Experimental Verification of the WLF - Doolittle Equation for Supercooled Liquid Dynamics by Pals Method: Polyisobutylene

The WLF equation is one of the most important formulae in polymer physics and for a long time is used to describe the universal non - Arrhenius behavior of viscosity η or of relaxation time τ, Williams et al (1955). Originally, the WLF eq. for the shift factor a (T) was interpreted within the free volume concept using the Doolittle equation, Doolittle(1951): $$ln{\text{ }}a(T) = \ln \eta (T)/\eta (T_g ) = \ln \tau (T)/\tau (T_g ) = B_f [l/f(T) - l/f(T_g )]$$ (1)