Poly[4(5)-vinylimidazole]/polyvinylidene fluoride composites as proton exchange membranes

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Title: Poly[4(5)-vinylimidazole]/polyvinylidene fluoride composites as proton exchange membranes
Author: Pan, Jingjing
Abstract: In the present research, the morphology and thermal chemical characteristics of composite films comprised of poly(vinylidene fluoride) (PVF2) and poly[4(5)-vinylimidazole/vinylimidazolium trifluoromethylsulfonylimide] (PVIm/VIm+TFSI-]) were studied. In these composites, conditions such as choice of solvent and drying and annealing conditions can affect the crystal habit, crystallite size and degree of crystallinity of PVF2 as well as the distribution of the minor component, poly[4(5)-VIm/VIm+TFSI-]. Such composites may have potential in fuel cells as high-temperature proton exchange membranes. When cast from either dimethylformamide (DMF) or dimethylacetamide (DMAC) at ambient temperature and dried at temperatures below 100°C, PVF2 homopolymer films and PVF2//PVIm/VIm+TFSI- composite films were obtained in which the crystallites of PVF2 were beta-phase. The films initially obtained were white, opaque films with limited strength and mechanical integrity. After heating to 200°C, both the PVF2 films and the PVF2//PVIm/VIm+TFSI- films became stronger and more transparent. X-ray diffraction showed that prior to heating the PVF2 homopolymer film was beta-phase and after heating to 200°C PVF2 was alpha-phase. This was also the case with the PVF2//PVIm/VIm+TFSI- composites. In other words, the crystalline polymorph of PVF2 in the non-heated composite films was identical to that of non-heated homopolymer PVF2, and the PVF2 polymorph in the heated composite films was identical to that of heated homopolymer PVF2 films. DSC analysis showed that the melting point of crystals in heated PVF2 was lower than that in the non-heated PVF2. This difference in Tm (melting temperature) is attributed to the fact that PVF2 beta-phase crystals have a higher melting temperature than that of the alpha-phase crystals. PVF2 composites cast from DMAC have a higher onset Tc (onset crystallization temperature) than do PVF2 composites cast from DMF. The crystallinity of PVF2 in the heated homopolymer films was lower than that in the non-heated films. The percent crystallinity in the composites was variable and depended, to some degree on the level of TFSI. In the composites cast from DMAC, the crystallinity is maximal at 15 mol% TFSI and decreases somewhat as the TFSI level is increased to 50 mol%. The crystallinity of PVF2 in the composites cast from DMF is minimal at 15 mol % TFSI and increases in concert with the TFSI level. The intimacy of the phases in the composites was assessed by evaluating the amount of PVIm/VIm+TFSI- that could be extracted with ethanol/water. It was found that, after heating, the amount of imidazole polymer that could be extracted dropped from about 81% to less that 16% of the imidazole polymer originally in the mixture. This observation coupled with the substantial transparency of the heated composites, is indicative of a nanoscopic composite in which the PVIm/VIm+TFSI- phase is intimately mixed with the amorphous phase of the majority PVF2 component.
Record URI: http://hdl.handle.net/1850/10285
Date: 2009-04

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