Concentration and Temperature Dependence of the Interaction Parameter and Correlation Length for Poly(benzyl methacrylate) in Ionic Liquids

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 polymer-rich compositions. To better understand these phenomena, small-angle 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.