Proton-Sweeping Role of Oligomeric Electrolyte Chains Grown on Silica Nanospheres
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0923-V05-08
Proton-Sweeping Role of Oligomeric Electrolyte Chains Grown on Silica Nanospheres Xinhui Zhang1, Liang Hong1,2, and Zhaolin Liu2 1 Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore, Singapore 2 Institute of Materials Research & Engineering, 3 Research Link, Singapore, Singapore
ABSTRACT Oligomeric ionmer-grafted silica nanoparticles were synthesized via atom transfer radical polymerization (ATRP) and applied to dope Nafion to investigate proton-sweeping effect of these hybrid nanoparticles. Two types of vinyl monomers, sulfopropyl acrylate, potassium (SPA) and N,N’-methyl-(6-hexylcarbamatoethylmethacrylate) imidazolonium bromide (EMACI), were employed to construct the grafted ionmer chains. The ionmer-grafted silica particles present a thick polymer layer surrounding the SiO2 core according to TEM investigation. The DSC analysis shows that the grafted ionmer oligomers ( DP < 10 ) exhibit a stronger ionic attractive interaction than the corresponding long chain counterparts. The other structural characterizations of the ionmer-grafted SiO2 nanoparticles also include FT-IR and EDX. The most interesting feature of the grafted ionmer shrub lies in its proton-conduction enhancing effect when combined with Nafion® polymer. A series of such composite membranes were prepared through dispersing the ionmer-grafted SiO2 nanoparticles in Nafion solution and followed by casting. The grafted copolymer (SPA-co-EMACI) demonstrated a greater promoting effect on proton conduction than its homopolymer counterparts of either (SPA) or (EMACI) in the matrix of Nafion. This outcome could be attributed to the adjacent proton-sweeping effect of the pendant imidazolonium group. INTRODUCTION Recent focuses on the proton exchange membrane (PEM) have been sifted towards pursuit of high operation temperatures (150-200oC) conductivity or long run stability when methanol is used as fuel [1]. Understanding the proton transport mechanism in PEM is therefore the basis for the design and synthesis of high performance PEM. At a molecular level the proton transport mechanism is usually described by either the hopping mechanism or the diffusion mechanism [2] in hydrated system. On the contrary, the study of proton-sweeper based on specific organic functional groups, which are considered imperative for low humidity proton migration, has still been quite limited. In this work, we investigated proton-sweeping effect of nanoparticles with grafted oligomeric ionmer chains, which comprise co-monomer units containing sulfonic acid and imidazolonium as end group, respectively. As imidazolonium is a weak Lewis base, it is expected to be a suitable proton hopping site. Experimentally this proton-sweeping action was indeed observed, but on the condition that sulfonic acid group must be located nearby through the random copolymerization structure.
EXPERIMENT Synthesis of cationic monomer (EMACI) 6-Bromo-1-hexanol, 2-isocynatoethyl methacrylate (molar ratio is 1:1) and a small amount of catalyst,
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