Dynamics of glass-forming liquids. XII. Dielectric study of primary and secondary relaxations in ethylcyclohexane.

The dynamics of ethylcyclohexane are investigated by high resolution dielectric spectroscopy aiming to characterize the relevant relaxational features of this simple system in its fluid, supercooled liquid, and glassy states. The dielectric signature of structural relaxation is a primary loss peak with amplitude Deltaepsilon=0.01, and a secondary loss process is found in the glassy state. This beta relaxation is compared with a "slow" process revealed by ultrasonics and with previously found gamma and chi processes in similar materials containing the cyclohexyl group. The results suggest that this secondary process is an intramolecular mode rather than a Johari-Goldstein process, consistent with its persistence in the liquid state at slow relaxation times which exceed those of the alpha process. The dielectric activity of such a slow process requires that the dipole magnitude changes with the intramolecular transition, whereas a change in dipole direction only would be masked by the faster structural relaxation.

Multiple mechanical relaxations in ethylcyclohexane above the glass transition temperature.

The mechanical response of ethylcyclohexane has been investigated at ultrasonic frequencies in a large temperature range from 300 K down to the glass transition region. The results indicate the existence of a secondary relaxation not yet reported for this system. The comparison with literature data leads to a rather complex dynamic behavior. In fact, this molecular liquid exhibits three different mechanical relaxations above the glass transition temperature: a main structural process and two additional processes, both having a possible intramolecular origin.

Thermodynamics, structure, and dynamics in room temperature ionic liquids: the case of 1-butyl-3-methyl imidazolium hexafluorophosphate ([bmim][PF6]).

A detailed investigation of the phase diagram of 1-butyl-3-methyl imidazolium hexafluorophosphate ([bmim][PF(6)]) is presented on the basis of a wide set of experimental data accessing thermodynamic, structural, and dynamical properties of this important room temperature ionic liquid (RTIL). The combination of quasi adiabatic, continuous calorimetry, wide angle neutron and X-ray diffraction, and quasi elastic neutron scattering allows the exploration of many novel features of this material. Thermodynamic and microscopic structural information is derived on both glassy and crystalline states and compared with results that recently appeared in the literature allowing direct information to be obtained on the existence of two crystalline phases that were not previously characterized and confirming the view that RTILs show a substantial degree of order (even in their amorphous states), which resembles the crystalline order. We highlight a strong connection between structure and dynamics, showing the existence of three temperature ranges in the glassy state across which both the spatial correlation and the dynamics change. The complex crystalline polymorphism in [bmim][PF(6)] also is investigated; we compare our findings with the corresponding findings for similar RTILs. These results provide a strong experimental basis for the exploration of the features of the phase diagram of RTILs and for the further study of longer alkyl chain salts.

Thermodynamic study of alkyl-cyclohexanes in liquid, glassy, and crystalline states.

The specific heat of some alkyl-cyclohexanes in their liquid, supercooled liquid, crystalline, and (for the first time) glassy states has been measured by quasiadiabatic calorimetry. Thermodynamic properties as well as the glass forming ability have been studied as a function of systematic changes of the molecular structure. Only one stable crystalline phase is observed experimentally for ethylcyclohexane, propylcyclohexane, and butylcyclohexane. In the case of methylcyclohexane, experimental evidence is provided of a crystal-to-crystal transition at temperatures just below the melting.

Dynamics of glassy and liquid m-toluidine investigated by high-resolution dielectric spectroscopy.

The glass-former m-toluidine displays the characteristic properties of a fragile supercooled liquid, which suggest the existence of a slow secondary relaxation process. In view of the recently realized importance of such a secondary relaxation feature, we have conducted a dielectric search for the secondary process in viscous and glassy m-toluidine. Based on high-resolution experiments on the distilled liquid, a secondary process can be identified which has the properties typical of a Johari-Goldstein beta relaxation. As a result, the previous hypothesis that the methyl group might be responsible for suppressing the secondary dynamics in glassy m-toluidine no longer holds.

Slow dynamics of supercooled m-toluidine investigated by mechanical spectroscopy.

Dynamics of supercooled m-toluidine close to the glass transition have been investigated by dynamic shear modulus measurements and stress relaxation experiments. The viscoelastic response of this material follows time-temperature-superposition in the temperature range investigated. Comparison with results at ultrasonic frequencies suggests the existence of a secondary relaxation. A change of the temperature dependent viscosity from a Vogel-Fulcher-Tammann behavior to another regime at low temperatures is also found. Compared to most inorganic glass formers, the viscosity of m-toluidine at the glass transition is approximately two orders of magnitude lower. The shear relaxation times are characterized by the same temperature dependence as the viscosity. They are in reasonable agreement with the results of previous ultrasonic measurements. The conclusions of the present work agree with recent results obtained by high resolution dielectric spectroscopy.