AC and DC conductivity of ionic liquid containing polyvinylidene fluoride thin films

Abstract

Polarisation processes and charge transport in polyvinylidene fluoride (PVDF) with a small amount (0.01–10 wt%) of the ionic liquid (IL) 1-ethyl-3-methylimidazolium nitrate ( $$\hbox {[EMIM]}^+[\hbox {NO}_3]^-$$ ) are investigated by means of dielectric spectroscopy. The response of PVDF that contains more than 0.01 wt% IL is dominated by a low-frequency relaxation which shows typical signatures of electrode polarisation. Furthermore, the $$\alpha _{{\rm a}}$$ relaxation, related to the glass transition, disappears for IL contents of more than 1 wt%, which indicates that the amorphous phase loses its glass-forming properties and undergoes structural changes. The DC conductivity is determined from the low-frequency limit of the AC conductivity and from the dielectric loss peak related to the electrode polarisation. DC conductivities of $$10^{-10}$$ to $$10^{-2}\,\hbox {S}/\hbox {m}$$ are obtained—increasing with IL content and temperature. The dependence of the DC conductivity on the IL content follows a power law with an exponent greater than one, indicating an increase in the ion mobility. The temperature dependence of the DC conductivity shows Vogel–Fulcher–Tammann behaviour, which implies that charge transport is coupled to polymer chain motion. Mobile ion densities and ion mobilities are calculated from the DC conductivity and the dielectric loss related to electrode polarisation, with the results that less than one per cent of the total ion concentration contributes to the conductivity and that the strong increase in conductivity with temperature is mainly caused by a strong increase in ion mobility. This leads to the conclusion that in particular the ion mobility must be reduced in order to decrease the DC conductivity.