rejection level was calculated to be 99.7%, and a high permeate flux of approximately 6.7 LMH was maintained for 16 h.

Superhydrophobic membranes are essential for enhanced desalination by utilizing MD.     

Casting of a superhydrophobic membrane composed of polysulfone/Cera flava for improved desalination using a membrane distillation process

Superhydrophobic membranes are necessary for effective membrane-based seawater desalination. This paper presents the successful fabrication of a novel electrospun nanofibrous membrane composed of polysulfone and Cera flava, which represents a novel class of enhanced performance membranes consisting of a superhydrophobic nanofibrous layer and hydrophobic polypropylene (PP). Cera flava, which helps lower the surface energy, was found to be the ideal additive for increasing the hydrophobicity of the polysulfone (PSF) polymeric solution because of its components such as long-chain hydrocarbons, free acids, esters, and internal chain methylene carbons. In the fabricated membrane, consisting of 10 v/v% Cera flava, the top PSF–CF nanofibrous layer is active and the lower PP layer is supportive. The hybrid membrane possesses superhydrophobicity, with an average contact angle of approximately 162°, and showed high performance in terms of rejection and water flux. This work also examined the surface area, pore size distribution, fiber diameter, surface roughness, mechanical strength, water flux, and rejection percentage of the membrane. The salt rejection was above 99.8%, and a high permeate flux of approximately 6.4 LMH was maintained for 16 h of operation.

Superhydrophobic membranes for effective MD desalination.     

Novel fabrication of PSSAMA_Na capped silver nanoparticle embedded sodium alginate membranes for pervaporative dehydration of bioethanol

Polystyrene-4-sulfonic acid co maleic acid sodium salt (PSSAMA_Na) capped silver nanoparticle (Ag_Np) embedded sodium alginate (Na-Alg) nanocomposite membranes have been developed to improve the pervaporation (PV) dehydration of bioethanol. The effect of PSSAMA_Na capped Ag_Nps on the micro-morphology, physicochemical properties and separation performance of the derived membranes was analyzed as a function of temperature at the azeotropic composition of the bioethanol–water mixture. WAXD analysis shows a decrease in crystalline domains with the increase in PSSAMA_Na capped Ag_Nps content and confirms the presence of Ag_Nps. DSC analysis demonstrated that the hydrophilic nature enhances as the PSSAMA_Na capped Ag_Nps content increases in the membrane matrix. Further, both total permeation flux and separation selectivity were increased with an increase in PSSAMA_Na capped Ag_Nps content. The results revealed that the membrane with 3 mass% of PSSAMA_Na capped Ag_Nps exhibited the highest permeation flux (13.40 × 10⁻² kg m⁻² h⁻¹) and separation selectivity (11 406) at 30 °C which indicate its better PV performance. The total permeation flux and permeation flux of water values were close to each other, which confirms that the membranes can be efficiently used to remove the water from azeotropic aqueous bioethanol.

Polystyrene-4-sulfonic acid co maleic acid sodium salt (PSSAMA_Na) capped silver nanoparticle (Ag_Np) embedded sodium alginate (Na-Alg) nanocomposite membranes have been developed to improve the pervaporation (PV) dehydration of bioethanol.     

Fabrication of a PVDF membrane with tailored morphology and properties via exploring and computing its ternary phase diagram for wastewater treatment and gas separation applications

We report a simple approach for tailoring the morphology of poly(vinylidene fluoride) (PVDF) membranes fabricated using a nonsolvent induced phase separation (NIPS) method that sustains both the hydrophilic and hydrophobic properties. Various membrane structures, i.e. skin layers and whole membrane structures as well, were obtained via an experimental method based on the obtained and computed ternary phase diagram. The nonsolvent interactions with polymer solution resulted in the different forms and properties of a surface layer of fabricated membranes that affected the overall transport of solvent and nonsolvent molecules inside and outside the bulk of the fabricated membranes. The resulting morphology and properties were confirmed using the 3D optical profiler, SEM, FT-IR and XRD methods. The effect of binary interaction parameters on the morphology of the fabricated membranes and on their separation performance was tested using water/oil mixture and gas separation. Both hydrophobic and hydrophilic properties of PVDF showed the excellent durable separation performance of the prepared membranes with 92% of oil separation and the maximum flux of 395 L h⁻¹ m⁻² along with 120 min of long-term stability. CO₂ separation from H₂, N₂, CH₄ and SF₆ gases was performed to further support the effect of tuned PVDF membranes with different micro/nanostructured morphologies. The gas performance demonstrated ultrahigh permeability and a several-fold greater than the Knudsen separation factor. The results demonstrate a facile and inexpensive approach can be successfully applied for the tailoring of the PVDF membranes to predict and design the resulting membrane structure.

We report a simple approach for tailoring the morphology of poly(vinylidene fluoride) (PVDF) membranes fabricated using a nonsolvent induced phase separation (NIPS) method that sustains both the hydrophilic and hydrophobic properties.     

Flexible electrospun PVDF–BaTiO3 hybrid structure pressure sensor with enhanced efficiency

Ceramic doped-polymer structures as organic and inorganic hybrid structures constitute a new area of advanced materials for flexible and stretchable sensors and actuators. Here, uniform ceramic-polymer composites of tetragonal BaTiO₃ and polyvinylidene fluoride (PVDF) were prepared using solution casting to improve the pressure sensitivity. By introducing Ba–TiO₃ nanoparticles to PVDF nanofibers, piezoelectricity and pressure sensitivity of hybrid nanofiber mats were significantly improved. In addition, we proposed a novel flexible and stretchable multilayered pressure sensor composed of electrospun nanocomposite fibers with high electrical sensitivity up to 6 mV N⁻¹ compared to 1.88 mV N⁻¹ for the pure PVDF sensors upon the application of cyclic loads at 2.5 Hz frequency and a constant load of 0.5 N. Indeed, this work provides a composition-dependent approach for the fabrication of nanostructures for pressure sensors in a wide variety of wearable devices and technologies.

A hybrid structure composed of organic and inorganic piezoelectric fibrous material was developed as a flexible and stretchable pressure sensor. A separately sprayed configuration has the best performance for low frequency and low-pressure conditions.     

A highly hydrophilic benzenesulfonic-grafted graphene oxide-based hybrid membrane for ethanol dehydration

A new type of hybrid membrane was prepared by blending sodium alginate (SA) with benzenesulfonic-grafted graphene oxide (BS@GO), which showed higher hydrophilicity and more defects or edges than GO to create channels for the transfer of water molecules. BS@GO was synthesized by reacting aryl diazonium salts with graphene oxide (GO). The BS@GO sheets were aligned parallelly to the membrane surface and affected the interactions between the SA chains. BS@GO could improve the hydrophilicity and pervaporation properties of SA-based hybrid membranes. Also, compared to GO fillers, BS@GO fillers could supply higher water permeance to improve the pervaporation flux and separation factor. For the pervaporation of 90 wt% aqueous ethanol at 343 K, the optimum hybrid membrane with 1.5 wt% BS@GO in the SA matrix showed the maximum permeate flux of 703 ± 89 g m⁻² h⁻¹ (1.4 times higher than that of an SA membrane), and the highest separation factor was 5480 ± 94 (5.6 times higher than that of the SA membrane). Moreover, the hybrid membrane exhibited good stability and separation ability during long-term testing.

A new type of hybrid membrane was prepared by blending sodium alginate with benzenesulfonic-grafted graphene oxide, which showed higher hydrophilicity and more defects or edges than GO to create channels for the transfer of water molecules.     

Dopamine incorporating forward osmosis membranes with enhanced selectivity and antifouling properties

A new type of polyamide thin-film composite forward osmosis (FO) membranes were prepared by controlling dopamine self-polymerization in the aqueous phase during interfacial polymerization. The as-prepared membranes were investigated by attenuated total reflection Fourier transform infrared, X-ray photoelectron spectroscopy, field-emission scanning electron microscopy, atomic force microscopy and water contact angle measurements. The influence of the dopamine self-polymerization degree with different polydopamine particle sizes on membrane morphologies and chemical properties was studied by regulating dopamine concentrations in the aqueous phase. FO performance of the membrane was evaluated under two different modes, i.e. active layer facing draw solution (AL-DS) and active layer facing feed solution (AL-FS). The optimized FO membranes achieved a doubly enhanced water flux (22.08 L m⁻² h⁻¹) compared with the control membrane without dopamine incorporation, and a half-reduced reverse salt flux (32.77 mmol m⁻² h⁻¹) with deionized water as the feed and 1 M NaCl as the draw in the AL-FS mode. The optimized FO membrane showed a significantly reduced structural parameter (176 μm) compared with the control membrane (635 μm), indicating the minimised internal concentration polarization. Moreover, the new FO membranes had less flux decline than the control membrane, suggesting the improved antifouling performance of the membrane. Incorporation of dopamine during interfacial polymerization can be an effective strategy to fabricate high-performance FO membranes with excellent antifouling properties.

Incorporation of dopamine enhanced selectivity and antifouling properties of novel TFC polyamide FO membranes.     

Enhanced performance of porous forward osmosis (FO) membrane in the treatment of oily wastewater containing HPAM by the incorporation of palygorskite

Since the emergence of forward osmosis (FO), low energy requirements, low fouling propensity and high-water recovery have made it one of the most promising water purification technologies. However, there have been few reports focusing on the treatment of polymer flooding produced water (PFPW) using FO technology up to now. In the present work, porous FO membranes with/without palygorskite (Pal) nanoparticles were utilized as the separation membrane to evaluate the potential of a porous FO membrane in the treatment of oily wastewater containing HPAM and the effect of Pal nanoparticles on the FO performance was investigated. When the loading concentration of Pal in the membrane was 0.75 wt%, the water flux could reach 37.67 L m⁻² h⁻¹ by using 4 g L⁻¹ poly(sodium-p-styrenesulfonate) (PSS) as draw solution under a cross-flow rate of 18.5 cm s⁻¹, which was much higher than that for pure polysulfone (PS) membranes. Besides, the comparison between ultrafiltration (UF) and FO performance in treating HPAM solution indicated that FO possessed better antifouling capacity, since less decline and higher recovery of water flux were observed during the FO process. Furthermore, recycling the draw solution gave an almost unchanged water flux, which suggested the feasibility of draw solute regeneration in the FO process. This work broadens the application field of porous FO technology and may pave a new way in the treatment of PFPW.

Porous forward osmosis (FO) membranes with/without palygorskite (Pal) nanoparticles were utilized as the separation membrane to evaluate the potential of porous FO membrane in the treatment of oily wastewater containing HPAM.     

A graphene oxide polymer brush based cross-linked nanocomposite proton exchange membrane for direct methanol fuel cells

Functional polymer brush modified graphene oxide (FPGO) with functional linear polysiloxane brushes was synthesized via surface precipitation polymerization (sol–gel) and chemical modification. Then, FPGO was covalently cross-linked to the sulfonated polysulfone (SPSU) matrix to obtain novel SPSU/FPGO cross-linked nanocomposite membranes. Meanwhile, SPSU/GO composite membranes and a pristine SPSU membrane were fabricated as control groups. Reduced agglomeration of the inorganic filler and better interfacial interaction, which are benefit to increase diffusion resistance of methanol and to generate continuous channels for fast proton transportation at elevated temperature, were observed in SPSU/FPGO cross-linked membranes. Moreover, the enhanced membrane stability (thermal, oxidative and dimensional stability) and good mechanical performance also guaranteed their proton conducting durability. It is noteworthy that the SPSU/FPGO-1 cross-linked membrane possesses the best comprehensive properties among all the prepared membranes and Nafion®117, it acquires the highest proton conductivity of 0.462 S cm⁻¹ at 90 °C under hydrated conditions together with a low methanol permeability of 1.71 × 10⁻⁶ cm² s⁻¹ at 30 °C. The resulting high membrane selectivity displays the great potential of the SPSU/FPGO cross-linked membrane for DMFCs application.

A novel proton exchange nanocomposite which was cross-linked by functional graphene oxide polymer brushes shows interesting and comprehensive advantages for DMFCs.     

Design of a novel poly(aryl ether nitrile)-based composite ultrafiltration membrane with improved permeability and antifouling performance using zwitterionic modified nano-silica

Zwitterionic nano-silica (SiO₂ NPs) obtained by lysine surface modification was used as a hydrophilic inorganic filler for preparing a poly(aryl ether nitrile) (PEN) nanocomposite membrane via an immersion precipitation phase inversion method. The effects of zwitterionic SiO₂ NPs addition on the morphology, separation and antifouling performance of the synthesized membranes were investigated. Zwitterionic surface modification effectively avoided the agglomeration of SiO₂ NPs. The PEN/zwitterionic SiO₂ NPs composite membranes exhibited improved porosity, equilibrium water content, hydrophilicity and permeability due to the introduction of hydrophilic SiO₂ NPs in the casting solution, and the optimal pure water flux was up to 507.2 L m⁻² h⁻¹, while the BSA rejection ratio was maintained at 97.4%. A static adsorption capacity of 72.9 μg cm⁻² and the FRR up to 85.3% in the dynamic antifouling experiment proved that the introduction of zwitterionic SiO₂ NPs inhibited irreversible fouling and enhanced the antifouling ability of the PEN membrane.

Zwitterionic nano-silica (SiO₂ NPs) obtained by lysine surface modification was used as a hydrophilic inorganic filler for preparing a poly(aryl ether nitrile) (PEN) nanocomposite membrane via an immersion precipitation phase inversion method.     

Study on thermal behavior and kinetics of Al/MnO2 poly(vinylidene fluorine) energetic nanocomposite assembled by electrospray

To explore the effect of the addition of poly(vinylidene fluorine) (PVDF) to a nanothermite system, an Al/MnO₂/PVDF energetic nanocomposite was prepared using an electrospray method, Al/MnO₂ nanothermite was prepared as a control group. Then, the energetic nanocomposite and nanothermite were tested and analyzed by XRD, FE-SEM and TG-DSC, and the reaction products were collected. The results show that energetic nanocomposite would have three obvious exothermic peaks in the range of room temperature to 800 °C with a total more than 1700 J g⁻¹ heat release while the control experiment, Al/MnO₂ nanothermite, could be found one exothermic peak with a 1100 J g⁻¹ heat release. The residues are mainly MnAl₂O₄, MnF₂ and AlF₃ which indicates that Al/MnO₂/PVDF energetic nanocomposite could make full use of manganese oxide. Finally, thermal analysis experiments were carried out under different heating rates to calculate the activation energy. The calculation results show that the addition of PVDF could significantly reduce the activation energy, which would help spark the thermite at comparatively low energy and temperature.

To explore the effect of the addition of poly(vinylidene fluorine) (PVDF) to a nanothermite system, an Al/MnO₂/PVDF energetic nanocomposite was prepared using an electrospray method, Al/MnO₂ nanothermite was prepared as a control group.     

Design of ultrathin hybrid membranes with improved retention efficiency of molecular dyes

Ultrathin layers of 2,2,6,6-Tetramethyl-1-piperidinyloxy (TEMPO) Oxidized Cellulose Nanofibers (TOCNF) embedded with Graphene Oxide nanosheets (GOs) in different ratios were built, via the blade coating technique, on a polyvinylidene difluoride (PVDF) substrate to obtain superior membranes for separating water pollutants from aqueous media. Cellulose nanofiber–graphene oxide hybrid materials have shown a great potential for water purification due to their active microporous structure with extended areas rich in negatively charged carboxyl functional groups capable of adsorbing positively charged contaminants efficiently. In contrast to the pristine free-standing TOCNF films, which are completely impermeable, the ultrathin (68 nm thick) hybrid coating with a 100 : 1 TOCNF : GO ratio showed a stable water permeability (816 ± 3.4 L m⁻² h⁻¹ bar⁻¹) higher than that of common polymeric membranes, and a very efficient size selectivity during filtration of water contaminated by various types of dyes. The membranes had high retention efficiency (82–99%) for dyes with hydrated radii greater than ≈0.5 nm due to the favorable combination of electrostatic/hydrophobic interactions with the hybrid matrices and steric entrapment controlled by the pore size. This was confirmed by theoretical calculations that revealed both the structure and dynamic behavior of the dyes in the complex environment of the membranes.

Nanocellulose–graphene oxide ultrathin coatings for water purification membranes with excellent swelling resistance, permeability and dyes retention are presented.     

Preparation of super-hydrophilic polyphenylsulfone nanofiber membranes for water treatment

Electrospun nanofiber membrane-supported thin film composite (TFC) membranes exhibit great potential in water purification. In this work, electrospun polyphenylsulfone (PPSU) nanofiber membranes were prepared and modified by heat and plasma treatments. The resulting membranes were used as support layers for biomimetic TFC-based forward osmosis membranes. Thermal treatment transformed a loose non-woven nanofiber structure into a robust interconnected 3-dimensional PPSU network displaying a 930% increase in elastic modulus, 853% increase in maximum stress, and two-fold increase in breaking strain. Superior hydrophilicity of PPSU nanofiber membranes was achieved by low-pressure plasma treatment, changing the contact angle from 137° to 0°. The fabricated exemplary TFC-based forward osmosis membrane showed an osmotic water flux J_w > 14 L m⁻² h⁻¹ with a very low reserve salt flux J_s (J_s/J_w = 0.08 g L⁻¹) demonstrating the potential for making high quality membranes for water treatment using PPSU-based support layers for TFC membranes.

A 3-dimensional nanofiber membrane with superior hydrophilicity and mechanical properties significantly improves flux and salt rejection in thin film forward osmosis.