More thermal studies on the PVOH/H3PO2/H2O solid proton conductor gels

Polymer blends based on poly(vinyl alcohol) (PVOH), hypophosphorous acid (H 3PO2) and water behave as solid protonic conductors. The ionic conductivity at room temperature depends strongly on OH/P ([number of mols of vinyl alc ohol monomeric units]/[number of mols of H3PO2]) ratios, x, showing a sensi tive variation between 10 (-6) and 10(-1) S cm(-1), as the acid concentrati on is increased. Using the membranes as electrolyte separator in a fuel cel l, voltages up to 726 mV have been obtained. Using high-resolution AC calor imetry and differential scanning calorimetry, we studied the phase behavior of the blends. We found that, independent of x, the glass transition (T-g) is about 167 K, compared with that of PVOH/H2O at 298 K and pure PVOH at 3 18 K. However, the step change at the glass transition and other two therma l events (peaks) at higher temperatures (at about 220 and 255 K, respective ly) representing the cold crystallization and the corresponding crystalline melting of the samples, increase with increasing acid content in the sampl es. These results are consistent with the interpretation of a two-phase mix ture in the blends: one of them with constant composition PVOH/H3PO2/H2O an d a separate second phase with composition H3PO2/H2O. (C) 2001 Elsevier Sci ence Ltd. All rights reserved.