Is the λ-transition in liquid sulfur a fragile-to-strong transition?

Abstract

The abrupt and large increase in the viscosity of liquid sulfur above the λ-transition temperature Tλ corresponds to a reversible structural transformation in the form of a ring-to-chain polymerization reaction. The mechanistic connection between this structural transformation and viscosity is investigated by studying the compositional dependence of the shear relaxation behavior of supercooled SxSe100-x liquids as their structural evolution mimics that of liquid sulfur across the λ-transition. The results of steady and oscillatory shear parallel-plate rheometry indicate that the viscosity of these liquids is controlled by the S/Se-S/Se bond scission/renewal dynamics and the time scale of these dynamics rapidly increases as the relative concentrations of rings and chains in the structure become comparable. The coexistence of these two types of topological units in these liquids lowers the conformational entropy of the chain elements due to a steric hindrance from the ring elements, resulting in a rapid drop in the fragility as S is added to Se. The same topological effect resulting from the ring-to-chain transformation in liquid S renders the highly fragile molecular liquid below Tλ, a strong polymerized liquid above Tλ. Therefore, it is argued that the λ-transition of liquid S corresponds to a fragile-to-strong liquid-liquid transition.