Pressure-induced change in the relaxation dynamics of glycerol

Glycerol is one of the best studied and most widely used glass-forming liquids; however, its dynamic properties are still under discussion. The dielectric spectra of glycerol are studied in detail over wide ranges of temperatures and pressures up to 4.5 GPa. Starting from the pressures of 2–3 GPa, qualitative change in the dynamics of structural relaxation processes in glycerol has been revealed. It is accompanied by the appearance of secondary relaxation and a change in the asymptotic behavior of the pressure dependence of the fragility. The relation between the parameters for different relaxation mechanisms is analyzed.     

On the Viscoelastic and Dielectric Behavior of Some Organic Compounds and Their Binary Mixtures

The dependence of dielectric relaxation time on the viscosity of the medium is being extensively used to draw certain quantitative conclusions regarding molecular motion and inter-molecular forces in liquids, liquid mixtures, dilute solutions, and multi-component polar solutes in dilute solution. In the absence of proper empirical or theoretical equations for the variation of dielectric relaxation time with viscosity, only the experimental investigations on different systems can give an insight. In the present study, the results of dielectric measurements carried out on pure samples of bromohexane, bromooctane and bromodecane in dilute solutions in different mixed solvents (benzene + paraffin) and on binary mixtures (1 : 1) of (bromohexane + bromodecane); (bromodecane + propyl alcohol) and (propyl alcohol + methyl alcohol) are reported. For comparison, the results of bromodecane + propyl alcohol and propyl alcohol + methyl alcohol are chosen as they form examples of mixture of non-associative + associative and associative + associative liquids, respectively. Different parameters determined using these dielectric measurements are also presented using different models. These studies indicate that the dielectric behavior at microwave frequencies favor the concept of dynamic viscosity and a single viscoelastic relaxation time for the systems under study.     

Application of a new perturbation theory to evaluate the shear and bulk viscosity coefficients

Three liquids, CH₃F, CH₃Cl and CHF₃, have been submitted to systematic investigation in the microwave and far infra-red frequency domains.

The studies have been extended over a gradual variation of temperature and density scanning the whole liquid range beneath the critical region. Accordingly, the relative location in time of absorption processes were observed to evolve towards each other as the liquids expand when approaching their critical region.

We discuss the dielectric relaxation processes and their characteristic times, together with some very important features from the very intense absorption in the far infra-red. The correlative dispersion is fitted by a distribution of resonances model. The occurrence of specific alignment effects are shown to be responsible for local enhancement of the dipole moments, resulting in excess of integrated intensities.

Throughout this report a direct presentation of the experimental features displays already a number of characteristics of molecular dynamics. Their analysis will be completed and refined through the correlation functions formalism developed in a following paper.     

Ultrasonic spectroscopy of pure cyclic compounds

Ultrasonic studies of the group of halogens of benzene and particularly dependence of acoustic parameters on the structure of organic liquids, demonstrate some interesting regularities in the group of these compounds in gas and liquid states. In this paper results for five cyclic liquids: bromo-, chloro-, fluoro- and jodobenzene are discussed and compared to benzene. Vibrational relaxation was observed in all the compounds. The studies reported here as well as other experimental for great number of compounds support the conclusion that almost all vibrational relaxation processes in liquids can be described using a single relaxation time. It also seems that all vibrational degrees of freedom of the molecule take part in this vibrational process. It seems that differences in transition probabilities between molecules studied could be caused by additional attraction between of molecules having a significant dipole moments.     

Evolution of solid state systems containing mutually coupled dipolar and quadrupole spins: perturbation treatment

Perturbation approach to time evolution of multi-spin systems containing quadrupole and dipolar spins has been presented and discussed. The treatment comprises polarization transfer effects, field-dependent relaxation processes of dipolar as well as quadrupole spins and combined results of both of them. Complete theories dealing with various aspects of the spin dynamic processes have been proposed. Because of an educational character of this paper, relevant assumptions, limitations and even particular steps of the proposed treatments have been discussed in detail. Special emphasis is put on understanding of validity regimes of the perturbation treatment, depending on relative strengths of spin interactions and timescales of relevant motional processes affecting them. Motional regimes required for spins to be involved in essentially different evolution pathways like polarization transfers or relaxation have been illustrated by experimental examples.     

Extracting energy and structure properties of glass-forming liquids from structural relaxation time

A comprehensive examination of the kinetic liquid model (Wang et al 2010 J. Phys.: Condens. Matter 22 455104) is carried out by fitting the structural relaxation time of 26 different glass-forming liquids in a wide temperature range, including most of the well-studied materials. Careful analysis of the compiled reported data reveals that experimental inaccuracies should not be overlooked in any 'benchmark test' of relating theories or models (e.g. in Lunkenheimer et al 2010 Phys. Rev. E 81 051504). The procedure, accuracy, ability, and efficiency of the kinetic liquid model are discussed in detail and in comparison with other available fitting methods. In general, the kinetic liquid model could be verified by 17 of the 26 compiled data sets and can serve as a meaningful approximative method for analyzing these liquids. Nonetheless, further experimental examinations in a wide temperature range are needed and are called for. Through fitting, the microscopic details of these liquids are extracted, namely, the enthalpy, entropy, and cooperativity in structural relaxation, which may facilitate further quantitative analysis to both the liquidus and glassy states of these materials.     

Sizes and series of m35–atomic clusters in icosahedral quasicrystals and in m35–approximant crystals. Theory and examples

Abstract

A critical examination of the behaviour of three fragile glass forming liquids viz. Salol, α-phenyl-o-cresol and o-terphenyl, has been carried out using differential scanning calorimetry and dielectric relaxation technique (frequency: 10⁶-10^?3 Hz). Our study reveals two sub-T_g processes designated as β- and γ-processes, in addition to the primary (α-) process. The β-process has an activation energy of 36–50 kJ/mol. in these liquids, and is found to be intramolecular in nature. Our results along with the published viscosity data on these liquids indicate a decoupling of the viscous modes from the α-modes in the region of T_g, with the latter still retaining its non-Arrhenius character. The nature of this decoupling has been discussed in detail.     

Nagel scaling, relaxation, and universality in the kinetic ising model on an alternating isotopic chain

The dynamic critical exponent and the frequency and wave-vector dependent susceptibility of the kinetic Ising model on an alternating isotopic chain with Glauber dynamics are examined. The analysis provides a connection between a microscopic model and the Nagel scaling curve originally proposed to describe dielectric susceptibility measurements of several glass-forming liquids. While support is given to the hypothesis relating the Nagel scaling to multiple relaxation processes, it is also found that the scaling function may exhibit plateau regions and does not hold for all temperatures.     

Measurements on polar liquids at a microwave frequency — evaluation of molecular parameters — a new method

A simple method of estimating the dipole moment (μ) and relaxation time (τ) of polar molecules in liquid state, from a single set of dielectric measurements at high frequency, is proposed by using the concept of dielectric virial coefficients and employing Onsager model. The proposed method is tested in a number of polar liquids and the results are discussed.     

Frequency-dependent conductivity in ionic glasses: A possible model

The frequency-dependent (ac) conductivity behaviour of ionically conducting glasses is discussed. A critique is given of the treatment of Almond and West for this behaviour. Two models are proposed instead to account for the observed ac loss data. One involves parallel relaxation processes and is a modification of a model originally developed for the case of large-polaron transport in amorphous semiconductors. The other is based on series relaxation, and is based on a model originally proposed by Glarum for the case of dielectric relaxation in molecular liquids. In the development given here, this model is modified so as to describe ionic transport, and a new microscopic transport mechanism is proposed—“diffusion-controlled relaxation”—which satisfies the requirements of the Glarum theory and which naturally incorporates cooperative ionic motions. For this reason, it is intuitively more appealing.     

Dielectric relaxation and related studies of 4-ethylphenol-methanol mixtures using time domain reflectometry

The dielectric relaxation studies of 4-ethylphenol-methanol mixtures have been carried out at various temperatures ranging from 10°C to 40°C using time domain reflectometry in the frequency range 10 MHz to 10 GHz. The relaxation mechanism in these systems is explained by Cole-Davidson model. The excess dielectric parameters, Kirkwood correlation factor and activation energy have been calculated and discussed with respect to molecular arrangements, and microdynamics of the binary mixture composed of both the associative type of liquids.     

A CNDO/2 calculation on the radical formed on electron irradiation of mesitylene

Mori's method is applied to the calculation of the collective molecular orientation correlation functions which are of interest in light scattering and dielectric relaxation. The case of liquids composed of asymmetric top molecules is discussed. For the collective variables, the usual ‘rotational diffusion’ assumptions do not yield the single-particle rotational diffusion results unless there exists no orientational order in the medium. This point is discussed with regard to the results of some recent light scattering and dielectric relaxation experiments.     

Investigation on crystalline structure and dielectric relaxation behaviors of hot pressed poly(vinylidene fluoride) film

The dielectric relaxation behaviors of hot pressed poly(vinylidene fluoride) (PVDF) film have been studied using dielectric spectroscopy in the frequency domain from 20 Hz to 5 MHz at temperatures between 20 °C and 200 °C. Crystalline/amorphous interphase is suggested with methods of FTIR, XRD, and DSC. Frequency and temperature dependence of dielectric spectroscopy reveals the relaxation behavior and structural dynamics of the samples, and three types of relaxation processes are suggested, α_Ac relaxation process contributed by the hopping transport process near the periphery of conduction band or valence zones at Fermi energy, α_c relaxation process related to the structure change of crystal lattice trapped dipoles in crystalline regions, and α_a relaxation process arising from segmental dipole rearrangement of interphases in amorphous regions. Cole-Cole and Havriliak-Negami experimental equations were utilized to analyze these relaxation processes, and differences of Arrhenius parameters for α_Ac and α_c relaxation processes obtained from Cole-Cole and Havriliak-Negami equations were discussed in detail. Activity energy of different relaxation processes obtained from Arrhenius equation and VFT equation indicates non-single thermal activation mechanism for hot pressed PVDF film.     

Light scattering and dielectric manifestations of secondary relaxations in molecular glassformers

Photon correlation data of the molecular glass-forming materials 2-picoline, dimethylphthalate (DMP) and salol are compared with their dielectric loss spectra in the time-frequency range where the dielectric data reveal secondary relaxations. Slow secondary relaxation processes in molecular liquids are commonly studied by dielectric spectroscopy (DS) and, based on such studies, believed to be characteristics of the deeply super-cooled liquid state. However, there has been no direct experimental evidence that they are similarly detected by other experimental techniques. In the present study, we experimentally address this question for the anisotropic (depolarized) light scattering (LS). In the first approximation, DS and LS probe the same molecular reorientation dynamics, and therefore are expected to provide qualitatively similar spectra. We find however that this is not the case, namely i) the magnitude of the slow secondary relaxations is much less in LS than in DS data, which is the opposite to expectations; ii) the shape of the relaxation spectrum is qualitatively different, concerning both the main and secondary processes. We discuss possible sources of these differences in the context of related data from the literature.Received: 21 July 2003, Published online: 23 December 2003PACS: 64.70.Pf Glass transitions - 77.22.Gm Dielectric loss and relaxation - 78.35. + c Brillouin and Rayleigh scattering other light scatteringJ. Mattsson: Presently at: DEAS and Physics Department, Harvard University, Cambridge, MA 02138, USA     

Fundamental aspects of light scattering and optical Kerr effect spectroscopy

We describe the principles of two spectroscopic methods of non-resonance light scattering (LS) and optical-Kerr-effect (OKE) spectroscopies in detail, and review basic but truly important phenomena observed by these two methods. Particularly, we focus on the following three experiments: 1) Response functions determined by frequency-domain LS and time-domain OKE spectroscopies, almost completely agree with each other, indicating that the quantum-mechanical fluctuation-dissipation theorem (QM-FDT) holds well in this system. 2) Femtosecond time responses of liquids clearly show an initial rise process even though they show an exponential response at a later time. This indicates that the Debye relaxation model does not hold in such an early time region. 3) From the measurement of the Stokes and anti-Stokes LS intensity ratios, it is found that the LS spectra of relaxation modes in liquids and solids are symmetric with respect to the spectral origin of the scattered light and hence the ratio does not satisfy the Boltzmann distribution rule expected from QM-FDT. These experimental results which contain apparently contradictory data are closely related with the nature of relaxation modes examined, which more or less assume a macroscopic character, and also with the physical basis of relaxation, which is inevitably connected to the observation problem of quantum mechanics. We discuss these points in relation to the physical reality of macroscopic quantity.     

Thermal behaviour of two room-temperature ionic liquids containing the methylimidazolium cation

We have investigated the glass transition relaxation of two room-temperature ionic liquids using Modulated Temperature Differential Scanning Calorimetry (MTDSC). Furthermore, conventional DSC was used to clarify their crystallisation behaviour. One of the liquids avoids crystallisation, while the other shows cold crystallisation. A step-by-step temperature scanning experimental procedure was used to analyse in detail the thermal behaviour of the latter in the crystallisation and melting temperature regions. The existence of polymorphism is discussed.     

Relation between dielectric and low-frequency Raman spectra of hydrogen-bond liquids

We analyzed the complex dielectric and Raman spectra of hydrogen-bond liquids in the microwave to terahertz frequency range. As for water and methanol, the high-frequency component of the dielectric spectrum, i.e., the small deviation from the principal Debye relaxation, clearly corresponds to the Raman spectrum. This indicates that the cooperative relaxation, accompanied by huge polarization fluctuation, is virtually not Raman active, whereas the faster processes reflect common microscopic dynamics. For ethylene glycol, the shape of the Raman spectrum also resembles that of the high-frequency deviation of the dielectric spectrum, but, additionally, a weak manifestation of the cooperative relaxation arising from quadrupolar conformers is detected.     

Statistical mechanical approach to secondary processes and structural relaxation in glasses and glass formers: a leading model to describe the onset of Johari-Goldstein processes and their relationship with fully cooperative processes

The interrelation of dynamic processes active on separated time-scales in glasses and viscous liquids is investigated using a model displaying two time-scale bifurcations both between fast and secondary relaxation and between secondary and structural relaxation. The study of the dynamics allows for predictions on the system relaxation above the temperature of dynamic arrest in the mean-field approximation, that are compared with the outcomes of the equations of motion directly derived within the Mode Coupling Theory (MCT) for under-cooled viscous liquids. By varying the external thermodynamic parameters, a wide range of phenomenology can be represented, from a very clear separation of structural and secondary peak in the susceptibility loss to excess wing structures.     

Statistical mechanical approach to secondary processes and structural relaxation in glasses and glass formers A leading model to describe the onset of Johari-Goldstein processes and their relationship with fully cooperative processes

The interrelation of dynamic processes active on separated time-scales in glasses and viscous liquids is investigated using a model displaying two time-scale bifurcations both between fast and secondary relaxation and between secondary and structural relaxation. The study of the dynamics allows for predictions on the system relaxation above the temperature of dynamic arrest in the mean-field approximation, that are compared with the outcomes of the equations of motion directly derived within the Mode Coupling Theory (MCT) for under-cooled viscous liquids. By varying the external thermodynamic parameters, a wide range of phenomenology can be represented, from a very clear separation of structural and secondary peak in the susceptibility loss to excess wing structures.