Glass Transition Dynamics of Room-Temperature Ionic Liquid 1-Methyl-3-trimethylsilylmethylimidazolium Tetrafluoroborate

The conductivity relaxation dynamics of the room-temperature ionic liquid 1-methyl-3-trimethylsilylmethylimidazolium tetrafluoroborate ([Si-MIm][BF4]) have been studied by broadband conductivity relaxation measurements at ambient pressure and elevated pressures up to 600 MPa. For the first time, several novel features of the dynamics have been found in a room-temperature ionic liquid. In the electric loss modulus M '' (f) spectra, a resolved secondary beta-conductivity relaxation appears, and its relaxation time tau(beta) shifts on applying pressure in concert with the relaxation time tau(alpha) of the primary alpha-conductivity relaxation. The spectral dispersion of the alpha-conductivity relaxation, as well as the fractional exponent (1 - n) of the Kohlrausch-Williams-Watts function that fits the spectral dispersion is invariant to various combinations of pressure and temperature that keep tau(alpha) constant. Moreover, tau(beta) is unchanged. Thus the three quantities, tau(omega) tau(beta), and n, are coinvariant to changes in pressure and temperature. This strong connection to the alpha-conductivity relaxation shown by the beta-conductivity relaxation in [Si-MIm][BF4] indicates that it is the analogue of the Johari-Goldstein beta-relaxation in nonionically conducting glass-formers. The findings have fundamental implications on theoretical interpretation of the conductivity relaxation processes and glass transition in ionic liquids. It is also the first time such a secondary conductivity relaxation or the primitive conductivity relaxation of the coupling model has been fully resolved and identified in M ''(f) in any ionically conducting material that we know of.