Crystallization and Dielectric Spectrum of Polyethylene Terephthalate

For polyethylene terephthalate, relationships between the dielectric properties and the dynamic mechanical properties have been studied. And effects of crystallinity and crystallization temperature on anomalous dispersions (absorptions) in the dielectric properties have been surveyed. Dielectric measurements have been carried out over the frequency range of 0.1 to 10⁶c/s and temperature range of -35 to 110°C. Amorphous polyethylene terephthalate have been crystallized at 118°C (series 1) or 202°C (series 2) and 10 samples having different degree of crystallinity have been prepared. If partially crystallized samples of series 1 and 2 are compared at the same degree of crystallinity, as judged by density, the spherulites in the sample of series 2 are larger in size but smaller in number. The dynamic mechanical data have been adopted from Thompson and Woods' paper.<br>For all samples, two dielectric disperions (absorptions) are observed. A main absorption (α) locates in high temperature and low frequeucy range and another absorption (β) locates in low temperature and high frequency range. For amorphous samples as well as highly crystalline sample, position and apparent activation energy ΔH^* of the dielectric α absorption are nearly identical with those of the mechanical α absorption. In both dielectric and mechanical properties, α absorption of the crystallized sample shifts towards higher temperature and lower frequency and has smaller ΔH^* than that of the amorphous sample. Distribution of dielectric relaxation time τ or mechanical retardation time for α loss process of the crostallized sample is broader than that of the amorphous sample. ΔH^* for the dielectric, β absorption which is independent of crystallinity is nearly equal to that for the mechanical, β absorption.<br>In series 2 as well as series 1, increase of crystallinity has many effects upon α absorption, such as decrease of the magnitude, shift of the position towards higher temperature and lower frequency, decrease of ΔH^* and ΔS^* (but ΔF^* is independent of crystallinity) and broadening of the distribution of τ. These effects are more remarkable in series 1. Therefore it is suggested that molecular motions in amorphous region are strained more markedly in series 1. The magnitude of α absorption for samples of series 2 changes linearly with crystallinity and may become zero at 100% crystallinity. However that for samples of series 1 does not change linearly with cystallinity. On the other hand, the position, ΔH^*, ΔS^*, ΔF^* and the distribution of τ for β absorption are independent of crystallinity and only the magnitude decreases with increase of crystallinity. From the magnitude and quantitative analysis of end groups of the polymer chains, mechanism of β absorption is discussed.