Ionic Liquids: An Ideal Solvent for Tuning the UCST Phase Behavior of Polymer Gels

Liquid-liquid phase separation (LLPS) is a common stimulus-responsive phenomenon widely studied and applied in constructing intelligent systems such as microfluidic valves, smart windows, and biosensors. However, LLPS in an aqueous solution has limited applicability confined to a narrow temperature range within 0-100 °C. In addition, for easy exploitation of thermoresponsive behavior, phase separation must be stable and accurately predictable under varying conditions. This study proposes a gel system exhibiting UCST phase behavior using ionic liquids (ILs) and hydrophilic polymers, whose phase transition temperature can be linearly tuned within a wide range (from subzero to over 100 °C) by varying the mixing ratio of ILs in their blends. Similar to the mixing of ILs with structurally similar cations, mixing ILs containing different anions proved to be an effective ideal random mixing method based on experimental results and molecular dynamics simulations. This mixing mechanism of ILs accounts for the linear regulation of the UCST of the ionogels when the mixing ratio of ILs in their blends varies. Moreover, the unique feature of ILs was further demonstrated using other hydrophilic polymer networks and multiple combinations of ILs, suggesting the generality of this strategy for UCST regulation in the ionogels.