Significance of the compensation law in dielectric and mechanical relaxations of polyethylene

It is well known that there is a compensation law between the change of enthalpy and change of entropy associated with the α, β, and γ relaxations of polyethylene (PE). We have already suggested that the temperatures of transition from one relaxation to another correspond to a thermodynamic transition. It is confirmed that the slope of the compensation law is the inverse of the transition temperature. The influence of crystallinity on the activation energy for the different relaxations is also discussed. It is often suggested that dielectric and mechanical relaxation measurements give similar information on the studied relaxations. A detailed analysis of the Gibbs free activation energy of several results indicate that this is not entirely true. The activation energy (not to be confused with the so-called energy of activation deduced from Arrhenius plots) of the dielectric measurements are systematically lower than those deduced from mechanical measurements. This is not surprising since these latter measurements involve nonnegligible stress or pressure to activate the relaxation.