Solid-that-Flows Picture of Glass-Forming Liquids

Abstract

This perspective article reviews arguments that glass-forming liquids are different from those of standard liquid-state theory, which typically have a viscosity in the mPa•s range and relaxation times on the order of picoseconds. These numbers grow dramatically and become 10 12 − 10 15 times larger for liquids cooled toward the glass transition. This translates into a qualitative difference, and below the "solidity length" which is roughly one micron at the glass transition, a glass-forming liquid behaves much like a solid. Recent numerical evidence for the solidity of ultraviscous liquids is reviewed, and experimental consequences are discussed in relation to dynamic heterogeneity, frequency-dependent linear-response functions, and the temperature dependence of the average relaxation time.

Microscopic Observation of the Anisotropy of the Johari–Goldstein-β Process in Cross-Linked Polybutadiene on Stretching by Time-Domain Interferometry

“…The cooperativity of the JG-β process in metallic glasses is similar to that of our polymer system. The connections between JG-β relaxation with cage-breaking dynamics and the formation of a percolating cluster of more mobile molecules have also been reported in supercooled molecular liquids and related materials. − According to this theory, “uncaged” molecules participating in JG-β relaxation form a highly mobile and spatially connected percolating cluster that spreads throughout the entire sample. In our polymer system, we propose that the collective motion of the cluster, consisting of several units, corresponds to their cage-breaking dynamics.…”