Structure development in poly(ethylene terephthalate) quenched from the melt at high cooling rates:: X-ray scattering and microhardness study

The structure and microhardness of poly(ethylene terephthalate) (PET) cooled front the melt, using a wide range of cooling rates, was studied. PET thin films rapidly cooled from the melt (cooling rates larger than 5 degrees C/s) show a continuous variation of structure and properties depending on cooling rate. Results highlight differences in the micro-mechanical properties of the glass suggesting the occurrence of amorphous structures with different degrees of internal chain ordering. The comparative X-ray scattering study of two glassy PET samples (7500 and 17 degrees C/s) reveals the occurrence of frozen-in electron density states giving rise to an excess of scattering for the amorphous sample solidified at a lower cooling rate. The initial glassy structure and its evolution, during isothermal cold crystallization at 117 degrees C of these two samples can be interpreted by assuming an improvement in the state of internal order. The differences in the incipient molecular ordering, which are detected by SAXS but not by WAXS, could be responsible for the hardening observed in the glassy PET samples. (C) 2000 Elsevier Science Ltd. All rights reserved.