Activation volume in superpressed glass-formers

In pressurized glass-forming systems, the apparent (changeable) activation volume V(a)(P) is the key property governing the previtreous behavior of the structural relaxation time (τ) or viscosity (η), following the Super-Barus behavior: [Formula: see text] , T = const. It is usually assumed that V(a)(P) = V(#)(P), where [Formula: see text] or [Formula: see text] . This report shows that V(a)(P) ≪ V(#)(P) for P → P(g), where P(g) denotes the glass pressure, and the magnitude V(#)(P) is coupled to the pressure steepness index (the apparent fragility). V(#)(P) and V(a)(P) coincides only for the basic Barus dynamics, where V(a)(P) = V(a) = const in the given pressure domain, or for P → 0. The simple and non-biased way of determining V(a)(P) and the relation for its parameterization are proposed. The derived relation resembles Murnaghan - O’Connel equation, applied in deep Earth studies. It also offers a possibility of estimating the pressure and volume at the absolute stability limit. The application of the methodology is shown for diisobutyl phthalate (DIIP, low-molecular-weight liquid), isooctyloxycyanobiphenyl (8*OCB, liquid crystal) and bisphenol A/epichlorohydrin (EPON 828, epoxy resin), respectively.