Molecular dynamics of a series of nematic polyesters

The molecular dynamics of a family of nematic main chain liquid crystalline polyesters with methylenic flexible spacers of different lengths has been studied by means of dielectric and dynamic mechanical spectroscopies. All polymers show three different relaxational processes: the dynamic glass transition (alpha-relaxation) and two secondary relaxations attributed to reorientations of the dipoles located at the mesogenic group (beta-relaxation) and at the ends of the methylenic spacer (gamma-relaxation). The influence of the number of methylenic units in the spacer on the parameters of the relaxations was analyzed. The alpha and beta processes appear at lower temperatures when length of the spacer increases, whereas the gamma provess seems to be independent on the spacer length. The possible cooperative character of the beta process was studied based on the values of its Arrhenius prefactor, the activation energy and the dielectric relaxation strength. The cooperativity tends to increase when increasing the number of methylenic units in the spacer