Difference in Lower Critical Solution Temperature Behavior between Random Copolymers and a Homopolymer Having Solvatophilic and Solvatophobic Structures in an Ionic Liquid^†

Abstract

The solubility and phase behavior of poly(benzyl methacrylate) (PBzMA) and poly(styrene-co-methyl methacrylate) (P(St-co-MMA)) in a hydrophobic ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethane sulfone)imide ([C2mim][NTf2]), have been explored as a function of temperature. Although both polymers have solvatophobic phenyl groups and solvatophilic methacrylate groups in the structure, their distribution on the polymer chains is quite different. In PBzMA, both structures are incorporated in each monomer unit, whereas in P(St-co-MMA)s the distribution is statistically determined by the monomer reactivity ratio of St and MMA. Both polymer solutions in [C2mim][NTf2] become turbid with an increase in temperature (lower critical solution temperature (LCST) behavior). The turbidity change occurs sharply at 100 °C for PBzMA, whereas it is sluggish for P(St-co-MMA)s. The LCST-type phase-separation temperature for P(St-co-MMA)s decreases with an increase of the St composition. The sluggish phase separation for P(St-co-MMA)s has been explained in terms of the presence of the MMA sequences along the polymer chain, which inhibits the St aggregation to a certain extent. The dynamic light scattering (DLS) measurements for PBzMA reveal that the hydrodynamic radius of PBzMA suddenly changes at 100 °C; below this temperature, no aggregation is observed. This result strongly implies that the coil-to-collapse transition is of the first-order type. It has been demonstrated that the LCST-type phase separation of the polymers in an ionic liquid is greatly affected by the distribution of the solvatophilic and solvatophobic groups on the polymer chains