Synthesis and non-isothermal crystallization behaviors of poly(ethylene isophthalate-co-terephthalate)s

A series of random copolyesters with reasonably high molecular weights were synthesized with varying composition by the bulk copolycondensation of ethylene glycol with dimethyl isophthalate (DMI) and dimethyl terephthalate. Compositions and molecular weights of the copolyesters were determined by H-1 NMR spectroscopy and viscometry, respectively. The copolyesters containing DMI of less than or equal to 20 mol% and greater than or equal to 90 mol% are crystallizable, whereas the copolyesters with DMI greater than or equal to 20 mol% and less than or equal to 90 mol% are amorphous. For the copolyesters containing DMI of less than or equal to 10 mol%, crystallization behaviors were non-isothermally investigated by calorimetry and analyzed by both modified Avrami and Ozawa approaches. Regardless of the composition, the value of the Avrami exponent is 2.8-3.5, depending on the cooling rate, whereas the value of the Ozawa exponent is 2.1-2.6, depending on the temperature. These results indicate that the nucleation and growth mechanisms of the copolyesters are apparently identical to those of poly(ethylene terephthalate). However, the crystallization rate is decreased by incorporating the DMI unit into the polymer backbone and also by lowering the cooling rate. In addition, the crystallization activation energy is found to be increased by incorporating the DMI unit into the polymer backbone. However, both of the analytical approaches show a big discrepancy in the estimation of the exponent which is critical to identify the type of nucleation and growth mechanism in the non-isothermal crystallization even though they have been derived from the same Avrami approach. (C) 1999 Elsevier Science Ltd. All rights reserved.