Three stages of elastic behavior were observed during cyclic deformations for poly(ether-b-amide) (PEBA) segmented copolymers based on crystalline hard segments of polyamide 12 (PA12) and amorphous soft segments of poly(tetramethylene oxide) (PTMO). The underlying microstructural evolution was characterized by a combination of in situ Fourier transform infrared spectroscopy (FTIR), wide-angle X-ray diffraction (WAXD), and small-angle X-ray scattering (SAXS) technologies. The gamma-alpha '' phase transition of crystalline PA12 occurred upon stretching, and the orientation of the alpha '' phase was less reversible under larger strains. PTMO chain orientation cannot be restored to the initial state, contributing to plastic deformation. Driven by the entropy effect, the strain-induced crystallization of PTMO can fuse during sample retarding, exerting little influence on the residual strain. For PEBA with a shore D hardness of 35D, the long period (L) can be restored to the initial L after the sample was unloaded until system fibrillation. The tie molecules between adjacent oriented lamellae can be by drawn out high stress in a PEBA material with a shore D hardness of 40D, and the relaxation led to a second long period. (C) 2018 Wiley Periodicals, Inc.