The mesomorphic phase structure in PET films: A conformational analysis

The conformational analysis of the mesomorphic phase structure in poly(ethy lene terephthalate) (PET) films based on the determination of the amount of Ta (trans in the amorphous phase), Te (trans in the crystalline phase), an d G (gauche) isomers present in samples of PET films heat treated between 1 10 degrees C and 230 degrees C is presented. The change in the amount of is omers was related to the mesomorphic phase determined from a correlation am ong wide-angle X-ray diffraction (WAXD), Fourier transform infrared (FTIR), density, and sorption properties. The reduction in sorption in semicrystal line samples, with respect to completely amorphous samples, at low penetran t activity allowed determination of the fraction of impermeable phase. This fraction, when compared with crystallinity determined from WAXD and densit y techniques, is higher, indicating the presence of mesomorphic fractions t hat probably have a bidimensional order. Separation of the contribution of isomers Ta and Tc and its relation with the mesomorphic phase indicate that the mesomorphic phase in PET films is related to the amount of Ta isomer. Up to 180 degrees C, the mesomorphic phase in PET films is composed predomi nantly of Ta isomer. Above 180 degrees C, the Ta isomer lends to be transfo rmed in Tc, and the difference in the values of the impermeable phase obtai ned by WAXD, density, and sorption suggests that there is a fraction of ori ented noncrystalline phase that is stable and contributes to the mesomorphi c phase. A thermal interchange diagram among Ta, G, and Tc isomer is propos ed. Degradation aspects were considered in samples heat treated at temperat ures higher than 180 degrees C in O-2 atmosphere.