SANS study on the solvated structure and molecular interactions of a thermo-responsive polymer in a room temperature ionic liquid

Abstract

We have utilized small-angle neutron scattering (SANS) to quantitatively characterize the LCST-type phase behavior of the poly(benzyl methacrylate) (PBnMA) derivative poly(2-phenylethyl methacrylate) (PPhEtMA) in the deuterated ionic liquid (IL) d₈-1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide (d₈-[C₂mIm⁺][TFSA⁻]). The SANS curves showed a discontinuous change from those characteristics of dispersed polymer chains to those of large aggregates of PPhEtMA chains suspended in the IL solution, indicating that phase separation occurs discontinuously at T_c. Furthermore, we evaluated the enthalpic and entropic contributions to the effective interaction parameter χ_eff of PPhEtMA in [C₂mIm⁺][TFSA⁻] and compared them with those of PBnMA. The absolute value of the enthalpic contribution observed for PPhEtMA was smaller than that for PBnMA. This difference in the enthalpic term can be attributed to the unfavorable interaction between the IL and the alkyl group in the side chain of PPhEtMA. In addition, the temperature dependence of χ_eff was smaller than the previously reported value for a thermo-responsive polymer in an aqueous system. It was found out that the strong dependence of T_c on the chemical structure in IL systems originated from the relatively smaller temperature dependence of χ_eff.