Molecular mobility in amorphous and partially crystalline PEN after isothermal crystallization

Poly(ethylene-2,6-naphthalene dicarboxylate) (PEN), a new aromatic polyester, presents high-performance physical and chemical properties and may be considered as a worthy substitute for polyethylene terephthalate (PET). Dynamic dielectric measurements were performed under isochronal conditions for twelve different frequencies between 1 and 10⁵ Hz. The aim was to follow the dielectric properties and their dependence on temperature, ranging from -100 to 200°C, with a heating rate of 2°C/min. Based on experimental considerations by using differential scanning calorimeter (DSC), a thermal cycle of crystallization was carried out. Different specimens were obtained in this way starting with as-received amorphous polymers. Crystallinity saturation, accompanied by a microstructure change, was observed with a second melting peak and a dual lamellar stack model was adopted. Amorphous and semi-crystalline PEN samples were compared. The thermal instability of PEN may be shown through dielectric relaxation before melting. A study was undertaken to understand the different dielectric relaxations present in PEN and the effect of thermal treatment on these relaxations. PEN mobility has been characterized by the presence of four relaxations: β, β*, α and ρ. When frequency increased, the β and α relaxations moved towards higher temperature, while the β* process disappeared gradually under the a maximum. The two secondary relaxations B and β* were found to obey Eyring´s law while the primary one obeyed the empirical Vogel-Fulcher-Tamman (VFT) relation. Characteristic of the amorphous PEN relaxation is the presence of the ρ peak, at a temperature>T_g. In this temperature range, it was concluded that this peak, not observed in semicrystalline specimens, was due to cold crystallization of the amorphous structure. To assign the occurrence of this peak to the mechanism of detrapping of free charges in the material seems inappropriate