Calorimetric and Neutron Scattering Studies on Glass Transitions and Ionic Diffusions in Imidazolium-based Ionic Liquids

摘要

Glass transition is one of the central research issuesof ionic liquids (ILs). In particular, the most typical ILs, imidazolium-basedones (ImlLs) are readily supercooled and exhibit glass transitions below room temperature. We have measured the heat capacities of several ImILs,encoded as CnimX (n alkyl carbon number, n = 2-8, X: anion, X = CI, I, FeCl4, TFSI)using an adiabatic calorimeter. We found that most of ImlLs exhibit glass transitions with large C_p jumps in a temperature range between 170 K and 230 K. The large C_p jumps reflect that these ILs are fragile liquids that exhibit large structural change depending on temperature near the glass transition temperature T_s. It is also revealed that T_g does not depend much on n but on the anion radius. We have investigated the dynamics of CnmimX (n= 2-8, X = Cl, NO3, PF6, TF, FSI, TFSI) by means of a quasielastic neutron scattering (QENS) technique. It was clarified that the ionic diffusion is directly associated with the viscosity and glass transition. The activation energy ΔE_a of the ionic diffusion increases with decreasing anion size but remains almost unchanged with was found for T_g. These systematic change of T_g and ΔE_a can be explained well by taking account the nano-domain structure which is the most characteristic feature of ImlLs.