Scaling out the density dependence of the α relaxation in glassforming polymers

We show that the density and temperature dependences of the $\alpha$-relaxation time of several glassforming polymers can be described through a single scaling variable $X=e(\rho)/T$, where $e(\rho)$ is well fitted by a power law $\rho^x$, $x$ 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.