We show that the density and temperature dependences of the α-relaxation time of several glassforming polymers can be described through a single scaling variable X = e(ρ)/T , where e(ρ) is well fitted by a power law ρ x , x being a species-specific parameter. This implies that "fragility" is an intrinsic, density-independent property of a glassformer characterizing its super-Arrhenius slowing down of relaxations, and it leads us to propose a modification of the celebrated Angell plot.

We present a consistent picture of the respective role of density (ρ) and temperature (T ) in the viscous slowing down of glassforming liquids and polymers. Specifically, based in part upon a new analysis of simulation and experimental data on liquid ortho-terphenyl, we conclude that a zeroth-order description of the approach to the glass transition should be formulated in terms of a temperature-driven super-Arrhenius activated behavior rather than a density-driven congestion or jamming phenomenon. The density plays a role at a quantitative level, but its effect on the viscosity and the α-relaxation time can be simply described via a single parameter, an effective interaction energy that is characteristic of the high-T liquid regime; as a result, ρ does not affect the "fragility" of the glassforming system.

No abstract