Novel PVDF Ultrafiltration Membranes with Tailor-made Zwitterionic Copolymer Additives

Adsorptive fouling and “trade-off” relationship between membrane permeability and selectivity largely limit the long-term application of polymer ultrafiltration (UF) membrane in industrial area, especially in the water treatment for clean drinking water. The work of this dissertation is a part of NANOPUR project, targeting preparation of UF membrane with high throughput, high selectivity and low fouling. A number of Polyvinylidenfluorid (PVDF) UF membranes containing synthesized zwitterionic additive PMMAm-co-PSPEn and PMMAm-b-PSPEn were prepared via nonsolvent induced phase separation (NIPS) in order to research their performance improvement on the fouling resistance as well as “trade-off” phenomenon. PMMA is polymethylmethacrylate. PSPE is poly (sulfobetaine methacrylate). Additive PMMAm-co-PSPEn was directly synthesized via free radical copolymerization of monomer MMA and SPE. Macroinitiator PMMA-Br with various molecular weights were synthesized via Atom transfer radical polymerization (ATRP). The molecular weights of PMMA-Br were designed as ~10 kg/mol, ~14 kg/mol, 30 kg/mol and ~53 kg/mol in order to examine the various effects arisen from different PMMA lengths. Then they were used to reinitiate the N, N-dimethylaminoethyl methacrylate (DMAEMA) to synthesize a number of PMMAm-b-PDMAEMAn with diverse molecular weights and various ratios of PMMA/PDMAEMA under different conditions for sequential ATRP. Finally after the post-treatment, PMMAm-b-PDMAEMAn were readily converted to zwitterionic PMMAm-b-PSPEn. The additive PMMAm-b-PSPEn with precise control of molecular weight as well as polydispersity plus tailor-made ratios of PMMA/PSPE can be achieved in this manner. By comparison of three UF PVDF membranes which were prepared by dope solutions containing 17.5 wt.% PVDF and respective 2 wt.% PMMAm-co-PSPEn, PMMAm-b-PSPEn and commercial Polyvinylpyrrolidon (PVP) via NIPS, zwitterionic PMMAm-co-PSPEn and PMMAm-b-PSPEn largely improved the bovine serum albumin (BSA) sieving performance as well as BSA fouling resistance for the corresponding PVDF membranes. However PMMAm-b-PSPEn remarkably reduced water permeability due to its strong facilitation effect on the precipitation rate of PVDF matrix polymer. A series of UF PVDF membranes were prepared by dope solutions possessing 16 wt.%PVDF and 0.5-1.5 wt.% PMMAm-b-PSPEn respectively with different molecular weights and various ratios of PMMA/PSPE via NIPS. These PVDF membranes showed distinct properties and performance from reference PVDF membranes with single PMMAm-b-PDMAEMAn or PVP. The additional PMMAm-b-PSPEn apparently also improved the BSA sieving performance of all the corresponding PVDF membranes. Some selected PVDF membranes also displayed much lower BSA fouling than reference membrane. The PVDF membrane containing PMMAm-b-PSPEn with higher ratios of PMMA/PSPE generally showed better fouling resistantce. Water permeability for all the PVDF membranes with PMMAm-b-PSPEn were generally still low. The higher precipitation rate of PVDF matrix resulted from PMMAm-b-PSPEn can be supported firmly. Another series of UF PVDF membranes were prepared with dope solutions containing 16 wt.% PVDF and combination of 1 wt.% PVP and 1-2 wt.% PMMAm-b-PSPEn with different molecular weights and various ratios of PMMA/PSPE via NIPS. The combination of PVP and PMMAm-b-PSPEn originated apparent synergy effect on the performance of corresponding PVDF membranes, the high water permeability and high BSA rejection being achieved at the same time. The “trade-off” phenomenon was overcome for all the PVDF UF membranes. The additional PVP properly offset part of the increased precipitation rate which brought by PMMAm-b-PSPEn so that a balanced water permeability and sieving performance was achieved. Compared with reference PVDF membranes with commercial PVP, selected PVDF membrane with combination of PVP and PMMAm-b-PSPEn also displayed stronger BSA fouling resistance and higher BSA sieving performance. One prepared UF PVDF membrane with combination of additives that showed performances meeting the target values of NANOPUR project was selected as prototype membrane. PVDF UF membranes which were prepared by dope solutions containing 16 wt.% PVDF and combination of PMMAm-b-PSPEn and respective PEG, glycol and PMMA via NIPS were also characterized and compared. The above combination of additives didn’t show satisfying effect on performance of corresponding PVDF membranes. Two UF PVDF membranes were prepared by dope solutions possessing 17.5 wt.% PVDF and 2 wt.% PMMAm-b-PDMAEMAn with different molecular weights and various ratios of PMMA/PDMAEMA via NIPS. Then through the surface modification the PDMAEMA groups exposing on PVDF membrane surface were converted to PSPE groups. The surface modification can be available but the complete conversion was difficult. These PVDF membranes after surface modification showed decreased water permeability but improved BSA sieving performance in contrast to ones before surface modification. Overall, synergy effect of PVP and PMMAm-b-PSPEn with high ratio of PMMA/PSPE exhibited strong potential to improve PVDF membrane performance. The largely enhanced fouling resistance to protein and properly controlled porosity can be readily donated to PVDF membranes via practical NIPS of the blending solution which involved PMMAm-b-PSPEn, PVP and PVDF.
Die adsorptive Verschmutzung und das "Trade-off"-Verhältnis zwischen der Permeabilität und der Selektivität begrenzen die Leistungsfähigkeit und die Nutzungsdauer von etablierten Polymermembranen in der Ultrafiltration, insbesondere bei der Wasseraufbereitung für sauberes Trinkwasser. Die Arbeit dieser Dissertation ist ein Teil des NANOPUR-Projekts und konzentriert sich auf die Herstellung von UF-Membranen mit hohem Durchsatz, hoher Selektivität und geringer Tendenz zur Verschmutzung. Eine Reihe von Membranen aus Polyvinylidenfluorid (PVDF), die im Rahmen der Dissertation neu synthetisierte zwitterionische polymere Additive wie statistische Copolymere (PMMAm-co-PSPEn) und Diblockcopolymere (PMMAm-b-PSPEn) enthielten, wurden über die Nichtlösungsmittel-induzierte Phasentrennung hergestellt (PMMA ist Polymethylmethacrylat, PSPE ist Poly(sulfobetainmethacrylat)). Zusammenhänge zwischen der Struktur der Additive und der Membranleistung einschließlich Verschmutzungsbeständigkeit wurden analysiert und eine Reihe von Membranen identifiziert, die im Vergleich zum "Trade-off" deutliche Verbesserungen der Trennleistung aufweisen.



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