Inducing thermoreversible optical transitions in urethane-acrylate systems via ionic liquid incorporation for stretchable smart devices

Abstract

Hydrogels are able to exhibit optical transitions in the presence of external stimuli such as temperature, driven by the lower critical solution temperature (LCST) phenomena. However, they suffer from inherent thermal instability, requiring reswelling for repeated utilisation. Ionogels possess greater thermal stability over conventional hydrogels. However, thermally driven optical transitions via LCST phenomena in ionic liquid incorporated polymer networks have not been studied in-depth. In this work, we incorporated a low amount of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide in a polymer matrix, to trigger the desired reversible optical transitions based on the LCST phenomenon. The composition of the ionic liquid elastomer hybrid is in contrast to conventional hydrogels which possess high amounts of liquid. We present NMR and UV-vis spectroscopic studies to reveal the underlying reversible hydrogen bonding based mechanism behind this optical transition. Unlike conventional hydrogels, our hybrids show excellent thermal and ambient stability along with repeatable optical transitions with comparable response time, indicative of their long term use in harsher environments. Improvements in the mechanical properties with the inclusion of ionic liquid in our hybrids were also observed (40% increase in ultimate strain, 34% decrease in Young's modulus). The enhanced properties and optical transition of the ionic liquid elastomer hybrids allowed them to serve as a patternable smart display and a stretchable & flexible device.