The influence of ionic comonomer units and ionic aggregation on the crystallization behavior and crystallite surface energies of poly(butylene terephthalate-co-5-sodiosulfoisophthalate) (PBTi) ionomers containing 3 and 5 mol% dimethyl-5-sodiosulfoisophthalate monomer units is compared to that of pure poly(butylene terephthalate). The rate of crystallization of the ionomers, as determined from an Avrami analysis, was shown to decrease with an increase in the ionic comonomer unit content. This behavior was attributed to the presence of non-crystallizable comonomer units and the reduced mobility of the crystallizable chain segments due to ionic aggregation in the PBT ionomers as evidenced by the broad peak centered about -9.5 ppm in the Na-23 NMR spectrum. The surface free energies of the polymer crystallites were determined using the Avrami rate constants in a modified Laurizten-Hoffman method. The PBT ionomers showed a 35-70% increase in the product of the surface energies (sigmasigma(e)) over that observed for pure PBT. By assuming a constant lateral surface energy, this behavior was attributed to a 30-70% increase in the work associated with chain folding. Relative to crystallites formed in pure PBT, these data suggest that there is a more disordered crystalline fold surface in the ionomers due the perturbed development of crystallites in the presence of a strong electrostatic network. (C) 2004 Elsevier Ltd. All fights reserved.