| dc.description.abstract | In addition to sharp molecular weight cut and high flux an ideal ultrafiltration (UF) membrane requires high mechanical strength, good chemical resistance as well as anti-fouling characteristics. Poly(vinylidene fluroride) (PVDF) is often chosen to prepare UF membranes owing to its good mechanical properties and excellent chemical resistance. To retain the merits of PVDF UF membrane properties, we try to graft antifouling poly sulfobetaine onto the surface of the PVDF UF membrane. The zwitterionic sulfobetaine methacrylate (SBMA) was grafted on the surface of PVDF membrane via ozone surface activation and surface-initiated atom transfer radical polymerization (ATRP). The steady adsorption of bovine serum albumin (BSA) and γ-globulin were performed to test the antifouling character after SBMA grafting. Hardly any albumin adsorption was found as the grafting density exceeded 0.4 mg/cm2 of polySBMA. The adsorption of?γ-globulin was also greatly reduced. To investigate whether the method, ozone surface activation plus ATRP, was able to graft SBMA inside the pores of the membrane cyclic filtration tests were performed and the UF membrane of wider pore size was used. The cyclic filtration test for BSA yielded an extremely low irreversible membrane fouling ratio (Rir) of 13% in the first cycle and apparently no irreversible fouling was found in the second cycle. A more stringent test is carried out by passing the γ-globulin solution. It was found that the virgin PVDF membrane was continuously fouled by γ-globulin after 3 cyclic operations, but the polySBMA modified membrane had a Rir value as low as 4.7% in the third cycle. The results indicated that the surface modification via ozone surface activation and ATRP could actually penetrate into the pores of an ultrafiltration membrane. The polySBMA grafted PVDF membrane could effectively resist plasma protein adsorption and exhibited an extremely low biofouling characteristic during filtration.
Four nanofiltration membranes, two negatively and two positively charged, were fabricated by interfacial polymerization. Three different amines, ethylendiamine (EDA), diethylentriamine (DETA), and hyperbranched polyethylenimine (PEI) were selected to react with two acyl chlorides, trimesoyl chloride (TMC) and terephthaloyl chloride. The two membranes containing hyperbranched PEI, PEI/TPC and PEI/TMC, are positively charged at the operational pH. But the other two membranes, EDA/TMC and DETA/TMC, are negatively charged. It is found that the two PEI membranes own special rejection characters during nanofiltration. The PEI/TPC membrane has a similar pore size to the EDA/TMC membrane but owns simultaneously the higher salt rejection and permeation flux. The PEI/TMC has a pore size as big as 1.5 nm and still has a higher NaCl rejection than the EDA/TMC membrane of which the pore size is as small as 0.43 nm. We consider that the special rejection characters are derived from the special structure of PEI. The hyperbranched structure allows some of the charged amine groups drifting inside the pores and interacting with the ions in the pathway. The drifting amines increase salt rejection but have little effect on water permeation. It imply that a high flux and high rejection membrane for desalting can be obtained by attaching freely rotating charged groups.
Membrane fouling is a fatal problem in membrane filtration operation. Biofouling is especially notorious. In order to reduce the biofouling on a nanofiltration membrane, we tried to create a hydrophilic surface fixed with balanced positive and negative charges. Nanofiltration membranes were fabricated by interfacial polymerization of Trimesic acid Trichloride (TMC) and Diethylenetriamine (DETA). The surfaces were then modified via N-alkylation of the secondary amine with iodopropionic acid (IPA). The resulting tertiary amines were further quaternized by iodomethane. Negatively charged Bovine Serum Albumin (BSA) and positively charged lysozyme (LYS) were used to test the protein fouling probability. The membranes of various degree of N-alkylation or quarternation exhibited different levels of protein adsorption. The DETA/TMC nanofiltration membrane adsorbed moderate amount of both BSA and LYS. After reacting with iodopropionic acid the BSA adsorption greatly decreased but the adsorption of LYS raised. After quarternization the membrane moderately modified by iodopropionic acid adsorbed little LYS but large amount of BSA. Only the membrane highly modified by IPA and moderately quarternized by iodomethane exhibits excellent resistant against both positively and negatively charged proteins. The protein resistant neutral membrane(QDETA-IPA25%) also showed reduced adsorption of E. coli and S. epidermidis. The results indicated the importance of charge balance on the membrane surface in view of protein fouling and bacteria adhesion.
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