Persistent internal polarizations in poly(ethylene terephthalate) films. I. stress-induced polarizations

When heated under zero voltage, commercial polyester film exhibits current peaks manifesting glass transition and glycol linkage motions, as well as a broad, low background. The peaks are due to the depolarization of dipoles oriented by local stresses during manufacture. Their variations with sample thickness and heating rate permit the calculation of “thermodynamically reversible” values of T_g (58.7ddot] ± 0.5ddot] C) and T_glycol (87.3ddot] ± 0.2ddot]C). The broad background current is a manifestation of the alignment of paracrystalline order in a thermal gradient.