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

For a liquid above its melting point (T-m), the activation energy determined 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 of the relative importance of activation dynamics. If E-infinity(rho) much greater than T-m, the liquid is called "strong," and for strong liquids it appears that E-infinity(rho)/E-infinity( P) > 0.5; these inequalities taken together indicate relaxations dominated by activated dynamics. If E-infinity(rho) approximate to T-m, the liquid is called "weak," and for weak liquids E-infinity(rho)/E-infinity( P) < 0.5; these inequalities taken together indicate relaxations dominated by nonactivated processes. Although E-infinity(rho) can be big or small, E-infinity(P) seems to be appreciably larger than T-m. Many glass-formers are quite "strong," even some such as orthoterphenyl, which is normally classified as "fragile," and, consequently, the relaxations of these are likely to be dominated by activated dynamics over the entire temperature range from well above melting down to the glass transition; 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].