Fabrication And Anti-oil-fouling Property Of Polymer Membranes With Amphiphilic/Special Wettability

Oily wastewater originated from oil-related industries and frequent oil spill accidents presents a major challenge for ecological environment and human health. Due to the high separation efficiency, low energy consumption and easy scale-up, pressure-driven membrane technology has been emerging as one of promising candidates for oily wastewater treatment. Ultrafiltration (UF) membrane has shown excellent advantage as an efficient method in the treatment of oil/water mixtures (especially oil/water emulsion) owing to its suitable pore size and the capability of removing emulsified oil droplets without any de-emulsification processes. However, oil droplets and other foultants commonly tend to deposit on the membrane surfaces and block the inner pores, leading to the membrane fouling associated with significant decline of permeation flux and selectivity of the membranes, which is recognized as the major bottleneck for the extensive application of membrane technologies. Thereby, the construction of antifouling polymer membranes with low/non fouling for effective oil/water separation has important theoretical significance and practical value.Fabrication of effective antifouling membrane materials is mainly relied on improving the surface hydrophilicity of membrane surfaces. The compact hydration layer barrier and repulsive force caused by hydrophilic components can dramatically reduce the nonspecific adsorption interactions between the foultants and membrane surfaces, thus render the membrane with excellent antifouling property. The purely hydrophilic modification could effectively inhibit the biologic and/or organic fouling caused by protein and microbe, however, it is difficult to effectively resist the intractable oil fouling. Compared to common organic foulants (BSA and polysaccharide), oil foulants with low surface tension and higher viscosity have more complicated fouling process. The oil droplets tend to coalesce, spread and migrate to form continuous fouling layer on the surface, leading to the significant permeation flux decay and reducing the persistent-service performance of membranes.In order to effectively control or minimize the polymer membrane fouling during the oil/water separation process, in this study, the polymer membrane materials with excellent anti-oil-fouling properties were fabricated based on two unique design strategies. On the one hand, on the basis of the common hydrophilization modification, the low surface free energy components (such as fluorinated polymers) with fouling-release property were simultaneously introduced onto the membrane surfaces. The low surface free energy microdmains can effectively weaken the interaction strengths of polar or hydrogen bonding between foulants and membrane surfaces, so as to endow the membranes with outstanding self-cleaning capacity. First, amphiphilic fluorinated copolymers containing various micro-sequence structures were prepared by RAFT controlled radical polymerization method. Subsequently, the amphiphilic polymer membrane surfaces with excellent anti-fouling properties were constructed via a facile coupled process of non-solvent induced phase separation (NIPS) and surface segregation. On the other hand, inspired by the anti-wetting behavior of oil droplets on fish scales and the mussel adhesion characteristics, highly hydrophilic and underwater superoleophobic PVDF membrane surfaces have been fabricated via the high hydrophilicity of chitosan and oxidation self-polymerization of dopamine. The surface compositions and micro-structures of the membranes constructed from the above two distinct strategies were elaborately manipulated, whose influences for membrane surface wettability, separation effeciency and antifouling performance were also investigated in detail, in order to achieve the highly efficient and stable application of the modified polymer membranes in oil/water separation.The main research work of this present work was described as follows: 1 The preparation of amphiphlic fluorinated gradient copolymers and their application for PES membrane modificationBased on the design aspect of "binary-synergy fouling resistance", the amphiphilic fluorinated gradient copolymers were prepared. Then the amphiphilic copolymers was used as modifiers and incorporated into the poly(ether sulfone) (PES) to fabricate PES blend membranes, leading to amphiphilic membrane surfaces composing mosaic hydrophilic and low surface energy components.Amphiphilic fluorinated gradient copolymer P(PEG-,grad-TFOA)m was prepared by reversible addition-fragmentation chain transfer (RAFT) method. The composition, sequence structure distribution, and glass transition temperature of these gradient copolymers were measured by FTIR,1H NMR, gel permeation chromatography (GPC), and DSC characterizations. The resultant amphiphilic copolymers were then incorporated into the Poly(ether sulfone) (PES) to fabricate amphiphilic PES blend membranes via the in situ blending modification method. SEM measurement showed that the PES blend membranes had the typical asymmetrical structure. AFM results confirmed that the surface roughness of investigated membranes revealed an increasing trend with the incorporation of amphiphilic fluorinated copolymers (increased from the 4.1nm for PES membrane to 12.8 nm for PES/G26.5-1.0 membrane). During the phase inversion process, both hydrophilic (PEGMA) and low surface energy (TFOA) segments significantly enriched on the membrane surface by free and forced surface segregation, respectively, to form an amphiphilic surface, which was demonstrated by X-ray photoelectron spectroscopy (XPS) measurement. The results of membrane surface static contact angles indicated that the oleophobicity property of modified membranes was significantly increased. According to the filtration experiments of oil-in-water emulsion, the heterogeneous membranes exhibited superior oil-fouling resistant properties. Especially PES/G26.5-1.0 membrane achieved the best anti-oil-fouling property:the total flux decay was as low as 15.4% and high flux recovery (99.5%).2 The preparation of amphiphlic fluorinated copolymers with different sequence structures and their application for PES membrane modificationBased on the blending modification of PES membrane by adding with amphiphilic fluorinated gradient copolymers, in this chapter, we focused on the effects of sequence structure of the modifiers on the surface segregation, membrane surface composition, surface wetting properties and anti-fouling performances.Three types of amphiphilic random, gradient, and block copolymers with similar molecular weights and chemical compositions were synthesized through batch/semibatch RAFT methods. The chemical structures, compositions, and glass transition temperature of these copolymers were characterized by FTIR,’H NMR, and DSC measurements. These resultant modifiers were then used to modify Poly(ether sulfone) (PES) membranes via a blending method. The effects of sequence structural differences of the copolymer modifiers on the surface chemical compositions, the surface properties and the antifouling capabilities of the membranes were investigated in detail. The results showed that the surface segregation capacities of all the three copolymers with different sequence structures during the phase inversion process is in the following order:P(PEG-co-TFOA)< P(PEG-grad-TFOA)< P(PEG-b-TFOA). In contrast, the amphiphilic block copolymer achieved the highest surface enrichment on the corresponding membrane surface. Accordingly, during the oil-in-water emulsion dynamic filtration test, the PES/Blo membrane displayed the best anti-oil-fouling performance:only 16.6% reversible flux-decline and nearly no irreversible flux-decline (0.2%), corresponding to a 16.8% total flux-decline and as high as 99.8% permeation flux recovery.3 The preparation of amphiphlic fluorinated copolymers with zwitterionic segments and their application for PES membrane modificationA novel amphiphilic copolymer poly(carboxyl betain methyl acrylamide-co-3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl acrylate) P(CBMA-co-TFOA) with zwitterionic and fluorinated moieties was synthesized by the simple two-step method. Subsequently, the synthesized copolymers were acted as additives and blended with poly(ether sulfone) (PES). An amphiphilic ultrafiltration membrane with excellent anti-oil-fouling property was obtained by the surface segregation process of amphiphilic segments.Amphiphilic precursor P(DMAPMA-co-TFOA) based on N-[3-(Dimethylamino) propyl] methacrylamide and 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl acrylate (TFOA) was firstly synthesized via a facile free radical polymerization. Then PDMAPMA chains were quaternized by reacting with bromoacetic acid (BAA), and thus the amphiphilic zwitterionic copolymers P(CBMA-co-TFOA) were obtained. The chemical structures, compositions, and molecular weight of these novel copolymers were characterized by FTIR,1H NMR, and GPC measurements. The FESEM measurement results revealed that all modification membranes exhibited a typical asymmetric structure. The amphiphilic copolymer additives (PI and P2) with relative higher content of hydrophilic zwitterionic segments showed pore-forming ability to some extent. The results of zeta potential measurement demonstrated that the zeta potential of amphiphilic PES blend membranes dramatically decreased, which is advantageous to the improvement of antifouling ability of PES blend membranes. The excellent oil-fouling resistant capacity of these modified membranes was demonstrated by oil/water emulsion separation tests. Especially the membrane PES/P3 with the optimized hydrophilic and hydrophobic ratio achieved the highest anti-oil-fouling property:the total flux decay was as low as 17.4%(nearly no irreversible flux-decline) and high flux recovery (99.3%) after simple water flushing.4.The fabrication of multifunctional PVDF membrane with anti-oil-fouling and the adsorption of water-soluble contaminants performancesInspired by the anti-wetting behavior of oil droplets on fish scales and the mussel adhesion characteristic, the PVDF membrane surfaces with underwater superoleophobic/ultralow oil-adhesion properties as well as the adsorption capacity of heavy metal ions and dyes have been fabricated via one co-deposition method of chitosan and dopamine.The mixed solutions containing specific amount of chitosan, dopamine, and tyrosinase were prepared and then the PVDF substrate was immersed into the above solution. Under the catalytic action of tyrosinase (at acidic condition), dopamine would undergo the oxidation self-polymerization, leading to the PDA layer that adhered tightly onto the PVDF membrane surface. Besides, the coating can be cross-linked through the Michael addition reaction between chitosan’s amino groups and dopaminoquinone, resulting in the improved mechanical properties of the chitosan coating. Compared to the modification of PVDF membrane with individual dopamine or chitosan, the PVDF membrane modified with both the dopamine and chitosan showed an increased surface hydrophilicity and underwater superoleophobicity, with static contact angle of 160.1°for 1,2-dichloroethane. Dynamic adhesion test proved that the modification membrane had a lower oil-adhesion force. PVDF/CS&DA membrane had a much higher pure water flux than that of PVDF membrane, meanwhile it also showed a dramatically increased anti-oil-fouling property, that is, the total flux decay was as low as 14.8% and flux recovery rate could almost fully recovered (99.8%) after simple water flushing. PVDF/CS&DA membrane could also effectively adsorb the water-soluble contaminants, such as, heavy metal ions and dyes. It is believed that the adsorption mechanisms for this multifunctional coating are derived from the chelation adsorption and hydrogen bonding interaction between the chitosan and dopamine with the heavy metal ions and dyes, respectively.