https://doi.org/10.1351/goldbook.12821
Equation describing the temperature dependence of the shift factor, \(a_{T}\), for time–temperature superposition \[\log_{10}a_{\rm{T}} = \frac{-C_{1}(T_{1} - T_{0})}{C_{2} + (T_{1} - T_{0})}\] \(C_{1}\) and \(C_{2}\) are empirical constants, \(T\) is the temperature of measurement, and \(T_{0}\) is the reference temperature.
Note: The WLF equation is usually applied with the reference temperature, \(T_{0}\), equal to the glass-transition temperature, \(T_{\rm{g}}\). The values of the constants \(C_{1}\) and \(C_{2}\) depend on the type of polymer and are valid over a limited range of temperature, from \(T_{\rm{g}}\) to about \(T_{\rm{g}}\) + \(\pu{50 K}\).