Why liquids are fragile

TL;DR

This paper compares the fragility of 33 glass-forming liquids under different conditions, revealing a linear correlation between isobaric and isochoric fragilities and linking fragility to the interplay of temperature and density effects.

Contribution

It establishes a quantitative relationship between isobaric and isochoric fragilities and interprets fragility in terms of intermolecular potential and the balance of temperature and density effects.

Findings

Linear correlation between mP and mV fragilities.

Fragility reflects the relative influence of temperature and density.

Strong liquids are density-controlled, fragile liquids are thermally-activated.

Abstract

The fragilities (Tg-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 +/- 0.5)mV. This result has obvious 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 Tg. Specifically, strong behavior reflects a substantial contribution from density (jammed dynamics), while the relaxation of fragile liquids is more thermally-activated. Drawing on a scaling law, a physical interpretation of this result in terms of the intermolecular potential is offered.