Structure formation and characterization of PVDF hollow fiber membranes by melt‐spinning and stretching method

Melt‐spinning and stretching (MS‐S) method was proposed for preparing poly(vinylidene fluoride) (PVDF) hollow fiber membranes with excellent mechanical properties. The morphology and properties of PVDF fibers and membranes were investigated by small angle X‐ray scattering (SAXS), differential scanning calorimeter (DSC), field emission scanning electron microscope, mercury porosimeter, and tensile experiment. SAXS results indicated that the stacked lamellar structure aligned normal to the fiber axis was separated and deformed when the fibers were strained, and the long period of the strained fibers increased accordingly. Factors affecting the membrane properties were mainly spin‐draw ratio, annealing temperature, time, and stretching rate. Experimental results showed that the average pore size, porosity, and N₂ permeation of the membranes all increased with the increasing spin‐draw ratios and annealing temperatures. Annealing the nascent PVDF hollow fibers at 145°C for 12 h was suitable for attaining membranes with good performance. In addition, the amount and size of the micropores of the membrane increased obviously with stretching rate. Tensile experiment indicated PVDF hollow fiber membranes made by MS‐S process had excellent mechanical properties. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Preparation of PVDF ultrafiltration membranes using PVA as pore surface hydrophilic modification agent with improved antifouling performance

Novel polyvinylidene fluoride (PVDF) ultrafiltration (UF) membranes were facilely fabricated using polyvinyl alcohol (PVA) aqueous solution as the coagulation bath through phase inversion method. In the process, PVA was introduced into the pore surfaces of the PVDF membranes via the interdiffusion of the non‐solvent water and the solvent. The effects of PVA content in the coagulation bath on membrane properties were systematically discussed. The results indicated that the increase of PVA content in coagulation bath resulted in the formations of the more sponge‐like structures and the higher surface hydrophilicity. Smaller pore size led to lower water flux and higher bovine serum albumin rejection. Fouling resistance measurement indicated that the membranes made in PVA/water coagulation bath had higher flux recovery ratio (92.1%) than the membrane made in a pure water bath (71.0%). Furthermore, mechanical property test revealed that the resulting membranes had high tensile strength and Young's modulus. In this work, we found that the morphology and the property of the novel PVDF membranes could be determined by the PVA content in the coagulation bath. POLYM. ENG. SCI., 59:E384–E393, 2019. © 2018 Society of Plastics Engineers     

Preparation and properties of PVDF/PVA hollow fiber membranes

On principle of polymer blend phase separation, PVDF/PVA hollow fiber membranes were prepared using phase inversion method. The membrane morphology and performance varied with the blending ratio. The PVDF/PVA blends showed incompatibility by the results of dynamic mechanical analysis (DMA) and infrared attenuated total reflection (FTIR-ATR) sampling technique. Based on bursting pressure and tensile strengths results, we suggest that the mechanical properties of PVDF/PVA blend membranes are worse than that of PVDF membrane. PVA can improve the hydrophilicity of PVDF/PVA hollow fiber membranes, which could be illuminated by the decrease in contact angle, the increase in equilibrium water content (EWC) and the variety in dynamic moisture regain. The pure water flux increases while the rejection ratio decreases with PVA content increasing. Moreover, PVA can improve the anti-fouling property of PVDF/PVA hollow fiber membranes, which could be illuminated by the result of increase coefficient of resistance.     

PVDF树脂的物性对中空纤维膜性能的影响

采用了不同特性粘度的PVDF树脂,通过非溶剂致相分离法(NIPS)制备了相应的PVDF中空纤维膜。通过力学性能、纯水通量、牛血清白蛋白(BSA)截留率等性能测试发现,不同特性粘度的PVDF树脂制备的中空纤维膜的性能差异较大。随着PVDF特性粘度的增加,PVDF中空纤维膜的拉伸强度及断裂伸长率逐渐增加,纯水通量逐渐降低,BSA截留率先降低后增加。通过扫描电镜(SEM)进一步发现,随着PVDF特性粘度的增加,PVDF树脂制备的中空纤维膜,其海绵层上的孔状结构逐渐减少且变小。     

凝固浴组成和温度对PVDF疏水微孔膜结构与性能的影响

利用非溶剂相转化法(NIPS),以聚偏氟乙烯(PVDF)/磷酸三乙酯(TEP)-N,N-二甲基乙酰胺(DMAc)为铸膜液体系,乙醇水溶液为凝固浴制备高性能的PVDF疏水微孔膜。考察了凝固浴中乙醇(EtOH)含量及凝固浴温度对PVDF成膜分相速率、膜结构和膜疏水性的影响。实验结果表明,在20℃的凝固浴温度下,凝固浴中乙醇含量的升高减慢了铸膜液体系的分相速率,提高了PVDF膜的孔隙率;在凝固浴中添加60%(wt)的乙醇,可形成表面荷叶状结构和截面对称的海绵状结构,膜表面的接触角为130.3°,呈很强的疏水性,并具有较优的膜强度。     

Hydrophilic modification of high‐strength polyvinylidene fluoride hollow fiber membrane

The polyvinylidene fluoride (PVDF)/polyvinyl alcohol (PVA) polymer solutions were coated on the outer surface of PVDF matrix hollow fiber membrane. On the principle of the homogeneous‐reinforced (HR) membrane technology, the reinforced PVDF/PVA (RFA) hollow fiber membranes prepared through the dry‐wet spinning method. The performance of the RFA membranes varies with the PVA concentration in the polymer solution and is characterized in terms of pure water flux (PWF), porosity, a mechanical strength test, and morphology observations by a scanning electron microscopy (SEM). The results of this study indicate that PVA can apparently improve the hydrophilicity of the PVDF hollow fiber membranes. The growing enrichment of the hydrophilic components PVA on the membrane surface is determined by X‐ray photoelectron spectroscopy. The RFA membranes have a favorable interfacial bonding between the coating layer (PVDF/PVA) and the matrix membrane (PVDF hollow fiber membrane), as shown by SEM. The elongation at break of the RFA membranes increases much more than that of the matrix membrane that is endowed with the better flexibility of the membrane performance. PWF decreases much more compared with that of the matrix membrane. The RFA membranes have a lower flux decline degree during the process of protein solution and ink solution filtration compared with that of the matrix membrane. POLYM. ENG. SCI., 54:276–287, 2014. © 2013 Society of Plastics Engineers     

Preparation of PVDF hollow‐fiber membranes via immersion precipitation

Porous polyvinylidene fluoride (PVDF) hollow‐fiber membranes with high porosity were fabricated using the immersion precipitation method. Dimethylacetamide (DMAc) and N‐methyl‐2‐pyrrolidone (NMP) were used as solvent, respectively. In addition, polyvinylpyrrolidone (PVP), lithium chloride, and organic acids were employed as nonsolvent additives. The effects of the internal and external coagulation mediums on the resulting membrane properties were also investigated. The resulting hollow‐fiber membranes were characterized in terms of maximum pore radius, mean pore radius, effective surface porosity as well as wetting pressure. The structures of the prepared hollow fibers were examined using a scanning electron microscope. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 1643–1653, 2001     

Preparation of a heterogeneous hollow‐fiber affinity membrane modified with a mercapto chelating resin

A high‐quality, heterogeneous hollow‐fiber affinity membranes modified with mercapto was prepared through phase separation with blends of a chelating resin and polysulfone as membrane materials, poly(ethylene glycol) as an additive, N,N‐dimethylacetamide as a solvent, and water as an extraction solvent. The effects of the blending ratio and chelating resin grain size on the structure of the hollow‐fiber affinity membrane were studied. The effects of the composition of the spin‐cast solution and process parameters of dry–wet spinning on the structure of the heterogeneous hollow‐fiber affinity membrane were investigated. The pore size, porosity, and water flux of the hollow‐fiber affinity membrane all decreased with an increase in the additive content, bore liquid, and dry‐spinning distance. With an increase in the extrusion volume outflow, the external diameter, wall thickness, and porosity of the hollow‐fiber affinity membrane all increased, but the pore size and water flux of the hollow‐fiber affinity membrane decreased. It was also found that the effects of the internal coagulant composition and external coagulant composition on the structure of the heterogeneous hollow‐fiber affinity membrane were different. The experimental results showed that thermal drawing could increase the mechanical properties of the heterogeneous hollow‐fiber affinity membrane and decrease the pore size, porosity, and water flux of the heterogeneous hollow‐fiber affinity membrane, and the thermal treatment could increase the homogeneity and stability of the structure of the heterogeneous hollow‐fiber affinity membrane. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Stability of acrylic acid grafted poly(vinylidene fluoride) hollow fiber membrane prepared by high‐energy electron beam

In this article, poly(vinylidene fluoride) (PVDF) hollow fiber membrane and acrylic acid (AA) were co‐irradiated by high‐energy electron beam to introduce hydrophilic carboxylic groups on the membrane surface. Thermal capability, mechanical performance, pore size, and permeation property were investigated to determine the stability of the membrane pore structure before and after irradiation polymerization. The decomposition temperature, melting point, glass transition temperature, and breaking force of the PVDF‐g‐AA membrane increased slightly because of irradiation grafting polymerization. After 15 months of storage, the pore size distribution of the PVDF‐g‐AA membrane became smaller and more dispersive. The pure water flux and the rejection to bovine serum albumin of the PVDF‐g‐AA membrane increased significantly with the increase in hydrophilicity and decrease in pore size. The results indicated that the structure and properties of the PVDF hollow fiber membrane were stable after high‐energy electron beam irradiation grafting polymerization, even after 15 months of storage. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41165.     

Effects of mixed solvents and PVDF types on performances of PVDF microporous membranes

Polyvinylidene fluoride (PVDF) microporous flat membranes were cast with different kinds of PVDFs and four mixed solvents [trimethyl phosphate (TMP)–N,N‐dimethylacetamide (DMAc), triethyl phosphate (TEP)–DMAc, tricresyl phosphate (TCP)–DMAc, and tri‐n‐butyl phosphate (TBP)–DMAc]. The effects of different commercial PVDFs (Solef® 1015, FR 904, Kynar 761, Kynar 741, Kynar 2801) on membrane morphologies and membrane performances of PVDF/TEP–DMAc/PEG200 system were investigated. The membrane morphologies were examined by scanning electron microscopy (SEM). The membrane performances in terms of pure water flux, rejection, porosity, and mean pore radius were measured. The membrane had the high flux of 143.0 ± 0.9 L m^?2 h^?1 when the content of TMP in the TMP–DMAc mixed solvent reached 60 wt %, which was 2.89 times that of the membrane cast with DMAc as single solvent and was 3.36 times that of the membrane cast with TMP as single solvent. Using mixed solvent with different solvent solubility parameters, different morphologies of PVDF microporous membranes were obtained. TMP–DMAc mixed solvent and TEP–DMAc mixed solvent indicated the stronger solvent power to PVDF due to the lower solubility parameter difference of 1.45 MPa^1/2 and the prepared membranes showed the faster precipitation rate and the higher flux. The less macrovoids of the membrane prepared with TEP (60 wt %)–DMAc (40 wt %) as mixed solvent contributed to the higher elongation ratio of 96.61% ± 0.41%. Therefore, using TEP(60 wt %)–DMAc (40 wt %) as mixed solvent, the casting solution had the better solvent power to PVDF, and the membrane possessed the excellent mechanical property. The microporous membranes prepared from casting solutions with different commercial PVDFs exhibited similar morphology, but the water flux increased with the increment of polymer solution viscosity. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Pore structure and properties of poly(ether ether ketone) hollow fiber membranes: influence of solvent‐induced crystallization during extraction

Poly(ether ether ketone) (PEEK) hollow fiber membranes were prepared by a thermally induced phase separation method with polyetherimide as diluent, and N‐methyl pyrrolidone (NMP), dichloromethane and a composite extractant composed of NMP, ethanolamine and water as extractant. The effects of the different solvents induced crystallization on the pore structure during extraction and the properties of the PEEK hollow fiber membranes were investigated in detail. The crystallization behaviors of the membranes were characterized by DSC and XRD. The effect of the extractants on the microscopic morphologies, pore structures, water fluxes and mechanical properties of the membranes were investigated. The results showed that the extraction ability of the composite extractant was the most significant, followed by NMP and dichloromethane. The crystallinity of the hollow fiber was 39.0% before extraction and was elevated to 39.2% after the extraction with NMP, 46.6% with dichloromethane and 46.7% with the composite extractant, which shows that dichloromethane and the composite extractant have strong ability to induce the crystallization of PEEK. The inner and outer surfaces of the membranes obtained after extraction by the composite extractant had the largest pore size and the highest surface porosity. The most probable pore diameter of the membranes obtained after extraction by NMP, dichloromethane and the composite extractant was 23.26 nm, 24.43 nm and 24.43 nm, respectively, which indicated that solvent‐induced crystallization was beneficial for the formation of larger pores. The pure water flux of the PEEK membrane prepared by the composite extractant was the largest, but the tensile strength was the lowest. © 2019 Society of Chemical Industry     

Removal of Benzene/Toluene from Water by Vacuum Membrane Distillation in a PVDF Hollow Fiber Membrane Module

Abstract

Asymmetric polyvinylidene fluoride (PVDF) hollow fiber membranes were prepared by a phase inversion method using dimethylacetamide (DMAc) and a mixture of water/LiCl as solvent and a nonsolvent additive, respectively. The prepared membranes were characterized by scanning electron microscopy (SEM) for observing its microstructures and by a gas permeation method for measuring its surface porosity, pore size, and pore size distribution. Wetting pressures of the dry hollow fiber membranes were also measured. Using the prepared PVDF hollow fiber membranes, a membrane module was fabricated for removal of benzene/toluene from water. Effects of various operating parameters such as downstream vacuum levels, feed temperatures, and feed flow rates on performances of the module were investigated experimentally. The benzene/toluene removal was achieved over 99% under an optimal operating condition. Mass transfer of benzene or toluene removal is controlled not only by the liquid phase resistance but also by the membrane and gas phase resistances. Benzene and toluene can be removed from water simultaneously with no adverse coupling effects.     

Preparation of PET threads reinforced PVDF hollow fiber membrane

PET threads were incorporated in the support layer of hollow fiber membrane in axial direction as a special reinforcement material for the purpose of improving the mechanical properties of PVDF hollow fiber membranes. It was found that the reinforcement threads had a limited effect on the separation-related properties of the membrane, such as porosity and pore size, but the tensile strength of the reinforced membrane was improved several folds. Also, the criterion of choosing reinforced fiber materials was suggested.     

Structure and performance of temperature‐sensitive poly(vinylidene fluoride) hollow fiber membrane fabricated at different take‐up speeds

Graft copolymer (PVDF‐g‐PNIPAAm) having poly(vinylidene fluoride) (PVDF) backbones and poly(N‐isopropylacrylamide) (PNIPAAm) side chains was synthesized via radical copolymerization and its hollow fiber membrane was fabricated from dry–wet spinning technique with N, N‐dimethylformamide as the solvent and poly(ethylene glycol) (10,000) as the additive. The effects of spinning condition (take‐up speeds) on the structures and performances of resulting fiber membranes were systematically considered. The structures and performances of fiber membranes were characterized by element analysis, X‐ray photoelectron spectroscopy, water contact angle measurement, scanning electron microscope, atom force microscope, and filtration experiments. The results indicate that PNIPAAm side chains tended to enrich on the membrane surface and pore surface and especially tended to aggregate on the inner surface due to the effect of bore fluid. The hollow fiber membrane exhibits an obvious temperature‐sensitive property. The pure water flux increases remarkably around 32°C, while the retention of albumin egg decreases accordingly, when the permeation temperature rises from 20 to 45°C. As the take‐up speed increases, both the inner and outer diameters of fiber membranes decrease. A higher take‐up speed favors higher pure water permeation flux, which allows larger molecules to permeate through the fiber membrane. POLYM. ENG. SCI. 2013. © 2012 Society of Plastics Engineers     

不同中空纤维膜材料对烟气中二氧化硫的吸收性能影响

膜法烟气脱硫能耗低、传质面积大、分离效率高,可以有效地解决传统塔器内的液泛、漏液、夹带等问题。本文采用自制的中空纤维膜接触器,通过改变烟气流量、水流量和水温对比了聚四氟乙烯（PTFE）、聚偏氟乙烯（PVDF）和聚丙烯（PP）这3种中空纤维膜对烟气中二氧化硫的吸收性能,并通过电镜和接触角仪表征,对比了3种膜的参数和疏水性。结果表明：在不同烟气流量、水量和水温下,3种膜的吸收性能都表现为PTFE＞PP＞PVDF,120min时二氧化硫吸收浓度,PTFE最大,是PP的1.68倍,是PVDF的4.62倍;烟气流量的改变对二氧化硫的吸收浓度有显著影响,当烟气流量由60mL/min提高到140mL/min时,120min时PTFE膜二氧化硫的吸收浓度提高了2.14倍;影响膜性能的主要因素为疏水性,PTFE浸泡前后的表面接触角为105°和97°,疏水性远大于PP和PVDF。PTFE中空纤维膜孔径大、孔隙率高,具有极强的疏水性,在烟气脱硫及相关吸收过程中表现出较好的应用前景。     

Poly(vinyl chloride) hollow‐fiber membranes for ultrafiltration applications: Effects of the internal coagulant composition

Poly(vinyl chloride) (PVC) hollow‐fiber membranes were spun by a dry/wet phase‐inversion technique from dopes containing 15 wt % PVC to achieve membranes with different pore sizes for ultrafiltration (UF) applications. The effects of the N,N‐dimethylacetamide (DMAc) concentration in the internal coagulant on the structural morphology, separation performance, and mechanical properties of the produced PVC hollow fibers were investigated. The PVC membranes were characterized by scanning electron microscopy, average pore size, pore size distribution, void volume fraction measurements, and solubility parameter difference. Moreover, the UF experiments were conducted with pure water and aqueous solutions of poly(vinyl pyrrolidone) as feeds. The mechanical properties of the PVC hollow‐fiber membranes were discussed in terms of the tensile strength and Young's modulus. It was found that the PVC membrane morphology changed from thin, fingerlike macrovoids at the inner edge to fully spongelike structure with DMAc concentration in the internal coagulant. The effective pores showed a wide distribution, between 0.2 and 1.1 μm, for the membranes prepared with H₂O as the internal coagulant and a narrow distribution, between 0.114 and 0.135 μm, with 50 wt % DMAc. The results illustrate that the difference in the membrane performances was dependent on the DMAc concentration. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Preparation of dual‐layer acetylated methyl cellulose hollow fiber membranes via co‐extrusion using thermally induced phase separation and non‐solvent induced phase separation methods

Dual‐layer acetylated methyl cellulose (AMC) hollow fiber membranes were prepared by coupling the thermally induced phase separation (TIPS) and non‐solvent induced phase separation (NIPS) methods through a co‐extrusion process. The TIPS layer was optimized by investigating the effects of coagulant composition on morphology and tensile strength. The solvent in the aqueous coagulation bath caused both delayed liquid–liquid demixing and decreased polymer concentration at the membrane surface, leading to porous structure. The addition of an additive (triethylene glycol, (TEG)) to the NIPS solution resolved the adhesion instability problem of the TIPS and NIPS layers, which occurred due to the different phase separation rates. The dual‐layer AMC membrane showed good mechanical strength and performance. Comparison of the fouling resistance of the AMC membranes with dual‐layer polyvinylidene fluoride (PVDF) hollow fiber membranes fabricated with the same method revealed less fouling of the AMC than the PVDF hollow fiber membrane. This study demonstrated that a dual‐layer AMC membrane with good mechanical strength, performance, and fouling resistance can be successfully fabricated by a one‐step process of TIPS and NIPS. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42715.     

Poly(vinylidene fluoride‐co‐hexafluoropropylene) (PVDF‐co‐HFP) hollow fiber membranes prepared from PVDF‐co‐HFP/PEG‐600Mw/DMAC solution for membrane distillation

Poly(vinylidene fluoride‐co‐hexafluoropropylene) (PVDF‐co‐HFP) hollow fiber membranes were prepared by using the phase inversion method. The effect of polyethylene glycol (PEG‐600Mw) with different concentrations (i.e., 0, 5, 7, 10, 12, 15, 18, and 20 wt %) as a pore former on the preparation and characterization of PVDF‐co‐HFP hollow fibers was investigated. The hollow fiber membranes were characterized using scanning electron microscopy, atomic force microscopy, and porosity measurement. It was found that there is no significant effect of the PEG concentration on the dimensions of the hollow fibers, whereas the porosity of the hollow fibers increases with increase of PEG concentration. The cross‐sectional structure changed from a sponge‐like structure of the hollow fiber prepared from pure PVDF‐co‐HFP to a finger‐like structure with small sponge‐like layer in the middle of the cross section with increase of PEG concentration. A remarkable undescribed shape of the nodules with different sizes in the outer surfaces, which are denoted as “twisted rope nodules,” was observed. The mean surface roughness of the hollow fiber membranes decreased with an increase of PEG concentration in the polymer solution. The mean pore size of the hollow fibers gradually increased from 99.12 to 368.91 nm with increase of PEG concentration in polymer solution. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Stability of PVDF hollow fibre membranes in sodium hydroxide aqueous solution

The stability of PVDF hollow fibre membranes in sodium hydroxide (NaOH) aqueous solutions were investigated in this study. PVDF hollow fibre membranes were prepared from each of the three commercial raw PVDF materials (Kynar 761, Solef 1015 and Solef 6010) from two major suppliers (Atofina Chemicals Inc., USA and Solvay, Belgium) for comparison purposes. The effect of NaOH concentration, treatment time and temperature on mechanical properties, thermal properties and crystalline structure of the PVDF hollow fibre membranes were investigated through mechanical strength measurement, surface area analysis, XRD, FTIR and DSC analyses. The obtained results indicate that the reaction between PVDF and NaOH was initiated even at low concentrations of NaOH and was aggravated with the extended treatment time, resulting in the decrease in mechanical strength and crystallinity of PVDF hollow fibre membranes. The reaction was accelerated and intensified by increasing the concentration of NaOH and/or treatment temperature. At 70 °C, the mechanical integrity of the PVDF membranes was completely destroyed in 4 wt% NaOH solution within 24 h or in 10 wt% NaOH solution within 8 h. The deterioration of stability in NaOH solutions is considered universal for all PVDF employed in this study, irrespective of the raw materials or the corresponding hollow fibre membranes.     

Modified poly(vinylidene fluoride) hollow fiber composite membranes reinforced by hydroxyapatite nanocrystal whiskers

The modified poly(vinylidene fluoride) (PVDF) hollow fiber composite membranes reinforced by hydroxyapatite (HAP) nanocrystal whiskers were fabricated with wet‐spinning method. The PVDF/HAP/N‐methyl‐2‐pyrrolidone dope solutions experienced delayed demixing mechanism, and the precipitation rate slightly increased as the HAP whisker content increased. The cross sections of PVDF‐HAP and neat PVDF hollow fiber composite membranes were composed of five distinct layers: two skin layers, two finger‐like sublayers, and a sponge‐like layer. The Young's modulus of and tensile strength of the PVDF‐HAP hollow fiber membranes gradually increased with the addition of nano‐HAP whiskers. The elongation ratio was also improved, which was different from the polymeric membranes modified by other inorganic nanofillers. The permeation flux of the PVDF‐HAP hollow fiber membranes slightly increased with the increase of HAP content in the composite membranes as its hydrophilicity was improved. The crystallization behaviors of PVDF in the composite membranes were also investigated. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013