Statistical terpolymers with thermo-responsive fluorescence response in an ionic liquid: effects of solvatophilicity on LCST phase separation and reversibility

Abstract

Well-defined terpolymers of benzyl methacrylate (BzMA) and 9-(4-vinylbenzyl)-9H-carbazole (VBK) with either methyl methacrylate (MMA) or oligo(ethylene glycol)methacrylate (OEGMA) were synthesized to study their LCST-type phase behavior (due to BzMA) and fluorescence properties (due to VBK) in a hydrophobic ionic liquid IL, 1-ethyl-3-methylimidazolium bis(trifluoromethane sulfone)imide ([C₂mim][NTf₂]) as a function of solvatophilicity and the difference in terpolymer chain mobility (via glass transition differences due to using MMA versus OEGMA). Specifically, MMA and OEGMA provided solvatophilic groups (the ester in MMA or ethylene glycol units in OEGMA) to the terpolymers and their composition was varied to study the effect of solvatophilicity on the terpolymer phase separation and re-dissolution processes in the IL. The concentration of solvatophilic groups in the terpolymer was not only shown to affect phase separation temperature significantly, but also demonstrated to be crucial in driving the re-dissolution process. A minimum concentration of solvatophilic groups to facilitate reversible phase separation was in the range of 20–30 mol% for both terpolymer systems. Solution concentration (1–10 wt%) also showed significant effects on phase separation temperature but lower concentration did not guarantee improved re-dissolution. Phase separation temperature was also demonstrated to be inversely proportional to the polymer molecular weight (7.3–22.5 kg mol⁻¹). Finally, the reversibility of phase separation and fluorescence responses in terpolymers with sufficient solvatophilicity was confirmed to be consistent by multiple heating/cooling cycles.