Polyurethane-Based Ionogels Exhibiting Durable Thermoresponsive Optical Behavior Under High-Temperature Conditions

Polyurethane (PU)-based transparent and flexible ionogels, showing unusual thermo-responsive optical properties, were successfully prepared by mixing PU-precursor and a hydrophobic ionic liquid, 1-ethyl-3-methylimidazolium bis( trifluoromethanesulfonyl) imide (EMIM-TFSI). Although the initial ionogels were transparent at room temperature, significant increases in opacity were observed with increasing temperature up to 120 degrees C, because of macroscopic phase separation of the PUmatrix and hydrophobic EMIM-TFSI. In addition, the optical transition temperature could be arbitrarily controlled simply by varying the mixing ratio of EMIM-TFSI within the PU-matrix. As confirmed by UV-Vis spectra acquired at different temperatures, this thermo-responsive optical behavior was found to be reversible, repeatable and durable even after 30 cycles of a thermal-stress testing between 30 and 100 degrees C.