Influence of charge placement on the thermal and morphological properties of sulfonated segmented copolyesters

A new synthetic strategy using melt transesterification enabled selective control of ionic charge placement into the hard segment or soft segment block of segmented copolyesters. Dynamic mechanical analysis of sulfonated soft segment copolyesters revealed an increase in the low-T_g transition and rubbery plateau storage modulus as a function of sulfonation level. Sulfonated hard segment copolyesters displayed similar rubbery plateau moduli and decreasing flow temperature from 145 to 90 C. Dynamic melt rheology and in situ, variable temperature small-angle X-ray scattering determined the non-sulfonated copolyester order-disorder transition at 150 ± 5 C, which was ∼60 C above the hard segment T_g. Atomic force microscopy revealed sulfonated hard segment copolyesters contained more disordered surface morphologies. Sulfonated soft segment copolyesters demonstrated increased tensile stress and improved tensile strain from 18 to 170% with increasing ion content. The enhanced mechanical performance of sulfonated soft segment was attributed to the preserved microphase-separated morphology and the presence of physical crosslinks from electrostatic interactions.