Preparation and characterization of PVDF microporous membrane with highly hydrophobic surface

Using the mixture of triethyl phosphate (TEP) and N,N‐dimethylacetamide (DMAc) as solvent, PVDF microporous membranes with highly hydrophobic surface were prepared by a modified NIPS method with a dual coagulation process. The effects of the exposure time on these membranes before being immersed into the coagulation bath and the composition in the coagulation bath on precipitation rate, membrane morphology, membrane hydrophobicity, membrane mechanical property, and membrane performance were studied. The morphologies and hydrophobicities of PVDF microporous membranes were investigated by scanning electron microscopy (SEM) and contact angle (CA) measurement. The precipitation processes were observed by light transmittance measurement. The pore size distribution was determined by liquid permeation technique. PVDF microporous membrane obtained by passing evaporation period of 60 min before being immersed into the water bath showed a high water CA of 122.1°. Using ethanol (EtOH) as coagulation bath, the water CAs of the top surface and bottom surface of the membrane increased to 125.9 and 132.6°, respectively. To further improve PVDF membrane hydrophobicity, a dual coagulation process was used and the mixed solvent (TEP–DMAc) was added into the first coagulation bath for 30 sec. Increase in the TEP–DMAc content led to the change in the morphology type of the membrane, that is, from an asymmetric structure with a dense top surface to a symmetric structure with a skinless top surface, and the pore size distribution widened greatly. By increasing the mass ratio of TEP to DMAc, the denseness of the membrane surface decreased significantly. Adding 60 wt% of TEP–DMAc to the first coagulation bath and the mass ratio of TEP to DMAc was 60:40, the CA reached to a maximum as high as 136.6°, and PVDF microporous membrane showed a high porosity of 80% and an excellent mechanical property of 3.14 MPa tensile strength and 61.79% elongation ratio. Copyright © 2009 John Wiley & Sons, Ltd.     

溶剂对PVDF铸膜液相转化和微孔膜皮-亚两层结构的不同影响

采用皮-亚分步凝固成膜机理分析了3种不同溶剂对聚偏氟乙烯(PVDF)铸膜液相转化和膜结构的影响,采用浊度法测定铸膜液体系的热力学性质,沉淀速度采用光透射仪测定.结果显示,3种膜的皮层分相主要由热力学性质控制,均发生延时液固分相,生成了相互融合的球粒组成的致密皮层.3体系的亚层分相行为由动力学扩散过程控制;对于二甲基亚砜(DMSO)、N,N-二甲基乙酰胺(DMAc)体系亚层发生瞬时液液分相,结晶化对动力学过程影响小,表现为光透射曲线上分相时间t2短,生成了大孔结构为主的亚层,膜厚度、孔隙率和气通量均高、结晶度低;N,N-二甲基甲酰胺(DMF)体系亚层发生延时液液分相,结晶化对动力学过程影响大,t2长,生成蜂窝状孔结构亚层,其膜厚度、孔隙率和气通量较低,但膜的结晶度高.     

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     

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.     

Poly(vinylidene fluoride) membranes by phase inversion: the role the casting and coagulation conditions play in their morphology, crystalline structure and properties

Flat membranes with controlled morphology, pore dimensions, mechanical properties and crystal structure were prepared by wet and dry wet phase inversion from polyvinylidene fluoride (PVDF). The effects of several parameters such as precipitation temperature, composition of the polymer solution (concentration, type of solvent), exposure time before immersion in the coagulation bath, type of coagulant on the sequence and the extent of the two phase separation processes, i.e. liquid-liquid and liquid-solid demixing (crystallization), were studied.Using solvent/nonsolvent pairs with different mutual affinity (DMA/water, DMA/C1-C8 alcohols), different morphologies were obtained. High casting solution temperature plays important role to increase the rate of the liquid-liquid demixing on the crystallization, i.e. the type of crystallites formed (α-type) also by using a soft coagulation bath. Exposure time before immersion favours the first type of phase separation and therefore once again crystallites of α type were observed. At room temperature, using C1-C8 alcohols as nonsolvents, the presence of crystallites of α type can be related to molar volume of the coagulant.     

溶剂浓度对PVDF相转换膜大孔结构的影响

提出决定大孔能否发展的初始分相点处溶剂浓度临界点的概念 ,认为初始分相点处较高的溶剂浓度有利于大孔的发展 ,溶剂浓度低于一定的界限后 ,大孔停止发展 ,转为海绵状结构 .实验考察了不同凝胶液组成下制得的PVDF中空膜的结构 ,建立了相应的传质模型 ,模拟不同制膜条件下初生态膜内的组成分布情况 ,根据初始分相点处溶剂浓度临界点的概念 ,预测膜的形态结构 .模拟结果与相应制膜条件下的电镜照片有很好的对应关系 ,证明了上述大孔形成机理的正确性 .     

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).     

添加剂对PVDF相转化过程及膜孔结构的影响

研究了PVP、PEG及LiCl 3种成孔添加剂下PVDF DMAc H2 O 添加剂体系的成膜机理 .无论那种添加剂的铸膜液相转换成膜过程中都存在凝胶分相和液液分相两种相变方式 ,在 30～ 6 0℃时凝胶分相在较低的非溶剂浓度下先于液液分相发生 ,LiCl作为添加剂较PEG、PVP对铸膜液有较强的致凝胶作用 ,成膜过程中凝胶分相段时间依PVP、PEG、LiCl的顺序延长 ,导致液液分相初始分相点处聚合物浓度增大 ,阻止了大孔结构的充分发展 .制得的膜依PVP、PEG、LiCl的顺序有效孔隙率和通量降低 ,结晶度升高 .以LiCl为添加剂制得的膜几乎不改变PVDF膜的疏水性 ,而以PVP或PEG为添加剂的膜隔水压差降低约 2 0kPa .     

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.     

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.     

小分子醇对PVDF／PFSA共混膜凝胶动力学、膜结构及膜性能的影响

以小分子醇为添加剂,研究了小分子醇对聚偏氟乙烯(PVDF)/全氟磺酸(PFSA)共混超滤膜凝胶动力学、结构与渗透性能的影响.结果表明:在PVDF/PFSA铸膜液中添加乙醇时,其凝胶速率随乙醇浓度的增加而加快,超滤膜通量随乙醇浓度的增加而增大;而当添加相同浓度的4种小分子醇时,初始凝胶速率差别不大,20 s后添加乙醇的铸膜液的凝胶速率最大;添加小分子醇后超滤膜的水通量大小顺序为:异丙醇<甲醇<乙醇<正丁醇.SEM照片显示在铸膜液中添加5%乙醇所制备的共混超滤膜具有较为圆整的内腔结构与均匀的孔结构.     

Phase behavior of a water/2‐propanol/poly(methyl methacrylate) cosolvent system

A phase diagram of poly(methyl methacrylate) in mixtures of water and 2‐propanol, individually nonsolvents for the polymer, was studied at 25 °C. For this system, there were two liquid–liquid demixing regions separated by a miscible region. This cosolvent phenomenon was thought to be a joint effect of the nonsolvents. The phase behavior was modeled according to modified Flory–Huggins chemical‐potential equations, which accounted for the possible contribution from a ternary interaction in terms of a lumped parameter, χ₁₂₃. The calculated phase‐equilibrium curves (binodals) agreed well with the measured results. By contrast, if only binary interaction parameters were considered, computations yielded binodals whose compositions departed significantly from the measured data. Using the wet phase inversion method with casting dopes selected on the basis of the phase diagram, we prepared membranes with microporous structures in various coagulation baths. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 747–754, 2000     

Molecular elucidation of morphology and mechanical properties of PVDF hollow fiber membranes from aspects of phase inversion, crystallization and rheology

This study explores the fundamental science of fabricating poly(vinylidene fluoride) (PVDF) hollow fiber membranes as well as elucidates the correlation among membrane morphology, crystallinity and mechanical properties as functions of non-solvent additives and dope rheology in the phase inversion process. A series of non-solvents (i.e. water, methanol, ethanol, isopropanol) are used either as non-solvent additives in the dope or as a component in the external coagulant. Depending on the strength of the non-solvent, the phase inversion of semi-crystalline PVDF membranes is dominated by liquid–liquid demixing or solid–liquid demixing accompanying crystallization. As a result, the membrane morphology transforms from an interconnected-cellular type to an interconnected-globule transition type with lower mechanical strengths when adding water, methanol, ethanol, or isopropanol into the spinning dopes or into the coagulation bath. The crystallinity and size of spherulitic globules in the morphology are controlled by the amounts of non-solvents presented in the systems. The rheological behavior of dope solutions is explored and the relationship between elongation viscosity and mechanical properties has been elaborated. Analytical methods and molecular dynamics simulations are employed to provide insights mechanisms from the views of thermodynamic and kinetic aspects as well as the state of polymer chains involved in the phase inversion process.     

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.     

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.     

Effect of the preparation conditions on the formation of asymmetric poly(vinylidene fluoride) hollow fibre membranes with a dense skin

Integrally skinned asymmetric poly(vinylidene fluoride) hollow fibre membranes were prepared and characterized. The effects of phase inversion methods (dry-wet or wet) and spinning conditions, such as the type of solvent (NMP, DMAc), the concentration of polymer in dope solution, temperature of the external coagulation bath and the composition of the inner coagulant on the morphology and on the formation of a dense skin layer were investigated. The structure of the membranes was analyzed by scanning electron microscopy and the gas permeation properties with six different gases (He, H₂, N₂, O₂, CH₄ and CO₂) were measured at 25 °C to confirm the integrity of the selective skin layer. Under the proper conditions highly selective and permeable PVDF hollow fibre membranes were thus obtained by dry-wet spinning of a 30 wt.% PVDF solution in DMAc, using hot water (50 °C) as the external coagulant and a bore fluid of pure water as the internal coagulant. The best membrane had a selective outer skin with an effective thickness of approximately 0.2 μm. The ideal selectivity of the hollow fibres approached or even exceeded the intrinsic ideal selectivity of a dense PVDF film, for instance the selectivity for He over N₂ was 86.2 for the hollow fibre, whereas it was 83.5 for a dense PVDF reference film. DSC and FT-IR/ATR analysis indicated a higher fraction of the β-crystal phase in the selective skin and a high overall crystallinity than in the melt-processed film. The latter explains the relatively high selectivity and low permeability of the membranes. Intrinsic polymer properties make the membranes also suitable for vapour transport than for gas separation.     

THE INFLUENCE OF PEG MOLECULAR WEIGHT ON MORPHOLOGIES AND PROPERTIES OF PVDF ASYMMETRIC MEMBRANES

The preparation and properties of asymmetric poly(vinyldiene fluoride)(PVDF)membranes are described in this study.Membranes were prepared from a casting solution of PVDF,N,N-dimethylacetamide(DMAc)solvent and water- soluble poly(ethylene glycol)(PEG)additives by immersing them in water as coagulant medium.Experiments showed that when PEG molecular weight increased,the changes in the resultant membranes' morphologies and properties showed a transition point at PEG6000.This indicated that PEG with a relati...     

ULTRAFILTRATION MEMBRANE FORMATION OF PES-C, PES AND PPESK POLYMERS WITH DIFFERENT SOLVENTS

Ultrafiltration membranes were prepared using phenolphthalein polyarylethersulfone (PES-C),polyethersulfone (PES) and poly(phthalazinone ether sulfone ketone) (PPESK) as polymers and NMP,DMAc,DMF and DMSO as solvents by immersion precipitation via phase inversion.Experimental data of thermodynamic properties of the polymer solutions and kinetic process of membrane formation were reported.For polymer solutions with good solvents,the sequence of the viscous flow activation energy (E_η) was coincident with ...     

Formation and gas flux of asymmetric PMMA membranes

In the present work, PMMA membranes were prepared by wet phase immersion methods to improve their gas fluxes. It is found that different membrane structure can be obtained by using different nonsolvent-solvent pairs. To completely describe the membrane formation process, the nonsolvent-solvent miscibility and the interfacial polymer concentration in casting solution should be considered accompanied by the ternary phase diagram. A simplified solution-diffusion model was developed to estimate the interfacial polymer concentration. In addition, the effects of adding solvent into the coagulation bath and adding nonsolvent into the casting solution are discussed.     

Fine structure of poly(vinylidene fluoride) membranes prepared by phase inversion from a water/N‐methyl‐2‐pyrollidone/poly(vinylidene fluoride) system

Poly(vinylidene fluoride) (PVDF) membranes were prepared by the isothermal immersion and precipitation of PVDF/N‐methyl‐2‐pyrollidone dope solutions in either harsh or soft nonsolvent baths. Low‐voltage field emission scanning electron microscopy imaging of the formed membranes at high magnifications (e.g., 300,000×) revealed their nanoscale fine structures, particularly dendrites observed on the surfaces of the macrovoids, cellular pores, and the membrane skin, which have never been successfully presented in the literature. Evidence of crystallization was also demonstrated by X‐ray diffraction and differential scanning calorimetry measurements. The phase diagram at 25 °C, including a binodal, tie lines, and a crystallization‐induced gelation line, was determined both experimentally and theoretically. These results were further used in mass‐transfer calculations to obtain diffusion trajectories and concentration profiles for the membrane region, which were useful for elucidating the relationship between the membrane preparation conditions and the obtained membrane morphologies. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 830–842, 2004