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

By now it is well established that the structural alpha-relaxation time, tau(alpha), of non-associated small molecular and polymeric glass-formers obey thermodynamic scaling. In other words, tau(alpha) is a function Phi of the product variable, rho(gamma)/T, where rho is the density and T the temperature. The constant gamma as well as the function, tau(alpha) = Phi(rho(gamma)/T), is material dependent. Actually this dependence of tau(alpha) on rho(gamma)/T originates from the dependence on the same product variable of the Johari-Goldstein beta-relaxation time, tau(beta), or the primitive relaxation time, tau(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 tau(alpha) and tau(beta) or tau(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-Wahnstrom model of orthoterphenyl, 1,4 polybutadiene, a room temperature ionic liquid, 1-ethyl-3-methylimidazolium nitrate, and a molten salt 2Ca(NO3)(2)center dot 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 rho(gamma)/T with the same gamma. (5) In polymers, the chain normal mode relaxation time, tau(N), is another function of rho(gamma)/T with the same gamma as segmental relaxation time tau(alpha). (6) While the data of tau(alpha) 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. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4736547]