Résumé
In this work we investigated polyvinyl alcohol films which we crosslinked at various crosslinking degrees by oxalic and citric acids for the fuel cell applications. We studied the effects of the crosslinking on structure, thermal properties, electric conductivity and dielectric relaxations. The glass transition temperature is increased indicating the lowering of the local mobility of the polymer chains. X-ray diffraction showed the presence of non-dissociated acid phases in dried samples. The Dc conductivity increases by increasing both the relative humidity and the temperature obeying to an Arrhenius law with activation energy compatible with proton conductivity type. The Ac conductivity studied between 10(-1) Hz and 1 MHz showed a power law response in the high frequency range. This behavior characterizes the charge transport in disordered materials. At low frequencies the Ac conductivity is governed by the electrode/sample interface polarization. The use of the dielectric permittivity indicates the presence of a relaxation process attributed to electrode/sample interface polarization and another relaxation process attributed to alpha relaxation of the polyvinyl alcohol chains.