Understanding the difference in the stretched structural relaxations probed by dielectric and enthalpic studies of...

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.