Reversible–Irreversible Transition of Strain-Induced Crystallization in Segmented Copolymers: The Critical Strain and Chain Conformation

The formation, growth, and melting of the strain-induced crystallization (SIC) structures of poly(tetramethylene oxide) (PTMO) were studied during the step-cycle deformation of poly(ether-b-amide) (PEBA) elastomers by Fourier transform infrared (FTIR) spectroscopy, wide-angle X-ray diffraction (WAXD), and small-angle X-ray scattering (SAXS). Whether SIC structures can either completely fuse or partially persist upon full relaxation of the stress depends on the maximum strain before sample retraction. After small and medium strains, the SIC can fully melt, i.e., reversible SIC. SIC can be partially retained after larger strain, i.e., irreversible SIC, which resulted from not only a higher equilibrium melting point but also a substantially higher plastic strain. For the reversible–irreversible transition, at the critical strain, PTMO chains of the soft segment were extended fully with all-trans zig-zag conformation after the lamellae of the hard segment completely fractured and greatly aligned. The three-stage microscopic model about the PEBA deformation proposed in our previous paper [Zhu, P. Strain-induced Crystallization of Segmented Copolymers: Deviation from the Classic Deformation Mechanism. Macromolecules 2017, 50, 3911 3921] was improved by adding a more detailed description of the SIC behaviors for the stage between the onset strain and the critical strain, as well as for the stage after the critical strain. The findings could help design elastomers with high tensile strengths and low residual strains.