Study On Organic-Inorganic Composite Membranes Based On Poly(Vinylidene Fluoride)

Poly(vinylidene fluoride)(PVDF) is the most extensively used materials for ultrafiltration and micro filtration membranes because of their sufficient chemical stability as well as fine mechanical strength. However, their hydrophobic nature has hindered the use of the membrane in water treatment as they tend to be fouled and hence show rapid reduction in permeability. Much attention has been paid to hydrophilic organic-inorganic composite membranes which are often fabricated through blending polymers with inorganic nanoparticles. But these inorganic nanoparticles are easy to agglomerate, resulting in non-uniform distribution in the membranes because of their high specific surface area and high surface free energy. Furthermore, the inorganic nanoparticles are easy to leach out during long term use, which results from the instability of the interface of inorganic particles and hydrophobic polymers. In this thesis, organic-inorganic composite membranes with high surface hydrophilicity were fabricated by introducing inorganic particles that was grafted with polymers or by biomineralization. The surface modification and biomineralization processes were explored in detail. Our specific studies are concentrated on the following aspects:PVDF ultrafiltration membranes were prepared by immersion precipitation method using poly(hydroxyethyl methacrylate)-block-poly(methyl methacrylate) grafted silica (PHEMA-b-PMMA@SiO2) nanoparticles as hydrophilic additives. The hybrid nanoparticles were synthesized by the surface initiated atom transfer radical polymerization (SI-ATRP). After blending polymer with the hybrid silica nanoparticles, these hybrid nanoparticles influence the structure and performance of the composite membranes obviously. Nanoparticles with appropriate molecular weight of polymer brushes increase the pure water flux, improve the BSA rejection to a high level (>90%), and reduce the membrane fouling at the same time.Organic-inorganic composite membranes were prepared via calcium carbonate (CaCO3) mineralization induced by PVDF/poly(acrylic acid)(PAA) blend membranes. PAA was used as a polyanionic macromolecule in the blend membranes to generate CaCO3particles by an alternate soaking process (ASP). The mineralization condition was optimized based on the concentrations of calcium chloride (CaCl2) and sodium carbonate (Na2CO3) solutions used for ASP, the number of ASP cycles, and the PAA content in the blend membranes. Structures and surface hydrophilicity of the composite membranes were characterized in detail by FTIR-ATR, FESEM, EDX, XRD and water contact angle. Results confirm that CaCO3particles consisting of calcite and vaterite dispersed uniformly in/on the membranes. The membrane hydrophilicity increased dramatically due to the intrinsic wettability of these CaCO3particles. In addition, pure water fluxes of the membranes were improved about three times. Furthermore, the mineralized membranes even showed a high rejection (99.85%) of Congo red with long-term stability, which makes them potential in dye-polluted wastewater treatment.PVDF/PAA/CaCO3composite membranes were prepared by in-situ mineralization method. The mineralization condition was optimized based on CaCl2content in the casting solution and carbonate source selection. Structures and surface hydrophilicity were characterized in detail by FESEM, FT-IR/ATR, EDX, XRD and water contact angle for the prepared composite membranes. Results confirm that CaCO3particles consisting of calcite and vaterite disperse uniformly on the membrane surface and in the membrane cross-section, and the membrane hydrophilicty increases with the increase of CaCl2content in the casting solution. The pure water fluxes of the membranes were improved about ten times. Due to the electrostatic repulsion between the positively charges lysozyme and CaCO3particles, the mineralized membranes possess a high rejection (85.13%) and flux recovery ratio (77.21%), indicating good antifouling property.