Preparation of porous membrane by combined use of thermally induced phase separation and immersion precipitation

By immersion in the cooled nonsolvent, PMMA porous membrane was prepared by the combined use of thermally induced phase separation (TIPS) and immersion precipitation. As nonsolvent, water with low mutual affinity with cyclohexanol (diluent) and methanol with high affinity were used. In the case of water, the porous structure was formed by TIPS immediately after the immersion. Near the top surface contacted with the nonsolvent, the thin skin layer was formed due to the outflow of the diluent. After the long immersion period, macrovoids were formed near the top surface due to the penetration of the nonsolvent. Thus, TIPS and the immersion precipitation occurred serially. On the contrary, TIPS and the immersion precipitation occurred simultaneously in the case of methanol because the inflow of methanol was fast. Therefore, the membrane obtained after the short immersion period had the larger pores near the top surface due to the nonsolvent induced phase separation and the smaller pores near the bottom surface due to TIPS. These two modes of the phase separations were confirmed by the changes in light transmittance through the polymer solutions.     

Light‐scattering study on porous membrane formation by dry‐cast process

The phase‐separation mechanism during porous membrane formation by the dry‐cast process was investigated by the light‐scattering method in poly(methyl methacrylate)/ethyl acetate (EA)/2‐methyl‐2,4‐pentanediol system. The evaporation of EA from the cast solution induced the phase separation and thus the porous membrane was obtained. By the light‐scattering measurement on the phase‐separation kinetics, the phase separation was found to occur by a spinodal decomposition mechanism. As the amount of nonsolvent in the cast solution decreased, the structure growth rate decreased and the growth stopped soon. The obtained porous structure was isotropic rather than asymmetric. The average interpore distances obtained from the SEM observation roughly agreed with the final constant interphase periodic distances measured by the light‐scattering method. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 10: 3205–3209, 2002     

Fabrication of three‐dimensionally ordered microporous membrane by wet phase separation

We report a novel porous fluorinated polyimide membrane with a cylinder structure fabricated by a wet phase inversion process, which is formed by a ternary system, polyimide/solvent/water. The porous polyimide membranes consisted of a thin top porous layer and three‐dimensionally ordered cylinder micropores. The porous membrane‐forming solvents were N‐methylpyrrolidone containing nonsolvent additives such as alcohol, and the height and width of the cylinder structure were controlled by the solvents. Water fluxes through the porous polyimide membranes were measured using a stirred dead‐end filtration cell, and the fluxes of the porous membrane with the cylinder‐type structure were approximately three times greater than those of the membrane with the finger‐type structure. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3016–3021, 2004     

热诱导相分离法制备高分子微孔膜的原理与进展

简述了热诱导相分离 (TIPS)法制备高分子微孔膜的相平衡热力学及相分离动力学原理 ,并对国内外研究进展进行了评述     

GO-TSC/聚酰亚胺混合基质膜的制备及气体分离性能研究

使用氨基硫脲（TSC）对氧化石墨烯（GO）进行改性,制备GO-TSC层状复合材料。随后,将该复合材料加入到Matrimid^®5218（PI）基质中,制备用于二氧化碳分离的混合基质膜（MMMs）。通过TGA、SEM及气体分离性能测试考察了GO-TSC对膜热稳定性、结构和气体分离性能等的影响。SEM结果显示GO-TSC可均匀分散在聚合物基质上并与基质紧密结合;TGA结果显示混合基质膜在250 ℃以上仍保持稳定。与纯PI膜相比,MMMs显著增强了二氧化碳的渗透性。GO-TSC中所含的氨基与二氧化碳具有良好的亲和力,增加的碱性位点可以有效地转运二氧化碳。GO-TSC的层状结构增加了气体的传输路径,不利于大动态直径气体（甲烷、氮气）的通过,从而提高了分离性能。GO-TSC负载量为0.75%（质量分数）时混合基质膜的分离性能最佳。相比较纯PI膜,混合基质膜的二氧化碳渗透系数和二氧化碳/甲烷、二氧化碳/氮气分离系数分别提高了42.16%、95.79%和83.72%。     

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     

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     

Formation of polyurethane membranes by immersion precipitation. i. liquid–liquid phase separation in a polyurethane/DMF/water system

Liquid–liquid phase separation phenomena of polyurethane/DMF/water were studied. Two polyurethanes having different hydrophobicity were synthesized by varying the polyol components. The cloud-point curves for the ternary system of polyurethane/DMF/water were determined by the titration method. A small amount of water is needed to induce liquid–liquid demixing, and the region of the homogeneous phase is enlarged with increased hydrophilicity of the polyurethane. We measured the interaction parameters, and calculated the binodal and spinodal curves based on the thermodynamics of polymer solutions. The light transmission experiment showed that precipitation time increased with the higher content of DMF in a coagulation bath. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 2377–2384, 1999     

Membrane formation via thermally induced phase separation in polypropylene/polybutene/diluent system

Porous membranes were prepared from a polymer blend system by the thermally induced phase separation (TIPS) process. The polymer blend system was isotactic polypropylene (iPP)/polybutene (PB) and the diluent was diphenyl ether (DPE). Two types of porous membranes were prepared by the extractions of DPE alone and both DPE and PB after the phase separation. The effect of the addition of PB to the iPP solution on the phase diagram was investigated and the phase separation kinetics was measured by the light scattering method. The addition of PB resulted in the higher solute rejection property and lower water permeance. By the further extraction of PB from the porous iPP/PB membrane prepared by the extraction of DPE, the water permeance was approximately doubled, maintaining almost the same rejection property. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 1701–1708, 2002; DOI 10.1002/app.10550     

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.

Open image in new window

     

Preparation of thermal stable porous polyimide membranes by phase inversion process for lithium-ion battery

The porous polyimide membranes were prepared by a wet phase inversion process based on the organo-soluble polyimide. The influence of coagulation bath composition and casting polymer solution concentration on the morphology of membranes was investigated. A series of spongy-like porous polyimide membranes with different porosity were obtained and characterized. These porous polyimide membranes exhibited excellent thermal stability and dimensional stability with the glass transition temperature of 274 °C and thermal shrinkage less than 1% after stored at 200 °C. All the porous polyimide membranes exhibited good wettability with electrolyte uptake of 190–378% due to their high surface polarity and high porosity. The discharge curves for the lithium-ion cells using porous polyimide membranes as separator displayed relatively flatter voltage plateaus than that for Celgard 2400 membrane and gave the discharge capacity of 129–131 mAh/g. The thermal stable porous polyimide membranes are favorable to be applied as separator in the lithium-ion cell and can be expected to provide excellent battery performance at elevated temperature.     

Formation of microporous membrane of isotactic polypropylene in dibutyl phthalate‐soybean oil via thermally induced phase separation

Isotactic polypropylene (iPP) microporous membranes were prepared via the thermally induced phase separation (TIPS) process with the diluents being dibutyl phthalate (DBP) and soybean oil mixture. By changing the weight ratio of DBP to soybean oil systematically, it was determined experimentally that the cloud‐point curves were influenced to a great extent, while the crystallization curves showed much less dependence on the diluents composition. Scanning electron microscopy (SEM) showed that the resulting membrane morphologies changed significantly by varying the composition of the diluents, i.e., by changing the interaction parameter and other characteristics of diluents, the interwoven or celluar structure can be fabricated successfully at a fixed polymer concentration under the same cooling conditions. Different growth rates of iPP spherulite were obtained in the diluents with different composition. It is shown that the spherulites growth rates may be also attributed to the great variations of the final microporous morphology to a certain extent. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Supercritical carbon dioxide extraction in membrane formation by thermally induced phase separation

In this study, a novel and environmentally friendly extracting method, supercritical carbon dioxide (SC‐CO₂) extraction, was investigated in the thermally induced phase separation (TIPS) process for making microporous membranes. In the SC‐CO₂ extraction, the effects of extraction time, pressure, and temperature on the extraction fraction, membrane morphology, and membrane performance were investigated. It was concluded that with extraction conditions of 18 MPa, 35°C and 2 h, the porous membrane had the highest extraction fraction. There was a close relationship between membrane performance and the extraction conditions of SC‐CO₂, and it is possible to tailor membrane performance through the choice of extraction conditions. Compared with traditional solvent extraction, a dry membrane treated by SC‐CO₂ extraction has much less shrinkage and greater water permeability, whereas the degree of crystallization of a membrane extracted by SC‐CO₂ is slightly greater than that extracted by ethanol. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1632–1639, 2007     

Real‐time temperature and photon transmission measurements for monitoring phase separation during the formation of poly(N‐isopropylacrylamide) gels

Phase separation during the formation of poly(N‐isopropylacrylamide) (PNIPA) hydrogels was investigated using real‐time photon transmission and temperature measurements. The hydrogels were prepared by free‐radical crosslinking polymerization of N‐isopropylacrylamide (NIPA) in the presence of N,N′‐methylenebisacrylamide (BAAm) as a crosslinker in an aqueous solution. The onset reaction temperature T₀ was varied between 20 and 28°C. Following an induction period, all the gelation experiments resulted in exothermic reaction profiles. A temperature increase of 6.5 ± 0.6°C was observed in the experiments. It was shown that the temperature increase during the formation and growth process of PNIPA gels is accompanied by a simultaneous decrease in the transmitted light intensities I_tr. The decrease in I_tr at temperatures below the lower critical solution temperature of PNIPA was explained by the concentration fluctuations due to the inhomogeneity in the gel network. At higher temperatures, it was shown that the gel system undergoes a phase transition via a spinodal decomposition process. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 3589–3595, 2002     

Time-resolved light scattering of the phase separation in polymer-dispersed liquid crystals formed by photo-polymerization induced phase separation

Time-resolved light scattering is utilized to monitor the phase separation of photo-initiated polymer-dispersed liquid crystals. At the lowest cure intensities studied, the system undergoes spinodal decomposition and the results are analyzed with Cahn-Hilliard theory. As the cure intensity increases, the rate of phase separation increases such that the early stages of spinodal decomposition are no longer observable. These systems are analyzed using the Debye-Bueche model, which provides the time evolution of the number and size of LC domains. These results indicate that an increase in cure beam intensity initially increases the rate of domain growth, but this effect is overwhelmed by the fast vitrification and cross-linking that can occur at highest cure beam intensities.     

Effect of diluents on membrane formation via thermally induced phase separation

The effect of diluents on isotactic polypropylene (iPP) membrane formation via thermally induced phase separation was investigated. The diluents were methyl salicylate (MS), diphenyl ether (DPE), and diphenylmethane (DPM). The cloud-point curve was shifted to a lower temperature in the order iPP–MS, iPP–DPE, and iPP–DPM, whereas the crystallization temperature was not influenced so much by diluent type. Droplet-growth processes were investigated under two conditions: quenching the polymer solution at the desired temperature and cooling at a constant rate. Although droplet sizes were in the order iPP–MS, iPP–DPE, and iPP–DPM in both cases, the difference was more pronounced with the constant cooling rate condition. Scanning electron microscopy indicated that interconnected structures were obtained when the polymer solution was quenched in ice water. The effect of the diluents on these structures was observed. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 169–177, 2001     

Surface modification of polyimide and polysulfone membranes by ion beam for gas separation

The surface carbonization of polyimide (PI) and polysulfone (PSf) by ion beam has been performed to adapt the carbon molecular sieve properties on the skin of the polymeric membranes without the deformation of the membrane structure. In order to control the structure of membrane skin and to improve gas transport properties, the irradiation conditions, such as the dosage and the source of ion beams, have been varied. The ideal separation factor of CO₂ over N₂ through the surface‐modified PI and PSf membranes increased threefold compared to those of the untreated, pristine membranes, whereas the permeability decreased with almost two orders of magnitude. This appears to be due to the fact that the structure of membrane skin has been changed to a barrier layer. The formation of barrier layer was confirmed by comparing the calculated values of a simple resistance model with the experimental results, and the estimated permeability of this barrier was 10⁻⁴ barrer. It was concluded that ion beam irradiation could provide a useful tool for improving selectivity for gas separation membranes. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 1554–1560, 2000     

Phase behavior and morphological studies of polyimide/PVP/solvent/water systems by phase inversion

The solubility gaps for poly(vinyl pyrrolidone) (PVP) in four polyimide solutions (NMP, DMF, GBL, DMSO) were determined by cloud point measurement and correlated with χ_PI/Solvent and Δδ_PVP/Solvent. Membranes prepared with NMP and DMF systems showed a tendency of suppressing fingerlike structure with addition of PVP. On the other hand, membranes prepared with GBL and DMSO systems showed an inclination toward inducing macrovoid formation. These effects of PVP on the membrane morphology were explained by means of miscibility gap, viscosity of the polymer solution, polymer–polymer phase separation, and overall porosity. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 3481–3488, 2001     

Phase separation in polyetherimide/solvent/nonsolvent systems and membrane formation

Phase separation phenomena of polyetherimide (PEI)/solvent/nonsolvent systems were investigated by measuring their precipitation values over the temperature range from 20 to 50°C. The solvents used are N‐methyl‐2‐pyrrolidone (NMP), dimethylacetamide (DMAC), and dimethylformamide (DMF). Nine nonsolvents were employed including water, methanol, ethanol, 1‐propanol, 2‐propanol, acetic acid, propionic acid, ethylene glycol, and diethylene glycol. Based on the measured precipitation values, critical solubility parameters for PEI were calculated, and the partial solubility boundary for PEI was obtained in a two‐dimensional solubility parameter coordinate graph. The relationship between solvent strength and membrane structure was examined using PEI hollow‐fiber membranes prepared from binary polymer solutions containing NMP, DMAC, and DMF as solvents. Water was used both as internal and external coagulants. The cross‐sectional structure and gas permeation properties of these hollow fibers were examined. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1789–1796, 1999     

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