Temperature and pressure dependences of the relaxation dynamics of supercooled systems explored by dielectric spectroscopy

Secondary Relaxation in Supercooled Liquid Propylene Glycol under Ultrahigh Pressures Revealed by Dielectric Spectroscopy Measurements

1,2-Propanediol (propylene glycol) is a well-known glassformer, which easily vitrifies under wide range of cooling rates. An interesting feature of propylene glycol is that, similar to glycerol, it retains one-mode primary relaxation (slow α process) under a wide range of external P- T conditions. It was demonstrated that the emergence of secondary (β) relaxation requires the application of very high pressures P > 4.5 GPa. In this pressure range, the observation of secondary relaxation is partially obfuscated by the presence of strong decoupling of the static (ionic) conductivity and primary relaxation (the fractional Debye-Stokes-Einstein effect). However, secondary relaxation can be unambiguously extracted from experimental data by the correlation procedure of the imaginary and real parts of the dielectric response by means of Cole-Cole plots. This is the second (after glycerol) example of observation of Johari-Goldstein relaxation under ultrahigh pressures P > 2 GPa.

Fractional Debye-Stokes-Einstein behaviour in an ultraviscous nanocolloid: glycerol and silver nanoparticles

One of the major features of glass forming ultraviscous liquids is the decoupling between translational and orientational dynamics. This paper presents studies of this phenomenon in glycerol, an accepted molecular glass former, concentrating on the impact of two exogenic factors: high pressures (P) up to the extreme 1.5 GPa and silver (Ag) nanoparticles (NPs). The analysis is focused on the fractional Debye-Stokes-Einstein (FDSE) relationship: σ(T,P)(τ(T,P))(S) = const, linking DC electric conductivity (σ) and primary (alpha, structural) relaxation time (τα). In glycerol and its nanocolloid (glycerol + Ag NPs) at atmospheric pressure only negligible decoupling (S ∼ 1) was detected. However, in the compressed nanocolloid, a well-defined transformation (at P = 1.2 GPa) from S ∼ 1 to the very strongly decoupled dynamics (S ∼ 0.5) occurred. For comparison, in pressurized 'pure' glycerol the stretched shift from S ∼ 1 to S ∼ 0.7 took place. This paper also presents the general discussion of FDSE behavior in ultraviscous liquids, including the new link between the FDSE exponent, fragility and the apparent activation enthalpy and volume.

Fluctuation-dissipation theorem and the dielectric response in supercooled liquids

We consider the correlation between static conductivity and dynamic dielectric relaxation in a number of polar organic liquids. Experimental evidence suggests that in the simple cases the linear dependence between characteristic frequency of relaxation process and the value of static susceptibility is observed. However, this proportionality can be broken due to the appearance of additional relaxation processes (secondary or high-frequency ones) so it can be confused with the "fractional" variant of Debye-Stokes-Einstein relation.

Molecular dynamics study of the thermal and the density effects on the local and the large-scale motion of polymer melts: scaling properties and dielectric relaxation

Results from a molecular dynamics simulation of a melt of unentangled polymers are presented. The translational motion, the large-scale and the local reorientation processes of the chains, as well as their relations with the so-called "normal" and "segmental" dielectric relaxation modes are thoroughly investigated in wide temperature and pressure ranges. The thermodynamic states are well fitted by the phenomenological Tait equation of state. A global time-temperature-pressure superposition principle of both the translational and the rotational dynamics is evidenced. The scaling is more robust than the usual Rouse model. The latter provides insight but accurate comparison with the simulation calls for modifications to account for both the local chain stiffness and the nonexponential relaxation. The study addresses the issue whether the temperature or the density is a dominant control parameter of the dynamics or the two quantities give rise to comparable effects. By examining the ratio /alphatau//alphaP between the isochronic and isobaric expansivities, one finds that the temperature is dominant when the dynamics is fast. If the relaxation slows down, the fluctuations of the free volume increase their role and become comparable to those of the thermal energy. Detectable cross-correlation between the "normal-mode" and the "segmental" dielectric relaxations is found and contrasted with the usual assumption of independent modes.