Investigation of a relationship between Free Volume and Ionic Conductivity of PVdF-co-HFP with Ionic Liquid

Abstract

Polymer matrix-ionic liquid (IL) complexes are of growing interest due to viable transport propertiesand potential applications in fuel cells and such other ionic devices as polymer electrolytes.Polymer electrolytes contain local free volumes which are formed by irregular molecular packingand affect thermal, mechanical, and transport properties of polymers. Their conductivity isrelated to the hole fraction as a measure of free volume, calculated from the Simha-Somcynskytheory. [1] In this work, we purpose to establish a correlation between the hole fraction and ionicconductivity of polymer-based IL electrolytes. For this aim, polymer electrolytes containing 1,3, 10, 30% of (weight) amounts of IL (1-etil 3-metil imidazolyum tetrafloroborat) were preparedby solution casting method. All these electrolytes were based on PVdF-co-HFP. The conductivitymeasurements of the samples were run by AC impedance analyzer from 273 K to 373 K temperatureintervals. The hole fraction of the samples was measured by ortho-positronium (o-Ps) lifetime andintensity using Positron Annihilation Lifetime Spectroscopy (PALS). [2] The changes of nanoscopichole free volume and hole fraction were investigated with respect to temperature which hadan increasing effect on them. However the hole fraction increased sharply to about 1% of ILfraction. It is the fact that the formation of polymer and salt complications which occur betweenthe fluorine groups in the polymer backbone and Li+ ion in salt retards the ordering of polymercrystalline and causes the formation of amorphous phases. As the weight ratio was greater than1%, the hole fraction decreased because of the IL occupation in the holes and then saturated.