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Novel Cross-linkable Polymeric Membranes for Organophilic Ultra- and Nanofiltration
Membrane technology is being applied in more and more processes involving organic solvent, examples are re-use of catalytic species, purification of synthesized products, edible oil processing, solvent recovery and solvent exchange. To meet the requirements of these applications, the membranes are required to have the following properties: they should have the desired selectivity combined with a high permeability, as well as good stability in the organic solvents and under the required conditions of temperature and pressure.
Two novel copolymers that can be applied to prepare solvent resistant nano-/ultrafiltration (SRNF/UF) membranes have been developed: block copolymers of acrylonitrile (AN) and polyethylene glycol (PEG), this block copolymer can be used either directly to prepare ultrafiltration membranes that are resistant to many kinds of organic solvents, or cross-linked via post-treatment and converted to nanofiltration (NF) membranes and they are stable even in extremely aggressive organic solvents; copolymers of AN and 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS), which can directly form nanofiltration membranes for aqueous conditions, it is also possible to cross-link the membranes via post-treatment thus applicable in non-aqueous conditions or to improve the rejection of salts.
For the synthesis of block copolymers of PAN and PEG, it was unconventional free radical copolymerization initiated by cerium(IV)-PEG redox system. The reaction conditions have been optimized. The composition of the copolymers was characterized by attenuated total reflection infrared spectroscopy (ATR-IR), 1H-NMR, element analysis, and the molecular weight information was studied by gel permeation chromatography (GPC). The free radical concentration in the initiation step was studied with the help of stable free radical. From the compositions of the copolymers and the experiments on the free radicals, the proposed mechanism of the reaction was proved to be true. Furthermore, because of the mechanism of the polymerization, the copolymers were exclusively block copolymers. Membranes from the block copolymers were prepared via non-solvent induced phase separation. Because of low solubility of the block copolymers in dimethylformamide (DMF), dimethyl sulfoxide (DMSO) or N-methylpyrrolidone (NMP), it was not possible to prepare polymer solutions with concentration higher than 16%, therefore, only ultrafiltration (UF) membranes were formed. From the research on the membrane performance and pore morphologies as well, the concentration had strong influence on the properties of the resulting membranes. Variation of other parameters in the preparation steps was made to find out the optimized conditions for membranes and also to derive membranes with distinct separation performances. The membranes were also tested in several normal organic solvents, and the results showed that the membranes were stable in these solvents; therefore they are applicable in some SRUF processes. After cross-linking as post-treatment, the membranes were stable in almost all the organic solvents including DMF, NMP and so on, so application of the membranes can be greatly broadened. With another post-treatment to convert the membranes to NF types, the membranes became largely densified, the finally resulting membranes were endowed with excellent pure solvents permeability, solvent resistance and good rejection of polymers in both aqueous and organic conditions. Besides, the membranes were also capable to reject salts in aqueous solutions at an acceptable level. It was also possible to adjust the separation properties of the membranes by changing the conversion conditions, therefore, the membranes can be used in most SRNF applications as well as in aqueous NF applications. For the copolymers of AN and AMPS, they were synthesized via typical free radical polymerization initiated by azobisisobutyronitrile at elevated temperature. The reaction conditions have been optimized. The composition of the copolymers was characterized by ATR-IR, 1H-NMR, element analysis, and the molecular weight information was studied by gel permeation chromatography (GPC). Reactivity ratio of these two monomers were studied in real condition synthesis, and found to be similar to each other and close to 1. Therefore, copolymers tended to be random copolymers. Because of the similar reactivity ratios, it was possible to change the compositions of the copolymers freely, thus to influence the separation properties. It was not possible to prepare polymer solutions with concentration higher than 6%, but the resulting membranes were rather dense even from lower concentration. From the filtration experiments, it was found that all the membranes were dense as well in application. The composition of copolymers also had clear influence on the separation performance and influence of other parameters during the preparation step was also studied to get the optimal conditions for that. The resulting membranes were endowed with both high water permeability and high rejection of PEGs, but the rejection of salts was relatively low, which was probably because of the low charge density in the membranes, since the charge was provided by sulfonic acid groups in AMPS, and the mole ratio of AMPS in the copolymers was low. In order to improve the salt rejection, content of AMPS have to be increased, and then a cross-linking process would be needed, otherwise the copolymers were water soluble. Therefore, this part of work needs and is worth further research.
This study provides a novel method to synthesize SRNF/UF membranes, and also develops a new type of NF membrane for aqueous condition using conventional method.