Polymer/ionic liquid systems are being increasingly explored, yet those exhibiting lower critical solution temperature (LCST) phase behavior remain poorly understood. Poly(benzyl methacrylate) in certain ionic liquids constitute unusual LCST systems, in that the second virial coefficient (A2) in dilute solutions has recently been shown to be positive, indicative of good solvent behavior, even above phase separation temperatures, where A2 < 0 is expected. In this work, we describe the LCST phase behavior of poly(benzyl methacrylate) in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide for three different molecular weights (32, 63, and 76 kg/mol) in concentrated solutions (5–40% by weight). Turbidimetry measurements reveal a strong concentration dependence to the phase boundaries, yet the molecular weight is shown to have no influence. The critical compositions of these systems are not accessed, and must therefore lie above 40 wt% polymer, far from the values (ca. 10%) anticipated by Flory-Huggins theory. The proximity of the experimental cloud point to the coexistence curve (binodal) and the thermo-reversibility of the phase transitions, are also confirmed at various heating and cooling rates.

Polymers in ionic liquids (ILs) are a fascinating class of materials that exhibit unusual behavior in comparison to more traditional polymer solutions. Previous work characterizing the lower critical solution temperature (LCST) phase behavior of poly(benzyl methacrylate) (PBnMA)/IL mixtures demonstrated that the second virial coefficient is consistently positive, even at temperatures above the observed phase separation boundary, and that the critical composition is shifted strongly toward polymerrich compositions. To better understand these phenomena, smallangle X-ray scattering was utilized along with Ornstein−Zernike analysis to determine the interaction parameter and correlation length of PBnMA in four ILs (one pyrrolidinium-based and three imidazolium-based) as a function of temperature (25−170 °C), concentration (5−30 wt%), and molecular weight (32−76 kDa). The interaction parameter was shown to increase with polymer volume fraction, contrary to the concentration-independent behavior anticipated by Flory-Huggins theory, which clarified the unusual phase diagram of these solutions. The semidilute correlation length of PBnMA in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide was shown to obey a temperature-concentration master curve; however, no such universal behavior was exhibited among the four ILs. Additionally, the concentration dependence of the correlation length was shown to decrease as the solvent quality worsened.

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