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

Abstract

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.et al. 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.

Recent advances on the structure–properties relationship of multiblock copolymers

“…In fact, reversible crystallization ensures a high flexibility at rest and a strong resistance under loading while permanent crystallization, possibly generated by a prestretching step, can provide MBCs with a much higher modulus and yield point directly from a low deformation level. This question is notably addressed in a very recent paper by Zhu et al 121 where the authors provide a general vision of the reversible–irreversible character of SIC.…”

Fatigue Characteristics of Poly(ether-b-amide) Elastomers during Cyclic Dynamic Tests and the Underlying Microstructural Evolution

“…The melting point of the PTMO SS (T m.SS ) for P35 is remarkably lower than that of homogeneous PTMO, i.e., 35−43 °C, since the PTMO segment can hardly develop into the same perfect lamellae crystals as in the homo PTMO. 7,40,41 The relative crystallinity of PA12 HS (X c.HS ) of E47 is evidently higher than that of P35 (Table 1). The T c.HS values of PA12 HS are 68.2 and 133.3 °C for P35 and E47, respectively.…”

“…Thereafter, the microscopic structures arrived at a new equilibrium, and consequently C R was less than the previous values. Note that strain hardening was observed for the sixth and seventh stages of E47, which result from the remarkable microscopic orientation, i.e., the highly aligned tie molecules . Shortly after the beginning of the seventh stage of P35, the strain reached 250%; the tests could not be carried out further under the premise of setting R = 0.1, and therefore, the experiment was terminated.…”

“…During the cooling scans, P35 with lower W PA displays two exothermic peaks as 16.7 and 146.5 °C, one at the higher temperature for the PA12 HS and the other at the lower temperature for the crystallization of PTMO SS (Figure a). , Consequently, two melting peaks can be seen in the second heating scans, which is a typical characteristic of microphase-separated copolymers. The melting point of the PTMO SS ( T m.SS ) for P35 is remarkably lower than that of homogeneous PTMO, i.e., 35–43 °C, since the PTMO segment can hardly develop into the same perfect lamellae crystals as in the homo PTMO. ,, The relative crystallinity of PA12 HS ( X c.HS ) of E47 is evidently higher than that of P35 (Table ). The T c.HS values of PA12 HS are 68.2 and 133.3 °C for P35 and E47, respectively.…”

Dynamic Monte Carlo simulations of strain-induced crystallization in multiblock copolymers: effects of dilution

“…20 Three stages of elasticity during the cyclic deformation of segmental polyether and polyamide copolymers have been observed, 21 and the reversible and irreversible mechanical responses have been discussed with the possible crystallization in soft blocks. 22,23 The asynchronous fracture of polyurethane elastomers has been attributed to a heterogeneous distribution of hydrogen bonding in the hard block domains, implying the diverse stabilities of crystallites. 24 Fast stretching also enhances the toughness of polyurethane 25 and poly(dimethylsiloxane)- b -poly(propylene glycol) (hard segment terminated).…”