An experimental study of the mechanical strains induced by a dc electric field in amorphous and partially crystallized PEN films

The aim of this study is the quantification of the mechanical strain induced by a high dc electric field in Poly (Ethylene-2, 6-Naphthalene dicarboxylate) PEN films by using an optical measurement method (tracking). This aromatic polyester used recently in electrical engineering presents praiseworthy physical performances which make it an interesting material to investigate. Several PEN samples with different crystallinity rates and thickness have been considered. Differential Scanning Calorimetry (DSC) was used to ensure the morphology status of the samples provided. The tracking method allowed measuring induced mechanical strains on the flat gold-metallized surfaces of amorphous and semi-crystalline PEN thin films. Several protocols of dc electric stress application were tried; the differences consisted mainly in the duration of dc voltage application and/or the existence of a depolarization period. The experimental results showed that the level of the induced mechanical deformation depended on the strength of the applied electric stress, the linear dimensions of the area under study, the thickness and the morphology of the samples. Plots of induced mechanical deformation dependence on the electric field indicated the existence of two zones of mechanical deformation. The first one corresponded to the linear zone, in which the conductivity of the sample is characterized by a linear response to the applied electric field [1]. The second region associated with relatively higher fields is characterized by a strong non-linear behaviour. Our experimental results showed that amorphous samples were more vulnerable to the field-induced mechanical deformation compared to similar semi-crystalline samples.