Dielectric relaxation behaviour of poly(ethylene naphthalene 2,6 dicarboxylate) (PEN)

Dielectric spectroscopy has been used to study poly(ethylene naphthalene 2,6 dicarboxylate) (PEN) samples of different morphologies obtained by thermally treating bi-axially stretched PEN films. Neat and thermally treated samples of PEN films have been characterised by differential scanning calorimetry in order to measure the glass transition and melting temperatures as well as the degrees of crystallinity. Dielectric analysis has allowed the observation of the evolution of molecular relaxation phenomena with morphology changes and has revealed three relaxation processes: β-, β*- and α-relaxation (increasing temperature). The β-relaxation is associated with local motions of ester and the β*-relaxation with partially cooperative motions of naphthalene groups. The latter has been shown to be related to the morphology of the materials under study. The α-relaxation associated to the glass transition of PEN corresponds to cooperative motions induced by conformational rearrangements of the main chain and depends also on the morphology of the PEN films. Dielectric relaxation behaviours were compared using the activation energies calculated from Arrhenius equation formalism for the two sub-glass processes. Vogel–Fulcher–Tammann fits were performed on the α-relaxation. Our important contribution for the bi-axially stretched PEN films study is related to the assignment of the β*-relaxation that can be attributed to naphthalene aggregates.