Poly(ethylene-co-ethyleneoxyethylene terephthalate)s: synthesis and non-isothermal crystallization behavior

Abstract

A series of random copolyesters of reasonably high molecular weight was synthesized with varying composition by melt copolycondensation of dimethyl terephthalate (DMT) with ethylene glycol (EG) and diethylene glycol (DEG). Composition and molecular weight of the copolyesters were determined by 1H NMR spectroscopy and viscometry, respectively. The copolyesters containing DEG of ≤ 40 mol-% are crystallizable, whereas those with DEG of > 40 mol-% are amorphous. For the copolyesters containing DEG of ≤ 13 mol-% which were crystallized isothermally, the melting behavior was investigated by means of differential scanning calorimetry (DSC). Three endotherms were found to be influenced by the crystallization temperature and composition, and the equilibrium melting temperatures were determined. In addition, the non-isothermal crystallization behavior was investigated by DSC and analyzed by both Ozawa and modified-Avrami approaches. Regardless of the composition, the value of the Ozawa exponent ranges from 2.4 to 2.6, depending on the temperature, whereas the value of the Avrami exponent ranges from 2.5 to 3.2, depending on the cooling rate. These results indicate that the nucleation and growth mechanisms of the copolyesters are independent of their composition. However, the crystallization rate is decreased by incorporating DEG units into the polymer backbone and also by lowering the cooling rate. Furthermore, incorporating DEG units into the polymer backbone increases the crystallization activation energy.