title: Why liquids are fragile.
abstract: The fragilities (T(g)-normalized temperature dependence of alpha-relaxation times) of 33 glass-forming liquids and polymers are compared for isobaric, mP, and isochoric, mV, conditions. We find that the two quantities are linearly correlated: mP = (37+/-3) + (0.84+/-0.05)mV. This result has obvious and important consequences, since the ratio mV/mP is a measure of the relative degree to which temperature and density control the dynamics. Moreover, we show that the fragility itself is a consequence of the relative interplay of temperature and density effects near T(g). Specifically, strong behavior reflects a substantial contribution from density (jammed dynamics), while the relaxation of fragile liquids is more thermally activated. Drawing on the scaling law log(tau) = I(T upsilon(gamma)), a physical interpretation of this result in terms of the intermolecular potential is offered.
authors.affiliation: Naval Research Laboratory, Code 6120, Washington, DC 20375-5342, USA. casalini@nrl.navy.mil
authors.forename: R
authors.initials: R
authors.lastname: Casalini
authors.forename: C M
authors.initials: CM
authors.lastname: Roland