Structural relaxation in glasses: numerical exploration of variables of Narayanaswamy and Moynihan's model

Abstract

The accurate description of the relaxation phenomenon near the glass transition temperature allows a better interpretation of differential scanning calorimetry (DSC) curves, since it can be separated from other thermal effects such as crystallization, melting, and recrystallization. The structural relaxation model of Narayanaswamy and Moynihan for thermorheological simple glasses was numerically explored for obtaining a grasp of the functional influence of the different parameters: i.e. relaxation enthalpy Δ h, non-linear parameter x, amplitude exponential distribution parameter b, and pre-exponential parameter A. It is known that when the relationship between the velocity of heat transfer in the cooling run ( q c) and the velocity of heating in the heating run ( q h) is equal to unity, the curves obtained in a DSC suffer only an horizontal translation for different absolute values of cooling and heating rates. It was found in this work that this criterion can be generalized, and curves also overlap by means of a horizontal translation when q c/ q h is a constant different from one, whichever the particular values of q c or q h will be. Finally, changes in the shape of the curves when using different values of q h and q c with a fixed q c/ q h relationship can be used as an indicator of the occurrence of other phenomena (i.e. recrystallization, melting, etc.) in addition to structural relaxation.