Viscous Liquids and the Glass Transition: A Potential Energy Barrier Picture

TLDR

Recent studies have compared free‑volume and entropy theories of viscous flow in glass‑forming liquids by measuring viscosity across wide temperature ranges and testing the derived equations. The authors propose that progress requires accepting crude, tractable models and recognizing their limits, and they introduce a new picture of viscous flow aimed at bridging molecular reality with predictive tractability. They model viscous liquids as dominated by high potential barriers relative to thermal energy, incorporating lattice‑defect concepts and Orowan’s continuum theory to predict flow and relaxation behavior. The authors argue that testing theories against precise viscosity data is misguided, that such theories are crude, and that a qualitative comparison across phenomena favors the entropy theory; their model yields qualitative predictions for flow and relaxation.

Abstract

Recent attempts have been made to assess the relative merits of the free volume and entropy theories of viscous flow in glass-forming liquids by accurate measurement of viscosity over wide temperature ranges, and subsequent comparison with the equations derived from these theories. In the author's view, this effort is misguided. The theories are crude and qualitative, and such tests are too stringent. It is better to make qualitative or semiquantitative comparison of a wide variety of physical phenomena; judged by this criterion, the entropy theory appears more successful. It is conjectured that further progress can be made by accepting the crude, naive character of any model we are likely to find tractable for the foreseeable future, and recognizing both the values and limitations of such models. A picture of the flow process in viscous liquids is proposed, in an attempt to answer certain questions about the molecular steps in flow either answered unsuccessfully or ignored by present theories, in the hope that it will lead a model closer to molecular reality but still sufficiently tractable to allow some range of predictive value. The model is based on the idea that in “viscous” liquids (shear relaxation time ≥10−9 sec) flow is dominated by potential barriers high compared to thermal energies, while at higher temperature, this will no longer be true. Certain concepts borrowed from the continuum theory of lattice defects are joined to a qualitative description of viscous flow due to Orowan, to provide a picture which leads to some qualitative predictions about flow and relaxation in the liquid and glassy states.