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

Abstract

The conductivity relaxation dynamics of the room-temperature ionic liquid 1-methyl-3-trimethylsilylmethylimidazolium tetrafluoroborate ([Si-MIm][BF(4)]) 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 β-conductivity relaxation appears, and its relaxation time τ(β) shifts on applying pressure in concert with the relaxation time τ(α) of the primary α-conductivity relaxation. The spectral dispersion of the α-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 τ(α) constant. Moreover, τ(β) is unchanged. Thus the three quantities, τ(α), τ(β), and n, are coinvariant to changes in pressure and temperature. This strong connection to the α-conductivity relaxation shown by the β-conductivity relaxation in [Si-MIm][BF(4)] indicates that it is the analogue of the Johari-Goldstein β-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.

Revealing the rich dynamics of glass-forming systems by modification of composition and change of thermodynamic conditions

“…Recently in a study of the conductivity relaxation dynamics of the roomtemperature ionic liquid, 1-methyl-3-trimethylsilylmethylimidazolium tetrafluoroborate ([Si-MIm][BF4]), a prominent and well resolved conductivity β-relaxation was observed [19]. The broadband conductivity (σ) relaxation measurements at ambient pressure and elevated pressures up to 600 MPa found an approximate co-invariance of τ σβ , τ σα , and (1 − n) in this neat ionic liquid glass-former.…”

“…Here τ σα and τ σβ stand for the conductivity α-and β-relaxation times. The CM is applicable to ion conductivity relaxation, and successful in accounting for experimental data in many ionically conducting materials [4,18,19,[47][48][49] with the same Eq. (3) and relation (4), but with τ σ0 , τ σβ , and τ σα replace the corresponding quantities in there.…”

Molecular dynamics and the translational–rotational coupling of an ionically conducting glass-former: amlodipine besylate

“…6, we verify the approximate agreement between s s0 (T g ) and s sb (T g ), and thus conrm that the conductivity b-relaxation present is in accord with the CM prediction as found before in other ionic conductors. 15,25,26,36,46 From the good agreement of the secondary conductivity relaxation and the secondary dipolar relaxation times shown in Fig. 6, we have concluded that there is translation-rotation coupling.…”

“…To resolve the issue and conrm the presence of the secondary conductivity relaxation, we used the Coupling Model (CM). 15,25,26,36,46 Originally, the CM was applied to secondary relaxations having properties strongly connected to the structural a-relaxation. The existence and universal presence of this kind of secondary relaxation, called the Johari-Goldstein (JG) b-relaxation, was suggested by its analogy to the primitive relaxation of the CM, which started in a paper 47 in 1998, and has since been extensively developed.…”

“…For this reason, the CM also has long since been applied successfully to the ion conductivity relaxation of crystalline, glassy and molten ionic conductors. 15,25,26,36,46 There is also the CM equation relating the conductivity arelaxation time s sa (T) to the conductivity primitive relaxation time s s0 (T), which is…”