Scaling out the density dependence of the a relaxation in glass-forming polymers

We show that the density and temperature dependences of the alpha-relaxation time of several glass-forming 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 glass-former characterizing its super-Arrhenius slowing-down of relaxations, and it leads us to propose a modification of the celebrated Angell plot.