Lower Critical Solution Temperature Phase Behavior of Polymers in Ionic Liquids

Polymer/ionic liquid systems have been widely explored, yet those exhibiting lower critical solution temperature (LCST) phase behavior remain poorly understood. Few complete compositional phase diagrams of these systems have been published, and to our knowledge, no phase diagrams involving pyrrolidinium-based ionic liquids have been explored. In this work, we describe the LCST phase behavior of poly(benzyl methacrylate) in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide within the solvent-rich regime of its phase diagram. The phase transition temperatures (T_CP) were assessed via cloud point measurements, where phase separation is taken to be the transition from a clear, homogenous phase into a cloudy, biphasic solution. This sharp phase transition (on the order of seconds) suggests rapid de-solvation of the polymer at T_CP. The observation of a narrow hysteresis in the transmittance curves upon heating and cooling, as well as the reversibility of the phase separation, further indicate that T_CP is a good approximation for the equilibrium transition temperature (binodal curve). These exciting results have revealed that molecular weight has no influence on the phase transition temperature of this system, and its critical composition lies above 40 wt% polymer, far away from the theoretical 5 – 7 wt% predicted by Flory-Huggins solution theory.