Dynamic process of cold crystallization of poly(butylene terephthalate) solids revealed by fluorescence spectroscopy

The crystallinity of poly(butylene terephthalate) (PBT) films was investigated by comparing measurements of fluorescence and density. It was found that the ratio of the fluorescence intensity at 320 nm to that at 365 nm can be an effective indicator of crystallinity. The dynamic process of cold crystallization was directly observed for PBT films prepared using a spin-casting method on quartz disks by means of fluorescence measurements at several temperatures between 360 and 400 K. The cold crystallization of such films was found to proceed one-dimensionally under the mechanism of heterogeneous nucleation, since the Avrami exponent, n, was determined to be one. The apparent activation energy of cold crystallization was determined to be 24 kJ/mol, indicating that rotation around carbon-carbon bonds in a butylene unit of PBT participates in reorientation and crystallization. The present work should provide a quick and nondestructive method for determining the crystallinity of PBT factory products.