Andrea Mandanici

Dynamic shear modulus of glycerol: Corrections due to instrument compliance

A recent article by Shi et al. [J. Chem. Phys.123, 174507 (2005)] reports results from mechanical measurements on three simple inorganic glass formers: glycerol, m-toluidine, and sucrose benzoate. The experiments carried out were stress relaxation, aging, and dynamic (all in shear) using a torsional rheometer, an advanced rheometric expansion system (TA Instruments). The original force rebalance transducer (2KFRT) supplied with the system was replaced with a custom-made load cell (Sensotec) that had a capacity of 20 000 g cm in torque and 5000 g in normal force. The replacement of the load cell was done due to the belief that the main source of compliance in this instrument was from the 2KFRT. With this assumption, the authors published their results for the three materials of interest and compared their results with the techniques of Schroter and Donth [J. Chem. Phys.113, 9101 (2000)] for the measurements on glycerol and reported important differences. These differences were disputed by one of the present authors (Schroter), and the present report shows that the results from Schroter and Donth are correct. We show that the reasons have to do with the instrument compliance being greater than originally thought by Shi et al. Here we examine the effects of platen diameter/geometry on the glycerol dynamic moduli, describe a means to correct dynamic data, present a revised comparison of the corrected data with that of Schroter and Donth, and provide a discussion of future work and conclusions.

Shear stress relaxation and physical aging study on simple glass-forming materials

Relaxation and aging behaviors in three supercooled liquids: m-toluidine, glycerol, and sucrose benzoate have been studied by shear stress relaxation experiments in the time domain above and below their nominal glass transition temperatures. For the equilibrium state, the current study provides new data on the behavior of organic complex fluids. The shape of the relaxation function as characterized by the stretching exponent beta is discussed considering that a time-temperature master curve can be constructed even though the betas for the individual response curves at each temperature vary systematically. In the nonequilibrium state, isothermal physical aging experiments at different glassy structures reveal that the effect of the aging process on the mechanical shear relaxation in these simple glass formers is similar to that observed in polymeric and other systems. Departure from the Vogel-Fulcher-Tamman behavior after the samples have aged back to equilibrium in the glassy state is observed for m-toluidine and, less strongly, for glycerol but not for sucrose benzoate. An inherent structure-based energy landscape concept is briefly discussed to account for the slow dynamics during the physical aging process.

The α-relaxation process in simple glass forming liquid m-toluidine. I. The temperature dependence of the dielectric response

Wide band dielectric spectroscopy has been used to study the bulk dynamics of the simple supercooled liquid m-toluidine. Following the temperature and frequency dependence of the dielectric response we find a stretched relaxation process, the stretching parameter of which is temperature dependent. The observed decoupling between rotational and translational diffusion parameters at low temperatures may be the result of heterogeneous dynamics.

Dynamics of glass-forming liquids. VIII. Dielectric signature of probe rotation and bulk dynamics in branched alkanes.

We have measured the dielectric relaxation of several glass forming branched alkanes with very low dielectric loss in the frequency range 50 Hz-20 kHz. The molecular liquids of this study are 3-methylpentane, 3-methylheptane, 4-methylheptane, 2,3-dimethylpentane, and 2,4,6-trimethylheptane. All liquids display asymmetric loss peaks typical of supercooled liquids and slow beta relaxations of similar amplitudes. As an unusual feature, deliberate doping with 2-ethyl-1-hexanol, 5-methyl-2-hexanol, 2-methyl-1-butanol, 1-propanol, or 2-methyltetrahydrofuran at the 1 wt % level generates additional relaxation peaks at frequencies below those of the alpha relaxation. The relaxation times of these sub-alpha-peaks increase systematically with the size of the dopant molecules. Because these features are spectrally separate from the bulk dynamics, the rotational behavior and effective dipole moments of the probes can be studied in detail. For the alcohol guest molecules, the large relative rotational time scales and small effective dipole moments are indicative of hydrogen bonded clusters instead of individual molecules.

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.

Dielectric modulus analysis of mixed alkali LixRb1−xPO3 glasses

Abstract Dielectric data of mixed alkali LixRb1−xPO3 glasses have been analysed in the modulus formalism. The imaginary part of the complex modulus was fitted with a KWW function. The stretching parameter β is larger in the mixed glasses as compared to the single alkali glasses, which indicates that the mixed alkali glasses behave as single alkali glasses of effectively lower concentrations. The finding is consistent with the random ion distribution model, which assumes that the two kinds of alkali ions are moving in distinctly different conduction pathways.

Ionic conduction in silver phosphate glasses doped with silver sulphide

Abstract The dielectric properties of (Ag2S) x ( AgPO3)1‐x and AgPO3 ionic glasses have been investigated at low frequencies (10 Hz‐10 MHz) by complex impedance measurements in the temperature range between 20 and 330 K and at microwave frequencies (8.2–18 GHz) using a waveguide transmission line and also a cavity resonator at 9 GHz from 77 to 300 K. Analysis on the basis of the jump relaxation model has shown that translational and reorientational hopping motions of mobile Ag+ ions are responsible for the observed frequency‐dependent ac conductivity. The temperature dependence of the ac conductivity at fixed frequencies can be fitted using three relaxation processes with characteristic activation energies which differ from each other by an order of magnitude and are closely similar to those obtained from previous ultrasonic attenuation studies on the same glasses. The highest energy (about 0–4 eV) has been associated with the translational hopping motion, responsible for the long‐range ionic diffusion th...

Ionic conduction and dynamical regimes in silver phosphate glasses

Abstract Dielectric measurements on ion conducting silver metaphosphate glasses, pure and doped with silver sulphide or silver sulphate, have been performed in the microwave frequency region up to 60 GHz in a wide temperature range between 77 and 413 K. With the current extension of previous measurements, a frequency range larger than 10 decades has been investigated and several distinct conductivity regimes have been observed. Above the low frequency dc conductivity value, the progressive increase of conductivity with frequency has been found to obey first a power law with exponent p ∼2/3 and later another power law with exponent q ∼4/3, nearly independent of temperature and of composition. In the low temperature region a nearly constant loss contribution to the conductivity has been also detected.

On the features of the dielectric response of supercooled ethylcyclohexane

The main relaxation observed in supercooled ethylcyclohexane in the 50 Hz–20 kHz frequency range using high-resolution dielectric spectroscopy displays an unusual relaxation pattern. The slope of the dielectric loss profile above the peak frequency, progressively tends to zero with increasing temperature. The observed features could be compatible with the existence of different underlying processes. A simple approach based on scaling is exploited to provide relevant information on the properties of the primary and secondary relaxation in ethylcyclohexane. The apparently anomalous behaviour is shown to be due to the superposition of a secondary process to the dielectric α-process and support the thesis that the β-relaxation observed in the glassy phase could have an intramolecular origin.