Secondary relaxations and the structure of glass

Abstract

The temperature dependence of relaxation processes in the supercooled region of glass-forming materials can be described more accurately by a power-law equation of the form Y∝(T–T′_g)^–r, rather than by any other commonly used functional form. The analysis of the relaxation data of a number of materials together with the experimental observations on the secondary (β) relaxations, suggests that two types of the cluster model of glass are appropriate. The first type of glass is characterized by the equality T′_g=T₂, where T₂ is the zero excess configurational entropy temperature. These glasses have a rigidly packed cluster structure at temperatures T⩽T′_g and hence do not show β-relaxation below T′_g. The second type of glass is characterized by the inequality T′_gT₂ and in these materials some ‘islands of mobility’ exist even below T′_g in an otherwise rigid matrix and hence secondary relaxations are present in this type of glass even below T′_g. Many of the experimental observations on β-relaxations are explained using the above model.