Hyun Chae Park

Zeolite-filled polyimide membrane containing 2,4,6-triaminopyrimidine

Abstract Interfacial void-free Matrimid polyimide (PI) membranes filled with zeolites were prepared by introducing 2,4,6-triaminopyrimidine (TAP). TAP enhanced the contact of zeolite particles with polyimide chains presumably by forming hydrogen bonding between them. The threshold amount of TAP, needed to depress totally the void formation, varied with zeolite type in the order of zeolite 4A≈13X

Membrane formation by water vapor induced phase inversion

The membrane formation by the phase inversion process was studied by coagulating a polysulfone/N-methyl-2-pyrrolidone solution with water vapor as a coagulant. The phase separation occurred when the relative humidity in the membrane casting atmosphere was higher than about 65%. The pore size was strongly affected by the relative humidity as well as the concentration of the polymer solution. It increased as both the relative humidity and the polymer concentration were decreased. The membranes produced showed a uniform structure composed of closed pores. The pure water flux measurement confirmed the closeness of the pores. The information on the late stage phase separation was obtained in situ by an optical microscope due to the slow phase separation. The pores seemed to grow very much at the late stage by coarsening which was observed to occur mainly by coalescence of polymer-lean droplets. As the relative humidity was lower, the coarsening continues longer ending up to a larger droplet size. The coarsening seems to enhance the interconnectivity of pores when the polymer concentration was low enough.

Phase behavior and mechanism of membrane formation for polyimide/DMSO/water system

Abstract A macrovoid-free, sponge-like porous membrane was prepared from polyimide/DMSO/water system, whereas a finger-like membrane was obtained from a polyimide/NMP/water system by phase inversion. The sponge-like membrane structure was investigated by the thermodynamics and kinetics of the phase separation process. The rates of phase separation for both systems were similar, but their phase diagrams were significantly different. The distinct features of the phase diagram for the polyimide/DMSO/water system are: (1) extremely narrow miscibility gap, (2) close gelation point to the binodal curve, and (3) almost parallel tie-line passing through the gelation point to the polymer–solvent axis. These properties hinder the phase separation process to proceed further, i.e. the growth of the polymer-lean phase is, thus frozen or stopped in the early stages. Therefore, the membrane morphology will be determined at an earlier stage in the polyimide/DMSO/water system than polyimide/NMP/water, resulting in a macrovoid-free, sponge-like membrane.

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.

Effect of poly(amic acid) imidization on solution characteristics and membrane morphology

Abstract The imidization effect of a poly(amic acid) dope solution on membrane formation has been investigated. Poly(amic acid) solution in N -methyl-2-pyrrolidione has been thermally imidized at 120°C at different curing times. The degree of imidization was determined by infrared spectroscopy. The solution properties have been studied as a function of concentration and curing time by dynamic light scattering. The quality of a solvent changed from good to poor with increasing imidization of poly(amic acid). The reduced polymer–solvent interaction diminishes the membrane formation time. The morphology of a membrane was able to be controlled by the characteristics of the dope solution.

Morphology control of asymmetric membranes by UV irradiation on polyimide dope solution

Abstract The membrane morphology was readily controlled by the gelation of dope solution before immersion into a coagulation medium. In particular, macrovoid-free, sponge-like integrally skinned asymmetric polyimide membranes with improved selectivity were obtained by the chemical gelation of cast solution film. Cross-linking was performed by ultraviolet irradiation for benzophenone containing polyimide dope solution. Cross-linking maintained the structure of cast solution film until the moment of immersion into a nonsolvent bath. Macrovoid decreases with irradiation time on the cast solution film. Sponge-like integrally skinned asymmetric membranes were obtained with sufficient irradiation. Gas permselectivity increased significantly with the progress of cross-linking on the cast solution film. Membrane morphology and subsequent transport properties can be controlled both by the extent of cross-linking and by the concentration of dope solution.

Light scattering and membrane formation studies on polysulfone solutions in NMP and in mixed solvents of NMP and ethyl acetate

The relationship between the characteristics of the polymer dope solution and the skin formation mechanism as well as the performance of the asymmetric membrane has been investigated. The solution characteristics have been studied on the polysulfone (PSf) dope solution as a function of the concentrations of both polymer and the cosolvent, ethyl acetate (EA), by dynamic light scattering. An anomalous light scattering was observed at small angles in both PSf/NMP and PSf/NMP:EA (6:4 by weight) solutions, indicating the inhomogeneity of the dope solutions. In the case of the PSf/NMP:EA (6:4) solution, an integrally skinned asymmetric membrane without defects having high gas selectivity was obtained while the membrane from the PSf/NMP solution had a defective skin. The scattered light intensity of the dope solution of PSf/NMP:EA (6:4) increased with the aging time while no notable change was observed in the PSf/NMP solution. The characteristics of the solution affect the final morphology of the membrane, particularly when phase separation occurred significantly before the immersion into the gelation medium.

The effect of dope solution characteristics on the membrane morphology and gas transport properties : PES/γ-BL/NMP system

The effect of heterogeneity of a dope polymer solution on membrane morphology has been investigated. Heterogeneity was developed in polyethersulfone solutions of either γ-butyrolactone (γ-BL) or a mixed solvent of γ-BL/N-methyl-2-pyrrolidone, which exhibit gelation and/or phase separation with aging time. Two different processes for membrane preparation have been employed; Process I, heterogeneity-developed dope solution was cast on a glass plate with a knife for uniform thickness, and immediately immersed into a nonsolvent form of the membrane. Process II, a dope solution was cast on a glass plate and then aged in a closed environment. After a certain aging time, it was immersed into a nonsolvent to be coagulated. The developed heterogeneity was destroyed markedly during casting in Process I, whereas the heterogeneity was maintained until the moment of immersion into a coagulation medium in Process II. Membranes prepared by Process I were finger-like regardless of the aging time, whereas Process II yielded macrovoid-free and sponge-like membranes having enhanced gas permeance and mechanical strength at long aging times. From these results, it was concluded that the morphology and transport properties of a membrane were able to be controlled by the solution structure, in particular, the solution heterogeneity in the cast film.

Estimation of penetrant transport properties through fixed site carrier membranes using the RC circuit model and sensitivity analysis

Abstract Gas permeability through fixed site carrier membranes (FSCMs) is predicted by the RC circuit models. If only one permeability value is available, other permeability values can readily be estimated as a function of applied pressure for a FSCM with given backward reaction rate and reaction equilibrium constants between carrier and penetrant, carrier concentration, and matrix permeability. The results were compared with experimental oxygen permeabilities through a PMMA membrane containing metallophorphyrin. The agreement between them was exceptional. In addition, the backward reaction rate constant between carrier and penetrant and the pressure fluctuation are found to be most sensitive in determining permeability in FSCMs according to the sensitivity analysis.

Effects of the nitrile group substitution on the gas separation properties of aromatic polyamide membranes

The effects of nitrile group substitution onto aromatic polyamide backbone on the gas permeability and permselectivity of the polymers are examined. The gas permeability of aromatic polyamides increase with increasing the content of nitrile group substitution, whereas the permselectivity decreases with increasing the nitrile group contents. The effects of chain linrearity on the permeability and permselectivity are also examined. The non-linearity of the polymers increases the permeability. These behaviors are interpreted in terms of chain packing and crystallinity of the aromatic polyamides.