Morphology and Formation Mechanism of Poly(Vinylidene Fluoride) Membranes Prepared with Immerse Precipitation: Effect of Dissolving Temperature

Poly(vinylidene fluoride) (PVDF) membranes were prepared by the immersion precipitation method. The effects of polymer dissolving temperature for the dopes on the morphology, crystallization and performance of prepared membranes were examined. Polymer dissolving temperature was varied from 50 to 120°C. N,N-dimethylacetamide (DMAc) and de-ionized water were used as solvent and non-solvent, respectively. Based on the membrane morphology and the schematic phase diagram of the ternary system, the membrane formation mechanism was analyzed theoretically. The binodal liquid-liquid demixing took place first for the nucleation and growth of droplets in the polymer poor phase; then consequently the spinodal liquid-liquid demixing occurred in the polymer rich phase. The demixings together resulted in the prepared membranes having a cross-section composed of interconnected globule-like particulates with bi-continuous structured surfaces. The dissolving temperature of the dopes had a remarkable effect on the morphology of the cross-section, even when the solution underwent a long time cooling before the demixing. The increase of the diameter of the particulates with the dissolving temperature was theoretically analyzed according to the conditions of the polymeric solution.     

Morphology,Crystallization and Formation Mechanism of Poly(Vinylidene Fluoride) Membranes Prepared with Immersion Precipitation: Effect of Maturation Time

Poly(vinylidene fluoride) (PVDF) membranes were prepared by the immersion precipitation method. Effects of the maturation time of dopes on the morphology and crystallization of the prepared membranes were investigated. The analysis showed that the maturation time played an important role in determining the morphology of the prepared membranes. For the dope prepared in the initial day, liquid–liquid demixing preceded solid–liquid demixing in the process of the membrane formation. The morphology of the cross section of the prepared membrane (M1) was finger-like structures with a sponge substrate beneath the porous skin. During the maturation, the dopes underwent a microscopic phase separation and the PVDF crystallized, which resulted in the existence of micro-liquid phases and micro-solid phase crystalline areas in the dopes. In the process of the membrane formation, liquid–liquid demixing took place by nucleation and growth of droplets of the polymer rich phase in the micro-liquid phase. The micro-solid phase crystallites were connected together by the polymer chains, and formed a three-dimensional network gelation morphology. The crystal structure of M1 was mainly β crystals. With increasing maturation time of the dopes, the proportion of β decreased crystals, but that of α crystals increased for the prepared membranes.     

Poly(vinylidene fluoride) Crystallization Behavior and Membrane Structure Formation Via Thermally Induced Phase Separation with Benzophenone Diluent

Microporous poly(vinylidene fluoride) (PVDF) membranes were prepared by thermally induced phase separation (TIPS) at different quenching temperatures with benzophenone as the diluent. The crystallization behavior and crystal structure of PVDF in PVDF/benzophenone systems were investigated by differential scanning calorimetry (DSC) and wide angle X‐ray diffraction (WAXD). The different PVDF concentrations had a remarkable effect on PVDF crystallization behavior and resulted in different membrane structures. Spherulitic structures were vague when the PVDF/benzophenone solution was quenched to ?8°C; however, discernable spherulitic structures were obtained when quenched to 34 and 49°C. Additionally, two phase separation mechanisms (solid–solid (S–S) and solid–liquid (S–L) phase separation) were observed during membrane preparation. It was revealed by scanning electron microscopy (SEM) that microporous membranes had more discernable spherulitic structures formed by S–L phase separation than by S–S phase separation, which induced macrovoids and irregular pores on the fracture surfaces of membranes.     

Investigation of Polyvinylidene Fluoride Membranes Prepared by Using Surfactant OP-10 Alone or with a Second Component,as Additives,via the Non-Solvent-Induced Phase Separation (NIPS) Process

Polyvinylidene fluoride (PVDF) flat-sheet membranes were prepared via a non-solvent-induced phase separation (NIPS) method at 60°C using a hydrophilic surfactant OP-10 (octylphenol polyoxyethylene ether) solely (Blank) or with a second additive [H₂O or lithium chloride (LiCl)] as pore-forming agents. The influence of OP-10 concentration on the surface tension, viscosity, and precipitation rate of PVDF/(H₂O, LiCl, or Blank) systems were investigated, and the ultrafiltration and mechanical properties of the resultant membranes were measured. It was found that an increased demixing rate during the coagulation process was the reason for the change in membrane morphology and properties. An obviously improved flux and slightly decreased mechanical properties and rejection were found in membranes prepared using a high concentration of OP-10 and the second component as additives. SEM pictures revealed an increased porous structure on the resultant membrane surface. A hypothesis was proposed to explain these phenomena; the reoriented surfactant molecules at the interface facilitated the water diffusion channels, which finally became the porous structure on the membrane surface. The weakened mechanical properties were due to the macrovoid structure in its membrane cross-section, which developed from the micelle structure in the casting solution. This hypothesis was further confirmed in a PVDF/OP-10/polyethylene glycol (PEG) system. A consistent conclusion was obtained.     

Preparation of superhydrophobic membranes by electrospinning of fluorinated silane functionalized poly(vinylidene fluoride)

Fluorinated silane functionalized poly(vinylidene fluoride) (PVDF) is synthesized by graft polymerization of 3-trimethoxylpropyl methylacrylate with PVDF followed by coupling of fluorinated silanes. Flat membrane prepared using this functionalized PVDF has a water contact angle of 140°. Superhydrophobic PVDF membrane with a contact angle larger than 150° is prepared by the electrospinning of the fluorinated silane functionalized PVDF. The morphologies of the membranes are characterized using scanning electron microscopy. The surface composition of the membranes is analyzed using FTIR and the contact angles and water drops on the surface of the membrane are measured using video microscopy.     

Poly(vinylidene fluoride) membranes by an ultrasound assisted phase inversion method

Poly(vinylidene fluoride) (PVDF) membranes were prepared by an ultrasound assisted phase inversion process. The effect of ultrasonic intensity on the evolution of membrane morphology with and without the addition of pore former LiCl during precipitation process was comprehensively investigated. Besides the inter-diffusion between the solvent and nonsolvent, the ultrasonic cavitation was thought to have significant influences on phase inversion and the resultant membrane morphology. The mutual diffusion between water and solvent during the ultrasound assisted phase inversion process was measured. The crystalline structure was detected by wide angle X-ray diffractometer (WAXD). The thermal behavior was studied by differential scanning calorimeter (DSC). The mechanical strength, forward and reverse water flux, rejection to bovine serum albumin (BSA) and pepsin were also investigated. By the ultrasound assisted phase inversion method, ultra-filtration membrane was successfully prepared, which exhibited more preferable morphology, better mechanical property and more favorable permeability without sacrificing the rejection and thermal stability.     

The influence of sulfonated polyethersulfone (SPES) on surface nano-morphology and performance of polyethersulfone (PES) membrane

In this research, firstly sulfonation of polyethersulfone (PES) was carried out and then polyethersulfone (PES)/sulfonated polyethersulfone (SPES) blend membranes were prepared with phase inversion induced by immersion precipitation technique. polyvinylpyrrolidone (PVP, 2 wt% concentration) was added in the casting solution as pore former. SPES was characterized by FT-IR and UV-visible spectra, ion exchange capacity and swelling ratio. The characterization of SPES polymer indicates that the sulfonic acid groups were produced on PES polymer. Also, the prepared PES/SPES blend membranes were characterized by contact angle, AFM, SEM and cross-flow filtration for milk concentration. The contact angle measurements indicate that the hydrophilicity of PES membrane is enhanced by increasing the SPES content in the casting solution. The SEM and AFM images show that the addition of SPES in the casting solution results in a membrane with larger surface pore size and higher sub-layer porosity. The mean pore size of the membrane increased from 98 nm for PES membrane to 240 and 910 nm for 50/50 and 0/100 PES/SPES blend membranes, respectively. The pure water flux and milk water permeation through the prepared membranes are increased by blending PES with SPES. Moreover, the protein rejection of PES/SPES blend membranes was lower than PES membrane.     

Preparation of PVDF Microfiltration Membranes with Thermo-responsive PNIPAM Hydrogel Composite Particles

Silica-core composite particles with poly(N-isopropylacrylamide) (PNIPAM) hydrogel-shell were prepared by using silica microparticle templates, which were modified with [3-(methacryloxy)propyl]trimethoxysilane (MPS). The thermo-responsive PNIPAM hydrogel microcapsules were prepared by soaking the core–shell composite particles in hydrofluoric acid solution. These hydrophilic PNIPAM hydrogel microcapsules were applied to poly(vinylidene fluoride) (PVDF) microfiltration membranes in order to control the hydrophobicity of membrane surface without sacrificing the permeability. PVDF/PNIPAM hydrogel composite membranes were made by phase inversion and diffusion in the mixed solvents of N-methyl-2-pyrrolidone (NMP) and ethylene glycol monomethyl ether (EGME) with polyethylene glycol 600 (PEG 600) as plasticizer.     

Effect of amphiphilic hyperbranched-star polymer on the structure and properties of PVDF based porous polymer electrolytes

An amphiphilic hyperbranched-star polymer (HPE-g-MPEG) was synthesized by grafting methoxy poly(ethylene glycol) to the end of the hyperbranched polyester (HPE) molecule using terephthaloyl chloride (TPC) as the coupling agent. The synthesized amphiphilic hyperbranched-star polymer was blended with poly(vinylidene fluoride) (PVDF) to fabricate porous membranes via typical phase inversion process, and then the membranes were filled and swollen by a liquid electrolyte solution to form polymer electrolytes. The influences of HPE-g-MPEG on the morphology, crystallinity, liquid electrolyte uptake, mechanical properties of the porous membranes and the electrochemical properties of the activated membranes were investigated. It was found that the addition of HPE-g-MPEG resulted in a significant increase in porosity and a considerable reduction in crystallinity of the blend membranes, which favored the liquid electrolyte uptake and, consequently, led to a remarkable increase in ion conductivity at ambient temperature. The maximum ion conductivity observed in this study was 1.76 × 10^? 3 S/cm at 20 °C for the blend membrane with a HPE-g-MPEG/PVDF ratio of 3/10 (w/w).     

Preparation of amino acid nanoparticles at varying saturation conditions in an aerosol flow reactor

In this article, we describe the synthesis of new and ion-selective nanofiltration (NF) membranes using polyvinylidene fluoride (PVDF) nanofibers and hyperbranched polyethylenimine (PEI) as building blocks. These new nanofibrous composite (NFC) membranes consist of crosslinked hyperbranched PEI networks supported by PVDF nanofibrous scaffolds that are electrospun onto commercial PVDF microfiltration (MF) membranes. A major objective of our study was to fabricate positively charged NF membranes that can be operated at low pressure with high water flux and improved rejection for monovalent cations. To achieve this, we investigated the effects of crosslinker chemistry on membrane properties (morphology, composition, hydrophobicity, and zeta potential) and membrane performance (salt rejection and permeate flux) in aqueous solutions (2,000?mg/L) of four salts (NaCl, MgCl₂, Na₂SO₄, and MgSO₄) at pH 4, 6, and 8. We found that an NFC?CPVDF membrane with a network of PEI macromolecules crosslinked with trimesoyl chloride has a high water flux (~30?L?m^?2?h^?1) and high rejections for MgCl₂ (~88 %) and NaCl (~65 %) at pH 6 using a pressure of 7?bar. The overall results of our study suggest that PVDF nanofibers and hyperbranched PEI are promising building blocks for the fabrication of high performance NF membranes for water purification.     

Influence of the processing method on the properties of a PVDF film

The main electrophysical properties of polyvinylidene fluoride (PVDF) films produced by two technologies: the solution casting method and the method of hot pressing from the melt are investigated. To analyze the PVDF films, methods of dielectric spectroscopy (DS), IR spectroscopy, TGA/DSC analysis, and x-ray diffraction (XRD) are used. It is demonstrated that the IR spectra of both PVDF films change weakly in comparison with the virgin PVDF film. The absorption bands characteristic for α-, β-, and γ-phases are observed for the virgin and both types of PVDF films. This testifies to the fact that the molecular structure of film samples is practically independent of their processing method. The only difference is that the new absorption band at 1723 cm^–1 arises in the IR spectra of the films produced by the method of hot pressing from the melt. The TGA/DSC analysis demonstrates that the beginning melting temperature, melting temperature, beginning decomposition temperature, and decomposition temperature for the film samples produced by the method of hot pressing from the melt decrease by 8, 2, 10, and 12°C, respectively, compared to the film samples produced by the solution casting method.     

Crystallizaion and surface morphology of poly(vinylidene fluoride)/poly(methylmethacrylate) films by solution casting on different substrates

The dependence of surface structure of the poly(vinylidene fluoride) (PVDF)/poly(methylmethacrylate) (PMMA) films by solution casting on properties of seven substrates was investigated by wide angle X-ray diffraction (WAXD), Fourier transform infrared (FTIR), scanning electron microscope (SEM) and differential scanning calorimetry (DSC). It was revealed that the polyblend films obtained by casting onto each substrate contained exclusively β phase PVDF. Higher crystallinity of the film was obtained by casting onto ceramic, polytetrafluoroethylene (PTFE), copper (Cu), stainless steel and glass substrates than that by casting onto aluminium (Al) and polypropylene (PP) substrates, depending on the degree of close lattice matching. The surface crystalline structure of PVDF was strongly affected by the wettability of substrate. The largest size of PVDF spherulitic crystal structure with about 6 μm presented in the casting film grown at the air/solution interface on glass substrate, while the smallest spherulite size with about 3 μm was generated by casting onto PTFE, stainless steel and PP substrates. It implied that the higher surface tension the substrate had, the larger PVDF spherulite grew at the air/solution interface.     

Iron oxide nanoparticle synthesis in aqueous and membrane systems for oxidative degradation of trichloroethylene from water

The potential for using hydroxyl radical (OH^?) reactions catalyzed by iron oxide nanoparticles (NPs) to remediate toxic organic compounds was investigated. Iron oxide NPs were synthesized by controlled oxidation of iron NPs prior to their use for contaminant oxidation (by H₂O₂ addition) at near-neutral pH values. Cross-linked polyacrylic acid (PAA) functionalized polyvinylidene fluoride (PVDF) microfiltration membranes were prepared by in situ polymerization of acrylic acid inside the membrane pores. Iron and iron oxide NPs (80–100 nm) were directly synthesized in the polymer matrix of PAA/PVDF membranes, which prevented the agglomeration of particles and controlled the particle size. The conversion of iron to iron oxide in aqueous solution with air oxidation was studied based on X-ray diffraction, Mössbauer spectroscopy and BET surface area test methods. Trichloroethylene (TCE) was selected as the model contaminant because of its environmental importance. Degradations of TCE and H₂O₂ by NP surface generated OH^? were investigated. Depending on the ratio of iron and H₂O₂, TCE conversions as high as 100 % (with about 91 % dechlorination) were obtained. TCE dechlorination was also achieved in real groundwater samples with the reactive membranes.     

Preparation and electroactive properties of a PVDF/nano-TiO2 composite film

In the present study, poly(vinylidene fluoride) (PVDF)/nano-TiO₂ electroactive film was prepared by coating a substrate with an acetone/DMF solution, which was evaporated at a high temperature (110 °C). The crystallisation behaviour, dynamic mechanical properties and electroactive properties of this PVDF/nano-TiO₂ electroactive film were investigated. The cross-section and surface of the film were observed with a scanning electron microscope (SEM). X-ray diffraction (XRD) results showed that the film containing the PVDF β phase, the desired ferroelectric phase, was obtained by crystallising the mixed solution of nano-TiO₂ and PVDF at 110 °C, while the film containing the α phase was obtained from the crystallisation of the pure PVDF solution at the same temperature. It was found that the storage modulus, the room-temperature dielectric constant and the electric breakdown strength of the composite films were much higher than those of a pure PVDF film. TiO₂ improved the mechanical properties and electroactive properties of the film. The results indicate that PVDF/nano-TiO₂ composite films can be applied to the fabrication of self-sensing actuator devices.     

Composite membranes of sulfonated poly(phthalazinone ether ketone) doped with 12-phosphotungstic acid (H3PW12O40) for proton exchange membranes

Sulfonated poly(phthalazinone ether ketone) (SPPEK) and 12-phosphotungstic acid (PWA) composite membranes were prepared by the casting procedure, using SPPEK solution blended with PWA. The physicochemical properties of these composite membranes were studied by means of field-emission scanning electron microscopy (FSEM), X-ray diffraction (XRD) analysis, thermogravimetry analysis (TGA) and Fourier transform infrared attenuated total reflection (FTIR-ATR) spectroscopy. The PWA particles in composite membranes are stable due to the interaction between SO₃H group of SPPEK and PWA particles. The proton conductivity of the composite membrane containing 10% PWA reaches the maximum of 0.17 S/cm at 80 °C under 100% relative humidity.     

Preparation and performance of polysulfone-sulfonated poly(ether ether ketone) blend ultrafiltration membranes. Part I

Ultrafiltration membranes were prepared from blends with polysulfone (PSf) and sulfonated poly(ether ether ketone) (SPEEK) by phase inversion technique. The blend membranes were prepared with polymer composition from 0 to15 wt%. Sulfonated poly(ether ether ketone) was used to improve the performance and permeability of blended membranes. The effects of polymer composition on compaction, pure water flux, water content, and membrane hydraulic resistance were studied. The membranes were also subjected to the determination of pore statistics and molecular weight cut-off (MWCO) determination studies by using different molecular weight of proteins. The porosity, pore size of the membranes increased with increasing concentrations of SPEEK in the casting solution. Similarly, the MWCOs of the blend membranes ranged from 20 to 45 kDa, depending on the various polymer blend compositions. The pure water flux of the PSf/SPEEK blend membranes increases from 16.7 to 61.5 l m⁻² h, when the concentration of SPEEK increased from 0 to 15 wt%. Scanning electron microscope (SEM) results qualitative evidence for the trends observed for the pore statistics and MWCO studies.     

Effect of PMMA on crystallization behavior and hydrophilicity of poly(vinylidene fluoride)/poly(methyl methacrylate) blend prepared in semi-dilute solutions

Films of poly(vinylidene fluoride) (PVDF)/poly(methyl methacrylate) (PMMA) blend were derived from a special procedure of casting semi-dilute solutions. Hydrophilic character and crystallization of PVDF were optimized by variation of PMMA concentration in PVDF/PMMA blends. It was found that a PVDF/PMMA blend containing 70 wt% PMMA has a good performance for the potential application of hydrophilic membranes via thermally induced phase separation. The films presented β crystalline phase regardless of PMMA content existed in the blends. Thermal analysis of the blends showed a promotion of crystallization of PVDF with small addition of PMMA which induced larger lamellar thickness of PVDF, leading to the largest spherulitic crystal of PVDF (10 wt% PMMA) is about 8 μm. SEM micrographs illustrated no phase separation occurred in blends, due to the high compatibility between PVDF and PMMA.     

Characterization and application of radiation grafted membranes in waste treatment

Ionic membranes were prepared by radiation-induced grafting of acrylic acid onto low density polyethylene films. To elucidate the possibility of practical use, a study has been made for the characterization of the grafted and chemically treated membranes. The selectivity of such prepared membranes towards the chelation or complexation of different alkali metals was investigated, to find that the higher affinity is observed for K⁺, Na⁺ and Li⁺ ions compared to other alkali metals used. The metal uptake percent was determined using different techniques: flame photometer and X-ray fluorescence (XRF). The uptake of metal from its feed solution by the grafted membrane increased as the degree of grafting increased, i.e., it is directly proportional to the content of functional carboxylic acid groups in the graft copolymer. As a consequence, the electrical conductivity of metal feed solution decreased during such process of metal chelation by the membrane. The higher the grafting degree of membrane, the lower the electrical conductivity of metal feed solutions observed. The changes in thermal properties of the prepared membranes were investigated and characterized using differential scanning calorimeter (DSC), and thermal gravimetric analysis (TGA). The thermal stability of these membranes increased with a degree of grafting due to the formation of crosslinked network structure via hydrogen bonding. Furthermore, such stability is enhanced for the alkali-treated membranes even at high elevated temperatures. The prepared membranes showed a great promise for possible use in the recovery of uranium from zirconium in their wastes.     

Preparation and characterization of asymmetric polyethersulfone and thin-film composite polyamide nanofiltration membranes for water softening

In this research, two types of nanofiltration membranes were prepared and evaluated for water softening. Their nanofiltration performance was evaluated by cross-flow filtration using NaCl (1 g/l) and MgSO₄ (1 g/l) solution at 5 and 10 bar, 25 °C and 10 l/min. The morphological studies were performed with SEM and AFM instruments. In addition, the hydrophilicity of membranes was examined by contact angle measurements. In the first type, asymmetric polyethersulfone (PES) nanofiltration membranes were prepared using phase inversion induced by immersion precipitation technique. Different components such as polyvinylpyrrolidone (PVP), polyethyleneglycole (PEG), acrylic acid and Triton X-100 were used as additive in the PES casting solution, which lead to the formation of new asymmetric nanofiltration membranes. Two concentrations of PES (20 and 25 wt%) and two different non-solvents (pure water and mixture of water (80 vol.%) and IPA (20 vol.%)) were used for preparing asymmetric nanofiltration membranes. The morphological studies showed that the membranes prepared with non-solvent containing 20 vol.% IPA have smoother surface and smaller pores in surface and sub-layer compared to membranes prepared with pure water as non-solvent. The flux was decreased when higher polymer concentration and mixture of water and IPA were employed for membrane formation. However, NaCl and MgSO₄ rejections were improved. In the second type, thin-film composite polyamide nanofiltration membrane was fabricated using interfacial polymerization of 1,3-phenylenediamine (PDA) with trimesoyl chloride (TMC). A rough and dense film was formed on the PES support membrane by interfacial polymerization. The water permeability of composite membrane was 7 and 21 kg m⁻² h⁻¹ at 5 and 10 bar, respectively. Moreover, the rejection to the MgSO₄ as divalent salt (85 and 90%) was high compared to the NaCl as monovalent salt (64 and 67%).     

Superhydrophobicity of polyvinylidene fluoride membrane fabricated by chemical vapor deposition from solution

Due to the chemical stability and flexibility, polyvinylidene fluoride (PVDF) membranes are widely used as the topcoat of architectural membrane structures, roof materials of vehicle, tent fabrics, and so on. Further modified PVDF membrane with superhydrophobic property may be even superior as the coating layer surface. The lotus flower is always considered to be a sacred plant, which can protect itself against water, dirt, and dust. The superhydrophobic surface of lotus leaf is rough, showing the micro- and nanometer scale morphology. In this work, the microreliefs of lotus leaf were mimicked using PVDF membrane and the nanometer scale peaks on the top of the microreliefs were obtained by the method of chemical vapor deposition from solution. The surface morphology of PVDF membrane was investigated by scanning electronic microscopy (SEM) and atomic force microscope (AFM). Elemental composition analysis by X-ray photoelectron spectroscopy (XPS) revealed that the material of the nanostructure of PVDF membrane was polymethylsiloxane. On the lotus-leaf-like PVDF membrane, the water contact angle and sliding angle were 155° and 4°, respectively, exhibiting superhydrophobic property.