From boiling point to glass transition temperature: Transport coefficients in molecular liquids follow three-parameter scaling

The phenomenon of the glass transition is an unresolved problem in condensed matter physics. Its prominent feature, the super-Arrhenius temperature dependence of the transport coefficients, remains a challenge to be described over the full temperature range. For a series of molecular glass formers, we combined {τ}(T) collected from dielectric spectroscopy and dynamic light scattering covering a range 10^-12 s {τ}(T) 10² s. Describing the dynamics in terms of an activation energy E(T), we distinguish a high-temperature regime characterized by an Arrhenius law with a constant activation energy E_∞ and a low-temperature regime for which Ecoop(T) {≡} E(T)--E_∞ increases exponentially while cooling. A scaling is introduced, specifically Ecoop(T)/E_∞ {∝} exp[--{λ}(T/TA--1)], where {λ} is a fragility parameter and TA a reference temperature proportional to E_∞. In order to describe {τ}(T) still the attempt time τ_∞ has to be specified. Thus, a single interaction parameter E_∞ describing the high-temperature regime together with {λ} controls the temperature dependence of low-temperature cooperative dynamics.