Unusual Lower Critical Solution Temperature Phase Behavior of Poly(benzyl methacrylate) in a Pyrrolidinium-Based Ionic Liquid

Abstract

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.

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

“…This latter conclusion, however, is not consistent with the extensive dynamic light scattering results of Kharel et al, which showed swollen chain conformations . Recent work from our group has demonstrated that the second virial coefficient of PBnMA in dilute IL solutions is consistently positive, even above temperatures where phase separation is observed at higher concentrations, and that the critical compositions of the phase diagrams are strongly displaced toward polymer-rich concentrations. , These observations deviate from the behavior anticipated by Flory–Huggins theory and observed in many other LCST systems; they may reflect a strongly concentration-dependent interaction parameter. These phenomena pose foundational questions about the solvation of PBnMA in ILs, which we explore here.…”

“…Additionally, profiles of PBnMA-32 and PBnMA-76 in [MMIM]­[TFSI], [EMIM]­[TFSI], and [BMIM]­[TFSI] were collected at 25 °C for 10–20 wt% and are reported in Figure S15. Upon crossing the cloud point temperature ( T CP ), as determined from previous work, solutions undergo phase separation into polymer-rich and solvent-rich phases . As such, measurements above T CP are out of equilibrium and are not considered in the subsequent analysis.…”

“…PBnMA was synthesized previously via atom transfer radical polymerization. , The polymers under study are denoted PBnMA- x , where x represents the polymer weight-average molecular weight in kDa. The number-average molecular weight ( M n ), weight-average molecular weight ( M w ), and dispersity ( Đ ) of the polymers are summarized in Table as determined by size exclusion chromatography performed in THF on an Agilent 1260 Infinity System (Agilent Technologies, Santa Clara, CA, USA).…”

Thermoresponsive Ionic Liquid/Water Mixtures: From Nanostructuring to Phase Separation

“…This macroscopic identification of demixing can be rendered automatic by measuring with a spectrophotometer the temperature and/or concentration dependence of the transmittance of a beam of light through the sample, which shows a drop at the cloud point of the mixture (see Figure 5 ). In practice, the cloud point is intermediate between the coexistence and the spinodal (stability) lines of the system [ 72 ]. The simultaneous measurement of the rotational power of the sample on a linearly polarised light beam allows to detect the formation of chiral liquid crystal phases, even for achiral molecules [ 73 ].…”

“…MD simulations [ 145 ], discriminate the role of the two type of hydrogen bonds, arguing that the one with oxygen is more directional and has an entropic cost higher than that with F, thus having a larger role in the demixing with increasing temperature. A recent experimental study extends the analysis to poly(benzyl metacrylate) in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonate)imide ([BMP][NTf ]) [ 72 ], also showing LCST behaviour, with a transition temperature that sensitively depends on the polymer/IL relative concentration, but, again, depends only weakly on the molecular weight of the polymer. The unusual aspect is that, even in the demixed temperature range, the second virial coefficient for the polymer in IL is positive, pointing to good solvent conditions.…”