PVA-polyelectrolyte Hybrid Ion Exchange Membranes

Organic-inorganic hybrid ion exchange membrane is a new type of membraneexhibiting ions selectivity and combining the excellent performances of organicmaterials and inorganic materials. They can potentially satisfy the requirements ofmembrane separation and selectivity, mechanical strength, thermal stability andswelling resistance and hence have been used in the areas of fuel cells, electrochemicalanalysis and sensors, diffusion dialysis, pervaporation and electrodialysis, etc.Polyvinyl alcohol (PVA) has advantages for diffusion dialysis application since itcontains a large number of hydroxyl groups on the branched chain, which can improvethe permeability of ions. Nevertheless, PVA contains no ion exchange groups and alsohas some shortcomings in terms of strength and stability. For this reason, manymodifications methodes such as chemical modification, blending with other copolymersand introducing hydrophobic groups have been tried. In the thesis, multisiliconcopolymers containing-SO3-and-N+(CH3)3groups have been selected aspolyelectrolytes to blend with PVA to yield a series of hybrid ion exchange membranesby sol-gel reaction, and the change of membrane microstructure and performances withdifferent type and content of multisilicon copolymers as well as preservation time areinvestigated.This thesis is divided into three chapters: The first chapter is the introduction,describing the advantages of the organic-inorganic materials, the properties andclassifications of hybrid ion exchange membranes. Then polyelectrolyte andpolyelectrolyte complexes are introduced, including the concept and properties ofpolyelectrolyte, the concept, formation method and mechanism, application ofpolyelectrolyte complexes. In addition, the preparation methods of hybrid membranesare presented, focusing on the sol-gel process, and putting forward that preservationtime may affect properties of sol-gel hybrid membranes.In the second chapter, PVA-hybrid cation exchange membranes have been preparedwith the multisilicon copolymer poly (SSS-co-γ-MPS) containing-SO3-groups.The impact of the preservation time of the coating solution or the membranes onmembrane microstructure and performances is investigated, including water uptake,mechanical properties,65oC water swelling and diffusion dialysis performances ofNaOH/Na2WO4solution. The results show that long time of the coating solution preservation has more significant influence as compared with the preservation ofmembranes. In addition, small molecular tetraethoxysilane (TEOS) is used as areference to prepare PVA-SiO2hybrid membranes, and the difference betweenmultisilicon copolymers and small moleculer silane is distinguished after comparison ofthe different membranes. Results show that the membranes from multisilicon copolymerare more stable than those from small molecular silane. Another reference is the poly(SSS-co-γ-MPS) solution for more than two years’ conservation. The change of themembranes from such solution and PVA with preservation time is not obvious.The third chapter describes the preparation of PVA-polyelectrolyte complexes(PECs/PVA) membranes by mixing and sol-gel reaction of PVA with the multisiliconcopolymer containing-SO3-groups as the anion polyelectrolyte, and the multisiliconcopolymer containing-N+(CH3)3groups as the cation polyelectrolyte. The influence ofthe cation and anion polyelectrolytes’ ratios on membrane morphology and properties isexplored. Compared with anion or cation exchange membrane, the PECs/PVAmembranes demonstrate significantly improved thermal stability and swelling resistance,which is very important for practical application. The PECs/PVA membranes are used inthe acid and alkali recovery processes through diffusion dialysis. Membrane separationfactor of HCl and FeCl2can be as high as89.9, dialysis coefficients of H+can reach to0.013m/h, which is significantly higher than the commercial DF-120membrane. Whenthe membranes are used for the separation of NaOH/NaWO4solution, the dialysiscoefficients of OH-are in range of0.014to0.019m/h, which is7to9times higher thancommercial SPPO membranes. Generally, PECs/PVA are more suitable forNaOH/NaWO4diffusion dialysis, since not only the S values are increased, but also UOHvalues maintain at a high level. In addition, the membranes are taked for treatment ofthe actual industrial acidic or alkali waste solution by diffusion dialysis, and excellentresults are obtained.The forth chapter is a summary of the whole thesis. The significances aboutpreparation of PVA-polyelectrolyte hybrid ion exchange membranes are summarizedthrough the experiments and theoretical analysis.