No abstract

Dense sodium alginate (SA) membranes crosslinked with glutaraldehyde (GA) have been prepared by the solution method, wherein a nonsolvent of SA (acetone) was used in a reaction solution instead of an aqueous salt solution. Through infrared radation, X-ray diffractometry, and the swelling measurement, the crosslinking reaction between the hydroxyl groups of SA and the aldehyde groups of GA was characterized. To investigate the selective sorption behavior of the crosslinked SA membranes, swelling measurements of the membranes in ethanol-water mixtures of 70-90 wt % ethanol contents were conducted by equipment that was able to measure precisely the concentration and amount of the liquid absorbed in the membranes. It was observed that the crosslinking could reduce both the solubility of water in the resulting membrane and the permselectivity of the membrane toward water at the expense of membrane stability against water. The pervaporation separation of a ethanol-water mixture was conducted with the membranes prepared at different GA contents in the reaction solution. When the membrane was prepared at a higher GA content, both flux and separation factor to water were found to be reduced, thus resulting from the more crosslinking structure in it. The pervaporation separations of ethanol-water mixtures were also performed at different feed compositions and temperatures ranging from 40 to 80ЊC. A decline in the pervaporative performance was observed due to the relaxation of polymeric chains taking place during pervaporation, depending on operating temperature and feed composition. The relaxational phenomena were also elucidated through an analysis on experimental data of the membrane performance measured by repeating the operation in the given temperature range.

For the purposes of the water-selective membrane material development for pervaporation separation, we crosslinked poly(vinyl alcohol) (PVA) with sulfursuccinic acid (SSA), which contains {SO 3 OH, by heat treatment and investigated the effect of the crosslinking density on the separation of water-alcohol mixtures by pervaporation technique. The crosslinking reaction between PVA and SSA was characterized through Fourier transform infrared spectroscopy and differential scanning calorimetry tests by varying the amount of the crosslinking agent, the reaction temperature, and the swelling measurements of each pure component. The separation performance of the water-methanol mixture is not good due to the existence of sulfonic acid, hydrophilic group, in the crosslinking agent. However, for the water-ethanol mixture, the flux of 0.291 kg/m 2 h and the separation factor of 171 were obtained at 70ЊC when PVAcrosslinked membrane containing 7 wt % SSA was used. The same membrane also showed flux of 0.206 kg/m 2 h and a separation factor of 1969 at the same operating temperature.

By blending a rigid polymer, sodium alginate ( SA ) , and a flexible polymer, poly ( vinyl alcohol ) ( PVA ) , SA / PVA blend membranes were prepared for the pervaporation separation of ethanol -water mixtures. The rigid SA membrane showed a serious decline in flux and a increase in separation factor due to the relaxation of polymeric chains, whereas the flexible PVA membrane kept consistent membrane performance during pervaporation. Compared with the nascent SA membrane, all of the blend membranes prepared could have an enhanced membrane mobility by which the relaxation during pervaporation operation could be reduced. From the pervaporation separation of the ethanol -water mixtures along with the temperature range of 50 -80ЊC, the effects of operating temperature and PVA content in membrane were investigated on membrane performance, as well as the extent of the relaxation. The morphology of the blend membrane was observed with PVA content by a scanning electron microscopy. The relaxational phenomena during pervaporation were also elucidated through an analysis on experimental data of membrane performance measured by repeating the operation in the given temperature range. SA / PVA blend membrane with 10 wt % of PVA content was crosslinked with glutaraldehyde to enhance membrane stability in water, and the result of pervaporation separation of an ethanol -water mixture through the membrane was discussed. ᭧ 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67: 949 -959, 1998

SYNOPSISThe pervaporation separation of a concentrated ethanol-water mixture with 90 wt % of ethanol content through a sodium alginate (SA) membrane was carried out to investigate both the relaxation process and permeation behavior of the membrane in pervaporation. From the swelling measurement of the membrane in 90 wt % of ethanol content of the aqueous solution, it was observed that SA has an excellent permselectivity toward water and a high solubility of water: about 1000 of sorption selectivity and 21 wt % of water content in the swollen membrane at 40°C. Its excellent sorption properties could result in the outstanding pervaporation performance for the aqueous solution; higher than 10,000 of separation factors and 120-290 g/(m2 h) of fluxes, depending on the operating temperature. However, a serious flux decline took place with operating time due to the occurrence of a significant relaxation process in SA. The flux decline was remarkable a t the beginning stage and then mitigated with operating time. The fluxes were reduced by about 40-50% because of the relaxation process during the measurement. To analyze these phenomena in more detail, the hysteresis behavior of membrane performance along with an operation temperature cycle was investigated. During the heating process in the cycle, the relaxation process was important to affect membrane performance while the formation of excess stress in the membrane might be a crucial factor during the cooling process. It was postulated from the experimental observations that in the heating process an increase in flux with temperature is restricted by a more rapid relaxation process at a higher operation temperature, while in the cooling process, the excess stresses in the membrane are formed and accumulated in and near the transition zone and near the area in the membrane because of slower relaxational consolidation in unequilibrium cooling than in relaxational dilation in the heating process, resulting in flux more sensitive to temperature than in the heating process. These phenomena were found diminished as the relaxation time increased with aging time. The relaxation phenomena were discussed through an analysis on permeation activation energy data obtained from the pervaporation experiments. A qualitative model was established to describe the relaxational phenomena in the membrane material during the pervaporation process by using the basic principles of polymeric relaxation and the experimental observations obtained in this study.