The William-Landel-Ferry equation is an empirical equation associated with the time-temperature superposition.
Tr is the reference temperature,
C1 and C2 are empirical factors,
log(aT) is the shift factor
The equation can be used to determine the factor of the temperature shift for temperatures other than those at which a material was tested. By using a time-temperature superposition on a multi-frequency DMA measurement, for example, one can establish a master curve which plots E’ (storage modulus) as a function of frequency in the range beyond what is measureable.