Thermodynamic scaling of α-relaxation time and viscosity stems from the Johari-Goldstein β-relaxation or the primitive relaxation of the coupling model

摘要

By now it is well established that the structural α-relaxation time, τ _α, of non-associated small molecular and polymeric glass-formers obey thermodynamic scaling. In other words, τ _α is a function Φ of the product variable, ρ ~γT, where ρ is the density and T the temperature. The constant γ as well as the function, τ _α Φ(ρ ~γT), is material dependent. Actually this dependence of τ _α on ρ ~γT originates from the dependence on the same product variable of the Johari-Goldstein β-relaxation time, τ _β, or the primitive relaxation time, τ _0, of the coupling model. To support this assertion, we give evidences from various sources itemized as follows. (1) The invariance of the relation between τ _α and τ _β or τ _0 to widely different combinations of pressure and temperature. (2) Experimental dielectric and viscosity data of glass-forming van der Waals liquids and polymer. (3) Molecular dynamics simulations of binary Lennard-Jones (LJ) models, the Lewis-Wahnstr?m model of ortho-terphenyl, 1,4 polybutadiene, a room temperature ionic liquid, 1-ethyl-3-methylimidazolium nitrate, and a molten salt 2Ca(NO _3) _2·3KNO _3 (CKN). (4) Both diffusivity and structural relaxation time, as well as the breakdown of Stokes-Einstein relation in CKN obey thermodynamic scaling by ρ ~γT with the same γ. (5) In polymers, the chain normal mode relaxation time, τ _N, is another function of ρ ~γT with the same γ as segmental relaxation time τ _α. (6) While the data of τ _α from simulations for the full LJ binary mixture obey very well the thermodynamic scaling, it is strongly violated when the LJ interaction potential is truncated beyond typical inter-particle distance, although in both cases the repulsive pair potentials coincide for some distances.