(250b) Highly Conductive Ionic Liquid-Homopolymer Mixtures

Polymer electrolytes are of great interest due to their application in membrane-based separations, fuel cells, sensors, and actuators, where high ionic transport is desired for process efficacy. However, ionic conductivity is usually a strong function of water content. Therefore, dry or high temperature operation results in dehydration and consequently severe performance losses. This project aims to replace water as the medium for ionic transport with a class of organic salts commonly known as ionic liquids. In particular, room temperature ionic liquids are of interest due to their negligible vapor pressures, high degradation temperatures, large electrochemical windows, and high ionic conductivities. Successful implementation of ionic liquids as a substitute for water will allow for water-free operation of current polyelectrolyte membranes, expanding their range of operating conditions and utility.

In this study, a number of ionic liquid-homopolymer mixtures were investigated as a function of ionic liquid content. Several ionic liquids were explored, where the chemical compatibility with the polymer has a significant effect on the resulting ionic conductivity. For compatible (miscible) ionic liquid-homopolymer systems, the room temperature conductivity increased by seven orders of magnitude with increasing ionic liquid volume fraction. Also, the ionic liquid acts as a plasticizer, where there is a step change in the ionic conductivity at the ionic liquid content that corresponds with the glass transition temperature. For partially miscible and immiscible ionic liquid-homopolymer mixtures, the room temperature conductivity did not significantly change with increasing ionic liquid volume fraction. The phase behavior of these systems was confirmed with differential scanning calorimetry and scanning electron microscopy. Transport measurements will be coupled to static and in situ morphological studies. These results provide insights into the interactions between ionic liquid and polymers that will help with the design of new ionic liquid-polymer systems and new poly(ionic liquid)s to address key questions about ion transport.