A test for the existence of isomorphs in glass-forming materials

Abstract

We describe a method to determine whether a material has isomorphs in its thermodynamic phase diagram. Isomorphs are state points for which various properties are invariant in reduced units. Such materials are commonly identified from strong correlation between thermal fluctuations of the potential energy, U, and the virial W, but this identification is not generally applicable to real materials. We show from molecular dynamic simulations of atomic, molecular, and polymeric materials that systems with strong U-W correlation cannot be pressure densified; that is, the density obtained on cooling to the glassy state and releasing the pressure is independent of the pressure applied during cooling.

Challenges of interpreting dielectric dilatometry for the study of pressure densification

“…The number density of dipoles between the electrodes as a function of temperature and pressure is illustrated for the normal (Glass 1), compressed (Glass 2) and post-compression (Glass 1bis) glasses. The existence of isomorphs in strongly correlating liquids gives a theoretical foundation for arguing that, on cooling, both normal and compressed glasses of 5PPE explore state points in the phase diagram that remain quasi-isomorphic [21,27]. Therefore, post-compression and normal glass should have similar densities that, together with the longitudinal expansion of the glass in the normal direction, would give rise to a lower value of the capacitance for the post-compression glass as indicated at the bottom of Fig.…”

“…Roland and colleagues recently proposed a new approach for examining pressure densification from a more fundamental point of view. They suggested examining the problem under the framework of the isomorph theory [16,21,22], which predicts the existence of isomorphs in the so-called Rsimple systems [23]. Pressure densification is also a longstanding topic in the physics of polymeric materials that has been investigated both experimentally and theoretically, for instance in polymethylmethacrylate (PMMA) and in other amorphous polymers [22,24,25].…”

“…Pressure densification is also a longstanding topic in the physics of polymeric materials that has been investigated both experimentally and theoretically, for instance in polymethylmethacrylate (PMMA) and in other amorphous polymers [22,24,25]. Pressure densification has also been studied in polymeric systems by computer simulations [21]. The fact that PMMA can be pressure densified is somewhat surprising since this polymer conforms with isochronal superposition and density scaling [26], and therefore it is likely to behave as a "Roskilde-simple system" (R-simple) according to the isomorph theory [23,27].…”

“…The recent interest in pressure densification from a more fundamental point of view is sparked by the prediction that R-simple liquids cannot be pressure densified [21,29]. Isomorph theory predicts that all glasses formed along the same glass-transition isochrone (for example formed with the same cooling rate at different pressures) will have the same microscopic structure, except for the average distance between the molecules which will scale with the qubic root of the inverse density.…”

Instability and thermal conductivity of pressure-densified and elastically altered orientational glass of Buckminsterfullerene

“…34 It is now of interest for investigating the elastic modulus, 35 the source of the JG relaxation, and of other features of the glassy state. 36,37,38,39 A recent paper 40 formally described the excess free energy of elastically expanded glasses and related it to increase in the solubility of a glass in a given solvent. As mentioned earlier here, in contrast to glasses and the OGs made from other ODICs, orientational disorder of C60 molecules in sc-C60…”