Molecular mobility in amorphous and partially crystalline PEN after isothermal crystallization

Poly(ethylene-2,6-naphthalene dicarboxylate) (PEN), a new aromatic polyester, presents highperformance 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 10(5) Hz. The aim was to follow the dielectric properties and their dependence on temperature, ranging from - 100 to 200 degreesC, with a heating rate of 2 degreesC/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: beta, beta*, alpha and rho. When frequency increased, the beta and alpha relaxations moved towards higher temperature, while the beta* process disappeared gradually under the alpha maximum. The two secondary relaxations beta and beta* 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 rho 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.