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)-EMIMTFSI 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.
Zhang et al. (Sun,) studied this question.