Dielectric study of the molecular motions in Bisphenol A Polycarbonate

The thermal transitions in amorphous and semicrystalline Bisphenol A Polycarbonate (BPA-PC) have been studied by the thermally stimulated depolarization currents technique. The dielectric relaxations at low temperature, β band, are carefully analyzed by the direct signal analysis in order to decompose the complex peak in elementary excitations in an energy interval, whose inverse frequency factors and contribution to polarization are best fitted to the experimental trace. The complex peak is shown to be composed by 3 main relaxations and their behavior is compared as the free volume in the material is varied, either by thermal treatments and/or by increasing the degree of crystallinity. It is shown that the observed behavior is compatible with a cooperative molecular motion of several repeating units along the chain for the first two relaxations, the third one being a more localized reorientation mechanism. The high temperature zone of the dielectric spectrum shows, in addition to the intense relaxation corresponding to the glass transition, a space charge peak due to the relaxation of the accumulated free carriers near the electrodes