Formation of porous poly(vinylidene fluoride) membranes with symmetric or asymmetric morphology by immersion precipitation in the water/TEP/PVDF system

The phase equilibrium boundaries of the membrane forming system, water/triethyl phosphate (TEP)/PVDF, at 25 °C were determined experimentally using cloud-point and equilibrium absorption methods. Based on the phase diagram, appropriate dope and bath compositions were selected to prepare microporous membranes by means of the isothermal immersion-precipitation technique. As a metastable casting dope with respect to crystallization was adopted, the formed membranes exhibited a uniform cross-section composed of interlocked crystal elements coexisting with the network of continuous pores, as was revealed by high resolution FESEM imaging. Morphologies of the membranes’ top surfaces were found to depend heavily on the bath strength, which was controlled by the TEP content. By changing the bath gradually from pure water to 70% TEP, the top surface evolved from a dense skin (asymmetric membrane) to a totally porous morphology (symmetric membrane). Wide angle X-ray diffraction analysis indicated that PVDF crystallized into α-type structure for all of the synthesized membranes. The crystallinity as determined from diffraction peak deconvolution was ≈65%, which value was confirmed by Differential Scanning Calorimetry (DSC). The obtained thermograms also showed a similar melting peak temperature (T_m ≈ 169 °C) for all membranes. Furthermore, water fluxes and tensile strengths of the membranes were measured. The results were found to correlate with the morphologies of the membranes.     

INFLUENCE OF ALCOHOL-BASED NONSOLVENTS ON THE FORMATION AND MORPHOLOGY OF PVDF MEMBRANES IN PHASE INVERSION PROCESS

Through the preparation of PVDF membranes using various nonsolvent coagulation baths following the phase inversion process, the influence of alcohol-based nonsolvents on the formation and structure of PVDF membranes were investigated. The light scattering and light transmission measurements were used to characterize the equilibrium phase diagram and the gelation speed, respectively. The locations of the crystallization-induced gelation boundaries for various systems and precipitation processes were explained from the corresponding thermodynamic and kinetic parameters. It was found that the better affinity between alcohol-based nonsolvents and DMAc solvent caused the gelation boundaries further away from the PVDF-DMAc axis with the coagulation bath varying from water, methanol, ethanol to iso-propanol. Due to the lower exchange rate of DMAc and alcohols, the delayed demixing took place for the membrane-forming using alcohols as baths, and the delayed time became longer when the coagulation bath was changed from methanol, ethanol to iso-propanol. The characterization results of membranes indicate that the influence of nonsolvents on the phase diagram and the precipitation process are in agreement with those on the membrane morphology. The better thermodynamic stability and a low exchange diffusion rate of PVDF/DMAc/alcohols favor the liquid-solid phase separation in gelation process, and therefore yield the membranes with a porous upper surface, a particular bottom surface and symmetrical structure.     

Microporous polyethersulfone membranes prepared under the combined precipitation conditions with non‐solvent additives

Using diethylene glycol (DegOH) as non‐solvent additive (NSA) and N, N‐dimethylacetamide (DMAc) as solvent (S), polyethersulfone (PES) flat sheet membranes were prepared via immersion precipitation combined with the vapor induced phase separation (VIPS) process. Light transmittance was used to follow the precipitation rate during the immersion process as well as during the VIPS stage. As the addition of the NSA, the viscosity of casting solutions increased, which led to a slow precipitation rate. Though the precipitation rate decreased, the instantaneous demixing type was maintained. High flux membranes were obtained only at a high mass ratio of NSA/S; producing membranes had cellular pores on the top surface and sponge‐like structure on cross section. The VIPS process prior to immersion precipitation was important for the formation of cellular pore on the surface. With the increase in exposure time, the liquid–liquid phase separation took place on the surface of casting solution; nucleation and growth induced the formation of cellular pore on the top surface. Coagulation bath temperature also had large effect on the precipitation rate; high temperature on coagulation bath mainly accelerated the transfer of solvent and non‐solvent. Higher flux membrane with a porous skin layer could be obtained at a high coagulation bath temperature, but at the same time the mechanism properties were weakened. Copyright © 2007 John Wiley & Sons, Ltd.     

Morphology changes of polyvinylidene fluoride membrane under different phase separation mechanisms

In this study, the strong morphology changes of polyvinylidene fluoride (PVDF) membrane were demonstrated by changing phase separation process from a diffusion induced phase separation (DIPS) to its combination with a thermally induced phase separation (TIPS) which can be attained via changing the diluent – dibutyl phthalate (DBP) content in solvent – N,N-dimethylacetamide (DMAc). The solvent became poor when it mixed with DBP, so TIPS could occur in the quenching process which resulted in a rapid crystallization process. In this process, the porous skin and interlocked small crystallite particle (or bi-continuous) morphologies were formed, while the porous skin and leaf-like network morphology came from the rapidly crystallizing in TIPS, the large spherulite and dense skin could be attributed to the relaxed crystallization in DIPS, the finger-like macro-void and dense skin resulted from the liquid–liquid phase separation in DIPS. Simply speaking, the different membrane morphologies can be obtained by changing the DBP content in DMAc and the coagulation bath temperature.     

TEP-DMAc混合溶剂对PVDF膜性能的影响

利用非溶剂相转化法(NIPS)制备聚偏氟乙烯(PVDF)膜,考察了聚乙二醇(PEG200)与N,N-二甲基乙酰胺(DMAc)的质量比对膜分相速率和膜性能的影响,讨论了以磷酸三乙酯(TEP)和DMAc的混合液作溶剂对PVDF膜凝胶速率、膜结构和膜通量的影响。结果表明:PEG200的加入减弱了溶剂对聚合物的溶解能力,但铸膜液的分相行为由延迟分相转变为瞬时分相,膜通量提高。随着混合溶剂中TEP含量的增大,铸膜液的黏度增大,分相速率减慢;在高质量比m(TEP)/m(DMAc)时,膜表面的孔增多,指状孔膜结构逐渐消失,整个膜截面呈海绵状,膜通量变大,力学性能提高。     

Porous membranes modified by hyperbranched polymers: I. Preparation and characterization of PVDF membrane using hyperbranched polyglycerol as additive

Porous membranes were prepared via phase inversion process from casting solution composed of poly(vinylidene fluoride) (PVDF), N,N-dimethylacetamide (DMAc), and hyperbranched polyglycerol (HPG). The membranes were characterized in terms of surface and bulk chemical compositions, morphology, water contact angle, porosity, and water flux. The effects of HPG content on membrane structures and properties were investigated. The effect of HPG addition on the hydrophilicity was discussed as well when the compositions of coagulation bath were changed. To better understand the special effects of HPG on the structures and properties of the membranes, PVDF membranes prepared using HPG as the additive were compared with those prepared using polyethylene glycol (PEG) as the additive.     

Enhanced antifouling performance of ZnS/GO/PVDF hybrid membrane by improving hydrophilicity and photocatalysis

In order to improve the antifouling performance of PVDF membrane, a novel zinc sulfide/graphene oxide/polyvinylidene fluoride (ZnS/GO/PVDF) composite membrane was prepared by immersed phase inversion method. The surface morphology, crystal structure, photocatalytic activity, and antifouling property of the as‐prepared membranes were systematically studied. Results showed that the ZnS/GO/PVDF hybrid membranes were successfully fabricated with uniform surface. The hybrid membrane surface possessed higher hydrophilicity with water contact angle decreasing from 77.1° to 62.2°. The permeability of the hybrid membrane was therefore enhanced from 222.9 to 326.1 L/(m² hour). Moreover, bovine serum albumin (BSA) retention experiment showed that the hybrid membrane separation was also promoted by 7.2%. The blending of ZnS and GO enhanced the hydrophilic and photocatalytic performances of PVDF membrane, which mitigated the membrane fouling effectively. This novel hybrid membrane could accelerate the practical application of photocatalytic technology in membrane separation process.     

Morphology of microporous poly(vinylidene fluoride) membranes studied by gas permeation and scanning electron microscopy

Microporous poly(vinylidene fluoride) (PVDF) membranes with asymmetric pore structure were prepared by a wet phase inversion process. The polymer was precipitated from a casting solution when immersed in a cold water (gelation) bath. The casting solution was, in most cases, composed of polymer, solvent, and nonsolvent. In this solvent-nonsolvent system, the solvents used were triethylphosphate (TEP) and dimethylsulfoxide (DMSO), and the nonsolvents used were glycerol and ethanol. Mean pore sizes and effective porosity of the microporous membranes were calculated using the gas permeation method. They were studied as a function of evaporation time of wet nascent film, polymer molecular weight, concentration of polymer, and concentration of nonsolvent. The morphology of the membranes was examined by scanning electron microscopy (SEM).     

Preparation and characterization of microporous PVDF/PMMA composite membranes by phase inversion in water/DMSO solutions

PMMA/PVDF composite membranes were prepared by isothermal immersion-precipitation of dope solutions consisting of PMMA, PVDF, and DMSO into both harsh and soft nonsolvent baths. The effects of PMMA and DMSO contents on the membrane morphology, crystal structure, thermal behavior and tensile strength of the formed membrane were investigated. For a PMMA-free casting dope immersed in a harsh bath, such as pure water, the formed membrane exhibited a typical asymmetric morphology characterized by skin, finger-like macrovoids, and cellular pores. In contrast, when a soft 70% DMSO bath was adopted, PVDF crystallized to form a membrane packed by spherulitic globules. Incorporation of PMMA gave rise to interesting morphological features; e.g., PVDF globules were observed to adhere to the interlocked polymer branches coexisting with the continuous porous channels, as revealed by high resolution FESEM imaging. XPS analysis of the surfaces of the composite membranes suggested the occurrence of a surface segregation phenomenon, wherein PVDF preferentially migrated to the top surface region of the membrane such as to minimize the interfacial energy. XRD analyses indicated that PVDF crystallized into ‘α’ structure in both PVDF and PMMA/PVDF composite membranes. The crystallinity of the membranes was found to decrease with increasing PMMA content, which was confirmed by DSC thermal analyses. The latter results also indicated a significant decrease in membrane’s melting temperature as the PMMA content was increased. Tensile strengths of the membranes were improved by inclusion of PMMA in either harsh or soft baths. However, elongation at break showed a reversed trend.     

PVDF亲水复合膜的制备及表征

聚偏氟乙烯(PVDF)膜材料存在强疏水性的缺陷,亲水化改性是解决该问题的主要途径。以PVDF为基膜材料、聚乙烯醇(PVA)为共混材料、N,N-二甲基乙酰胺(DMAc)为溶剂,采用相转化法制备PVDF/PVA复合膜。考察了复合膜的PVDF/PVA共混比、固含量、低分子化合物添加剂、聚合物添加剂等非溶剂添加剂对复合膜接触角的影响。结果表明,当PVDF/PVA共混比为7/3,固含量为13%时,制备的复合膜接触角为22.92°;当添加剂为无水氯化锂、纳米二氧化硅、聚乙烯吡咯烷酮(PVP)时,复合膜接触角分别从53.12°、30.51°和41.89°都降低到了0°,亲水性提高,其中纳米二氧化硅作为添加剂时复合膜亲水性最好;当添加剂为丙三醇、PMMA、PEG时,复合膜接触角都增大,亲水性变差。     

Regulation of the microstructure of polyvinylidene fluoride membrane via incorporation of nano-ZIF-7 for improving hydrophobicity and antiwetting performance

The design and fabrication of a membrane with super hydrophobicity and antiwetting property is of great importance for improving membrane performance in distillation, desalination, gas absorption, and separation. In this work, polyvinylidene fluoride (PVDF) membranes were modified by Zeolitic Imidazolate Framework-7 (ZIF-7) nanocrystals to improve the hydrophobic property and antiwetting performance. ZIF-7/PVDF hybrid membranes were prepared via the nonsolvent-induced phase separation (NIPS) method. Different concentrations of ZIF-7 nanocrystals (0, 0.5, 1, 2, 3, and 5 wt%) were introduced into the PVDF dope solution, and the physical structure of the resulting membranes were systematically characterized. Due to the hydrophobic nature of ZIF-7 nanocrystals, the solvent–nonsolvent exchange rate had been regulated effectively during phase inversion. The morphology of top and bottom surfaces, together with the inner structures of the hybrid membrane, has been changed obviously, showing a more twisted finger-like macrovoid layer and a thicker sponge-like layer compared to pristine PVDF membrane. Furthermore, the hydrophobicity and antiwetting properties of these hybrid membranes improved obviously when the incorporated concentration of ZIF-7 was higher than 1 wt%. The M(2) membrane, which possessed the highest surface roughness and water contact angles, showed the best antiwetting property and recovered gas permeance ratio (>95%) after being immersed in aqueous solution for 10 hr.     

PVDF membrane formation by diffusion‐induced phase separation‐morphology prediction based on phase behavior and mass transfer modeling

The equilibrium phase behavior of water (nonsolvent)‐DMF (solvent)‐PVDF system at 25°C was investigated via both theoretical and experimental approaches. Using binary interaction parameters, χ_ij, obtained previously, the theoretical phase boundaries were computed and were found to match closely the measured binodal and crystallization‐induced gelation data. Membranes were prepared using the isothermal immersion‐precipitation processes in various dope and bath conditions. The formed membranes demonstrated a broad spectrum of morphologies: At one extreme, asymmetric structure was obtained featuring a continuous tight skin and a sublayer composed of parallel macrovoids and cellular pores; at the other limit, skinless microporous membrane was produced with spherical particles packed into a bi‐continuous structure. The crystalline characters of PVDF gels and membranes were examined by small angle light scattering, scanning electron microscopy, and differential scanning calorimetry techniques. In addition, diffusion trajectories and concentration profiles in the membrane solution before precipitation were calculated for the immersion process. These results predicted reasonably the various morphologies observed in the membranes. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2079–2092, 1999     

Fabrication of Hydrophilic and Sponge-like PVDF/Brush-like Copolymer Blend Membranes Using Triethylphosphate as Solvent简

Porous PVDF blend membranes with good hydrophilicity and a symmetric structure were prepared by the phase inversion method using amphiphilic brush-like copolymers, P（MMA-r-PEGMA）, as hydrophilic additive and triethylphosphate （TEP） as solvent. P（MMA-r-PEGMA） was synthesized by radical polymerization in TEP. Then the obtained amphiphilic copolymer solution was mixed with PVDF and TEP to prepare the dope solution. The effects of P（MMA-r-PEGMA） content and coagulation composition on membrane morphologies were investigated using scanning electron microscopy （SEM）. The results demonstrated that, even blended with amphiphilic copolymers, a symmetric structure can be formed. Hollow fiber membranes with a mainly symmetric structure were also fabricated. The dry hollow fiber membranes showed good hydrophilicity, high flux and good rejection performance because of their hydrophilic surface and pores wall.     

Hierarchical metal-phenolic-polyplex assembly toward superwetting membrane for high-flux and antifouling oil-water separation

Superwetting membranes have emerged as promising materials for the efficient treatment of oily wastewater. Typically, superwetting membranes can be developed by ingeniously chemical modification and topographical structuration of microporous membranes. Herein, we report the hierarchical assembly of metal-phenolic-polyplex coating to manipulate membrane surface superwettability by integrating metal-phenolic (Fe^III-tannic acid (TA)) assembly with polyplex (tannic acid-polyethylenimine (PEI)) assembly. The proposed Fe-TA-PEI coating can be deposited on microporous membrane via simply dipping into Fe^III-TA-PEI co-assembly solution. Based on the catechol chemistry, the coordination complexation of Fe^III and TA develops metal-phenolic networks to provide hydrophilic chemistries, and the electrostatic complexation of TA and PEI generates nanoconjugates to impart hierarchical architectures. Benefiting from the synergy of hydrophilic chemistries and hierarchical architectures, the resulting PVDF/Fe-TA-PEI membrane exhibits excellent superhydrophilicity (～0°), underwater superoleophobicity (～150°) and superior anti-oil-adhesion capability. The superhydrophilicity of PVDF/Fe-TA-PEI membrane greatly promotes membrane permeability, featuring water fluxes up to 5860 L m^?2 h^?1. The underwater superoleophobicity of PVDF/Fe-TA-PEI membrane promises potential flux (3393 L m^?2 h^?1), high separation efficiency (99.3%) and desirable antifouling capability for oil-in-water emulsion separation. Thus, we highlight the reported hierarchical metal-phenolic-polyplex assembly as a straightforward and effective strategy that enables the synchronous modulation of surface chemistry and topography toward superwetting membranes for promising high-flux and antifouling oil-water separation.     

The microstructure and surface characteristics of fluorine-based copolymer coatings deposited by the static breath method

We constructed a coating with controllable microstructure on the substrate by static breath figure (BF) method. In the atmosphere of water, when the fluorine-containing 1H,1H,2H,2H-perfluorooctyl polyacrylate-block-polystyrene copolymer (PTFOA-b-PS (4 h)) synthesized via atom transfer radical polymerization for 4 h was dissolved in carbon disulfide (CS₂) and then coated on the silicon wafer, the concentrated pore size and uniform micro-pore structure was successfully constructed on the surface. However, the hydrophobicity of the resulting microporous coating was not so good. An interesting result is that when the copolymer solution of PTFOA-b-PS (2 h) in tetrahydrofuran (THF) was used under the MeOH/H₂O atmosphere (the methanol content was 60% or 80%), the microsphere coating can be produced on the substrate surface to obtain the excellent hydrophobicity. The water contact angles (WCAs) were 148.8 ± 1.8° and 150.3 ± 1.8°, respectively. Energy dispersive X-ray spectrometer (EDS) results showed that the fluorine element was enriched in the outerfield of the microsphere coating. The low surface energy of fluorine combined with the rough and complex structure of the microspheres can effectively improve the hydrophobicity of the coating.     

Enhanced Pervaporation Performance of Multi-layer PDMS/PVDF Composite Membrane for Ethanol Recovery from Aqueous Solution

Multi-layer PDMS/PVDF composite membrane with an alternative PDMS/PVDF/non-woven-fiber/PVDF/PDMS configuration was prepared in this paper. The porous PVDF substrate was obtained by casting PVDF solution on both sides of non-woven fiber with immersion precipitation phase inversion method. Polydimethylsiloxane (PDMS) was then cured by phenyltrimethoxylsilane (PTMOS) and coated onto the surface of porous PVDF substrate one layer by the other to obtain multi-layer PDMS/PVDF composite membrane. The multi-layer composite membrane was used for ethanol recovery from aqueous solution by pervaporation, and exhibited enhanced separation performance compared with one side PDMS/PVDF composite membranes, especially in the low ethanol concentration range. The maximum separation factor of multi-layer PDMS/PVDF composite membrane was obtained at 60 °C, and the total flux increased exponentially along with the increase of temperature. The composite membrane gave the best pervaporation performance with a separation factor of 15, permeation rate of 450 g/m²h with a 5 wt.% ethanol concentration at 60 °C.     

两亲性梳状聚醚硅氧烷对相转化法聚偏氟乙烯多孔膜的共混改性作用研究

以聚醚链段为侧链的两亲性梳状聚醚硅氧烷(ACPS)为改性剂,研究了相转化法制备聚偏氟乙烯(PVDF)多孔膜的改性效果与机理.采用SEM、XPS、接触角、水通量等考察了ACPS对膜结构与性能的影响.研究发现,ACPS在相转化成膜过程中不流失,随着制膜液中ACPS含量的增加,相分离速度降低,膜中微孔由指状结构向蜂窝状结构发展,膜强度提高,亲水性显著提高.提出了ACPS在膜表面的富集现象和在膜中的稳定性机理和模型.结果表明,两亲性梳状聚醚硅氧烷在原理上是一类适合于相转化法制备聚合物微孔膜表面亲水化改性的有效物质.     

Porous poly(vinylidene fluoride) membrane modified with hyperbranched poly(amine‐ester)

Poly(vinylidene fluoride) (PVDF) membranes were hydrophilic modified with hydroxyl group terminated hyperbranched poly(amine‐ester) (HPAE). Fourier transform infrared spectroscopy (FT‐IR) was used to study the chemical change of PVDF membranes. X‐ray photoelectron spectroscopy (XPS) indicated that some HPAE molecules were retained in PVDF membrane through polymer chain coiling. The presence of HPAE would improve the hydrophilicity of PVDF membrane. Scanning electron microscopy (SEM) was employed to characterize the morphology of different membranes. The thermodynamic stability for PVDF/DMAc/HPAE/Water system was characterized by the determination of the gelation values. Precipitation kinetics for PVDF/DMAc/HPAE/Water system was studied by precipitation time measurement. The water contact angle indicated that the hydrophilicity and the biocompatibility corresponding to protein adsorption of PVDF membrane were improved significantly after blending with hydrophilic HPAE molecules. Copyright © 2011 John Wiley & Sons, Ltd.     

Fine structure and formation mechanism of particulate phase‐inversion poly(vinylidene fluoride) membranes

The structure and formation mechanism of a microporous phase‐inversion poly(vinylidene fluoride) (PVDF) membrane exhibiting a relatively loosely packed agglomerate of semicrystalline globules are explored. The membrane has been prepared by the coagulation of a solution of PVDF in dimethylformamide by the action of 1‐octanol, which is a soft nonsolvent. Experimental observations pertain to the globule surface, which is dominated by a grainy nanostructure; the globular interior, which exhibits a range of fine structures (e.g., twisted sheets and treelike branches); and the globule–globule connections, which exhibit a sheetlike or ropelike structure. On the basis of the observed structural details and phase diagram considerations, it is proposed that the membrane structure is the result of a unique combination of a polymer crystallization and a liquid–liquid phase‐separation process, with end‐result globular structural features of remarkable uniformity. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1578–1588, 2003     

Acrylonitrile–maleic anhydride copolymer membrane (I): effects of casting conditions

The effects of casting conditions, including casting solution (composition and temperature) and coagulation conditions (pre‐evaporation time, temperature and concentration of coagulation bath) on the structure and performance of acrylonitrile–maleic anhydride copolymer membrane have been investigated. The results showed that the water flux decreased gradually while the rejection of bovine serum albumin (BSA) decreased as the concentration of copolymer increased. When the total solid concentration was kept unchanged, the water flux increased with additive polyvinylpyrrolidone (PVP), the rejection did not decrease until the ratio of PVP/copolymer was 60%. When the content of copolymer in the casting solution was kept constant, the water flux decreased rapidly while the rejection increased a little (compared with the case of no additive) as the ratio of PVP/copolymer increased. As to the temperature of casting solution, the water flux had a maximum at 45 °C and the rejection had a maximum and a minimum at 45 and 55 °C, respectively. The water flux had a maximum value when the pre‐evaporation time was 40 sec. The rejection of BSA was almost unchanged when the pre‐evaporation time was less than 40 sec. and then decreased and reached a minimum value at 60 sec. As the temperature of coagulation bath increased, the water flux reached a maximum at 35 °C while the rejection increased uniformly. With increasing the concentration of DMSO in the coagulation bath, the water flux decreased gradually and got to a minimum at 50 wt% as the concentration of dimethylsulfoxide in the coagulation bath increased, but no apparent effect on the rejection was observed. Copyright © 2000 John Wiley & Sons, Ltd.