Glass-liquid transition: The glass-liquid transition is a pseudo second order phase transition in which a solid amorphous material (glass) transforms, on heating, into a supercooled melt. The glass-liquid transition is characterised by the glass transition temperature, Tg. Below Tg amorphous materials are in glassy state and above Tg they are liquid.
Most of joining bonds in amorphous materials are intact in the glassy state (Tg). The viscosity of amorphous materials is characterised in the glassy state by a high activation energy. Thermal fluctuations break joining bonds: the higher temperature the higher concentration of broken bonds (termed configurons). At temperatures approaching Tg configurons begin to form large clusters which become macroscopic large above the Tg. Broken bonds facilitate the flow and the viscosity of amorphous materials is characterised in the liquid state by a low activation energy.
The bond system of an amorphous material changes its Hausdorff-Besikovitch dimensionality from Euclidian 3 below Tg (where the amorphous material is solid), to fractal 2.55±0.05 above Tg, where the amorphous material is liquid.
^The IUPAC Compendium of Chemical Terminology, 66, 583 (1997)
^ M.I. Ojovan, W.E. Lee. Topologically disordered systems at the glass transition. J. Phys.: Condensed Matter, 18, 11507-11520 (2006); http://eprints.whiterose.ac.uk/1958/