Polyether(ester amide)s with poly(propylene oxide) (PPO) and monodisperse poly(hexamethylene terephthalamide) segments were synthesized, and their structure-property relations were investigated. The length of the amide segments was varied from diamide to tetraamide to hexaamide segments, and therefore the number hydrogen bonds per amide segment increased from two to four to six. PPO was end-capped with 20 wt % ethylene oxide and had number-average molecular weights of 1000, 2300, and 4000 g/mol (including ethylene oxide tips). The morphology of the polyether(ester amide)s was studied with transmission electron microscopy and atomic force microscopy, the thermal properties were studied with differential scanning calorimetry and dynamic mechanical thermal analysis, and the tensile properties were studied with dumbbell samples. The elastic behavior of the block copolymers was investigated with tensile and compression tests. These segmented copolymers had two sharp transitions: a glass-transition temperature (T-g) of the PEO-PPO-PEO phase [where PEO is poly(ethylene oxide)] and a melting temperature (T-m) of the amide segments. The amide segments crystallized in nanoribbons with a high aspect ratio 1000. T-m increased with the amide segment length and with decreasing PEO-PPO-PEO content (solvent effect). The modulus increased strongly with the amide content. This modulus increase could be described by the Halpin-Tsai fiber composite model. Increasing the amide segment length surprisingly also improved the elasticity. (c) 2006 Wiley Periodicals, Inc.