Chapter 13 - Glass transition range behavior

Abstract

The physical properties of a glass vary with its thermal history. At any point during the cooling of a glass, the structural change through a small step of physical temperature change can be described by a small change in the fictive temperature on the equilibrium liquid line. Because of nonlinearity, however, on cooling through the glass transition range, glass retains a particular fictive history (or a memory) characteristic of the cooling path. The fictive history determines the value of the physical properties such as volume, specific heat, thermal expansion coefficient, etc., which is the value of the property upon complete relaxation less the portion which is yet to be relaxed. Calculations show that the relaxation function is described as a sum of at least two Maxwellian functions with two different relaxation times. Properties of specimens where all the relaxation processes have not proceeded to near completion, hence, display path sensitivity. A recent model of the relaxation kinetics of glass is the Mauro-Allan-Potuzak (MAP) model. Thermodynamics suggests that glass is an unstable state with respect to the equilibrium liquid. It was previously thought that glass had a residual entropy at absolute zero, in violation of the Third Law of Thermodynamics. However, even as a nonequilibrium material, glass must still obey all the laws of thermodynamics.