Understanding the difference in the stretched structural relaxations probed by dielectric and enthalpic studies of glass forming substances

Abstract

We investigated the stretched dynamics of the structural relaxation in molecular glass formers by using dielectric and thermal (or enthalpic) relaxations. The dielectric stretching exponents β die are determined by the Havriliak–Negami function, while the enthalpic β TNMH is quantified by using the Tool–Narayanaswamy–Moynihan–Hodge formalism. We found β TNMH is anticorrelated with the degree of freedom, a molecule addressed by the concept of beads. Referring to the reported relation of β die to the dipole moment μ, we proposed a combined parameter of μ2* beads, which can rationalize the difference in stretching exponents obtained by dielectric and enthalpic relaxations. For the majority of glass-forming molecules, the difference is trivial, but for those molecules with both unusually high dipole moments and flexibility, a large difference is obvious. The interplay of the degree of freedom and dielectric dipole–dipole interaction in molecular dynamics is addressed.