Uniaxially stretched poly(ethylene naphthalene 2,6-dicarboxylate) films studied by broadband dielectric spectroscopy

The thermal behaviors of amorphous PEN films uniaxially stretched above and below Tg have been characterized by Temperature Modulated DSC. When drawing is performed above Tg, the degree of crystallinity and the glass transition temperature Tg increase with increasing drawing ratio. PEN samples stretched below Tg are more crystalline than when stretched above Tg. Moreover the cold crystallization phenomenon disappears with drawing as a result of the mechanically induced crystallization. Broadband dielectric spectroscopy has also been used to study the influence of the orientation on the molecular mobility of PEN to gain insight into the microscopic origin of the relaxation processes in the transverse direction i.e. when the electric field is perpendicular to the stretching direction. Dielectric relaxation phenomena are thus discussed as a function of the microstructure (orientation, crystallinity). The α-relaxation dynamics associated with the glass transition are largely slowed down by the drawing process and the relaxation strength decreases. This can be related to a transformation of the amorphous phase into (i) crystalline, (ii) rigid amorphous fraction (RAF) and/or constrained amorphous phase with lower molecular mobility due to the crystallization induced by the orientation process. In contrast, the dynamics of the β*-relaxation are apparently increased and the activation energy for the uniaxially stretched samples decreases compared to the unstretched isotropic amorphous state. If the β*-relaxation is assigned to the presence of naphthalene aggregates, this leads to the conclusion that stretching is structurally altering the naphthalene aggregates (creation of defective aggregates that induces a higher molecular mobility) or orientation is selecting the faster motions active for β*. The β-relaxation which corresponds to more local motions implying reorientation of the ester groups seems to be broad and composed of two components as in the case of PET: the β1- and β2-relaxations. While the high frequency β1-component relaxation strength is increasing significantly with the deformation below Tg, that of the β2-process is more related to the presence of water.