Crossover region of dynamic glass transition: general trends and individual aspects

The dynamic glass transition of many substances is characterized by significant changes and peculiarities in a narrow frequency temperature range, the crossover region. Crossover temperatures and frequencies as obtained from different experimental methods are compiled for 38 glasses of a variety of substance classes. The crossover temperatures obtained from different methods (TB, Tt−r, Ton, Tc, Tβ) are usually similar for a given substance. We attempt to relate the crossover parameters for different substances to typical properties of conventional α relaxation and Johari–Goldstein-process β below the crossover. The parameters are discussed with respect to empirical concepts from the literature. An attempt is presented to quantify the old, intuitive idea that molecular cooperativity starts in the crossover region and increases for lower temperature.