| Pervaporation membrane surface plays an important role in its separation properties. By using a certain kind of chemical or physical modification methods, one can find it is easy to build a special structure on the membrane surface for the sake of improving its pervaporation performance. When adding surface modifier into polymer solution, certain components or functional groups would enrich at the obtained polymeric membrane surface because of environmental response, which has the potential application in the surface modification of polymeric membranes, especially the organic-permselective membranes. However, a hydrophobic conformation would be created on the surface of water-permselective membrane when such surface modifier is added into the polymer solution, which is unfavorable for improving its water selectivity. Therefore, it is of great importance to explore the contribution of membrane surface structure to its pervaporation performance when the effect of adding polymer additive on the surface structure of water-permselective membranes which are in the course of curing or in pervaporation separation is clearly understood.For this purpose, we added poly methylacrylic acid (PMAA) into the poly( vinyl alcohol)(PVA) solution, studied the change in surface structure as PMAA used as polymer additive. At the same time, the molecular conformation or functional groups arrangement on polymer solution and on membrane surface were detected by using sum frequency generation (SFG) bibrational spectroscopy, a very powerful tool to study surfaces and interfaces in situ at the molecular level with submonolayer sensitivity. And then, the pervaporation performance through PMAA modified PVA membrane for the separation of acetic acid/water, ethanol/water mixtures was investigated in detail. The results are given as follows:1) When a small amount of PMAA was added to the PVA solution, the PMAA would enrich at the air-side surface of PVA membrane. SFG indicated that methyl groups are regularly arranged on the membrane surface. This surface conformation will turn into a hydrophilic one when the membranes are treated in a polar environment. On the other hand, carboxyl groups would occupy the glass-side surface due to the minimization of interface free energy, accordingly improving the hydrophilic properties.2) The enrichment of PMAA at the air-side and glass-side surface of the PVA membrane would attain a steady state when a certain amount of PMAA is introduced. The enrichment on glass side levels off when PMAA content is at 2wt%, while at the air side, PMAA content is as highly as 4wt %.3) When the surface modified PVA membranes were employed to separate 90wt% acetic acid/water mixtures at 40℃, the water content in permeate is about 81wt%, higher than that (73wt%) of the unmodified PVA membrane, while the flux seems unchangeable. The results may be due to the fact that the PMAA molecules near the surface tend to reconstruct themselves on membrane surface, forming a hydrophilic conformation when the membrane is in contact with the feed liquid, accordingly increasing membrane separation performance.4) When PMAA was added in PVA (a alcoholysis of 88%) solution, the surface structure and property of the resultant membranes was changed. For the separation of 95wt% ethanol/water mixtures (azeotropic mixtures), the surface-modified PVA membrane shows a higher flux of 100g/m2·h, while its water content in permeate is still attained to above 87wt%. When separating other organic water mixture such as isopropanol/water and acetone/water mixtures, the membrane displays a desirable pervaporation performance when comparing other membranes reported in related literatures. |
PDF Full Text Request