For nonassociated, glass-forming liquids and polymers, thermodynamic scaling of structural relaxation times and viscosities is an empirical fact, demonstrated by various groups for dozens of materials. The $P$ and $T$ invariance of the isochoric fragility follows directly from this scaling. Apparent inconsistencies with these statements were reported recently by Grzybowski et al. [A. Grzybowski, K. Grzybowska, J. Zioło, and M. Paluch, Phys. Rev. E 74, 041503 (2006)]; however, the putative inconsistencies arise from use at higher pressures of parameters to correlate the isobaric and isochoric fragilities that are valid only for ambient pressure.

• Received 31 October 2006

DOI:https://doi.org/10.1103/PhysRevE.76.013501