Analysis of the activation energy spectrum for the enthalpy relaxation of a glassy liquid crystalline polymer

Abstract

The phenomenology of the physical ageing of glassy materials can be described by using the activation energy spectrum model. This model was originally developed for relaxations in metallic and oxide glasses and this paper applies it to a liquid crystalline polymer, poly(diethylene glycol p, p′-bibenzoate) (PDEB), in the glassy state.

PDEB samples were quenched from above Tg and isothermally annealed in a differential scanning calorimetry (DSC) equipment, and a description of the enthalpy relaxation of PDEB was achieved by means of the Kohlrausch–Williams–Watts (KWW) equation. The results show that the relaxation time decreases when the annealing temperature increases and can be explained in terms of an Arrhenius-type equation. The predicted value for the apparent activation enthalpy (0.98 eV) is lower than those reported for other polymers. This approach assumes that a unique elemental process with a relaxation time τ0=1.3×10-13 min controls the evolution of the system.

Energy spectra demonstrate that the activation energy values for the mechanisms controlling the relaxation are lower than 1.1 eV and the spectra present a maximum for an activation energy close to 0.98 eV. The total number of available relaxation processes (measured by the area below the curves) decreases if the sub-Tg annealing temperature increases. Moreover, the results show that the distributions widen if the shape parameter of the KWW equation decreases.