Jinghang Wu

Abstract

In order to demonstrate proton conductivity in an imidazole polymer system, novel composite proton exchange membranes were fabricated by casting films of poly[4(5)-vinylimidazole/vinylimidazolium trifluoromethylsufonylimide] and poly(vinylidene fluoride)], PVdF, from mixed dimethylformamide (DMF) solutions. The phase, composition and morphology of these composites were examined by differential scanning calorimetry (DSC), and hot stage polarized microscopy. Thermal stability was evaluated by thermal gravimetric analysis (TGA). Proton conductivity was in a fuel cell test fixture evaluated at GM Fuel Cell Activities in Honeoye Falls, NY. DSC thermograms were characterized by a crystalline melt for the PVdF component at ∼169°C. All composites displayed a well-form exothermic peak for recrystallization of PVdF at ∼121°C. The melting and recrystallization characteristics of PVdF in the composites were substantially identical to those of pure PVdF. In its homogeneous state, poly[4(5)-vinylimidazole/imidazolium trifluoromethylsulfonylimide] exhibited a glass transition temperature, Tg-mid, of -30°C. A glass transition temperature was not observed for the poly[4(5)-vinylimidazole/imidazolium trifluoromethylsulfonylimide] phase in the blends, because the temperature was not scanned below 0°C in the DSC thermograms of the blends. Incorporation of benzoyl peroxide resulted in a slight increase in crystallinity of the 3/1 and 4/1 compositions and a substantial increase in crystallinity of the 8/1 composite. Crystallinity increased slightly as the volume fraction of PVdF was increased. Classic crystalline spherulites were observed in films cast from DMF and dried at temperatures below the melting point of PVdF. On heating to 200°C on the hot stage microscope, crystals melted to reveal a rather amorphous dark field with thin worm-like inclusions which were presumed to arise from the poly[4(5)-vinylimidazole/imidazolium trifluoromethylsulfonylimide] phase. On cooling to ambient temperature, the background field became progressively brighter, however, no structure that might be associated with the reformation of crystallites was observed. This was presumably a result of submicroscopic size of the crystallites. TGA spectra of the all composite films were characterized by two transitions, one at 300°C corresponding to the decomposition of poly[4(5)-vinylimidazole/imidazolium], and one at 450°C which corresponds to the decomposition of PVdF. Mass loss corresponded well with the mass fractions of the two components of the composite. Proton conductivity was measured as a function of relative humidity at 80°C. Conductivity (0.05 S/cm) approaching that exhibited by Nafion® 112 (0.18 S/cm) was realized in the 4/1, PVdF/poly[4(5)-vinylimidazole/imidazolium] composite film. Substantial conductivity (0.02 S/cm) was also measured in the 3/1 composite films. No measurable proton conductivity was observed in films of the 8/1 composite. We believe that this is the first instance in which such high proton conductivity levels have been realized in a polymer system where a Grotthuss mechanism of proton transport might be invoked. These results are very exciting and may point the way to the preparation of membranes exhibiting high levels of proton conductivity at elevated temperature and low relative humidity.

Library of Congress Subject Headings

Polymers--Electric properties; Membranes (Technology); Composite materials--Electric properties; Fuel cells; Electric conductivity

Publication Date

12-7-2006

Document Type

Thesis

Department, Program, or Center

Center for Materials Science and Engineering

Advisor

Smith, Thomas - Chair

Advisor/Committee Member

Santhanam, K.S.V.

Comments

Note: imported from RIT’s Digital Media Library running on DSpace to RIT Scholar Works. Physical copy available through RIT's The Wallace Library at: QD381.9.E38 W8 2006

Campus

RIT – Main Campus

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