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) coole d front the melt, using a wide range of cooling rates, was studied. PET thi n films rapidly cooled from the melt (cooling rates larger than 5 degrees C /s) show a continuous variation of structure and properties depending on co oling rate. Results highlight differences in the micro-mechanical propertie s of the glass suggesting the occurrence of amorphous structures with diffe rent degrees of internal chain ordering. The comparative X-ray scattering s tudy of two glassy PET samples (7500 and 17 degrees C/s) reveals the occurr ence of frozen-in electron density states giving rise to an excess of scatt ering for the amorphous sample solidified at a lower cooling rate. The init ial glassy structure and its evolution, during isothermal cold crystallizat ion 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 incipie nt 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.