制膜条件对PVDF膜形态结构的影响

对干湿相转换法制备聚偏氟乙烯微孔膜进行了研究。利用光透射仪研究了不同制膜条件下成膜分相速度及其变化规律，用气体渗透法测定了膜的平均孔径和有效孔隙率，并结合电镜照片对不同制膜条件下膜的结构和性能进行了表征。实验结果表明较低的铸膜液温度和凝胶浴温度、较短的蒸发时间和较低聚合物浓度有利于增加膜的有效孔隙率。在铸膜液中加入非溶剂是提高膜性能的一种手段，但非溶剂的加入量需足够大，以抵消铸膜液温度提高引起的相反的效应。     

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.     

Selection criteria for solvent and coagulation medium in view of macrovoid formation in the wet phase inversion process

Sponge‐like and finger‐like structures are two distinct membrane structures commonly observed in membranes produced by the wet immersion process. An index Φ calculated solely from solubility parameters was defined as an indicator of the membrane structure. The Φ values of four polymers, poly(methyl metharylate), polysulfone, cellulose acetate, and poly(vinylidene fluoride), in various solvent‐nonsolvent pairs were calculated and compared with the corresponding membrane structures. It was found that the finger‐like structure often occurred at higher Φ values. Although the Φ value represents mostly the thermodynamics nature of a system, as an index for prior selection of solvent‐nonsolvent pairs for a particular polymer, a general rule of thumb was developed. Taking 15% polymer concentration and 300 μm casting thickness as a referential casting condition, selecting solvent‐nonsolvent pairs with Φ values higher than 0.25 is suggested, when a finger‐like structure is desired. The polymer concentration in the casting solution and the casting thickness will also affect the membrane structure. If a higher polymer concentration needs to be used, selection of a polymer‐solvent‐nonsolvent system with a Φ value much higher than 0.25 is suggested, or keeping the casting thickness lower than 300 μm to obtain a finger‐like membrane structure. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1495–1502, 1999     

聚乙二醇对聚醚砜微孔膜致孔作用的研究

以聚醚砜聚乙二醇溶剂为铸膜液体系、采用干湿相转化法制备微孔滤膜,研究了各种制膜条件对膜孔径结构的影响.实验发现聚乙二醇在体系中起到分散稳定的作用,只有到浓度大于70%时,才会对铸膜液的粘度产生明显影响,聚合物在铸膜液中的溶解状态也随之改变,进而影响膜的结构.不同溶剂NMP、DMF、DMAc、DMSO等极性溶剂或固体溶剂己内酰胺均可制得开孔率较高的微孔膜,但对膜的结构和性能影响差别不大.在本研究体系中,膜的结构取决于聚乙二醇、溶剂的浓度比例关系.     

Synthesis and Characterization of Poly(ether-block-amide) Membranes

Poly(ether-block-amide) membranes were made via casting a solution on a nonsolvent (water) surface. In this research, effects of different parameters such as ratio of solvent mixture (n-butanol/isopropanol), temperature, composition of coagulation bath (water) and polymer concentration, on quality of the thin film membranes were studied. The mechanism of membrane formation involves solution spreading, solvent–nonsolvent exchange, and partial evaporation of the solvent steps. Solvent- nonsolvent exchange is the main step in membrane formation and determines membrane morphology. However, at higher temperature of polymeric solution greater portion of solvent evaporates. The results showed that type of demixing process (mutual affinity between solvent and nonsolvent) has important role in film formation. Also, addition of solvent to the nonsolvent bath is effective on membrane morphology. The film quality enhances with increasing isopropanol ratio in the solvent mixture. This behavior can be related to increasing of solution surface tension, reduction of interfacial tension between solution and nonsolvent and delayed solvent-nonsolvent demixing. Uniform films were made at a temperature rang of 60–80 °C and a polymer concentration of 4–7 wt%. Morphology of the membranes was investigated with scanning electron micrograph (SEM). Pervaporation of ethyl butyrate/water mixtures was studied using these membranes and high separation performance was achieved. For ethyl butyrate/water mixtures, It was observed that both permeation flux and separation factor increase with increasing ethyl butyrate content in the feed. Increasing temperature in limited range studied resulted in decreasing separation factor and increasing permeation flux.     

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.     

Polyethersulfone/polyacrylonitrile blend ultrafiltration membranes with different molecular weight of polyethylene glycol: preparation,morphology and antifouling properties

Asymmetric ultrafiltration (UF) membranes were prepared from blends of polyethersulfone (PES)/polyacrylonitrile (PAN) via phase inversion method induced by immersion precipitation. Polyethylene glycol (PEG) with four different molecular weights was used as pore former and hydrophilic polymeric additive. N,N‐dimethylformamide (DMF) and water were used as solvent and coagulant (nonsolvent), respectively. The effects of different proportion of PES/PAN and molecular weight of PEG on morphology and performance of the prepared membranes were investigated. Performance of the membranes was evaluated using UF experiments of pure water and buffered bovine serum albumin (BSA) solution as feed. The contact angle measurements indicated that the hydrophilicities of PES/PAN membrane increase by increasing the PAN concentration in the casting solution. However, performance of the membranes improves by increasing the PAN concentration in the casting solution up to 20% and then decreases with further addition of PAN. It was found out that the rejection of BSA decreases with increasing the PAN concentration in the casting solution. Furthermore, it was found that the performance of the membranes increases by increasing the molecular weight of PEG up to 1500 Da and then decreases with the higher molecular weights. The morphology of the prepared membranes was studied by scanning electron microscopy. Copyright © 2011 John Wiley & Sons, Ltd.     

Interrelation between preparation conditions and structure of asymmetric membranes based on poly((diphenyl oxide amido)-N-phenylphthalimide)

Morphology of asymmetric membranes based on poly((diphenyl oxide amido)-N-phenylphthalimide) was studied by scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. The effect of preparation conditions on the morphology and transport properties of the membranes was investigated. The test membranes were prepared by the phase-inversion method under the conditions of “wet” casting in the polymer-solvent (N-methyl pyrrolidone)-nonsolvent (water, ethanol, 2-propanol, or aqueous alcohol solutions) systems. Feed casting solutions with a concentration of 10–14 wt %, including those containing a weak nonsolvent (ethyl acetate) were used. A comparative analysis of the structural features and water permeability of the test membranes allows the conclusion that some types of membranes can be used in nanofiltration processes.     

Synthesis and characterization of asymmetric polyethersulfone membranes: effects of concentration and polarity of nonsolvent additives on morphology and performance of the membranes

In this study, effects of methanol, ethanol and 1‐propanol as variable nonsolvent additives (NSAs) on the morphology and performance of flat sheet asymmetric polyethersulfone (PES) membranes were investigated. The membranes were prepared from PES/Polyvinylpyrrolidone (PVP)/N‐methyl‐2‐pyrrolidone (NMP) system via phase inversion. The obtained results indicate that with the addition of NSAs to the casting solution, the membrane morphology changes slowly from macrovoids to an asymmetric structure with finger‐like pores. By increasing the NSAs concentrations in the casting solution and decreasing their polarities, the membrane structure changes from finger‐like pores to sponge. The AFM and SEM images reveal that addition of NSA to the casting solution decreases the pore size of the prepared membranes and reduces the pure water flux and BSA solution flux, while increasing the protein rejection. Surface analysis of the membranes showed that mean pore size and surface porosity of the prepared membranes with NSAs in the casting solution are smaller compared with those of the membrane prepared with no NSA. Pure water flux and BSA solution flux through the membranes decrease and BSA rejection increases with increase in the concentration of NSAs and decrease in their polarity. Finally, it can be concluded that the T_g values of the PES membranes increase by addition of NSAs to the casting solution. Copyright © 2009 John Wiley & Sons, Ltd.     

Formation of porous flat membrane by phase separation with supercritical CO2

Microporous polystyrene membranes were prepared by the phase separation process using the supercritical CO₂ as a nonsolvent for the polymer solution. The thin polymer solution in a laboratory dish was located inside a cell and the supercritical CO₂ was introduced to induce the phase separation. The dry flat microporous membranes were obtained without collapse of the structure after the CO₂ pressure was diminished. Effects of the experimental conditions such as the CO₂ pressure, the polymer concentration and the temperature on the average pore size and membrane porosity were investigated.     

Control of the porosity of asymmetric TPX membranes

In the present work, the effect of adding nonsolvent in the casting solution on the porosity of asymmetric TPX (poly(4-methyl-1-pentene)) membranes was systematically investigated. A series of alcohols, with carbon number ranging from 2 to 14, was added in the casting solution (TPX/cyclohexane) to alter the porosity of two types of asymmetric TPX membranes, prepared by using ethanol and 1-propanol as the coagulation medium. It was found that the effect of nonsolvent on membrane porosity is different for the two types of membranes and the difference can be reasoned by considering the exchange rate between the polymer solvent and the coagulation medium during membrane formation. The results indicate that, for the membrane formation system with low exchange rate between coagulant and solvent, the membrane porosity is controlled by the coagulation value, defined as the volume of coagulant required to demix the casting solution. On the other hand, for the system with high exchange rate, the membrane porosity is not controlled by the coagulation value but by the penetration speed of the coagulant front moving through the casting solution.     

Effect of nonsolvents on properties of spinning solutions and polyethersulfone hollow fiber ultrafiltration membranes

The relationship among the presence of nonsolvent additives, the rheological behavior of spinning solutions and properties of hollow fiber membranes was studied. The additives tested were water, polyvinylpyrrolidone (PVP) and polyethylene glycol (PEG), and the base mixture was polyethersulfone/N-methyl-2-pyrrolidone (PES–NMP). In addition the effect of combining water and PVP or PEG was also studied. Membranes were prepared using a spinneret having two concentric orifices. The internal coagulant used as well as the nonsolvent from the coagulation bath were both water at 28°C and 30°C, respectively. Rheological properties of polymer solutions were evaluated using a rheometer Haake RV 20. Changes on composition of spin-solutions were also evaluated in terms of membrane water permeability, solute rejection and membrane structure observed using scanning electron microscopy (SEM). Experimental results from this work showed that spinning solutions containing any of the three additives behave as Newtonian fluids in the range of shearing rates tested. The addition of water, PVP or PEG to the base PES–NMP solution increased its viscosity and this effect was independent of the type of additive used. A direct relation between viscosity of casting solutions and membrane thickness was found. However, rheological properties (viscosity and normal stress difference) could not be used to explain differences on membrane water flux (MWF) when using different additives at the same concentration. The addition of any of the three additives generally increased MWF. The extent of this increment seemed to be more related to changes on membrane porosity than changes on pore sizes induced by the nature and concentration of the additive used.     

Factors affecting the nodule size of asymmetric PMMA membranes

Several factors that may affect the surface nodule size of a polymeric membrane were under investigation. The increase of polymer concentration and molecular weight were found to increase the surface nodule size. The increase of casting temperature also resulted in an increase in nodule size. These results supported that the radius of gyration and the collision frequency between polymer chains were the key factors affecting the nodule size. However, when the radius of gyration was reduced by the use of a poor solvent or by pre-adding nonsolvent in the casting solution, the surface nodule size increased. It suggested that there existed other factors affecting the nodule size on membrane surface besides the gyration radius and the collision frequency of polymer chains. In this study, we found in most cases that the surface nodule size decreased along with the surface tension difference between the casting solution and the coagulant. To demonstrate the effect of surface tension, we examined the nodule size inside the membranes where the nodule formation was not significantly affected by the interfacial tension. Opposite to what was observed on the surface, the nodule size increased with the solvation power of the solvent. This result suggested that it was the interfacial tension that overpowered the gyration radius in affecting the surface nodule size.     

A hierarchical porous carbon membrane from polyacrylonitrile/polyvinylpyrrolidone blending membranes: Preparation,characterization and electrochemical capacitive performance

Novel hierarchical porous carbon membranes were fabricated through a simple carbonization procedure of well-defined blending polymer membrane precursors containing the source of carbon polyacrylonitrile (PAN) and an additive of polyvinylpyrrolidone (PVP), which was prepared using phase inversion method. The as-fabricated materials were further used as the active electrode materials for supercapacitors. The effects of PVP concentration in the casting solution on structure feature and electrochemical capacitive performance of the as-prepared carbon membranes were also studied in detail. As the electrode material for supercapacitor, a high specific capacitance of 278.0 F/g could be attained at a current of 5 mA/cm² and about 92.90% capacity retention could be maintained after 2000 charge/discharge cycles in 2 mol/L KOH solution with a PVP concentration of 0.3 wt% in the casting solution. The facile hierarchical pore structure preparation method and the good electrochemical capacitive performance make the prepared carbon membrane particularly promising for use in supercapacitor.     

A hierarchical porous carbon membrane from polyacrylonitrile/polyvinylpyrrolidone blending membranes:Preparation,characterization and electrochemical capacitive performance

Novel hierarchical porous carbon membranes were fabricated through a simple carbonization procedure of well-defined blending polymer membrane precursors containing the source of carbon polyacrylonitrile （PAN） and an additive of polyvinylpyrrolidone （PVP）, which was prepared using phase inversion method. The as-fabricated materials were further used as the active electrode materials for supercapacitors. The effects of PVP concentration in the casting solution on structure feature and electrochemical capacitive performance of the as-prepared carbon membranes were also studied in detail. As the electrode material for supercapacitor, a high specific capacitance of 278.0 F/g could be attained at a current of 5 mA/cm2 and about 92.90% capacity retention could be maintained after 2000 charge/discharge cycles in 2 mol/L KOH solution with a PVP concentration of 0.3 wt% in the casting solution. The facile hierarchical pore structure preparation method and the good electrochemical capacitive performance make the prepared carbon membrane particularly promising for use in supercapacitor.     

扩散致相转化法制备结晶性聚合物多孔膜

介绍了扩散致相转化法制备结晶性聚合物多孔膜的研究现状。其三元等温成膜体系的相图包含液-液分相和固-液分相两种相分离方式,是理解成膜过程的重要工具,总结了成膜机理和膜的结构形貌：单纯S-L相分离生成粒子状对称膜结构;单纯L-L相分离生成蜂窝状非对称膜结构;两种相分离方式竞争发生将生成多样的混合膜结构。铸膜液浓度、非溶剂种类、铸膜溶剂组成、凝胶浴组成、制膜温度是影响膜结构形貌的主要因素。     

The influence of phase inversion process modified by chemical reaction on membrane properties and morphology

In this study, the chemical reaction between acetic acid (CH₃COOH) used as non-solvent additive of casting solution and sodium carbonate (Na₂CO₃) dissolved in water as coagulant was employed to modify the classical phase inversion process. By means of this method, the polyethersulphone (PES) ultrafiltration (UF) membranes were prepared. The influence of acetic acid on the properties of the polymer solution was examined by viscometry and related to the morphology of the membrane prepared from the casting solution. The membranes were characterized in terms of the pure water flux, solute transport and field emission scanning electron microscope (FESEM) observation. It was found that chemical reaction between the additive and coagulant increases membrane permeability and mean pore size while maintaining the relatively narrow pore size distribution. FESEM images also confirmed that the chemical reaction contributes to suppress the formation of macrovoid and enhance the interconnectivity of pore. Furthermore, the potential mechanism of membrane formation influenced by chemical reaction was explored tentatively.     

Effect of preparation variables on morphology and pure water permeation flux through asymmetric cellulose acetate membranes

In this study, cellulose acetate (CA) ultrafiltration (UF) membranes were prepared using the phase inversion method. Effects of CA and polyethylene glycol (PEG) concentrations in the casting solution and coagulation bath temperature (CBT) on morphology of the synthesized membranes were investigated. Based on L₉ orthogonal array of Taguchi experimental design 18 membranes were synthesized (with two replications) and pure water permeation flux through them were measured. It was found out that increasing PEG concentration in the casting solution and CBT, accelerate diffusional exchange rate of solvent 1-methyl-2-pyrrolidone (NMP) and nonsolvent (water) and consequently facilitate formation of macrovoids in the membrane structure. Increasing CA concentration, however, slows down the demixing process. This prevents instantaneous growth of nucleuses in the membrane structure. Hence, a large number of small nucleuses are created and distributed throughout the polymer film and denser membranes are synthesized. Rate of water flux through the synthesized membranes is directly dependent on the size and number of macrovoids in the membrane structure. Thus, maximum value of flux is obtained at the highest levels of PEG concentration and CBT (10 wt.% and 23 °C, respectively) and the lowest level of CA concentration (13.5 wt.%). Analysis of variance (ANOVA) showed that all parameters have significant effects on the response. However, CBT is the less influential factor than CA and PEG concentrations on the response (flux).     

Wet poly(vinyl chloride) membrane as a support: Sorption and transport of low-molecular-weight organic compounds and proteins

Characteristics of sorption and transport behavior of “wet” (PVC) membranes prepared by a poly(vinyl chloride) casting method were studied. It was found that wet PVC membranes adsorbed enzymes and proteins, whilst they did not adsorb low-molecular-weight compounds. The diffusivity of acetylcholine iodide as a penetrant through the wet PVC membrane was approximately 10^?7 cm²/sec, although it depended strongly on membrane thickness and slightly on PVC concentrations in the casting solution. Activation energy of acetylcholine iodide diffusivity was 2.6 kcal/mol. These results were explained by hydrophobic interaction and minute pore dimensions. Wet PVC membrane was suggested as a support for immobilization and was compared with collagen on the basis of transport behavior.     

非溶剂添加剂对共聚砜铸膜液体系及膜性能与结构的影响

以含二氮杂萘酮结构的新型共聚砜为膜材料 ,通过浊点滴定法和特性粘数的测定研究了几种非溶剂添加剂对铸膜液体系的影响 .并由共聚砜 NMP 添加剂三组分相分离曲线数据计算了共聚砜的Θ溶剂中NMP与添加剂的比例 ,考察了添加剂EGME用量对共聚砜超滤膜结构和性能的影响 .发现当EGME NMP接近Θ组成时膜性能发生突变 ,在EGME NMP低于Θ组成时 ,膜的水通量随添加剂用量的增大而增大 ,截留率基本不变 .