Study of dielectric relaxation modes in poly(ε-caprolactone): Molecular weight, water sorption, and merging effects

Abstract

The behavior of the various relaxation modes in poly(ε-caprolactone) (PCL), is studied by Broad Band Dielectric Spectroscopy in the frequency range from 3×10−3 to 1.8×109 Hz and from 133 to 313 K. The experimental trace of the dielectric loss as a function of the angular frequency, ε″(ω), is analyzed by best fitting a sum of Cole–Cole distributions corresponding to the γ and β local modes and to the α relaxation which is the dielectric manifestation of the dynamic glass transition. The kinetic parameters of the three predominant relaxations are determined and relaxation plots describing the temperature dependencies of the relaxation times are given as a function of 1/T. These relaxation plots are insensitive within experimental errors, either to the molecular weight or to the water concentration. The hydration level (<1%) only affects the intensities of the local processes and no plasticization effect is observed. At temperatures higher than those recorded for the α mode a fourth intense process, α′ is observed as a shoulder in the conductivity rise. The position of this peak is the only one that changes with the molecular weight but not as much as it should if it were only caused by a normal mode. As the temperature increases the local β relaxation and the cooperative α mode merge into an αβ mode. The traces can be fitted below the merging temperature with three Cole–Cole distributions which after 219 K reduce to 2, the αβ and γ modes. This decomposition is confirmed by extracting the relaxation time distribution with the Simulated Annealing Direct Signal Analysis. In PCL, there is no necessity of invoking the existence of a change in the relaxation mechanism, a simple superposition of the α and β modes is adequate. This α–β crossover affects the intensity and shape of the distant γ relaxation. At still higher temperatures this αβ mode merges with the more localized γ mode, thus evidencing a new crossover and the persistence of the γ mode in the whole temperature range. The activation energy of the γ process changes after the crossover temperature, together with the α Cole–Cole shape parameter and its dielectric strength.