Pristine Polyvinylidene Fluoride Membrane Research Articles

Anti-wetting polyvinylidene fluoride membrane incorporated with hydrophobic polyethylene-functionalized-silica to improve CO2 removal in membrane gas absorption

Membrane gas absorption (MGA), an attractive technology for carbon dioxide (CO2) separation was used in conjunction with polyvinylidene fluoride (PVDF) membranes. In order to improve the membrane’s CO2 absorption flux and gas selectivity, silica nanoparticles functionalized with low-density polyethylene (LDPE) were incorporated into PVDF matrix to produce an anti-wetting protection layer to overcome membrane wetting; a critical issue in regards to the MGA process. FESEM micrograph and EDX line-scan profile clearly show that f-silica were dispersed evenly in the composite membrane due to the better interactions with PVDF polymer chains in the presence of LDPE. LEPw, a measure of membrane wettability, was improved from 4.75 bar (pristine PVDF membrane) up to 13.55 bar for silica/PVDF composite membrane. More importantly, embedding silica nanoparticles into PVDF matrix improved both the CO2 absorption flux from 0.66 × 10−4 mol/m2 s to 2.32 × 10−4 mol/m2 s (3.5 times higher) and selectivity from 3.48 up to 29.4 (8.5 times higher) in comparison to the pristine PVDF membrane. In short, the results indicated that f-silica was effective in producing anti-wetting protection in PVDF membrane to improve CO2 removal in membrane gas absorption.

Preparation and Properties of Polyvinylidene Fluoride Nanocomposited Membranes based on Poly(N-Isopropylacrylamide) Modified Graphene Oxide Nanosheets

The nanomaterial of graphene oxide grafting poly (N-isopropylacrylamide) (GO-g-PNIPAAm) was synthesized and PVDF/GO-g-PNIPAAm blended membranes were fabricated by wet phase inversion. In this work, a hydrophilic nanomaterial GO-g-PNIPAAm with poly(N-isopropylacrylamide) (PNIPAAm) grafted on GO, was synthesized by the atom transfer radical polymerization (ATRP) method. The resulting nanomaterial was confirmed by scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), Raman spectrum, and X-ray photoelectron spectroscopy (XPS) analysis. The synthesized GO-g-PNIPAAm was incorporated with polyvinylidene fluoride (PVDF) via phase inversion, and investigated for its temperature sensitivity, porosity, contact angle, scanning electron microscopy, and permeate properties. The water contact angle measurements confirmed that GO-g-PNIPAAm nanomaterial-endowed PVDF membranes with better hydrophilicity and thermo-responsive properties compared with those of the pristine PVDF membranes. Bovine serum albumin (BSA) adsorption experiments suggested that excellent antifouling properties of membranes were acquired after adding GO-g-PNIPAAm. The modified membranes showed good performance when the doping amount of GO-g-PNIPAAm was 0.2 wt %.

Effects of multi-walled carbon nanotubes (MWCNTs) and integrated MWCNTs/SiO2 nano-additives on PVDF polymeric membranes for vacuum membrane distillation

Polyvinylidene fluoride (PVDF) was blended with multi-walled carbon nanotubes (MWCNTs) and MWCNTs/SiO2 nano-additives to fabricate nanocomposite vacuum membrane distillation (VMD) membranes for desalination. The effects of MWCNTs and MWCNTs/SiO2 integrated additives on the morphology and performance of the PVDF membrane were studied. When only MWCNTs were used, 2 wt% loading of MWCNTs to PVDF had the maximum effect on the improvement of VMD flux, due primarily to the enlargement and elongation of the finger-like pores.Then, one of three different SiO2 nanoparticles (NPs) (hydrophobic, superhydrophobic and hydrophilic) was further added to the membrane that already contained 2 wt% of MWCNTs. When the amount of SiO2 NPs was relatively low, VMD flux increased as a result of the synergistic effects between the two filler components, i.e. one is the increase of overall porosity by MWCNTs and the other is the growth of the macrovoids by SiO2 NPs. But among those two, the effect of MWCNTs was more dominant. Compared with the pristine PVDF membrane and the nanocomposite PVDF membrane with a single filler of MWCNTs, respectively, the flux increase by the addition of SiO2 was 292% and 55%. On the other hand, when a relatively large amount of SiO2 NPs was added, the effect of SiO2 NPs started to dominate. Especially, the asymmetric structure of the membrane disappeared, leading to low mechanical strength. All fabricated membranes exhibited great desalination potential with salt rejection as high as >99.97%.

Understanding fouling dynamics on functionalized CNT-based membranes: Mechanisms and reversibility

Herein, we explored resistance to fouling and the bactericidal potential of a functionalized carbon nanotube (CNT)-based polyvinylidene fluoride (PVDF) membrane. Humic acid (HA), a main constituent in natural organic matter was used as an indicator for the estimation of organic fouling, while Escherichia coli was employed as a model bacterium for biofouling investigation. In-situ fouling characterization was performed using a non-destructive optical coherence tomography. Results revealed that a thin CNT layer on the PVDF surface effectively prevented HA molecules from interacting directly with the porous structure of membrane, thereby averting the possibility of pore blockage (irreversible fouling). Moreover, the CNT membranes offered higher permeability, increased HA rejection, and better flux restoration capacity as compared with pristine PVDF membrane. This superior anti-HA-fouling performance of CNT membrane was accredited to the electrostatic repulsions between the CNT layer and HA molecules, both carrying a strong negative charge and high density of hydrophilic functional groups on the CNT surface. Antibacterial results from the ultrastructural examination of cell membrane integrity authenticate the bactericidal potential of CNT membrane toward biofouling causing bacteria. Finally, when tested with real sea water, CNT membrane with less organic and biofouling propensity outperformed commercial PVDF, indicating their suitability for seawater reverse osmosis (SWRO) pretreatment.

Preparation of omniphobic PVDF membranes with silica nanoparticles for treating coking wastewater using direct contact membrane distillation: Electrostatic adsorption vs. chemical bonding

In order to reduce the wetting and fouling propensity of polyvinylidene fluoride (PVDF) membrane during coking wastewater treatment by membrane distillation, two kinds of omniphobic PVDF composite membranes were prepared by coating SiO2 nanoparticles on PVDF membrane surface via electrostatic adsorption or chemical bonding respectively, and subsequently fluorination. The coated SiO2 nanoparticles observed as amounts of nano-scale protrusions by scanning electron microscope constructed a re-entrant structure and enhanced membrane surface roughness. The omniphobicity of the modified membranes was confirmed by high contact angles (> 150°) for water, diiodomethane and ethylene glycol on the membrane surface. The omniphobic layer with SiO2 nanoparticles coated via chemical bonding was more stable than electrostatic adsorption. Both the omniphobic PVDF membranes exhibited stable and high rejections of the inorganic and organic contaminants, which were determined by conductivity, total organic carbon and three-dimensional excitation-emission fluorescence spectroscopy of the distillate. The modified omniphobic membranes also showed much less flux decline than the pristine PVDF membrane in direct contact membrane distillation tests (120 h), suggesting their excellent anti-fouling performance. This study indicates that coating silica nanoparticles via chemical bonding is an effective and promising method to fabricate omniphobic PVDF membrane for coking wastewater treatment.

A new strategy of PVDF based Li-salt polymer electrolyte through electrospinning for lithium battery application

Polyvinylidene fluoride (PVDF) ultrafine fibers with different proportions of lithium nitrate (LiNO3) were fabricated by an electrospinning device. The processing parameters are optimized to 19 wt% PVDF to get a bead free structure. Scanning electron microscope (SEM) and atomic force microscope (AFM) showed the uniform and interconnected porous structure. With the addition of 2 wt% LiNO3, the fiber diameter of the electrospun membrane decreased from 371 to 222 nm. Furthermore, the addition of LiNO3 into the nanofibrous membrane enhanced the ionic conductivity from 0.97 × 10−3 S cm−1 to 1.61 × 10−3 S cm−1 at room temperature after soaking with 1 M LiPF6 (lithium hexafluoro-phosphate) in ethylene carbonate (EC) and diethyl carbonate (DEC) in (1:1 wt%). Compared with the conventional Celgard and pristine PVDF membrane, the salt doped PVDF membranes showed higher electrochemical stability window and lower interfacial resistance. The electrospun membrane separators (ES) were assembled into Lithium cobalt oxide (LiCoO2) as cathode and lithium metal as an anode. The salt doped membrane showed superior discharge, C-rate and stable cycle performance than the commercial Celgard membrane.

Engineered Slippery Surface to Mitigate Gypsum Scaling in Membrane Distillation for Treatment of Hypersaline Industrial Wastewaters.

Membrane distillation (MD) is an emerging thermal desalination process, which can potentially treat high salinity industrial wastewaters, such as shale gas produced water and power plant blowdown. The performance of MD systems is hampered by inorganic scaling, particularly when treating hypersaline industrial wastewaters with high-scaling potential. In this study, we developed a scaling-resistant MD membrane with an engineered "slippery" surface for desalination of high-salinity industrial wastewaters at high water recovery. A polyvinylidene fluoride (PVDF) membrane was grafted with silica nanoparticles, followed by coating with fluoroalkylsilane to lower the membrane surface energy. Contact angle measurements revealed the "slippery" nature of the modified PVDF membrane. We evaluated the desalination performance of the surface-engineered PVDF membrane in direct contact membrane distillation using a synthetic wastewater with high gypsum scaling potential as well as a brine from a power plant blowdown. Results show that gypsum scaling is substantially delayed on the developed slippery surface. Compared to the pristine PVDF membrane, the modified PVDF membranes exhibited a stable MD performance with reduced scaling potential, demonstrating its potential to achieve high water recovery in treatment of high-salinity industrial wastewaters. We conclude with a discussion of the mechanism for gypsum scaling inhibition by the engineered slippery surface.

Novel conductive membranes breaking through the selectivity-permeability trade-off for Congo red removal

The permeability-selectivity trade-off remains the major drawback limiting application of membrane technology. This study provided a new strategy to break through this trade-off. A novel conductive membrane prepared by electroless Ni plating of the polyvinylidene fluoride (PVDF) membrane was used for dyeing wastewater treatment by applying an electric field. Series analyses demonstrated success of immobilizing a Ni/NiO layer on the pristine PVDF membrane. The conductive membrane possessed improved hydrophilicity indicated by the water contact angle decrease from 75.5° to 45.0°. Moreover, the conductive PVDF membrane integrating electric field voltage of 20 V simultaneously possessed high selectivity (97.93% rejection) and extremely high permeability (202.51 ± 9.63 L m−2 h−1 at 0.1 MPa) for filtration of Congo red (CR) solution, significantly breaking through the permeability-selectivity trade-off. The filtration performance was quite stable even after 6 filtration cycles, indicating the feasibility of the conductive membrane for long-term treatment of CR wastewater. In addition, the conductive membrane showed high antifouling ability to methylene blue (MB), rhodamine B (RB), CR and microorganisms. The conductive membranes, together with the new strategy, offered in-depth insights into membrane fabrication.

A novel strategy to develop antifouling and antibacterial conductive Cu/polydopamine/polyvinylidene fluoride membranes for water treatment

Membrane fouling problem impedes widespread application of polyvinylidene fluoride (PVDF) membrane for water treatment. This study proposed a novel strategy to endow hydrophobic PVDF ultrafiltration (UF) membrane with good antifouling and antibacterial activity. The novel strategy includes 3 steps: polydopamine (PDA) modification, Ag catalytic activation and electroless Cu plating. Via this strategy, copper nanoparticles (CuNPs) were in situ immobilized on PVDF membrane. The adhesive PDA layer not only facilitated to firm attachment of CuNPs, but also increased surface hydrophilicity. Scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) analyses confirmed successful immobilization of CuNPs, which improved surface hydrophilicity and absolution value of negative zeta potential as compared with the pristine PVDF membrane. Meanwhile, the modified Cu/PDA/PVDF membrane had relatively high conductivity, significantly improving antifouling performance when 0.30 V cm−1 of external electrical field was applied. Antibacterial tests revealed that the modified membrane possessed significantly enhanced antibacterial activity against live E. coli. The novel modification strategy in this study gave important implications for membrane fabrication, and the modified membrane possessed attractive potential in water treatment.

High‐flux PVDF membrane incorporated with β‐cyclodextrin modified halloysite nanotubes for dye rejection and Cu (II) removal from water

The application of polyvinylidene fluoride (PVDF) membrane is impeded by its intrinsic hydrophobic property. In this work, a series of novel membranes were fabricated by blending different contents of β‐cyclodextrin modified halloysite nanotubes (β‐CD‐HNTs) into the casting solution. Elemental mapping images confirmed the good dispersion of β‐CD‐HNTs in the PVDF membrane matrix. The microstructure of membrane was improved as revealed by the morphological characterizations. Besides, the permeation and separation performances of all membranes were extensively studied. The results demonstrated that as‐prepared blend membranes exhibited better hydrophilicity, which included lower contact angle and higher water flux than pristine PVDF membrane. In the separation experiments, the dye rejection for the blend membranes was not declined (85%‐90%), even though the pore size of membranes was increased with the addition of β‐CD‐HNT nanoparticles. More importantly, because of the chelation reaction between β‐CD‐HNTs and heavy metal, the removal ratio of Cu (II) for 3 wt.% content of blend membrane could reach 20.9%, compared with unmodified membrane of 4.2%. After 3 filtration cycles, blend membranes showed superior antifouling property. Overall, this method demonstrated very promising characteristics for removing different scales of pollutants using membrane technology.

Polyvinylidene fluoride membrane modification via oxidant-induced dopamine polymerization for sustainable direct-contact membrane distillation

Porous hydrophobic polyvinylidene fluoride (PVDF) membranes have been extensively used in direct-contact membrane distillation (DCMD) processes. However, these PVDF membranes are vulnerable to membrane fouling and pore wetting in low surface tension feeds, restricting its application for water recovery from challenging industrial wastewaters. Therefore, it is of paramount importance to engineer fouling- and wetting-resistant MD membranes for robust long-term applications. In this study, a superoleophobic composite hollow fiber membrane with sandwich structure has been developed via accelerated oxidant-induced polydopamine (PDA) deposition on both the outer and inner surfaces of a commercial hydrophobic PVDF membrane under slightly acidic conditions (pH = 5). The modified surface prevents organics adhesion ascribing to its underwater superoleophobicity while the unmodified pores remain hydrophobic for vapor transport. The long-term robustness of the PDA-decorated membrane in highly saline feeds containing low surface tension contaminants has been evaluated via bench-scale DCMD experiments. In contrast to the pristine PVDF membrane, the PDA-decorated membrane exhibits excellent fouling- and wetting-resistant properties in different surfactant solutions as well as oil-in-water emulsion. The PDA-decorated membrane has also been used for seawater desalination, during which it maintains a stable flux and high salt rejection rate. Furthermore, the PDA-decorated membrane presents a flux enhancement of up to 70% over the pristine PVDF membrane in 3.5 wt% NaCl solution at 333 K. This study demonstrates the potential of the PDA-decorated membrane for extended DCMD applications such as water recovery from industrial wastewater containing low surface tension substances.

Bioinspired Peptide-Coated Superhydrophilic Poly(vinylidene fluoride) Membrane for Oil/Water Emulsion Separation.

Polyvinylidene fluoride (PVDF) membranes are limited in the field of oil-in-water emulsion treatment because the intrinsic hydrophobicity of PVDF can cause serious membrane fouling. Here, a superhydrophilic PVDF membrane (PVDF@PDA-GSH) was fabricated using a facile, versatile, mussel-inspired method. The pristine PVDF membrane was coated with dopamine under mild alkaline conditions by a dip-coating method, followed by addition of glutathione (GSH) via a simple reaction. GSH was successfully coated onto the membrane surface and confirmed by X-ray photoelectron spectroscopy and energy dispersive X-ray spectrometry. Hierarchical surface structure and superhydrophilicity were examined by scanning electron microscopy and contact angle, respectively, giving the PVDF@PDA-GSH membrane excellent wettability and antifouling ability. The water flux of PVDF@PDA-GSH was several-fold higher than conventional filtration membranes, and the oil rejection ratio was nearly 99%. The PVDF@PDA-GSH membrane also showed favorable reusability because the flux recovery ratio (FRR) remained above 90% after five cycles. In general, these results indicated that this modification might provide a good method for the fabrication of superhydrophilic PVDF membranes with good prospects for water filtration applications.

Enhancing water permeability and fouling resistance of polyvinylidene fluoride membranes with carboxylated nanodiamonds

Polyvinylidene fluoride (PVDF) is a popular membrane material, but it has the drawback of relatively high fouling potential. In this study, we prepared carboxylated nanodiamonds (CNDs) by thermal oxidation, and then incorporated CNDs into PVDF membranes during phase inversion. Transmission electron microscopy (TEM) showed that the CNDs had minimized aggregation and better dispersity compared with the raw nanodiamonds. The prepared membranes were characterized by scanning electron microscope (SEM), atomic force microscopy (AFM) and water contact angle analysis. The bulk porosities and pore sizes of the membranes were evaluated by the gravimetric method and the filtration method, respectively; surface pores were evaluated based SEM images. Compared with the pristine PVDF membrane, CNDs blended membranes had larger porosities and surface pore sizes, higher water permeabilities and hydrophilicities, smoother surfaces, and improved antifouling and cleaning performance due to the incorporation of hydrophilic carboxyl groups. In particular, the irreversible fouling ratio to the total fouling ratio (Rir/Rt) dropped from 85% for the pure PVDF membrane to 21% for the CNDs blended membrane. Our study demonstrates that CNDs are excellent pore-forming fillers in developing antifouling membranes, and they may open up a new avenue to the next generation of high performance membranes.

A comprehensive study on the performance and antifouling enhancement of the PVDF mixed matrix membranes by embedding different nanoparticulates: Clay, functionalized carbon nanotube, SiO2 and TiO2

Abstract To design a highly permeable and fouling resistance ultrafiltration (UF) membrane from polyvinylidene fluoride (PVDF), one approach is the incorporation of hydrophilic nanofillers in membrane matrix to modify the hydrophilicity, porosity and pore size of the membrane, and open up the channels for water transport. However, many nanoparticulates with different structures and characteristics were applied for these purposes. In this study, we tried to select the best nanomaterial among the used famous nanomaterials for modifying PVDF based membranes. We have fabricated PVDF mixed matrix membranes (MMMs) consisting of cloisite 30B clay, carboxyl-functionalized multi-walled carbon nanotubes (MWCNTs-COOH), SiO2 and TiO2. As a result, the pure water flux (PWF) across the MMMs was significantly improved. The PWF enhancement for clay, MWCNTs-COOH, TiO2, and SiO2 embedded MMMs as compared with the pristine PVDF membrane was 187%, 143%, 111%, and 50%, respectively. However, higher loading of the nanoparticulates may result in agglomeration and reduce the separation performance. The breaking elongation of the membranes was reduced by incorporating of the optimum amount of the nanoparticulates. The MMMs showed superior antifouling properties as compared with the pristine membrane owing to the hydrophilicity enhancement by the addition of the nanoparticles. Based on the results, the membrane containing 0.2 wt% MWCNTs-COOH showed the best performance with 135% PWP improvement, nearly 100% bovine serum albumin (BSA) rejection, 33% flux recovery ratio enhancement and lowest breaking elongation reduction. In addition, the used amount of this nanomaterial was lowest, which it could be important from an economical view.

Improvement in fouling resistance of silver-graphene oxide coated polyvinylidene fluoride membrane prepared by pressurized filtration

Abstract Polyvinylidene fluoride (PVDF) membranes are widely used in microfiltration and ultrafiltration because of their high chemical resistance, thermal stability, and mechanical strength. However, the hydrophobicity and high roughness of the PVDF membrane surface make it vulnerable to membrane fouling. Therefore, engineered nanomaterials such as graphene oxide (GO) have been coated to the membranes to improve its fouling resistance. In this study, PVDF membranes were coated with different concentrations of silver functionalized graphene oxide (Ag-GO) nanocomposite suspensions (0.01–0.3 mg ml−1) via pressurized filtration process. The fouling resistance of Ag-GO coated PVDF membranes was significantly enhanced due to increased hydrophilicity and smoother membrane surface. The initial feed water flux of modified membranes increased up to 53% when compared to that of the pristine PVDF membrane. Also, the flux recovery rate and antibacterial properties of Ag-GO/PVDF membranes were higher than that of the pristine membrane. Ag-GO was stably coated on the membrane surface and the enhanced performance resulting from modification with Ag-GO was maintained after the membrane backwash. We expect that our results could contribute to the development of highly reusable antifouling membranes coated with inorganic nanocomposites.

Preparation and characterization of novel poly (vinylidene fluoride) membranes using flower-like Bi 2 WO 6 for membrane distillation

Abstract In this study, hydrophobic flat-sheet composite membranes of polyvinylidene fluoride (PVDF) were fabricated using non-woven fabrics as a support for direct contact membrane distillation (DCMD) through non-solvent induced phase separation (NIPS). The effects of three-dimensional (3D) Bi 2 WO 6 on membrance morphology, porosity, pore size, pore size distribution, hydrophobicity, thermal properties and permeability were investigated. The flower-like Bi 2 WO 6 could optimize membrane structure and improve the membrane porosity by its unique 3D hollow structure. During the desalination process of 3.5% sodium chloride solution, the salt rejection with nano-Bi 2 WO 6 membranes was higher than the pristine PVDF membrane. The maximum transmembrane permeate flux (about 13.12 kg/m 2 h) was obtained with the feed solution at 60 °C and the cold distillate water at 20.0 °C, which was higher than the commercial PVDF membrane (9.43 kg/m 2 h) and the pure PVDF membrane (12.06 kg/m 2 h). Furthermore, the hybrid membrane exhibited outstanding stability in the 100 h continuous desalination experiments. Our results demonstrated that the prepared PVDF flat-sheet composite membrane with 0.5% flower-like Bi 2 WO 6 has a very good prospect to be utilized in membrane distillation process for desalination.

Preparation of PVDF/GO[sbnd]SiO2 hybrid microfiltration membrane towards enhanced perm-selectivity and anti-fouling property

Polyvinylidene fluoride (PVDF) microfiltration (MF) membranes with enhanced perm-selectivity and antifouling property were successfully prepared by incorporating GOSiO2 nanoparticles into PVDF matrix. Under the optimal GOSiO2/PVP ratio of 0.5/1(wt./wt.) in the casting solution, the prepared PVDF/GOSiO2 hybrid membrane exhibited improved perm-selectivity with a water flux of 850L/(m2h) and HA rejection of 92%. Compared with the pristine PVDF membrane, the optimized hybrid membrane exhibited 89% higher water flux without sacrificing its rejection, increased porosity and hydrophilicity. In addition, the PVDF/GOSiO2 hybrid membrane also showed satisfactory antifouling performance (the lower irreversible fouling ratio of 37.4% and high recovery ratio of 62%) and good stability in the long-term performance test.

Superoleophobic surface modification for robust membrane distillation performance

Inspired by mussels’ byssus with remarkable adhesive power that is neither degraded nor deformed in the marine environment, dopamine coating has emerged as an option for membrane surface modification. With its many advantages, direct-contact membrane distillation (DCMD) appears to hold potential for the recovery of high quality water from produced water. However, membrane fouling and pore wetting are challenging issues of long-term DCMD operations due to the presence of oils and surfactants in these waters. Hence, it is paramount to develop robust membranes with anti-fouling and anti-wetting properties for effective produced water treatment. In this study, we fabricated a composite hollow fiber membrane by single-step co-deposition of polydopamine (PDA)/polyethylenimine (PEI) onto the outer surface of a commercial hydrophobic polyvinylidene fluoride (PVDF) substrate. The successful co-deposition was verified using different characterization techniques. The composite membrane exhibited Janus wettability (Janus membranes have opposing properties at an interface) with its modified outer surface being in-air hydrophilic/underwater superoleophobic for preventing organics adhesion while insuring that unmodified pores beneath the surface remained hydrophobic for vapor transport. The anti-fouling and anti-wetting properties of the modified membrane were investigated via DCMD experiments by feeding a series of low surface tension solutions. In comparison to the pristine PVDF membrane, the modified membrane exhibited promising wetting resistant property against different surfactant types and excellent fouling resistant property against nonionic and cationic surfactants. Moreover, the modified membrane presented a promising long-term performance when feeding a cationic surfactant-stabilized petroleum-in-water emulsion mimicking produced water, during which a stable flux and excellent permeate quality were maintained throughout 7 days of operation. The efficacious combined effects of a hydration layer and protonated amine-functional groups on the PDA/PEI grafted layer could prevent membrane fouling and pore wetting. The results suggest that the mussel-inspired composite PVDF membrane could potentially be used for long-term water recovery from produced water via DCMD.

Comparison of antifouling behaviours of modified PVDF membranes by TiO2 sols with different nanoparticle size: Implications of casting solution stability

Widespread applications of membrane technology call for the development of antifouling membranes, and preparation of mixed matrix membranes (MMMs) by mixing nanoparticles (NPs) into polymeric membrane matrix has attracted much attention. In this work, polyvinylidene fluoride (PVDF) MMMs were prepared by two TiO2 sols with different NPs size (one from hydrolysis and condensation with an average size of 5nm, and the other from dispersion of TiO2 nanoparticles with a particle size of 21nm), and comparison of their antifouling behaviours was conducted. Compared to the pristine PVDF membrane (N0) and membrane (N1) modified by TiO2 sol from dispersion of TiO2 NPs, the surface pore size, porosity and permeability of modified membrane (N2) by TiO2 sol from hydrolysis and condensation were slightly decreased, while the hydrophilicity and the absolute value of Zeta potential were obviously improved. Moreover, the N2 membrane showed a higher surface energy barrier, smaller frequency shift in quartz crystal microbalance with dissipation monitoring, and higher relative flux than N1, indicating a better antifouling performance. The abundant hydroxyl groups and the increased Ti and O elements on the membrane surface accounted for the enhanced hydrophilicity and antifouling ability. Scanning electron microscope (SEM) images clearly show that TiO2 particles were uniformly distributed for N2; however, the severe agglomeration and sedimentation were observed for N1. Multiple light scattering spectroscopy (MLiSSP) analysis showed that N2 casting solution had a higher stability as revealed by the negligible backscattering change, decreased demixing thickness and low turbiscan stability index (TSI) value. The different stability of casting solutions might result in the different NPs’ behaviours during membrane formation process, accounting for the discrepancy in the physicochemical properties and antifouling ability between N1 and N2 membranes.

High performance triboelectric nanogenerators based on phase-inversion piezoelectric membranes of poly(vinylidene fluoride)-zinc stannate (PVDF-ZnSnO3) and polyamide-6 (PA6)

Vertical contact-separation mode triboelectric generator (TEG) based on lead-free perovskite, zinc stannate (ZnSnO3)-polyvinylidene fluoride (PVDF) composite and polyamide-6 (PA6) membrane is demonstrated. For the 5 wt% PVDF-ZnSnO3 nanocomposites, the facile phase-inversion method provides a simple route to achieve high crystallinity and β-phase with a piezoelectric coefficient d33 of −65 pm V−1, as compared to −44 pm V−1 for pristine PVDF membranes. Consequently, at a cyclic excitation impact of 490 N/3 Hz, the PVDF-ZnSnO3/PA6 based TEGs provide a significantly higher voltage of 520 V and a current density of 2.7 mA m−2 (corresponding charge density of 62.0 µC m−2), as compared to the pristine PVDF-PA6 TEG which provides up to 300 V with a current density of 0.91 mA m−2 (corresponding to a charge density of 55.0 µC m−2). This increase in the electrical output can be attributed to not only the enhanced polarisation of PVDF by ZnSnO3 leading to an increase in the β-phase content, but also to the surface charge density increase by stress induced polarisation of ZnSnO3, leading to the generation of stronger piezoelectric potential. The work thus introduces a novel method of enhancing the surface charge density via the addition of suitable high polarisation piezoelectric materials thus eliminating the need for prior charge injection for fluoropolymer membranes.