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 of the product variable, ργ /T, where ρ is the density and T the temperature. The constant γ as well as the function, τ α = (ργ /T), is material dependent. Actually this dependence of τ α on ργ /T 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–Wahnström model of orthoterphenyl, 1,4 polybutadiene, a room temperature ionic liquid, 1-ethyl-3-methylimidazolium nitrate, and a molten salt 2Ca(NO3)2 ·3KNO3 (CKN). (4) Both diffusivity and structural relaxation time, as well as the breakdown of Stokes-Einstein relation in CKN obey thermodynamic scaling by ργ /T with the same γ . (5) In polymers, the chain normal mode relaxation time, τ N, is another function of ργ /T 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.
Thermodynamic scaling of a-relaxation time and viscosity stems from the Johari-Goldstein beta-relaxation or the primitive relaxation of the coupling model
CAPACCIOLI, SIMONE;
2012-01-01
Abstract
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 of the product variable, ργ /T, where ρ is the density and T the temperature. The constant γ as well as the function, τ α = (ργ /T), is material dependent. Actually this dependence of τ α on ργ /T 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–Wahnström model of orthoterphenyl, 1,4 polybutadiene, a room temperature ionic liquid, 1-ethyl-3-methylimidazolium nitrate, and a molten salt 2Ca(NO3)2 ·3KNO3 (CKN). (4) Both diffusivity and structural relaxation time, as well as the breakdown of Stokes-Einstein relation in CKN obey thermodynamic scaling by ργ /T with the same γ . (5) In polymers, the chain normal mode relaxation time, τ N, is another function of ργ /T 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.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
