Membrane formation by isothermal precipitation in polyamide-formic acid-water systems II. Precipitation dynamics

An analysis is presented, which describes the isothermal ternary diffusion process encountered in the formation of the aliphatic polyamide membranes such as Nylon-66 by direct immersion-precipitation of a polymeric solution in a nonsolvent bath. A material coordinate is employed to derive the mass transfer equations for the membrane solution. The convective mass transfer in the coagulation bath is taken into account by solving the hydrodynamic boundary layer equations. Diffusion coefficients were measured and used to deduce ternary phenomenological coefficients. The computed results are found to agree with measured precipitation times and with membrane morphologies observed by scanning electron photomicrographs. © 1995 John Wiley & Sons, Inc.     

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.     

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。     

Poly(methyl methacrylate) Membrane: Dynamic Measurement of Concentrations During Water-Induced Phase Separation

The properties of membranes produced by nonsolvent-induced phase separation (NIPS) depend on the rate of mass transfer of mobile components. This paper reports the use of Fourier transform infrared-attenuated total reflectance (FTIR-ATR) using principle component regression (PCR) analysis to study the compositional change of polymer solution immersed into the water prior to the phase demixing. To correlate the compositional change with the macroscopic structure, the observation on the dynamics of NIPS is made by a low magnification optical microscope equipped with a video camera. The morphology of the films was also examined by scanning electron microscope. Acetone and N, N-dimethylformamide were chosen as solvents to present different behavior of poly (methyl methacrylate) (PMMA) solutions during the NIPS process. The ratio of solvent and nonsolvent exchange, flux ratio shows the mass transfer in the homogeneous polymer solution after immersion in a nonsolvent bath. The results show that the flux ratio ( ) is affected by solvents which probably affects the ultimate structures of the precipitated polymer.     

A new modeling of asymmetric membrane formation in rapid mass transfer system

Mass transfer process involved in the immersion precipitation of polyurethane/dimethylformamide (DMF)/water system was investigated. The set of diffusion equations describing the local composition of the membrane solution as a function of space coordinate and time were solved by numerical method, and the composition path in the phase diagram was obtained. Instead of boundary conditions based on the instantaneous equilibrium assumption between membrane solution and coagulation bath, new boundary conditions were set up by using mass transfer formalism at the interface which is especially valid in the condition that the mass transfer rate is extremely rapid. Phase separation phenomena during immersion precipitation were taken into account to continue the calculation after phase separation. The calculated results showed that the chance of phase separation via spinodal decomposition increases with the strength of nonsolvent, addition of nonsolvent to the dope solution, and the use of more hydrophobic polymer. The proposed model is the improvement of the previous works eliminating the equilibrium assumption at the interface and extending the calculation after phase separation.     

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.     

Effect of evaporation temperature on the formation of particulate membranes from crystalline polymers by dry-cast process

The membrane formation of crystalline poly(ethylene-co-vinyl alcohol) (EVAL), poly(vinylidene fluoride) (PVDF), and polyamide (Nylon-66) membranes prepared by dry-cast process was studied. Membrane morphologies from crystalline polymers were found to be strongly dependent on the evaporation temperature. At low temperatures, all the casting solution evaporated into a particulate morphology that was governed by the polymer crystallization mechanism. The rise in the evaporation temperature changed EVAL membrane structure from a particulate to a dense morphology. However, as the temperature increased PVDF and Nylon-66 membranes still exhibited particulate morphologies. The membrane structures obtained were discussed in terms of the characteristics of polymer crystallization in the casting solution theoretically. At elevated temperatures the crystallization was restricted for the EVAL membrane because the increase rate in the polymer concentration was fast relative to the time necessary for growth of nuclei. Nonetheless, the time available for PVDF and Nylon-66 with stronger crystalline properties was large enough to form the crystallization-controlled particulate structure that differed in particle size only. In addition, particles in the PVDF membrane were driven together to disappear the boundary, but those in the Nylon-66 membrane exhibited features of linear grain boundary. The difference in particle morphology was attributed to the Nylon-66 with the most strongly crystalline property. Therefore, the kinetic difference in the crystallization rate of the polymer solution play an important role in dominating the membrane structure by dry-cast process.     

Fine structure and crystallinity of porous Nylon 66 membranes prepared by phase inversion in the water/formic acid/Nylon 66 system

Microporous Nylon-66 polymeric membranes were prepared by immersion-precipitation from a ternary system, water/formic acid/Nylon 66. Depending upon the precipitation conditions, membranes with morphologies that reflect the sequence of liquid-liquid demixing (as characterized by cellular pores) and crystallization (as characterized by crystal particles) events during the course of precipitation were obtained. The details of the membrane morphologies were disclosed using a low voltage field emission scanning electron microscope (FESEM) at very high resolutions. In particular, nano-scale fine structures such as dendritic crystal elements, nano-pores, nano-grains, branching lamellae, etc., which were rarely presented in the membrane literature. Wide angle X-ray diffraction analyses indicated that Nylon-66 crystallized into ‘α’ structure in all prepared membranes. Crystallinities were determined by appropriate deconvolution of the diffraction peaks. The results indicated that membranes prepared by a well-dissolved casting dope had a somewhat higher crystallinity than those prepared by incipient dopes being in metastable states with respect to crystallization. This observation was confirmed by Fourier transform infrared spectroscopy and DSC thermo analyses.     

Effect of cooling baths on EVOH microporous membrane structures in thermally induced phase separation

The purpose of this work is to investigate the effect of cooling bath on the membrane preparation of crystalline polymer/diluent system via thermally induced phase separation (TIPS), when the cooling bath is compatible with the diluent. In this work, poly(ethylene-co-vinyl alcohol) (EVOH)/PEG300 system with water and methanol as the cooling baths was proposed. Results showed that when water was used as the cooling bath, the membrane presented an asymmetric structure consisting of a porous skin, macrovoids near the top and lacy structures near the bottom. In contrast, when cooled in the bath of methanol, it showed particulate morphology on the top surface and cellular pores near the bottom. The lacy and cellular structures were the typical structures resulted from liquid–liquid thermally induced phase separation, the novel macrovoids and particulate morphology were then supposed to be induced by the mutual diffusion between the diluent and the cooling bath. In the case of water, the diluent's outflow was comparative with the water's inflow into the membrane, so the penetrated water acted as a strong nonsolvent and induced macrovoids near the top. In the bath of methanol, the diluent's outflow was much faster than the methanol's inflow, which changed the solution composition from a liquid–liquid phase separation region to a solid–liquid phase separation region and resulted in particulate morphology near the top.     

Crystallization of polylactic acid under in situ deformation during nonsolvent‐induced phase separation

In this study, we investigate polylactic acid (PLA) crystallization under in situ biaxial extension in a nonsolvent‐induced phase separation foaming process. Our ternary system consists of PLA, dichloromethane (DCM) as solvent and hexane as nonsolvent. For the first time, the formation of a shish‐kebab crystalline morphology is observed in such a solution‐based foaming process in certain solid–liquid phase separated systems. The formation of shish‐kebabs is described based on the coil‐stretch transition concept. The rapid biaxial deformation caused by macropore growth uniaxially stretches the long chains that are tied with at least two single crystals which eventually leads to the formation of shish structures throughout the polymer‐rich phase. The kebab lamellae then form perpendicularly on the shish cores. The scanning electron microscopy (SEM) observations and our interpretation of the crystallization phenomena are confirmed by differential scanning calorimetry (DSC) analysis. The observation of various crystalline morphologies, particularly shish‐kebabs, and the elucidation of their formation mechanisms contribute to the understanding of phase separation and pore growth as well as crystallization in such polymer–solvent–nonsolvent systems. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1055–1062     

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。     

热致相分离法制聚偏氟乙烯微孔膜稀释剂的选择

依据聚偏氟乙烯(PVDF)、邻苯二甲酸二甲酯、水杨酸甲酯、卡必醇醋酸酯、三醋酸甘油酯、邻苯二甲酸二正丁酯(DBP)、苯乙酮和二苯甲酮(DPK)的Hansen溶度参数及其相对介电常数, 选择能与PVDF以液-液相分离机理进行热致分相的稀释剂, 制备了具有双连续结构的微孔膜. 通过比较PVDF-稀释剂间的溶度参数及相对介电常数的差异, 发现PVDF-DBP体系和PVDF-DPK体系有发生热致液-液相分离的可能. 实时观察上述两个体系的分相过程并测定其结晶温度, 当PVDF质量分数低于30%时, 随着温度的降低, PVDF-DPK体系发生液-液相分离. 根据PVDF-DPK体系相图, 通过控制PVDF含量和降温条件, 无须添加非溶剂或拉伸工艺, 就可以制备出具有双连续结构的PVDF微孔膜.     

Asymmetric membrane formation via immersion precipitation method. I. Kinetic effect

The kinetic effect of the phase inversion process on the membrane morphology is explored, with emphasis on the diffusion coefficient of the nonsolvent as a measure of the solvent/nonsolvent exchange rate. The diffusion coefficient is closely related to the nonsolvent tolerance of the polymer solution, which was estimated from a pseudo-ternary phase diagram of the following system: polymer: polysulfone; solvent system: a mixture of the solvent 1-methyl-2-pyrrolidinone and a solvent additive (formic acid, water or ethanol); and nonsolvent: ethanol. Regardless of the kind of solvent additive employed, when the diffusion coefficient of the nonsolvent is high for a given gelation medium, then the membrane consists of a smooth, defect-free surface and macrovoid-free cross section, and is highly permeable to oxygen. However, using a polymer solution with a low diffusion coefficient results in a membrane of a rather defective morphology. Therefore, it is concluded that the diffusion coefficient of the nonsolvent is a crucial parameter in controlling membrane morphology.     

链段刚性对非溶剂致相分离成膜过程影响的耗散粒子动力学模拟

通过分子动力学(MD)模拟映射方法构建了符合聚醚砜(PES)刚性结构的耗散粒子动力学(DPD)简谐力场, 并研究了PES链段刚性对PES/N-甲基-2-吡咯烷酮(NMP)/水体系非溶剂致相分离(NIPS)过程的影响. 结果表明, 由于非溶剂和溶剂在两相界面上发生的质量交换, 导致在相界面处PES链段发生堆积, 形成了薄而致密的聚合物表层, 在PES溶液内部, 由于非溶剂的侵入导致体系发生了旋节相分离, 从而在整体上得到了明显的非对称结构; 同时, PES链段刚性的提升能够明显加快体系的相分离速度, 导致相界面处的PES薄层形成得更加快速, 薄层更加致密、 孔径更小, 而对内部的疏松结构影响较小; 此外, 结合不同力场下聚合物浓度对相分离过程的影响可以发现, 不同PES浓度下, 链段刚性的提升对相分离过程的特征和演变趋势没有造成根本性的影响, 与经典的弹簧力场的模拟结果在整体趋势上有相似性. 研究结果表明, 简谐力场能提升PES链段的刚性, 从而能更真实地模拟实际体系的非溶剂相分离法成膜过程.     

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     

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.     

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.     

Equilibrium phase behavior of the membrane forming water-DMSO-EVAL copolymer system

The equilibrium phase behavior of ethylene vinyl alcohol (EVAL) copolymer in mixtures of DMSO (dimethylsulfoxide, solvent) and water (nonsolvent) was studied for different temperatures. Both crystallization-induced gelation and liquid-liquid demixing were observed. From the determined phase diagram of this system at 25°C, three regions may be identified, i.e., a homogeneous region, a gel region, and binodal region in which both types of phase transition take place. At higher temperatures, crystallization isotherm was found to intersect the binodal phase boundary, which is analogous to the phase behavior reported by Stokes and Berghmans for several binary systems.     

Solid scintillation proximity membranes: I: Characterization of polysulfone-inorganic fluor morphologies precipitated from NMP solutions

Solid scintillation proximity membranes were prepared by coagulation of polysulfone polymer solutions containing cerium-activated yttrium silicate particles (CAYS) as a fluor. Membranes were formed by three different solidification processes: precipitation of the polymer-rich phase after liquid–liquid phase separation, vitrification via solvent evaporation, or rapid polymer collapse due to fast exchange between solvent in the solution and nonsolvent in the coagulation bath. The results indicate that when the coagulation process includes the liquid–liquid phase separation due to nucleation of the polymer-lean phase, the inorganic fluor particles are expressed out of the polymer-rich phase and separated from the polymer matrix. On the contrary, solidification by the vitrification of the polymer solution through solvent evaporation results in the inorganic fluors being surrounded by the polymer matrix, much like the dispersed state in the solution. In contrast, rapid collapse of the polymer also induces entrapment of the fluor in the polymer structure. However, fluor impregnation in the polymer matrix is distinguished from that in the vitrified membrane in that the impregnation is due to localized impingement of the fluor on the polymer structure rather than envelopment by polymer molecules.     

High molecular weight syndiotacticity-rich poly(vinyl alcohol) gel in aging process

The properties of the aged gels of high molecular weight syndiotacticity-rich poly(vinyl alcohol)s (HMW S-PVAs) with different syndiotactic diad (s-diad) contents were investigated. HMW S-PVA gels with s-diad content of 61.5% and 58.2% showed the rapid increases of the syneresis and the turbidity from the early stage of aging time, which is ascribable to the phase separation, while that with s-diad content of 55.7% did not. From the morphological study, it was confirmed that the phase separation in HMW S-PVA gel with s-diad content of 61.5% occurred without the liquid-liquid phase separation in sol state, whereas both the liquid-liquid phase separation in sol state and the subsequent phase separation in gel state occurred in the case of HMW S-PVA gel with s-diad content of 58.2%. On the other hand, HMW S-PVA gel with s-diad content of 55.7% showed neither the liquid-liquid phase separation in sol state nor the phase separation in gel state in the long period of time. It was also confirmed from wide angle X-ray diffractogram that the crystallization was accompanied by the phase separation in gel state in the aging process of PVA gel. However, the crystallization was hindered by the fast network formation at the initial stage of time. Later the syndiotacticity promoted the crystallization. The tensile modulus of HMW S-PVA gel with higher syndiotacticity increased more significantly with time. Received: 2 December 1999/Accepted: 12 July 2000