A facile synthesis of hydroxyethyl-functionalized poly(ether sulfone) (PESu) oligomers permitted subsequent melt transesterification into segmented block copolymers with poly(butylene terephthalate). The unique solubility of the PESu oligomers in the melt with 1,4-butanediol and dimethyl terephthalate enabled a systematic study of segment length on thermomechanical properties of the resulting block copolymers. H-1 NMR spectroscopy revealed a compositional dependence on the average segment length of the PBT. Additionally, the concert of NMR spectroscopy, DSC, and DMA highlighted critical segment lengths for crystallization and phase separation. In agreement with a relatively constant T-m and phase separation observed with DSC and DMA, respectively, small-angle X-ray scattering identified a compositionally independent lamellar thickness, while the amorphous layer thickness increased with PESu incorporation. As a result, the complementary analytical techniques provided an understanding of the morphological influence on the thermomechanical behavior of an unprecedented family of high-T-g, semicrystalline, segmented block copolymers.