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Published on: October 31, 2019
Thermo-Optical UCST Transition in PHPA-[EMIM][TFSI] Ionogels Governed by Polymer-Ionic Liquid Interactions
Wenxi Zhang1, Yang Hou1, Xinyang Zhao1
1State Key Laboratory of Chemical Resource Engineering, Beijing Laboratory of Biomedical Material, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing, P. R. China.
Abstract:
Understanding the molecular origin of thermo-optical transitions in ionic-liquid ionogels is essential for rationally designing robust, nonvolatile thermoresponsive devices. Here, a UCST-type poly(hydroxypropyl acrylate) (PHPA)-[EMIM][TFSI] ionogel was constructed via a one-step UV-induced photopolymerization and displayed reversible switching between opaque and transparent states upon heating. By integrating variable-temperature Fourier transform infrared spectroscopy (VT-FTIR) with 2D correlation infrared spectroscopy (2D-COS IR), the transition was traced to a temperature-driven redistribution of noncovalent interactions within the polymer/ionic-liquid matrix. At low temperature, cooperative hydroxyl-centered association dominated the microstructure, while heating progressively weakened polymer-associated hydrogen-bond motifs and promoted polymer-ionic-liquid association involving both hydroxyl and carbonyl environments. This interaction rebalancing diminished microheterogeneity and reduced light scattering, thereby generating the UCST-type opaque-to-transparent transition. The ionogel maintained stable optical switching over repeated thermal cycles and exhibited high sensitivity near the transition region, highlighting its promise for high-resolution temperature sensing and adaptive optical modulation in nonvolatile soft materials.
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