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

The behavior of the various relaxation modes in poly(epsilon -caprolactone) (PCL), is studied by Broad Band Dielectric Spectroscopy in the frequency range from 3x10(-3) to 1.8x10(9) Hz and from 133 to 313 K. The experimental trace of the dielectric loss as a function of the angular frequency, epsilon(omega), is analyzed by best fitting a sum of Cole-Cole distributions corresponding to the gamma and beta local modes and to the alpha 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 alpha mode a fourth intense process, alpha' 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 beta relaxation and the cooperative alpha mode merge into an alpha beta mode. The traces can be fitted below the merging temperature with three Cole-Cole distributions which after 219 K reduce to 2, the alpha beta and gamma 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 alpha and < modes is adequate. This alpha-beta crossover affects the intensity and shape of the distant gamma relaxation. At still higher temperatures this alpha beta mode merges with the more localized gamma mode, thus evidencing a new crossover and the persistence of the gamma mode in the whole temperature range. The activation energy of the gamma process changes after the crossover temperature, together with the alpha Cole-Cole shape parameter and its dielectric strength. (C) 2001 American Institute of Physics.