12. Above Tg: dynamic heterogeneity and non-exponential relaxations

Above the melting point of a compound, the relaxation function of macroscopic properties f(t) generally exhibits exponential (Debye) behavior. However, when the liquid is undercooled and approaches the glass transition, f(t) deviates further and further from this exponential behavior. The system's α-type relaxations, in response to a disturbance, have a much more trailing behavior; f(t) is then well described by a "stretched exponential" law (called the Kohlraush-Williams-Watts function):