Geeta Chowdhury

Low pressure reverse osmosis performances of sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) thin film composite membranes: effect of coating conditions and molecular weight of polymer

Thin film composite (TFC) membranes with sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) (SPPO) as the top layer coated on ultrafiltration (UF) membranes were prepared under different preparation conditions including the polymer concentration, solvent used in the coating solution, drying temperature, and the polymer molecular weight. Membrane performances for nanofiltration that separated electrolytes such as MgSO4 and NaCl from aqueous solution were investigated using the composite membranes thus prepared. The concentration of SPPO both in H+-form and in Na+-form showed different tendencies for MgSO4 separation while it did not affect the separation of NaCl. Membrane permeation rate increased on increasing the molecular weight of the glycol ethers used as the solvent without sacrificing separation. TFC membranes prepared from high molecular weight SPPO exhibited better performances as compared to those prepared from lower molecular weight SPPO.

Development of poly(vinylidene fluoride) hollow-fiber membranes for the treatment of water/organic vapor mixtures

Attempts were made to spin hollow-fiber membranes from poly(vinylidene fluoride) (PVDF) material by the dry–wet phase inversion method. Hollow fibers so prepared were characterized for various parameters and by electron microscopic techniques. Membranes were also tested for the separation of water/1-propanol mixtures in vapor phase. It was found that the hollow fibers were water selective despite the fact that PVDF material is hydrophobic. Intrinsically organic selective property of PVDF material was proved by coating a porous polyetherimide membrane with a PVDF layer, which resulted in enhancement of 1-propanol permeation while suppressing the permeation of water.

A study on the preparation of polyvinyl alcohol thin-film composite membranes and reverse osmosis testing

Thin-film composite membranes were prepared by coating porous polysulfone membranes with a polyvinyl alcohol layer and further cross-linking its surface. Aldehydes, dialdehydes and malic acids were used as cross-linking agents. The effects of additives to the PVA solution, of solvents used for making PVA solutions, and of alcohols used for the membrane post-treatment on the reverse osmosis performance were investigated. The effect of PVA solution coating, drying and heat-treatment on the water permeability was also studied without cross-linking. It was found that heat-treatment had the strongest effect on the reduction of the water permeability. Hence, it was attempted to use cross-linking agents that could react at room temperature. Attempts were also made to carry out the cross-linking reaction at the cross-linking solution/PVA layer interface by dissolving cross-linking agents in hydrophobic solvents.

Characterization of membranes prepared from PPO by Raman scattering and atomic force microscopy

Abstract The surface structures of dense (homogeneous) and asymmetric (integrally skinned) membranes made from poly(2,6-di-methyl-1,4-phenylene oxide) (PPO) in chloroform and in 1,1,2-trichloroethylene were investigated by Raman spectroscopy and by tapping mode atomic force microscopy (TM AFM). Results revealed by Raman spectroscopy indicated that the “state of the polymer” in the polymer powder and in the membranes prepared from PPO by using different solvents was not identical. A significant perturbation in the Raman scattering was observed in the asymmetric membrane prepared from PPO by using CHCl3 (PPO-CHCl3). A difference in the morphology of the surfaces (top and bottom) was also observed by TM AFM. It is believed that faster evaporation rates resulting from the use of a more volatile solvent, results in preserving more of the polymer structure present in the solution. Therefore, nodules produced from polymer dissolved in more volatile solvents might contain more free volume entrapped inside nodules, which results in larger dimensions of nodules compared to those produced from polymer dissolved in a less volatile solvent. This was confirmed by the larger dimensions of nodules and the higher permeation rate of CO2 in the membranes prepared from PPO-CHCl3 solution compared to those prepared from PPO-TCE solution.

Surface morphology of homogeneous and asymmetric membranes made from poly(phenylene oxide) by tapping mode atomic force microscope

Surface morphology of asymmetric and homogeneous membranes prepared from poly(phenylene oxide) (PPO) was studied by tapping mode atomic force microscopy (TM AFM). As expected, a significant difference in the morphology between the top and the bottom surfaces of the asymmetric membrane was observed. The images of the top surface revealed a small variation in the vertical direction (6.7 nm), compared to the mean diameter of nodules (62 nm), while the images of the bottom surface were very porous (microfiltration structure). On the other hand, the observed difference in morphology between the top and the bottom surfaces of the membrane prepared by the complete evaporation of the solvent (homogeneous membrane) was rather unexpected. The nodules on the bottom surface were twice as large as those on the top surface. These studies also revealed some differences in the morphology of the top surface of asymmetric and homogeneous membranes. Both surfaces were made up of nodules having a similar size (62-64 nm) ; however, roughness parameters calculated for the top surface of the asymmetric membrane were approximately two times greater than those for the top surface of the homogeneous membrane.

Sulfonated polyphenylene oxide–polyethersulfone thin-film composite membranes: Effect of counterions on the gas transport properties

Abstract Thin film composite membranes based on sulfonated poly(phenylene oxide) (SPPO) as the top selective layer and PES ultrafiltration membrane as the support layer, were prepared. The effect of the support membrane pore size distribution, and the effect of exchanging the proton of the sulfonic acid groups with mono-, di- and trivalent cations on the gas transport properties of the TFC membranes were studied. It was found that the resistance to gas permeation imposed by the PES support layer increased significantly upon exposure to the solvent in the coating solution. The differences in the performance of the TFC membranes having different metal cation replacing the proton of the sulfonic acid groups were explained in terms of the metal cations’ electronegativities and sizes. Very high selectivity associated with reasonable permeance was obtained when the SPPO was in the Al 3+ -form.

Characterization of the PPO dense membrane prepared at different temperatures by ESR, atomic force microscope and gas permeation

Abstract Dense (homogeneous) membranes were prepared from poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) by using 1,1,2-trichloroethylene as a solvent at different solvent evaporation temperatures (22,4 and −10°C). The effect of temperature used during evaporation of solvent on the characteristics of the membrane was studied by using electron spin resonance, atomic force microscopy and gas permeation rate. The morphology of the surfaces of the membrane, the shape of spin probe in the membrane, and the selectivity of gases depend on the temperature of evaporation of solvent. The permeation rate of CO2 increased with the decrease in the temperature used for the preparation of the membrane. However methane permeation rate increased in the membrane prepared at −10°C. It is suggested that Langmuir sites could be favorable for the CH4 permeation.

Effects of polymer molecular weight and chemical modification on the gas transport properties of poly(2,6-dimethyl-1,4-phenylene oxide)

Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) of different intrinsic viscosities has been studied to understand the effect of polymer molecular weight on the permeability and permeability ratio of CO2/CH4 and O2/N2 gas pairs. The increase in permeability of dense films prepared from higher molecular weight PPO was explained in terms of increased free volume. Gas permeability for the high molecular weight was further improved by attaching bulky bromine groups to the phenyl ring of the PPO backbone. Permeability ratio of PPO was greatly improved by attaching polar groups such as —COOH or —SO3H. The loss in permeability because of the presence of the polar groups was compensated by using PPO that was brominated and sulfonated.

Sulphonated poly(2,6-dimethyl-1,4-phenylene oxide)-polyethersulphone composite membranes. Effects of composition of solvent system, used for preparing casting solution, on membrane-surface structure and reverse-osmosis performance

Abstract A further investigation on the effect of solvent-system composition on microscopic structure and reverse-osmosis performance of sulphonated poly(2,6-dimethyl-1,4-phenylene oxide) (SPPO)-polyethersulphone (PES) composite membranes is presented. Charged composite membranes are prepared by coating PES substrate ultrafiltration membranes with dilute solutions of hydrogen-form SPPO (SPPOH) having an ion-exchange capacity of 1.93 meq g−1. Methanol and methanol-chloroform mixtures containing 18, 42 and 66 mass% chloroform are used as solvents for making 1.0 mass% SPPOH coating solutions. Reverse-osmosis performance of the composite membranes is investigated by measuring the membrane permeation rates and rejection for various electrolyte solutions. The effect of the solvent, used in making the coating solution, is also studied through intrinsic-viscosity measurements. The microscopic structure of the SPPOH-PES composite membranes is explored by employing a scanning electron microscope (SEM) and an atomic force microscope operated in the tapping mode (TM AFM). The reverse-osmosis performance is explained in terms of the observed TM AFM skin-layer topographs, which are in turn correlated with the intrinsic-viscosity measurements.

Effect of metal cations on the gas separation performance of sulfonated poly (phenylene oxide) membranes

The polymer polarity and density of sulfonated poly (phenylene oxide) (SPPO) membranes were manipulated by exchanging the proton of the sulfonic groups with mono-, di-, and tri-valent metal cations. The performance of the TFC membranes coated with ion-exchanged SPPO, toward the separation of CO2/CH4 and O2/N2 gas systems, showed to correlate with the location of the metal in the universal periodic table of the chemical elements. Altering the metal cation that replaces the proton of the sulfonic groups has altered: the cross-linking forces, the hindrance effects due to the voluminous metal size, and the SO3-Metal groups polarity due to the change in the metal electronegativity.