Synthesis and Evaluation of Silica Particles Grafted with Electrolyte Unit (VI)-Improvement of Polymer Introduction Rate

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Next-generation PEFC electrolyte membranes will require high proton conductivity and durability under high temperature and low relative humidity conditions. Core-shell organic-inorganic hybrid materials are expected to utilize the characteristics of organic and inorganic materials to overcome the problems of conventional PEM materials. In this study, silica particles (SiPs) were selected as the inorganic core to increase mechanical properties and thermal resistance, and the polymers having sulfo groups were selected as the organic shell to develop proton conductivity. Surface-grafted polymerization was performed for two kinds of monomers, p-phenylene and thiophene derivatives, and the synthesis conditions such as monomer feed ratio to initiator groups, surface area of core particles, and type of catalysts were investigated to improve the polymer introduction rates and electrochemical properties.SiPs were synthesized by Stöber method with tetraethylorthosilicate (TEOS) followed by the reaction with p-bromophenyltrimethoxysilane (SiPs-initiator). Sphere-shaped particles were obtained, and the diameter was about 380 nm. XPS spectra of the SiPs-initiator showed signals at 182 and 69 eV assigned to Br 3p and 3d, respectively. The amount of initiator was determined by TG-DTA to be 7.58 - 23.7×10- 5 mol g- 1, which corresponds to a density of 1.92 - 6.01×10- 23 mol nm- 2. SiPs grafted with poly{2,5-di[4-(2,2-dimethylpropoxysulfonyl)phenyl]propoxy-1,4-benzene} (SiPs-g-NS) and SiPs grafted with poly[3-(6-bromohexyl)thiophene] (SiPs-g-P3BHT) were synthesized. Polymerization was started from the activation of initiator groups. Ni(bpy)(COD) was reacted with the p-bromophenyl groups of SiPs-initiator, followed by the ligand exchange with the bidentate phosphate derivatives, 1,2-bis(diphenylphosphino)ethane (dppe) or 1,3-bis(diphenylphosphino)propane (dppp). Ni atoms were observed in the EDX spectra of Ni-modified SiPs-initiator. SiPs-g-NS and SiPs-g-P3BHT were obtained by reacting the Grignardized monomers with Ni modified SiPs-initiator. SEM images of SiPs-g-NS and SiPs-g-P3BHT exhibited the surface became rough after the polymerization, and XPS spectra showed S 2p signals at 168 eV or 163 eV derive from the sulfonyl groups of SiPs-g-NS and the sulfur atoms in thiophene rings of SiPs-g-P3BHT, respectively. The amount of polymer was determined by TG-DTA to be 1.36×10- 5 mol g-1, 7.04×10- 5 mol g- 1, and the introduction rates over initiator groups was 5.77%, 93.3% for SiPs-g-NS and SiPs-g-P3BHT, respectively. Thiophene derivative, BHT showed higher introduction rate than that of p-phenylene derivative. SiPs-g-P3BHT was synthesized by changing the monomer feed ratio to improve the polymer introduction rate, which was 30, 60, and 160 eq. over initiator groups (SiPs-g-P3BHT-30, SiPs-g-P3BHT-60, SiPs-g-P3BHT-160). The amount of polymer and introduction rate over initiator groups were 8.23×10- 5 mol g- 1 and 108.5% for SiPs-g-P3BHT-30. UV-vis absorption spectra of SiPs-g-P3BHT in THF exhibited an absorption band around 426 nm for SiPs-g-P3BHT-30, which is attributed to the π-π* transition of polythiophene backbone. SiPs with diameters of 355, 260 and 200 nm (SiPs-355, SiPs-260, and SiPs-200) were synthesized from the TEOS solution with concentration of 0.279, 0.144, and 7.24×10- 2 mol L- 1, respectively. Surface-grafted polymerization and postfunctionalization were performed to convert to the sulfo group for both SiPs-g-NS and SiPs-g-P3BHT, and the sulfonated polymer, SiPs-g-S and SiPs-g-SHT were obtained. SiPs-g-S and SiPs-g-SHT-Nafion composites were fabricated by dispersing particles in Nafion solution. The composite membranes fabricated by bar-coat method were transparent, and the thickness of membranes were about 10 μm for SiPs-g-S. Proton conductivity and gas permeability measurements were performed to investigate the electrolyte properties of composite membranes.