Supercooled Ortho-terphenyl Research Articles

On the ergodicity of supercooled molecular glass-forming liquids at the dynamical arrest: the o-terphenyl case

The dynamics of supercooled ortho-terphenyl has been studied using photon-correlation spectroscopy (PCS) in the depolarized scattering geometry. The obtained relaxation curves are analyzed according to the mode-coupling theory (MCT) for supercooled liquids. The main results are: i) the observation of the secondary Johari-Goldstein relaxation (β) that has its onset just at the dynamical crossover temperature TB (TM > TB > Tg); ii) the confirmation, of the suggestion of a recent statistical mechanical study, that such a molecular system remains ergodic also below the calorimetric glass-transition temperature Tg. Our experimental data give evidence that the time scales of the primary (α) and this secondary relaxations are correlated. Finally a comparison with recent PCS experiments in a colloidal system confirms the primary role of the dynamical crossover in the physics of the dynamical arrest.

Translational and Reorientational Motion in Supercooled Ortho-Terphenyl : A Molecular-Dynamics Study

Translational and Reorientational Motion in Supercooled Ortho-Terphenyl : A Molecular-Dynamics Study

Dynamic coupling of a small rigid probe to viscous ortho-terphenyl

We have measured the dielectric relaxation of 2,2,6,6-tetramethyl-piperidine-1-oxyl (TEMPO) as a rotational probe in supercooled ortho-terphenyl (OTP). Due to the significant dipole moment of TEMPO compared with OTP, its contribution to the loss spectra can be identified already at moderate concentrations. For time scales ranging from 10 μs to 1 s, it is found that the tumbling mode of TEMPO is a true replica of the structural relaxation of OTP regarding average time constant, relaxation time dispersion, and the temperature dependence. While the present dielectric results are consistent with a decoupling of a spinning mode (about the nitroxyl dipole axis) of TEMPO from viscosity, they do not agree with the strong decoupling of the tumbling mode derived from electron spin resonance experiments.

Structural relaxation in supercooled orthoterphenyl.

We report molecular-dynamics simulation results performed for a model of molecular liquid orthoterphenyl in supercooled states, which we then compare with both experimental data and mode-coupling-theory (MCT) predictions, aiming at a better understanding of structural relaxation in orthoterphenyl. We pay special attention to the wave number dependence of the collective dynamics. It is shown that the simulation results for the model share many features with experimental data for real system, and that MCT captures the simulation results at the semiquantitative level except for intermediate wave numbers connected to the overall size of the molecule. Theoretical results at the intermediate wave number region are found to be improved by taking into account the spatial correlation of the molecule's geometrical center. This supports the idea that unusual dynamical properties at the intermediate wave numbers, reported previously in simulation studies for the model and discernible in coherent neutron-scattering experimental data, are basically due to the coupling of the rotational motion to the geometrical-center dynamics. However, there still remain qualitative as well as quantitative discrepancies between theoretical prediction and corresponding simulation results at the intermediate wave numbers, which call for further theoretical investigation.

Logarithmic decay of the orientational correlation function in supercooled liquids on the Ps to Ns time scale

Dynamics of supercooled ortho-terphenyl, salol, benzophenone, 2-biphenylmethanol, and dibutylphthalate have been studied using optical heterodyne detected optical Kerr effect experiments over a broad range of time, <1 ps to tens of ns. On time scales longer than those influenced by intramolecular vibrational dynamics, “intermediate” power law decays with temperature independent exponents close to −1 have been observed from ∼2 ps to 1–10 ns in all five samples. The intermediate power law decays occur over a wide range of temperatures from well above to somewhat below Tc, the mode-coupling theory (MCT) critical temperature. The intermediate power law corresponds to approximately a logarithmic decay of the polarizability–polarizability (orientational) correlation function. The amplitude of the intermediate power law increases with increasing temperature as [(T−Tc)/Tc]1/2. The intermediate power law decay is followed by a second longer time scale power law, and the final portion of the decay is exponential. As a framework for discussion, the results are compared to the quantitative predictions of the MCT. The observations are in contrast to the standard MCT for the longer time portions of the decays. The observed intermediate power law decays may be consistent with MCT if the dynamics occur in the part of the MCT parameter space near a high order singularity (end point case).

Orientational and induced contributions to the depolarized Rayleigh spectra of liquid and supercooled ortho-terphenyl

The depolarized light scattering spectra of the glassforming liquid ortho-terphenyl have been calculated in the low frequency region using molecular dynamics simulation. Realistic system configurations are produced by using a recent flexible molecular model and combined with two limiting polarizability schemes, both of them using the dipole-induced-dipole contributions at first and second order. The calculated Rayleigh spectral shape are in good agreement with the experimental results in a large temperature range. The analysis of the different contributions to the Rayleigh spectra emphasizes that the orientational and the collision-induced (translational) terms lie on the same time scale and are of comparable intensity. Moreover, the cross correlation terms are always found to be an important contribution to the scattering intensity.

Dynamics and configurational entropy in the Lewis-Wahnström model for supercooled orthoterphenyl.

We study thermodynamic and dynamic properties of a rigid model of the fragile glass-forming liquid orthoterphenyl. This model, introduced by Lewis and Wahnström in 1993, collapses each phenyl ring to a single interaction site; the intermolecular site-site interactions are described by the Lennard-Jones potential whose parameters have been selected to reproduce some bulk properties of the orthoterphenyl molecule. A system of N=343 molecules is considered in a wide range of densities and temperatures, reaching simulation times up to 1 micros. Such long trajectories allow us to equilibrate the system at temperatures below the mode coupling temperature T(c) at which the diffusion constant reaches values of order 10(-10) cm(2)/s and thereby to sample in a significant way the potential energy landscape in the entire temperature range. Working within the inherent structures thermodynamic formalism, we present results for the temperature and density dependence of the number, depth and shape of the basins of the potential energy surface. We evaluate the total entropy of the system by thermodynamic integration from the ideal-noninteracting-gas state and the vibrational entropy approximating the basin free energy with the free energy of 6N-3 harmonic oscillators. We evaluate the configurational part of the entropy as a difference between these two contributions. We study the connection between thermodynamical and dynamical properties of the system. We confirm that the temperature dependence of the configurational entropy and of the diffusion constant, as well as the inverse of the characteristic structural relaxation time, are strongly connected in supercooled states; we demonstrate that this connection is well represented by the Adam-Gibbs relation, stating a linear relation between logD and the quantity 1/TS(c). This relation is found to hold both above and below the critical temperature T(c)-as previously found in the case of silica-supporting the hypothesis that a connection exists between the number of basins and the connectivity properties of the potential energy surface.

Acoustic and relaxation processes in supercooled orthoterphenyl by optical-heterodyne transient grating experiment.

The dynamics of the fragile glass-forming orthoterphenyl have been investigated by transient grating experiments with an heterodyne detection technique. We measured the relaxation processes of this glass former over more than six decades in time with an excellent signal-to-noise ratio. Acoustic, structural, and thermal relaxations have been clearly identified in a time-frequency window not covered by previous spectroscopic studies and their characteristic dynamic parameters have been measured as a function of temperature and wave vector. A detailed comparison with the density response function, calculated on the basis of generalized hydrodynamic model, has been worked out.

Is the susceptibility minimum of glass-forming liquids due to the β-relaxation of the mode coupling theory or caused by the ubiquitous constant loss?

Observation of a minimum in the susceptibility spectra of glass-forming liquids has often been used as evidence of the fast β-relaxation in the mode coupling theory (MCT). However, recent experiments showing the absence of the knee predicted by MCT and the presence of the fast relaxation in crystalline as well as in glassy and supercooled ortho-terphenyl has cast doubt on the cause of the fast relaxation. A constant (approximately frequency independent) loss contribution to the dielectric susceptibility also can generate a minimum. The constant loss contribution is ubiquitous. I show that it is related to the fast process observed by light and neutron scattering and is possibly the cause of the susceptibility minimum. The properties of the constant loss have led to the suggestion that it originates from vibrational relaxation, a conclusion reached also by recent work on crystalline ortho-terphenyl. This suggestion is made plausible by theoretical results from studies of relaxation of non-linear (anharmonic) vibrations. Finally I show that if the constant loss contribution is the cause of the susceptibility minimum, then an explanation of the entire spectrum including the structural relaxation at all temperatures can be obtained by simply incorporating the constant loss into the coupling model of the author.

A nuclear magnetic resonance study of higher-order correlation functions in supercooled ortho-terphenyl

Using deuteron NMR techniques two-, effective three-, and various four-time correlation functions were recorded for supercooled ortho-terphenyl at 10–15 K above the calorimetric glass transition in order to characterize the heterogeneous nature of its primary response. The experimental results could successfully be described within various energy landscape models as well as via continuous time random walk simulations. These theoretical considerations provide a suitable basis for a definition of the term dynamic heterogeneity. We discuss the power but also some limitations of the present multidimensional NMR techniques when applied to amorphous materials.

Coherent dynamic structure factor of orthoterphenyl around the mode coupling crossover temperatureTc

The coherent dynamic structure factor of supercooled orthoterphenyl has been measured by time-of-flight neutron scattering. The data extend previous backscattering and spin-echo data towards shorter times. In the $\ensuremath{\beta}$ relaxation regime the two-step decay of density correlations can be described by the same master function as found before for self-correlations. We show that the mode-coupling crossover temperature ${T}_{c}$ can be determined from $\ensuremath{\beta}$ relaxation data alone. The wave-number dependence of the relaxation amplitude follows, to some extent, that of the static structure factor.

Translational and Reorientational Motion in Supercooled Ortho-Terphenyl: A Molecular-Dynamics Study

Translational and Reorientational Motion in Supercooled Ortho-Terphenyl: A Molecular-Dynamics Study

Relaxation of spatially heterogeneous dynamic domains in supercooled ortho-terphenyl

A photobleaching technique has been used to observe rotational dynamics of dilute probe molecules in supercooled o-terphenyl (OTP). The nonexponential rotational relaxation of the probe molecules is shown to be due, at least in part, to the presence of spatial heterogeneity in the host dynamics. Under appropriate photobleaching conditions, a nonequilibrium distribution of probe molecule mobilities can be created by preferentially bleaching the more mobile probe molecules in a sample. Near Tg this nonequilibrium distribution is observed to return to an equilibrium distribution of relaxation times over times on the order of 103 τc, where τc is the average rotational correlation time of a probe molecule. The time required to return to equilibrium is interpreted as a structural relaxation time for dynamic heterogeneities in OTP.

Translational and rotational diffusion in supercooled orthoterphenyl close to the glass transition

Self diffusion coefficients in supercooled orthoterphenyl (OTP) have been determined down toD t =3·10−14 m2s−1 using a1H-NMR technique applying static field gradients up to 53T m−1 In a range of more than two decades theD t values agree with those of photochromic tracer molecules of the same size determined by forced Rayleigh scattering down to the glass transition temperatureT g . A change of mechanism is found for translational diffusion atT c ≈1.2T g whereD t is proportional to the inverse shear viscosityη −1 atT>T c butD t ∼η ξ with ξ=0.75 atT<T c . Rotational correlation times determined by2H-NMR stimulated echo techniques in deuterated OTP remain proportinal toη −1 down toT g . Our results are discussed in relation with mode coupling theory and with models of cooperative motion at the glass transition.

2H-NMR study of the glass transition in supercooled ortho-terphenyl

The glass forming molecular liquid ortho-terphenyl has been investigated by 2H-NMR techniques providing spin-relaxation times (T1, T2), and spin-alignment data which yield information on the time scale and geometry of ultra-slow molecular reorientation. The main results are as follows: The primary glass transition (α process) is characterized by rotational molecular jumps with a jump size distribution weighted in favor of large jump angles, and by a distribution of correlation times. In addition intramolecular flip–flop jumps of the lateral phenyl rings are found which do not take part in the α process. Apart from this (secondary) intramolecular dynamics no residual small angle reorientation persists below Tg on the time scale (10−4 to 102 s) of the spin-alignment experiment.

Pressure effects on dielectric relaxation of supercooled ortho-terphenyl

Pressure effects on dielectric relaxation of supercooled ortho-terphenyl

Molecular motion in the supercooled liquid state: Small molecules in slow motion

It is shown that the motion of small molecules in the supercooled liquid state may occur at reorientation rates 10 8 to 10 11 sec slower than in normal low viscosity solvents. The detailed dielectric behaviour of anthrone and nitrobenzene in supercooled ortho-terphenyl is described, and interpreted as a cooperative process. The important implications of this work in relation to the behaviour of solid polymers is briefly discussed.