Constant density activation energies and the role of activated dynamics inliquids above their melting points

For a liquid above its melting point (T-m), the activation energy determine d at constant density, E-infinity(rho), is associated with "true" activated dynamics, while the ratio E-infinity(rho)/E-infinity(P), where E-infinity( P) is the activation energy determined at constant pressure, is a measure o f the relative importance of activation dynamics. If E-infinity(rho) much g reater than T-m, the liquid is called "strong," and for strong liquids it a ppears that E-infinity(rho)/E-infinity( P) > 0.5; these inequalities taken together indicate relaxations dominated by activated dynamics. If E-infinit y(rho) approximate to T-m, the liquid is called "weak," and for weak liquid s E-infinity(rho)/E-infinity( P) < 0.5; these inequalities taken together i ndicate relaxations dominated by nonactivated processes. Although E-infinit y(rho) can be big or small, E-infinity(P) seems to be appreciably larger th an T-m. Many glass-formers are quite "strong," even some such as orthoterph enyl, which is normally classified as "fragile," and, consequently, the rel axations of these are likely to be dominated by activated dynamics over the entire temperature range from well above melting down to the glass transit ion; the relaxation of others, such as toluene, seem not to be activated at temperatures above melting. (C) 1999 American Institute of Physics. [S0021 -9606(99)51622-X].