A broadband dielectric spectroscopic (BDS) study of poly(2,5-benzimidazole) revealed three sub-glass relaxations and the manner in which the time scales of these local molecular motions shift with annealing temperature and time. Also issuing from the BDS studies was a trend in the degree of connectivity of charge migration pathways and conductivity with annealing temperature and time. These studies were complemented with dynamic mechanical analysis, which showed the same relaxations, and solid state 1 H and 13 C NMR studies that showed hydrogen bonding group mobility versus temperature. Wide angle X-ray diffraction investigations indicated an increase in chain-packing efficiency that was used to rationalize the BDS and NMR results.

Hot (at 120 °C) and dry (20% relative humidity) operating conditions benefit fuel cell designs based on proton exchange membranes (PEMs) and hydrogen due to simplified system design and increasing tolerance to fuel impurities. Presented are preparation, partial characterization, and multi-scale modeling of such PEMs based on cross-linked, sulfonated poly(1,3-cyclohexadiene) (xsPCHD) blends and block copolymers with poly(ethylene glycol) (PEG). These low cost materials have proton conductivities 18 times that of current industry standard Nafion at hot, dry operating conditions. Among the membranes studied, the blend xsPCHD-PEG PEM displayed the highest proton conductivity, which exhibits a morphology with higher connectivity of the hydrophilic domain throughout the membrane. Simulation and modeling provide a molecular level understanding of distribution of PEG within this hydrophilic domain and its relation to proton conductivities. This study demonstrates enhancement of proton conductivity at high temperature and low relative humidity by incorporation of PEG and optimized sulfonation conditions.

Changes in macromolecular dynamics associated with the Tg -related β relaxation of Nafion®, with chemical degradation, were investigated using broadband dielectric spectroscopy. Loss permittivity vs. frequency spectra showed peak splitting reflecting microstructural heterogeneity. Spectra were analyzed using the conductivity-modified Havriliak-Negami equation and the Kramers-Krönig integral transform, both of which showed broadening and peak splitting for degraded samples. Peak broadening and bi and tri-modal character of the spectra of degraded samples was also manifest in distributions of relaxation times which reflected a broadening of molecular weight distribution and related microstructural heterogeneity induced by chemical degradation.

Broadband dielectric spectroscopy was used to identify sub-Tg macromolecular motions for undoped and phosphoric acid - doped polybenzimidazole (PBI) films. Loss permittivity vs. frequency spectra showed three secondary relaxation processes labeled beta, gamma and delta in order of decreasing temperature for undoped PBI. As acid content in the membrane increased, peak broadening was observed for all relaxations while the peak positions shift closer to each other. The loss and storage permittivity vs. frequency curves for PBI membranes are displaced upwards with increased acid content which reflects increasingly greater polarizability due to increasingly greater populations of polar phosphoric acid groups. The high storage permittivity values observed for acid doped samples in the low frequency regime could be due to membrane | electrode interfacial polarization relaxation. The Kramers-Krönig integral transform was used to calculate pure loss permittivity relaxation peaks from experimental dc-free storage permittivity curves to eliminate obscuring dc conductivity contribution.

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