Studies On The Preparation And Properties Of Al₂O₃/PU/PVDF Hybrid Membranes

Membrane separation is an economic, efficient and environmental-friendly technologywhich can be used for separation, concentration and purification. Polyvinylidene fluoride（PVDF） is one of the widely used membrane materials. However, the strong hydrophobicityof PVDF leads to the fouling of membranes, resulting in decreasing flux in the water filtrationprocess, which restricts its application in food industry. Moreover, the low mechanicalstrength of PVDF membranes is also a serious disadvantage for their widely applications. It isof great importance to increase the hydrophilicity and enhance mechanical properties ofPVDF membranes. To address these two issues, the blending of PVDF with some hydrophilicpolymers and the incorporation of some inorganic fillers into PVDF matrix seem to be verypromising solutions.Polyurethane （PU） is a kind of hydrophilic polymer which can be synthesized by thepolymerization of isocyanate monomer and polyether/polyester polyo monomer. Due to itsthree-dimensional network structure, thermosetting PU has strong hydrophilicity as well asexcellent mechanical properties. Inorganic nanomaterials such as SiO2, Al₂O₃, TiO2andzeolite particles exhibit excellent mechanical modulus and high strength. In this thesis,Al₂O₃/PU/PVDF hybrid membranes were prepared via a thermally induced phase separation（TIPS） method，in which the nano-Al₂O₃particles were synthesized by the sol-gel technique.The structures of the resulting membranes were characterized by atomic force microscipy（AFM）, differential scanning calorimetry （DSC）, Fourier transform infrared spectroscopy（FT-IR） and contact angle tests. The effects of the introduction of PU and nano-Al₂O₃inorganic filler on the morphology of the membranes were studied. The work includes fourparts as follows:1. Preparation and characterization of PU/PVDF blending MembranesPU/PVDF blending membranes were prepared by the TIPS method. The structures andproperties of the blending membranes were characterized by contact angle tests, FT-IR, AFMand DSC. Comparing with the direct addition of PU polymer or semi-finished PU product, theintroduction of PU by individually adding A and B monomers could achieve betterpermeation results. The PU/PVDF blending membrane obtained with16-18%of PVDF,50%of DMP and2%of PU show better hydrophilicity. FT-IR results indicated that new chemicalbonds were observed on the surface of the blending membranes. Comparing with pure PVDFmembrane, the PU/PVDF blending membrane exhibted slightly higher mechanical strength. AFM tests indicated that the surface roughness of the PU/PVDF blending membrane waslower than that of pure PVDF membrane. DSC tests showed that the crystallinity ofPU/PVDF blending membrane membrane decreased with the introduction of PU polymer.2. Preparation and characterization of Al₂O₃/PVDF hybrid membranesAl₂O₃/PVDF hybrid membranes were prepared via the in-situ hydrolysis of Aluminumiso-propoxide （AIP） in PVDF matrix by a sol-gel process. The effects of the introduction ofAl₂O₃inorganic filler on the properties of Al₂O₃/PVDF hybrid membranes were investigated.The results indicated that the properties of the resulting membranes were highly dependent onthe ratio of Al₂O₃, PVDF and that the Al₂O₃/PVDF hybrid membrane with12%of AIP and18%of PVDF had the best hydrophilicity and mechanical properties. FT-IR analysis showedthat the stress distribution of PVDF molecules were changed with the introduction of AIP intothe PVDF casting solution, thus can weaken the α-crystal shape absorption peak of PVDF andstrengthen the β-crystal shape absorption peak. A bonding between AIP and PVDF wasobeserved with the addition of AIP. AFM tests indicated that the surface roughness ofAl₂O₃/PVDF hybrid membranes decreased.3. Preparation of functionalized nano-Al₂O₃solsFunctionalized nano-Al₂O₃sols were prepared using AIP as a precusor by the sol-geltechnique. The effects of hydrolysis temperature, the ratios of n（H₂O）/n(Al³⁺) andn（H⁺）/n(Al³⁺), peptization agent, aging temperature and aging time on the particle sizes andviscosities of the sols were studied. A stable and transparent nano-Al₂O₃sols was obtained bythe synthetic conditions: hydrolysis temperature=80℃, n（H₂O）/n(Al³⁺)=105,n（H⁺）/n(Al³⁺)=0.24, peptization agent=HNO3, aging temperature=80℃and aging time=24h. The structure of the sol was characterized by FT-IR. It was indicated that the C=C bondsand the hydrophilic groups, such as hydroxyl groups and carboxyl groups were successfullyintroduced to the functionalized nano-Al₂O₃sols, which can enhance the interaction betweenAIP and PU and the strength of the sols.4. Preparation and characterization of Al₂O₃/PU/PVDF hybrid membranesAl₂O₃/PU/PVDF hybrid membranes were prepared by adding the functionalizednano-Al₂O₃sol into the casting solution. The structures of the hybrid membranes werecharacterized by SEM, FT-IR, AFM and DSC. The results indicated that the hydrophilicity,penetrating property and mechanical properties were superior to pure PVDF membrane,PU/PVDF blending membrane and Al₂O₃/PVDF hybrid membrane. FT-IR showed that-OH,-CHO and C=O were observed in the surfaces of Al₂O₃/PU/PVDF hybrid membranes. AFMtests indicated that the surface roughness of Al₂O₃/PU/PVDF hybrid membrane was minimized to5nm, which can further improve the antifouling property of the membrane.