Study of dc conductivity, transport mechanism, and dielectric relaxation in cassava starch membranes plasticized with glycerol

Abstract

In the current work, we studied dc conductivity and its relaxation in glycerol-plasticized starch membranes at various concentrations using impedance spectroscopy in frequency ranges between 20 Hz and 3 MHz and temperature ranging from 303 K to 423 K. We observed thermally activated dc conductivity following the Arrhenius model: \( \ln \sigma = \ln {\sigma_o} - {E_{\sigma }}/{k_B}T \), where the activation energy, E _σ , diminished as glycerol increased. Non-Debye relaxation phenomena were observed in the actual portion of the conductivity as a function of the frequency, \( \sigma \prime \left( \omega \right)\sim {\omega^n}\left( {n \approx 1} \right) \). Also, the symmetry of the relaxation peak observed in the imaginary part of the electric module remained unaltered as a function of temperature and glycerol content in the membranes, corresponding to a constant exponent, β = 0.80 ± 0.02, if we assume the \( \Phi (t) = \exp \left[ { - {{\left( {t/\tau } \right)}^{\beta }}} \right] \) correlation function in the ionic transport. The angular velocity, ω _p, registering the maximum peak, \( M_{{\max }}^{{\prime \prime }} \), was fitted to the Arrhenius model, \( \ln \omega = \ln {\omega_{{\rm o} }} - {E_{\omega }}/{k_{{\rm B} }}T \), finding great similarity between E _ω and E _σ values for each sample, indicating that the corresponding ionic processes have the same origin, in other words, due to the correlated jump of the charge carriers. The independence found between n and β indicates that the correlation processes or cooperative effects among ions in these materials deviate from the empirical description expressed by Ф(t). However, these processes were identical when varying temperature glycerol content in the membranes, as suggested by the M″(ω) spectra.