  Classically, the Prigogine-Defay (PD) ratio involves differences in isobaric
volumic specific heat, isothermal compressibility and isobaric thermal
expansion coefficient between a super-cooled liquid and the corresponding glass
at the glass transition. However, determining such differences by extrapolation
of coefficients that have been measured for super-cooled liquid and glassy
state, respectively, poses the problem that it does not take into account the
non-equilibrium character of the glass transition. In this paper, we asses this
old question by taking into account the gradual change of configurational
contributions to the three thermodynamic coefficients upon varying temperature
and pressure. Macroscopic non-equilibrium thermodynamics is applied to obtain a
generalized form of the PD ratio. The classical PD ratio can then be taken as a
particular case of this generalization. Under some assumptions, a
configurational PD ratio (CPD ratio) can be expressed in terms of fictive
temperature and fictive pressure what, hence, provides the possibility to
experimentally verify this formalism. Noteworthy and differing from previous
approaches towards the PD ratio, here, the glass transition is considered as
non-isoaffine.
