Rheology behavior of high‐density polyethylene/diluent blends and fabrication of hollow‐fiber membranes via thermally induced phase separation

The phase‐separation behavior of high‐density polyethylene (HDPE)/diluent blends was monitored with a torque variation method (TVM). The torque variation of the molten blends was recorded with a rheometer. It was verified that TVM is an efficient way to detect the thermal phase behavior of a polymer–diluent system. Subsequently, polyethylene hollow‐fiber membranes were fabricated from HDPE/dodecanol/soybean oil blends via thermally induced phase separation. Hollow‐fiber membranes with a dense outer surface of spherulites were observed. Furthermore, the effects of the spinning temperature, air‐gap distance, cold drawing, and HDPE content on the morphology and gas permeability of the resultant membranes were examined. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Microporous membranes prepared via thermally induced phase separation from metallocenic syndiotactic polypropylenes

Microporous membranes were prepared by thermally induced phase separation (TIPS) using different tailor-made syndiotactic polypropylenes (sPP) synthesized by metallocene catalysts. The phase diagrams of sPP samples in diphenyl ether (DPE) were determined. The polymer microstructure effect on the thermodynamic and kinetic properties of the sPP-DPE systems were also determined and correlated with membrane pore size. The crystal structure which developed in the matrix of the porous membranes was investigated by wide-angle X-ray diffraction (WXRD). The cloud points were found to be slightly affected by molecular weight (M_w) and the influence of syndiotacticy was negligible. The dynamic crystallization curves depended solely on the syndiotacticity of the samples, shifting to lower temperatures as the stereoregularity of the sPP decreased, and no relation with M_w was found. The viscosity of sPP-DPE solutions increased with M_w and stereoregularity of the sPP. Membrane pore sizes were correlated with droplet growth period, crystallization behaviour, and sample viscosity, the latter being an important parameter for low polymer solution concentration (15 wt%) but not so at higher concentration (40 wt%). The results showed that by controlling polymer microstructure it is possible to control membrane pore size.     

Effect of diluent on poly(ethylene‐co‐vinyl alcohol) hollow‐fiber membrane formation via thermally induced phase separation

Poly(ethylene‐co‐vinyl alcohol) hollow‐fiber membranes with a 44 mol % ethylene content were prepared by thermally induced phase separation. A mixture of 1,3‐propanediol and glycerol was used as the diluent. The effects of the ratio of 1,3‐propanediol to glycerol in the diluent mixture on the phase diagram, membrane structure, and membrane performance were investigated. As the ratio increased, the cloud point shifted to lower temperatures, and the membrane structure changed from a cellular structure due to liquid–liquid phase separation to a particulate structure due to polymer crystallization. Better pore connectivity was obtained in the hollow‐fiber membrane when the ratio of 1,3‐propanediol to glycerol was 50:50, and the membrane showed about 100 times higher water permeability than the membrane prepared with pure glycerol. For the prepared hollow‐fiber membrane, the solute 20 nm in diameter was almost rejected. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 219–225, 2005     

HDPE hollow‐fiber membrane via thermally induced phase separation. II. Factors affecting the water permeability of the membrane

Some major factors of the melt spinning of high‐density polyethylene (HDPE) and a liquid paraffin (LP) blend, which affect the water permeability of HDPE hollow‐fiber membrane obtained therefrom, were investigated. The water permeability of the membrane was found to increase as the membrane thickness decreases and as the melt‐flow‐rate value of HDPE and the LP content of the blend increases. The dependence of the water permeability on the major factors is also discussed in connection with the morphology of the membrane. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 1235–1242, 2000     

Structure and properties of poly(acrylonitrile‐co‐methyl acrylate) membranes prepared via thermally induced phase separation

Poly(acrylonitrile‐co‐methyl acrylate) [(P(AN‐MA)] microporous membranes were prepared via a thermally induced phase separation (TIPS) process by using γ‐butyrolactone (γ‐BA) and glyceryl triacetate (GTA) as the mixed diluent. The purpose of this work is to investigate the effects of the γ‐BA content, P(AN‐MA) concentration, and cooling rate on the structure and properties of P(AN‐MA) membranes. A lacy structure with high connectivity was formed with 50 wt % γ‐BA, and 50 wt % GTA comprising the mixed diluent. With an increase in the γ‐BA content, the pore structure acquires semi‐closed or completely closed cell‐like morphologies. The different phase separation mechanisms greatly influence the mechanical properties of the P(AN‐MA) membranes. P(AN‐MA) membranes with a lacy structure possess better tensile strength than those with semi‐closed or completely closed cell‐like structures. The membrane pore size grows larger when the TIPS process utilizes a higher γ‐BA content and a lower cooling rate. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43444.     

Preparation of dual‐layer acetylated methyl cellulose hollow fiber membranes via co‐extrusion using thermally induced phase separation and non‐solvent induced phase separation methods

Dual‐layer acetylated methyl cellulose (AMC) hollow fiber membranes were prepared by coupling the thermally induced phase separation (TIPS) and non‐solvent induced phase separation (NIPS) methods through a co‐extrusion process. The TIPS layer was optimized by investigating the effects of coagulant composition on morphology and tensile strength. The solvent in the aqueous coagulation bath caused both delayed liquid–liquid demixing and decreased polymer concentration at the membrane surface, leading to porous structure. The addition of an additive (triethylene glycol, (TEG)) to the NIPS solution resolved the adhesion instability problem of the TIPS and NIPS layers, which occurred due to the different phase separation rates. The dual‐layer AMC membrane showed good mechanical strength and performance. Comparison of the fouling resistance of the AMC membranes with dual‐layer polyvinylidene fluoride (PVDF) hollow fiber membranes fabricated with the same method revealed less fouling of the AMC than the PVDF hollow fiber membrane. This study demonstrated that a dual‐layer AMC membrane with good mechanical strength, performance, and fouling resistance can be successfully fabricated by a one‐step process of TIPS and NIPS. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42715.     

Preparation of PVDF hollow‐fiber membranes via immersion precipitation

Porous polyvinylidene fluoride (PVDF) hollow‐fiber membranes with high porosity were fabricated using the immersion precipitation method. Dimethylacetamide (DMAc) and N‐methyl‐2‐pyrrolidone (NMP) were used as solvent, respectively. In addition, polyvinylpyrrolidone (PVP), lithium chloride, and organic acids were employed as nonsolvent additives. The effects of the internal and external coagulation mediums on the resulting membrane properties were also investigated. The resulting hollow‐fiber membranes were characterized in terms of maximum pore radius, mean pore radius, effective surface porosity as well as wetting pressure. The structures of the prepared hollow fibers were examined using a scanning electron microscope. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 1643–1653, 2001     

Fouling reduction of a poly(ether sulfone) hollow‐fiber membrane with a hydrophilic surfactant prepared via non‐solvent‐induced phase separation

Membrane fouling is still a crucial problem, especially in applications for water treatment. When fouling takes place on membrane surfaces, it causes flux decline, leading to an increase in production cost due to increased energy demand. The selection of the right membrane material and a special treatment of the membrane are required to avoid membrane fouling. This article reports the fouling resistance of a poly(ether sulfone) (PES) hollow‐fiber membrane modified with hydrophilic surfactant Tetronic 1307. Experiments on several methods of fouling were carried out to investigate the effect of the addition of nonionic surfactant Tetronic 1307 on membrane fouling. The effectiveness of a chemical agent [sodium hypochlorite (NaClO)] in the reduction of bovine serum albumin (BSA) deposition on the membrane surface was also evaluated. Permeation results showed that the fouling of a PES blend membrane with Tetronic 1307 was lower than that of the original PES membrane in the case of BSA filtration. A treatment with a 100 ppm NaClO solution was capable of removing BSA cake formation and effective at improving the relative permeability. The permeability of a PES blend membrane with Tetronic 1307 was almost 2 times higher than the original permeability when the membrane was treated with a 100 ppm NaClO solution because both BSA and Tetronic 1307 could be decomposed. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Effects of process conditions on the formation of microporous membranes via solid-liquid thermally induced phase separation

Poly(phenylene sulfide) (PPS) membranes were formed via solid-liquid thermally induced phase separation. The effects of nucleation density () on final membrane structure were investigated. was varied by changing dissolution temperature (T _d, the temperature at which the melt-blend is formed) and polymer concentration in the initial polymer-diluent mixture.     

Preparation and characterization of cellulose triacetate membranes via thermally induced phase separation

A series of cellulose triacetate (CTA) membranes were prepared via thermally induced phase separation (TIPS) process with dimethyl sulfone (DMSO2) and polyethylene glycol (PEG400) as a crystallizable diluent and an additive, respectively. The phase separation behavior of CTA/DMSO2/PEG400 ternary system was investigated in detail by optical microscopy, differential scanning calorimetry and wide angle X‐ray diffraction. This ternary system dynamically undergoes solid‐solid phase separation and thus the CTA membranes possess cellular, lacy, plate‐, or even ellipse‐shaped pores. However, we can modulate the pore structure, porosity, water flux, and mechanical properties of the membranes by varying polymer concentration, composition of the mixed diluent, and cooling condition. Due to the intrinsic hydrophilicity, the prepared CTA membranes have better antifouling property than polysulfone membranes. These porous membranes were used as supports to fabricate thin‐film composite forward osmosis (FO) membranes, which show good water permeability and selectivity. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44454.     

稀释剂对TIPS法等规聚丙烯中空纤维微孔膜孔径及其连通性的影响

热致相分离（TIPS）法制备等规聚丙烯（iPP）中空纤维微孔膜,邻苯二甲酸二丁酯（DBP）与邻苯二甲酸二辛酯（DOP）的混合溶剂作为制膜稀释剂。干/湿氮气流量法测定了α（稀释剂中DBP的质量分数）和β（铸膜液中聚合物的质量分数）对膜样品的平均孔径及其分布的影响,并采用膜孔曲折因子定量表达膜孔连通性。发现全部膜样品均体现窄孔径分布特征。对于相同的β, α增加导致平均孔径及膜孔连通性下降。α=0.20时,β增加,膜的平均孔径先增加后降低,膜孔曲折因子稍下降; α=0.35或0.50时,β增加,膜的平均孔径降低,膜孔曲折因子下降。膜孔连通性体现了膜内部的拓扑结构,共溶剂组成和铸膜液固含量能够调节iPP中空纤维微孔膜的孔径及其连通性。     

Factors controlling surface morphology of porous polystyrene membranes prepared by thermally induced phase separation

A special device for preparing porous polymer membranes through a thermally induced phase separation (TIPS) process was designed and machined; it included a solution container, a membrane‐forming platform, a coldplate, a temperature‐decreasing system and a temperature‐supervising system. Polystyrene was selected as the model polymer from which to prepare porous membranes using the device due to its better understood TIPS and good biocompatibility with cells. The major factors controlling surface morphology and cell size, ie volume fraction of polystyrene (ϕ₂), quench rate and solvent‐removing methods, were studied. Fixing the coldplate temperature, when ϕ₂ is as low as 0.045, provokes the formation of round pores on both the bottom and top surfaces of the membrane; when ϕ₂ = 0.16 no pores are formed on either surface; when ϕ₂ = 0.087 pores form on the top surface, but not on the bottom surface. When ϕ₂ = 0.087 the cell size is very small or no pores are formed on the bottom surface, whereas the top surface shows a regular decrease of the pore sizes and an increase of the pore number and pore area, along with a decrease of the coldplate temperature. The side near the coldplate is dense, and the dense layer aligns along the coldplate, while the side away from the coldplate is like a porous foam, the shape of which is isotropic and the surfaces are interconnected with each other three dimensionally. On the top surface of a membrane obtained by ethanol extraction, the cell size is enlarged and the cell number reduced, but the surface morphology and the whole area remained almost the same when compared to samples obtained by freeze drying in the same membrane‐forming conditions. The isotropic, uniformly distributed and round pores suggest that the mechanism of phase separation is a spinodal liquid–liquid decomposition under our research conditions. © 2000 Society of Chemical Industry     

Formation of microporous polymeric membranes via thermally induced phase separation: A review

A review of recent research related to microporous polymeric membranes formed via thermally induced phase separation (TIPS) and the morphologies of these membranes is presented. A summary of polymers and suitable diluents that can be used to prepare these microporous membranes via TIPS are summarized. The effects of different kinds of polymer materials, diluent types, cooling conditions, extractants and additive agents on the morphology and performance of TIPS membranes are also discussed. Finally new developments in TIPS technology are summarized.

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Characteristics of poly(ether ether ketone) microporous membranes prepared via thermally induced phase separation (TIPS)

The morphology and bulk properties of microporous membranes based on poly (ether ether ketone) (PEEK) have been investigated as a function of initial casting composition and thermal and mechanical processing history. Membranes were prepared via solid—liquid phase separation of miscible blends of PEEK and polyetherimide (PEI), with subsequent extraction of the PEI diluent. Scanning electron microscopy studies revealed a microporous morphology with two distinct pore size scales corresponding to diluent extraction from interfibrillar and interspherulitic regions, respectively. The membrane structure was sensitive to both initial blend composition and crystallization temperature, with the resulting pore size distribution reflecting the kinetics of phase separation. For membranes prepared with lower initial diluent content or at lower crystallization temperatures, mercury intrusion porosimetry indicated a relatively narrow distribution of fine interfibrillar pores, with an average pore size of approximately 0.04 microns. Membranes prepared at higher diluent content or at higher crystallization temperatures displayed a broad pore distribution, with a sizeable population of coarse, interspherulitic pores (0.1 to 1 μm in size). Uniaxial drawing led to a fibrillated network structure with markedly higher water flux characteristics compared to the as-cast membranes. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 2347–2355, 1997     

Influences of extractant on the hydrophilicity and performances of HDPE/PE‐b‐PEG blend membranes prepared via thermally induced phase separation (TIPS) process

Blending the block copolymer into the membrane matrix is a convenient and efficient way for membrane modification. In this study, HDPE/PE‐b‐PEG membranes were prepared via TIPS process, and the extractant effect was investigated. An interesting finding was that a non‐polar extractant (n‐hexane) was more conducive to the surface enrichment of PEG chains than a polar solvent (ethanol). The reason was deemed to be the combined effect of entropy drive, interfacial energy, and the swelling behavior. Besides, membrane performances related to the surface chemical properties were studied. Results suggested that the prepared blend membranes extracted by n‐hexane showed enhanced hydrophilicity, anti‐fouling property and water flux. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 2680–2687, 2013     

Effect of polymer density on polyethylene hollow fiber membrane formation via thermally induced phase separation

Microporous high‐density polyethylene (HDPE) and low‐density polyethylene (LDPE) hollow fiber membranes were prepared from polyethylene–diisodecyl phthalate solution via thermally induced phase separation. Effect of the polyethylene density on the membrane structure and performance was investigated. The HDPE membrane showed about five times higher water permeability than the LDPE membrane because it had the larger pore and the higher porosity at the outer membrane surface. The formation of the larger pore was owing to both the initial larger structure formed by spinodal decomposition and the suppression of the diluent evaporation from the outer membrane surface due to the higher solution viscosity. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 471–474, 2004     

Preparation of ultrahigh‐molecular‐weight polyethylene membranes via a thermally induced phase‐separation method

Porous, flat membranes of ultrahigh‐molecular‐weight polyethylene were prepared as thermally resistant and solvent‐resistant membranes by the thermally induced phase‐separation method. Diphenyl ether and decalin were chosen as the diluents. The phase diagrams were drawn with the cloud‐point temperatures and the crystallization temperatures. According to the phase diagrams, scanning electron microscopy images, and porosities of the samples, the influential factors, including the polymer concentration, cooling rate, and viscosity, were investigated. Porous ultrahigh‐molecular‐weight polyethylene membranes with thermal and solvent resistance could be prepared with suitable diluents and cooling rates by the thermally induced phase‐separation method. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Effects of the extractant on the hydrophilicity and performance of high‐density polyethylene/polyethylene‐b‐poly(ethylene glycol) blend membranes prepared via a thermally induced phase separation process

The blending of a block copolymer into the membrane matrix is a convenient and efficient way to modify membranes. In this study, high‐density polyethylene/polyethylene‐b‐poly(ethylene glycol) (PEG) membranes were prepared via a thermally induced phase separation process, and the extractant effect was investigated. An interesting finding was that the nonpolar extractant (n‐hexane) was more conducive to the surface enrichment of the PEG chains than the polar solvent (ethanol). The reason was deemed to be the combined effect of the entropy drive, interfacial energy, and swelling behavior. In addition, the membrane performance related to the surface chemical properties was studied. The results suggest that the prepared blend membranes extracted by n‐hexane showed enhanced the hydrophilicity, antifouling properties, and water flux. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3816–3824, 2013     

Crystallization behavior of PVDF in PVDF‐DMP system via thermally induced phase separation

The crystallization behavior of PVDF (poly (vinylidene) fluoride) in PVDF‐dimethylphthalate(DMP) system was investigated in the liquid–liquid (L–L) phase separation region, solid–liquid (S–L) phase separation region and different quenching conditions via thermally induced phase separation (TIPS). Differential scanning calorimetry (DSC) indicated the crystallinity of PVDF in PVDF‐DMP system increased in the early stage of phase separation and polymer‐rich phase crystallized completely in the late stage of phase separation. The scanning electron microscopy (SEM) showed the different quenching temperatures had effects on the spherulite size of polymer rich phase and the ultimate membrane structure in the different phase separation regions. The wide angle X‐ray diffraction (WAXD) was used to quantify the crystal structure of PVDF in PVDF‐DMP system. The α‐phase PVDF was obtained when the system quenched to different temperatures above 40°C, and the area of diffraction peaks changed when quenching temperatures changed. While the β‐phase PVDF was formed when PVDF‐DMP system was quenched form liquid nitrogen and crystallized for 24 h in 25°C water bath. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3714–3719, 2006     

Porous cellulose acetate membrane prepared by thermally induced phase separation

Microporous cellulose acetate membranes were prepared by a thermally induced phase separation (TIPS) process. Two kinds of cellulose acetate with acetyl content of 51 and 55 mol % and two kinds of diluents, such as 2‐methyl‐2,4‐pentandiol and 2‐ethyl‐1,3‐hexanediol, were used. In all polymer‐diluent systems, cloud points were observed, which indicated that liquid–liquid phase separation occurred during the TIPS process. The growth of droplets formed after the phase separation was followed using three cooling conditions. The obtained pore structure was isotropic, that is, the pore size did not vary across the membrane. In addition, no macrovoids were formed. These pore structures were in contrast with those usually obtained by the immersion precipitation method. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 3951–3955, 2003