High throughput screening for rapid development of membranes and membrane processes

The objective of this study was to investigate the feasibility of high throughput (HT) screening techniques for pressure-driven membrane processes. For this purpose, a HT-filtration module, allowing to perform 16 pressure-driven separations simultaneously, was designed. The potential of the developed equipment and of the HT-screening concept in general was validated by demonstrating both the reproducibility of experimental flux and selectivity data, and the scalability of these data between the HT-module and a conventional dead-end filtration set-up. Data were obtained with two solvent resistant nanofiltration (SRNF) membranes: a laboratory-prepared polyimide (PI) and a commercial MPF-50 membrane. The reproducibility of the data was highly encouraging, proving that this HT-approach can be a useful tool to rapidly screen a large array of operational parameters in membrane processes and of synthesis parameters in the development of new membranes.     

Modeling of asymmetric membrane formation by dry-casting method

Many polymeric membranes are produced by phase inversion technique invented by Loeb and Sourirajan in 1962. The dry-casting method is one of the major phase inversion techniques in which a homogeneous polymer solution consisting of solvent(s) and nonsolvent(s) is cast on a support and then evaporation of the casting solution takes place under convective conditions. In this paper, we model membrane formation by the dry-casting method. The model takes into account film shrinkage, evaporative cooling, coupled heat, and mass transfer and incorporates practical and reliable diffusion theory as well as complex boundary conditions especially at the polymer solution/air interface. The predictions from the model provide composition paths, temperature, and thickness of the solution. By plotting the composition paths on the ternary phase diagram, we ascertain the general structural characteristics of the membranes prepared from particular casting conditions. The predictive ability of the model was evaluated by comparing the results with the experimental data obtained from gravimetric measurements for cellulose acetate (CA)–acetone–water system. In an attempt to illustrate the importance of diffusion formalism on the predictions, recently proposed multicomponent diffusion theory and its simplified forms were utilized in the model. The computational results show that the critical factor for capturing the accurate behavior of membrane formation is the diffusion formalism utilized in the model.     

Preparation, characterization and effect of annealing on performance of cellulose acetate/sulfonated polysulfone and cellulose acetate/epoxy resin blend ultrafiltration membranes

Polymeric membranes based on cellulose acetate (CA)--sulfonated polysulfone blends at three different polymer compositions were prepared by solution blending and phase inversion technique, characterized and subjected to annealing at 70, 80 and 90 °C. The permeate water flux, separation of bovine serum albumin and its flux by the blend membranes before and after thermal treatment, have been compared and discussed. Similarly, CA and epoxy resin (diglycidyl ether of bisphenol-A) were blended in various compositions, in the presence and in the absence of polyethyleneglycol 600 as non-solvent additive, using N,N^′-dimethylformamide as solvent, and used for preparing ultraflltration membranes by phase inversion technique. The polymer blend composition, additive concentration, casting and gelation conditions were optimized. Blend membranes were characterized in terms of compaction, pure water flux, water content and membrane resistance. The effects of polymer blend composition and additive concentration on the above parameters were determined and the results are discussed.     

Influence of binary interactions on phase behavior of water/ dimethylsulfoxide/ polyethersulfone casting solution: Thermodynamic modeling

Prediction and control of membrane morphology using multi‐phase thermodynamic knowledge are of growing interest. The water/dimethylsulfoxide/polyethersulfone ternary system is a widely used casting dope for the preparation of MF, UF, and NF membranes. In the current study, Flory–Huggins (F–H) model was applied to predict the behavior of this ternary system during phase inversion. Titration method was applied to generate cloud point data. The prediction accuracy of the F–H model was directly dependent on the binary interactions of the system components. The compressible regular solution (CRS) model predicts the binodal location using only the pure component properties as the input parameters. Accordingly, the influence of binary parameters on the location of the binodal curves was investigated. The predicted binodal points showed superior accordance with the experimental data, where the binary interaction between nonsolvent (water) and solvent (DMSO) was overlooked. In addition, the modelling results emphasized on the pivotal importance of the interactions between polymer (PES) and nonsolvent (water) on the phase inversion and thus, on the control of the membrane morphology. The CRS model offered a greater conformity with the experimental results in comparison with the F–H theory. Copyright © 2013 John Wiley & Sons, Ltd.     

Effect of temperature and membrane preparation parameters on gas permeation properties of polymethacrylates

Permeabilities of N₂, Ar, O₂, CO₂, and H₂ gases in PEMA (Polyethylmethacrylate) membranes have been measured above and below glass transition in the temperature range of 25–70 °C. The permeabilities of the gases were observed increasing with temperature. Arrhenius plot of permeability versus temperature data showed that there is a slope discontinuity at near to T_g of PEMA. In addition, the effects of membrane preparation parameters by solvent casting method (percentage of polymer in solvent, annealing temperature, annealing time, evaporation temperature, and evaporation time) have been investigated by using homogenous dense membranes of PEMA. It is observed that membrane preparation parameters strongly affect the membrane performance and the reproducibility of the permeability measurements. On the other hand, the effect of polymer structure on membrane performance has been investigated. Comparison of the permeabilities of N₂, Ar, O₂, CO₂, and H₂ gases in PEMA and PMMA membranes shows that PMMA membranes have smaller permeabilities and higher selectivities than PEMA membranes because of their higher glass transition temperature, T_g. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 3025–3033, 2007     

Novel cellulose acetate membrane blended with phospholipid polymer for hemocompatible filtration system

To improve the blood compatibility of cellulose acetate (CA) membranes for hemofiltration, a novel CA membrane blended with 2-methacryloyloxyethyl phosphorylcholine (MPC) copolymer was designed for a hemocompatible filtration system. The MPC copolymer (PMB30) was synthesized from MPC and n-butyl methacrylate. The polymer solution for making the membrane was prepared from a solvent mixture composed of N,N-dimethylformamide, acetone, and 2-propanol. The CA and CA/PMB30 blended membranes with an asymmetric and porous structure were prepared by a phase inversion process. The mechanical properties and solute permeability of the CA/PMB30 blended membrane could be controlled by preparation conditions such as the composition of the solvents and the solvent evaporation time. The CA/PMB30 blended membrane showed both good water and solute permeabilities in comparison with the CA membrane. Also, the molecular weight of the solute passed through the membrane was changed by the addition of PMB30, and good permselectivity could be obtained. Moreover, the CA/PMB30 blended membranes had excellent blood compatibility such as protein adsorption resistivity compared to the CA membrane due to location of the MPC units in the PMB30 at the surface.     

Structure formation of integral-asymmetric membranes of polystyrene-block-Poly(ethylene oxide)

For the first time the combination of solution casting and solvent–nonsolvent exchange (phase inversion) has been applied to generate asymmetric membranes with highly ordered hexagonally packed cylinders with perpendicular orientation composed of polystyrene-block-poly(ethylene oxide). The influence of parameters like solvent composition and evaporation time on the membrane formation is presented. The development is based on a study of the solution behavior by dynamic light scattering and the precipitation behavior of the cylinder forming diblock copolymer by turbidity measurements from different solvent and nonsolvent systems. The water flux properties, as an important membrane characteristic, show a time dependent behavior, due to swelling of the polyethylene oxide blocks. The morphologies of the membranes are imaged by scanning electron microscopy. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

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。     

耐溶剂纳滤膜*

纳膜分离过程是一种选择性高、操作简单、能耗低的分离技术,已在各工业领域和科学研究中得到广泛的应用。纳滤过程的诸多优点,使其在石油化工、医药、食品等领域的非水溶液体系中具有极大的潜在应用价值,而传统的纳滤膜难以拓宽到非水溶液体系中使用,为此进一步研究和发展耐溶剂纳滤膜,对于拓宽纳滤过程的应用极其重要。目前,耐溶剂纳滤膜已成为膜分离科学领域的研究热点,在现有报道的文献基础上,本文综述了有关在非水溶液体系中使用的耐溶剂纳滤膜制备的研究进展,并对将来的发展方向提出了建议。     

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。     

Comparison of surface segregation and anticoagulant property in block copolymer blended evaporation and phase inversion membranes

In our recent study, an ABA amphiphilic triblock copolymer poly(vinyl pyrrolidone)‐b‐poly(methyl methacrylate)‐b‐poly(vinyl pyrrolidone) (PVP‐b‐PMMA‐b‐PVP) was synthesized and directly blended with polyethersulfone (PES) to prepare membranes. To further investigate the effects of surface energy and miscibility on the near‐surface composition profile of the membranes, evaporation membrane and phase inversion membrane of PES/PVP‐b‐PMMA‐b‐PVP were prepared by evaporating the solvent in a vacuum oven, and by a liquid–liquid phase separation technique, respectively. The surface composition and morphology of the membranes were investigated using XPS and tapping mode atomic force microscopy, and the surface segregations of the membranes were compared and discussed. For the evaporation membrane, PVP blocks were buried below the lower surface energy PMMA blocks and PES substrate at the airside surface. For the phase inversion membrane, however, the hydrophilicity of PVP blocks were the biggest driving force because of the high speed exchange between water and solvent, and present at the membrane surface. Thus, the modified PES membrane prepared by using phase inversion method has a layer of PVP block brushes on its surface and has the better anticoagulant property, which might improve the blood compatibility of the membrane and has potential to be used in blood purification. Copyright © 2012 John Wiley & Sons, Ltd.     

NMR approach to properties of polymeric membranes. Kinetics of membrane formation

Nuclear magnetic resonance (NMR) is used to characterize properties of polyetherimide membranes obtained from a phase inversion process. At the end of the phase inversion and prior to the subsequent thermal treatments, the membrane is made of a porous structure filled of solvent and nonsolvent molecules embedded in a concentrated polymer–solvent matrix in the glassy state. The confinement of the small solvent and non solvent molecules in the porous system leads to a restriction of their mobility. The kinetics of membrane formation is observed from NMR. It is found that the penetration of non-solvent and the propagation of the glassy phase into the system obey simple diffusion laws. © 1993 John Wiley & Sons, Inc.     

Solvent‐Resistant Nanofiltration for Product Purification and Catalyst Recovery in Click Chemistry Reactions

The quickly developing field of “click” chemistry would undoubtedly benefit from the availability of an easy and efficient technology for product purification to reduce the potential health risks associated with the presence of copper in the final product. Therefore, solvent‐resistant nanofiltration (SRNF) membranes have been developed to selectively separate “clicked” polymers from the copper catalyst and solvent. By using these solvent‐stable cross‐linked polyimide membranes in diafiltration, up to 98 % of the initially present copper could be removed through the membrane together with the DMF solvent, the polymer product being almost completely retained. This paper also presents the first SRNF application in which the catalyst permeates through the membrane and the reaction product is retained.     

Effect of filler incorporation route on the properties of polysulfone–silver nanocomposite membranes of different porosities

Flat sheet porous polysulfone–silver nanocomposite membranes were synthesized by the wet phase inversion process. The effects of casting mixture composition and nanoparticle incorporation route on the morphological and separation properties of prepared membranes were studied by comparing nanocomposites of different preparations with silver-free controls. Silver nanoparticles were either synthesized ex situ and then added to the casting solution as an organosol or produced in the casting solution via in situ reduction of ionic silver by the polymer solvent. Nanocomposite membranes of three types differing in skin porosity and macrovoid structure were prepared. The structure and properties of nanocomposites were interpreted in terms of the coupling between the processes of nanoparticle formation and gelling of the polymer-rich phase during phase inversion. Larger nanoparticles preferentially located in the skin layer were observed in composites prepared via the ex situ method while in situ reduction of silver led to formation of smaller nanoparticles homogeneously distributed along the membrane cross-section. In some cases, incorporation of nanoscale silver formed ex situ resulted in macrovoid widening and an order of magnitude decrease in hydraulic resistance accompanied by only a moderate decrease in rejection. The accessibility of the silver nanoparticles embedded in the membrane was quantitatively assessed by the degree of the growth inhibition of a membrane biofilm due to the ionic silver released by the nanocomposites and was found to depend on the method of silver incorporation.     

Polyurethane and sulfonated polysulfone blend ultrafiltration membranes. I. Preparation and characterization studies

Polyurethane (ether type) and sulfonated polysulfone (sodium salt form) in the presence of polyethylene glycol 600 were blended in various compositions using N,N'-dimethylformamide as solvent and used for preparing ultrafiltration membranes by the phase inversion technique. Polymer blend composition, additive concentration, and casting and gelation conditions were optimized. Blend membranes were subjected to ultrafiltration characterizations such as compaction, pure water flux, water content, and membrane resistance. The membranes were also subjected to the determination of pore statistics and molecular weight cutoff determination studies using dextran of different molecular weights. Surface morphology of the membranes was analyzed using scanning electron microscopy at different magnifications. The effects of polymer composition and additive concentration on the above parameters were analyzed and the results are compared and discussed with those of pure sulfonated polysulfone membranes. The derived pore size, porosity, and number of pores have a remarkable interrelationship and also have a definite role and relationship with the molecular weight cutoff, morphology, and flux performance of the membranes.     

Phase inversion sulfonated polysulfone membranes

The use of two nonsolvents serving as a cosolvent system, replacing the traditional volatile solvent plus less volatile nonsolvent system, in the formation of asymmetric phase inversion membranes was investigated. Specifically, asymmetric membranes of sulfonated polysulfone were cast from a cosolvent system consisting of tetrahydrofuran and formamide. The nonsolvents and the proportions in which they are mixed to produce the cosolvent system, as well as the gelation medium isopropyl alcohol, were selected based on the three-component solubility parameter concept of Hansen. The structure of each membrane was evaluated using scanning electron microscopy; the performance was evaluated for use in pressure-driven membrane separation processes. The membranes were found to be dependent on the composition of the original casting solution and the composition of the nascent membrane at the instant of gelation. These ideas are clearly represented through the use of a triangular polymer solubility diagram.     

Modification effects of amphiphilic comb-like polysiloxane containing polyether side chains on the PVDF membranes prepared via phase inversion process

Amphiphilic comb-like polysiloxane (ACPS) containing polyether side chains was used as the modification reagent in the preparation of hydrophilic porous poly (vinylidene fluoride) (PVDF) membranes via a phase inversion process. The effects of ACPS on morphology, crystallinity, mechanical properties, reservation of ACPS in the phase inversion process, chemical structure, hydrophilicity and filterability performance of porous PVDF membranes were discussed. It was found that the addition of ACPS would result in the delayed demixing which yields “sponge-like” sublayers and longer crystallization time during the membrane formation process. It was revealed that O/F ratios of the bulk membrane were almost the same as those of the corresponding casting solutions which obviously indicated the high reservation of ACPS in the membrane formation process. The fact that the O/F ratios in the membrane surface layers were much higher than those in the bulk membrane proved the enrichment of ACPS on the surface. The filterability experiments and water contact angle testing proved the hydrophilicity of the blend membranes. Through a schematic model, the mechanism relating the membrane structure and performance was interpreted. From the observed results, it can be concluded that ACPS acts as a potential candidate material for preparing PVDF membranes with extraordinary hydrophilicity and filterability. __________ Translated from Acta Polymerica Sinica, 2007, 12: 1168–1175     

浸没凝胶相转化制备聚合物膜的孔结构及其形成机理

全面地综述了浸没凝胶相转化法制备的聚合物微孔膜的表面和膜中存在的各种孔的结构和形态,从制膜体系的热力学性质和成膜动力学角度解释了各种孔结构形态的形成和生长机理,即膜表面与膜中孔的结构形态由此时制膜体系发生的相分离类型决定,由此可推断出不同的膜层可能有不同的成膜机理。因此,只要掌握了各种膜孔结构形成的机理,通过改变膜的制备条件,控制体系的热力学性质与成膜时动力学扩散是可以实现相转化膜结构的控制。     

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.     

Remarkable pH-responsive Microfiltration Membrane from Well-defined PS-b-PDEAEMA Copolymers by ATRP Method

This work presented a detailed study on pH-responsive self-supporting microfiltration membranes via the non-solvent-induced phase separation (NIPS) process from mixtures of THF and DMF. The well-defined pH-responsive polymers polystyrene-block-poly (N,N-diethylaminoethyl methacrylate) (PS-b-PDEAEMA) were synthesized via atom transfer radical polymerization (ATRP) evidenced by ¹H-NMR and GPC studies. Two amphiphilic diblock copolymers were used, St₇₁-b-DEAEMA₃₁ and St₇₁-b-DEAEMA₈₂. The influence of pH value in the coagulation bath, the solvent composition, the “open-time” before immersion into the coagulation bath and the polymer composition onto the membrane morphology were investigated, and flux values obtained for the different membrane systems were compared. The SEM images and polarized optical microscopy revealed that the size and the number of pores in the membranes were larger along with the decreasing THF content and “open time”. For the copolymer with the longer PDEAEMA block, St₇₁-b-DEAEMA₈₂, the flux values were lower than that of St₇₁-b-DEAEMA₃₁ under different “open time”, which meant that the total area of the pores in the membranes was smaller, respectively. The optimum preparation conditions for microfiltration membrane were as follows: the polymer was St₇₁-b-DEAEMA₈₂, the “open time” was 20s, the non-solvent bath was the pH = 2 distilled water, and the solvent composition was 25% THF and 75% DMF. Both polymers were shown to form self-supporting membrane systems that were able to react onto pH stimuli in terms of water flux.