A series of poly(hexamethylene terephthalate- co -hexamethylene 5-sodium sulfoisophthalate) copolyesters containing from 5 to 50 mol % of sulfonated units as well as the two parent homopolymers are prepared by melt polycondensation. The polyesters are obtained with high molecular weights, which decrease with the increased content of sulfonated units in the copolymer. Polyesters with 5 and 10 mol % of sulfonated units are semicrystalline whereas for contents equal or above 20 mol % they are unable to crystal- lize from the melt. Thermogravimetric analysis show that the thermal stability decreases with the content in sulfonated units. Isother- mal crystallizations of semicrystalline copolyesters show that the insertion of the sulfonated units causes a reduction of crystallizabil- ity, most probably due to the occurrence of ionic aggregations. It is observed a synergistic effect on the mechanical properties for copolymers with contents of around 5 mol % where the elongation at break increases drastically. Moreover, the hydrolytic degrada- tion of the copolymer is enhanced with the content in sulfonated units

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