Porous polyphenylene sulfide membrane with high durability against solvents by the thermally induced phase‐separation method

Porous polyphenylene sulfide membranes were prepared as new solvent‐resistant membranes by the thermally induced phase‐separation (TIPS) method. Porous structures were either formed by solid–liquid phase separation (polymer crystallization) or liquid–liquid phase separation. The effects of solvents, cooling rates, and polymer concentrations on the porous structures were investigated. Various characteristics of pore structure can be obtained with suitable diluents and cooling rates using the TIPS method. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 2959–2966, 2006     

Formation of porous poly(ethylene‐co‐vinyl alcohol) membrane via thermally induced phase separation

Crystalline poly(ethylene‐co‐vinyl alcohol) (EVOH) membranes were prepared by a thermally induced phase separation (TIPS) process. The diluents used were 1,3‐propanediol and 1,3‐butanediol. The dynamic crystallization temperature was determined by DSC measurement. No structure was detected by an optical microscope in the temperature region higher than the crystallization temperature. This means that porous membrane structures were formed by solid–liquid phase separation (polymer crystallization) rather than by liquid–liquid phase separation. The EVOH/butanediol system showed a higher dynamic crystallization temperature and equilibrium melting temperature than those of the EVOH/propanediol system. SEM observation showed that the sizes of the crystalline particles in the membranes depended on the polymer concentration, cooling rate, and kinds of diluents. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 79: 2449–2455, 2001     

Calculation of alcohol‐acetone‐cellulose acetate ternary phase diagram and their relevance to membrane formation

ABSTRACT Alcohol‐acetone‐cellulose acetate phase diagrams incorporated with methanol, ethanol, and isopropanol as nonsolvents are calculated according to a new form of the Flory–Huggins equation. Nonsolvent–cellulose acetate interaction parameters are measured by swelling experiments. Concentration‐dependent nonsolvent–solvent interaction parameters are obtained by vapor–liquid equilibrium and the Wilson equation. It is shown that alcohol is a week coagulant compared with water, and water > methanol > ethanol > isopropanol for cellulose acetate. The phase diagrams characteristic of acetone‐cellulose acetate combined with water, methanol, ethanol, and isopropanol as nonsolvents is different, which leads to the different morphological structure of a cellulose acetate membrane. The structure of a water coagulated membrane has large macrovoids from liquid–liquid phase separation. A methanol coagulated membrane has a honeycomb‐like structure from spinodal microphase separation. An ethanol or isopropanol coagulated membrane has a thicker, dense top layer from the delay time phase separation. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1650–1657, 2001     

Optimization of structure and properties of polyphenylene sulfide porous membrane by controlling the process of thermally induced phase separation

Polyphenylene sulfide (PPS) porous membranes were successfully prepared from miscible blends of PPS and polyethersulfone (PES) via thermally induced phase separation followed by subsequent extraction of the PES diluent. The morphologies, crystalline structures, mechanical properties, pore structures and permeate fluxes of the PPS porous membranes obtained from different phase separation processes were characterized and are discussed. During the phase separation in the heating process, PPS and PES mainly underwent liquid–liquid phase separation, and then a nonhomogeneous porous structure with a mean pore size of 100 μm and a honeycomb‐like internal structure formed on the membrane surface. The phase separation of PPS/PES occurring in the cooling process was easier to control and the related pore diameter distribution was more regular. In the process of direct annealing, as the phase separation temperature decreased, the pore size distribution became more homogeneous and the mean diameter of the pores also decreased gradually. When the phase separation temperature decreased to 200 °C, PPS membranes with a network structure and a uniform as well as well‐interconnected porous structure could be obtained. In addition, the maximum permeation flux reached 1718.03 L m^–2 h^–1 when the phase separation temperature was 230 °C. The most probable pore diameter was 6.665 nm, and the permeate flux of this membrane was 2.00 L m^–2 h^–1; its tensile strength was 17.07 MPa. Finally, these PPS porous membranes with controllable pore structure as well as size can be widely used in the chemical industry and energy field for liquid purification. © 2020 Society of Chemical Industry     

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.     

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 a bicontinuous structure membrane of polyvinylidene fluoride in diphenyl ketone diluent via thermally induced phase separation

The polyvinylidene fluoride (PVDF)‐diphenyl ketone (DPK) mixture was studied as a new system to prepare PVDF membranes via thermally induced phase separation (TIPS). The phenomena of liquid–liquid phase separation was found in this mixture when the temperature of mixture was decreasing and the PVDF concentration was less than 30 wt %. Using DPK as diluent, PVDF membrane with bicontinuous structure was obtained without necessity to add a nonsolvent or a stretching process further. The phase diagram of PVDF‐DPK system was also constructed to help investigate the effect of PVDF concentration and coarsening temperature on morphology of resulting membrane. The experiments showed that high coarsening temperatures and low PVDF concentrations resulted in the formation of the large pore size membrane. The strength of the wet membrane was decreasing with decreasing PVDF concentration. On condition that the PVDF concentration was larger than 30 wt %, thermally induced solid–liquid separation occurred and bicontinuous structure disappeared. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Fabrication of cellulose acetate ultrafiltration membrane with diphenyl ketone via thermally induced phase separation

With diphenyl ketone as diluent, cellulose acetate (CA) ultrafiltration (UF) membrane with a bicontinuous structure was prepared via thermally induced phase separation (TIPS) method. The liquid–liquid phase separation region of CA/diphenyl ketone system was measured and the maximum corresponding polymer concentration was approximately 53 wt %. The effects of polymer concentration, coarsening time and coarsening temperature on the morphologies, and mechanical properties of CA membranes were investigated systematically. As the polymer concentration increased from 15 to 30 wt %, the bicontinuous structure could be obtained and the tensile strength of CA membranes increased from 3.92 to 30.17 MPa. With the increase of coarsening time, the thickness of dense skin layer and the asymmetry of cross‐section reduced. However, excess coarsening rendered the membrane morphology evolved from a bicontinuous structure to a cellular structure. When the coarsening time was 5 min, the bicontinuous structure in cross‐section showed good interconnectivity and the dense skin layer exhibited a thin thickness of 2 μm. The fabricated CA hollow fiber UF membrane exhibited a high tensile strength of 31.00 MPa and rejection of 96.10% for dextran 20 kDa. It is indicated that diphenyl ketone is a competitive diluent to prepare CA membranes with excellent performance via TIPS. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42669.     

Thermosensitive poly(n‐isopropylacrylamide) hydrogels bonded on cellulose supports

A two‐step initiation and polymerization process was developed for the preparation of two series of hydrogel–cellulose composites with distinctively different morphologies and swelling behaviors. Hydroentangled cotton cellulose fibers were optimally initiated in 20 mM aqueous ammonium cerium(IV) nitrate for 15 min and then polymerized in aqueous solutions of N‐isopropylacrylamide (NIPAAm) monomer and N,N′‐methylene bisacrylamide (BisA) crosslinker. The extents of hydrogels on the cellulose solids could be controlled by variations in the concentrations of the monomer and crosslinker as well as the NIPAAm/BisA solution‐to‐solid ratios. The two series of hydrogel–cellulose composites formed were hydrogel‐covered/filled cellulose (I) and cellulose‐reinforced hydrogel (II) composites. Series I composites were synthesized with NIPAAm/BisA solutions below the liquid saturation level of the cellulose; this led to pore structures (size and porosity) that depended on both the extent and swelling of the grafted hydrogels. Series II composites were polymerized in the presence of excessive NIPAAm/BisA solutions to produce cellulose solids completely encapsulated in the hydrogels. All the cellulose‐supported hydrogels exhibited lower extents of phase transition over a wider temperature range (28–40°C) than the free poly(N‐isopropylacrylamide) hydrogels (32°C). These findings demonstrate that hydrogels can be used to control the pore structure of cellulose and can be supported with cellulose fibers. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 999–1006, 2003     

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     

Preparation and characterization of poly(ethylene‐co‐vinyl alcohol) membranes via thermally induced liquid–liquid phase separation

Porous membranes were prepared through the thermally induced phase separation of poly(ethylene‐co‐vinyl alcohol) (EVOH)/glycerol mixtures. The binodal temperature and dynamic crystallization temperature were determined by optical microscopy and differential scanning calorimetry measurements, respectively. It was determined experimentally that the liquid–liquid phase boundaries were shifted to higher temperatures when the ethylene content in EVOH increased. For EVOHs with ethylene contents of 32–44 mol %, liquid–liquid phase separation occurred before crystallization. Cellular pores were formed in these membranes. However, only polymer crystallization (solid–liquid phase separation) occurred for EVOH with a 27 mol % ethylene content, and the membrane morphology was the particulate structure. Scanning electron microscopy showed that the sizes of the cellular pores and crystalline particles in the membranes depended on the ethylene content in EVOH, the polymer concentration, and the cooling rate. Furthermore, the tendency of the pore and particle sizes was examined in terms of the solution thermodynamics of the binary mixture and the crystallization kinetics. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 853–860, 2003     

Fabrication and characterization of chlorinated polyvinyl chloride microporous membranes via thermally induced phase separation process

Microporous chlorinated polyvinyl chloride (CPVC) membranes were prepared via thermally induced phase separation process for the first time using diphenyl ether (DPE) as diluent. The CPVC/DPE blends exhibit upper critical solution temperature (UCST)‐type phase behavior, which undergoes liquid‐liquid phase separation followed by sol‐gel transition during cooling process. Therefore, the resulting CPVC membranes presented symmetric morphology with uniformly distributed cellular pores. The cloud point (liquid‐liquid phase separation temperature) decreased with increasing CPVC content, while the sol‐gel transition temperature showed an opposite trend. Both the growth rate of diluent‐rich phase droplets and the gelation rate of the CPVC/DPE blends increased by decreasing CPVC concentration or cooling rate, leading to an increase of the pore size in the final membranes. Results of water permeation tests confirmed that the water flux of the membranes have a significant dependence on their porosity and pore size, that is the water flux increased with the increase of porosity and pore size. Moreover, the CPVC microporous membranes prepared by the TIPS process showed a high mechanical strength and excellent acid/alkali resistance, which has presented a great potential for application in the fields of water and wastewater treatment. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44346.     

Effect of diluents on the crystallization behavior of poly(4‐methyl‐1‐pentene) and membrane morphology via thermally induced phase separation

The effect of diluents on polymer crystallization and membrane morphology via thermally induced phase separation(TIPS) were studied by changing the composition of the mixed‐diluents systematically, in the system of poly(4‐methyl‐1‐pentene) (TPX)/dibutyl‐phthalate (DBP)/di‐n‐octyl‐phthalate (D‐n‐OP) with TPX concentration of 30 wt %. The TPX crystallization was observed with differential scanning calorimetry (DSC) and wide angle X‐ray diffraction (WAXD). The membranes were characterized with scanning electron microscopy (SEM), porosity, and pore size measurement. As the content of D‐n‐OP increased in mixed‐diluents, the solubility with TPX increased, inducing the phase separation changing from liquid–liquid phase separation into solid–liquid phase separation, which changed the membrane morphology and structure. When the ratios of DBP to D‐n‐OP were 10 : 0, 7 : 3; 5 : 5, and 3 : 7, membranes were formed with cellular structure and well connected pores, while the ratio was 0 : 10, discernable spherulities were found with not well‐formed pore structure. The effect of composition of the mixed‐diluents on membrane morphology was more remarkable in TPX/dioctyl‐sebacate (DOS)/dimethyl‐phthalate (DMP) system, since good cellular structure was formed when the ratios of DOS to DMP were 10 : 0, 7 : 3, while spherulites were observed when 5 : 5. Dual endotherm peaks behavior on DSC melting curves emerged for all the samples in this study, which was attributed to the special polymer crystallization behavior, primary crystallization, and secondary crystallization occurred when quenching the samples. As the content of D‐n‐OP increased, the secondary crystallization enhanced which induced the first endotherm peak on DSC melting curves moving to a lower temperature and the broadening of the overall melting peak, as well as the increasing of the overall crystallinity. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Mass transfer dynamics of the evaporation step in membrane formation by phase inversion

A ternary diffusion model has been developed for the evaporation step of the phase inversion process. The model is applied to the analysis of mass transfer dynamics of the evaporation step for the methanol–acetone–cellulose acetate (CA) ternary casting system. The combined analysis of quantitatively computational results from the ternary evaporation model and qualitative dynamic results during the quench process has shown that the evaporation step is essentially necessary to prepare the defect‐free, ultrathin skinned asymmetric CA membrane for the separation of CO₂/CH₄. The skin layer of high CA concentration obtained by evaporation has an ability to suppress liquid–liquid phase separation. And the skin layer with high tensile strength can resist the interfacial tension caused by spinodal decomposition from the substructure. Although the CA concentration in the skin layer increases considerably because of the evaporation step and the following delay time during the quench process, the substructure can still induce the spinodal decomposition because the strong coagulant, methanol, can diffuse rapidly across the ultrathin skin layer. Hence the defect‐free, ultrathin‐skinned asymmetric membrane for gas separation can be prepared from methanol–acetone–CA casting system by evaporation step and the wet phase inversion. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 1564–1571, 2002     

Phase‐separation phenomena in the polymerization of styrene in the presence of polyethylene wax

The demixing processes that occur during the polymerization of styrene in the presence of a low molar mass polyethylene wax were investigated. Quantitative information on the phase behavior of such a three‐component system was obtained through the investigation of the temperature‐induced phase separation and the observation of the phase separation during polymerization. Both techniques allow the construction of the same ternary phase diagram. Such phase behavior can be understood through the discussion of the interference of a liquid–liquid phase‐separation process and crystallization. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 2234–2243, 2004     

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.     

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.     

Phase separation mechanism during membrane formation by dry‐cast process

Phase separation mechanisms during the membrane formation by dry‐cast process were investigated by light scattering in the cellulose acetate/dimethylformamide (DMF)/2‐methyl‐2,4‐pentanediol system. Phase separation occurred by spinodal decomposition (SD) when paths of the composition changes due to the evaporation of DMF were close to the critical point in the phase diagram. Characteristic properties of the early stage of SD such as an apparent diffusion coefficient and an interface periodic distance were obtained from the Cahn theory. Phase separation occurred by nucleation and growth (NG) when paths of the composition changes were far from the critical point. SEM observation confirmed that the membrane formed by the SD mechanism had interconnected structure, whereas that by the NG mechanism had the closed cell porous structure. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 776–782, 2000     

Hemodialysis membrane prepared from cellulose/N‐methylmorpholine‐N‐oxide solution. II. Comparative studies on the permeation characteristics of membranes prepared from N‐methylmorpholine‐N‐oxide and cuprammonium solutions

Two kinds of regenerated cellulose membranes for hemodialysis were prepared from casting solutions of N‐methylmorpholine‐N‐oxide (NMMO) and cuprammonium (denoted NMMO membranes and cuprammonium membranes, respectively). The concentration of cellulose in the casting solution investigated was 6–8 wt %. The permeation characteristics of both membrane series were compared in terms of the ultrafiltration rate (UFR) of pure water, the sieving coefficient (SC) of dextran, and the solute permeabilities of urea, creatinine, and vitamin B₁₂. The UFR and SC of the NMMO membranes were strongly affected by the cellulose concentration of the casting solution, and NMMO was a preferable solvent for the production of cellulose membranes with high performance; the cuprammonium solution gave low‐performance membranes. The pore structures of both types of membranes were estimated with the Hagen–Poiseuille law. The results showed that the NMMO membranes had larger pore radius and smaller pore numbers than the cuprammonium membranes. The differences in the membrane pore structures led to the differences in the performance between the two membrane series. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 333–339, 2003     

Preparation and characterization of PES/CA microporous membranes via reverse thermally induced phase separation process

The blend polyethersulfone (PES)/cellulose acetate (CA) flat‐sheet microporous membranes were prepared by reverse thermally induced phase separation (RTIPS) process. The effects of CA content and coagulation bath temperature on membrane structures and properties were investigated in terms of membrane morphology, water contact angle, permeation performance, and mechanical properties. The cloud point results indicated that the cloud point decreased with the increasing content of CA. When the coagulation bath temperature was lower than the cloud point, the membrane formation process underwent nonsolvent induced phase separation (NIPS) process and dense skin layer and finger‐like structure were formed in membranes. These membranes had lower pure water flux and poor mechanical properties. But when the coagulation bath temperature was higher than the cloud point, the membrane formation process underwent RTIPS process. The porous top surface as well as porous cross‐section of the membranes were formed. Therefore, high pure water flux and good mechanical properties were obtained. The contact angles results indicated that the hydrophilicity of the prepared membranes improved obviously with the addition of CA. When the content of CA was 0.5 wt% and the membrane formation temperature was 323K, the PES/CA microporous membrane which was prepared via the RTIPS process displayed a optimal permeability of the pure water flux of 816 L m^?2 h^?1 and the BSA rejection rate of 49.5%, which showed an increase of 48.9% and 23.6% than that of pure PES membrane, respectively. Moreover, the mechanical strengths of the membranes obtained by RTIPS process were better than those membranes prepared by NIPS process. POLYM. ENG. SCI., 58:180–191, 2018. © 2017 Society of Plastics Engineers