Relaxation and flow mechanisms in ‘‘fragile’’ glass-forming liquids

Abstract

Fragile glass-forming liquids are characterized by strongly non-Arrhenius temperature dependence of shear viscosity, and by unusually high heat capacity in the supercooled liquid above the glass transition. Using the inherent structure formalism, a theoretical model is developed to explain these characteristics. The vitrifying liquid is viewed as a dynamic patchwork of relatively strongly bonded (but amorphous) molecular domains that are separated by irregular walls of weakened bonds. Mean domain diameter ξ rises as temperature declines to minimize wall free energy, but the process is self-limiting due to intradomain frustration energy. Shear flow occurs by a ‘‘tear and repair’’ mechanism involving elemental wall areas of mean size ξ2. The relation of the present model to the tiling models for glass formation is explored, and helps to establish the presence of anomalous heat capacity in the supercooled liquid. The analysis suggests in strongly supercooled fragile liquids that the Stokes–Einstein formula may underestimate self-diffusion constants, and that the Adam–Gibbs relation for mean relaxation time in terms of calorimetric entropy may display systematic errors.