Collective dynamics of liquid sulphur across the polymerisation transition temperature probed by inelastic x-ray scattering

Abstract

Inelastic x-ray scattering (IXS) experiments on liquid sulphur were carried out below (140 ^∘C) and above (180 ^∘C) the polymerisation temperatureT_λof about 159 ^∘C to investigate changes in the collective dynamics of this unique liquid, that exhibits a liquid-liquid transition. As reported earlier, broad longitudinal acoustic excitation signals were observed at both temperatures, and only the width of the quasielastic peaks slightly decreased when the temperature crossed the transition temperature. A model analysis was performed using a generalised Langevin formalism with a memory function having one thermal and two viscoelastic decay channels with the help of simple sparse modelling, and large positive deviations from the hydrodynamic sound velocity by 51%-54% were observed. The fast viscoelastic relaxation timeτ_µis close to the correlation times of intermolecular stretching and bending motions of local sulphur connections in both ring and chain structures, and is similar to those of other molecular liquids. The small contrasts in the IXS spectra across theλtransition result in large changes in only the slow viscoelastic decay timeτ_αof the memory function. Theτ_αvalue at 140 ^∘C matches the mixed internal/external torsional modes of S₈molecules well, whereas that at 180 ^∘C has no corresponding molecular motion mode. The kinematic viscosity values at theQ→0limit are much smaller than the minimum values of macroscopic shear viscosity, indicating that large changes in relaxation dynamics are expected withQin the GHz and MHz excitation regimes.

Keywords: collective; dynamics; liquid; polymerisations; sulphur; transition.