Nonlinear Mechano-Optical Behavior and Strain-Induced Structural Changes of L-Valine-Based Poly(ester urea)s

The uniaxial mechano-optical behavior of a series of L-valine-based poly(ester urea) (PEU) with varying diol lengths was studied. A custom, real-time measurement system was used to capture the true stress, true strain, and birefringence during deformation. PEUs exhibited similar temperature dependent behaviors at temperatures above the glass transition temperature (T_g). A characteristic temperature, defined as the liquid−liquid (T_ll) (rubbery−viscous) transition, was found at ~1.05 T_g (K) (T_g +15°C) under the strain rate of 0.017 s⁻¹. Above T_ll, the stress optical curve becomes linear and temperature independent. A stretched exponential form was applied to evaluate the isothermal relaxation of PEUs after the deformation. The mean relaxation time was found to exhibit a maximum at T_g +15°C. Real-Time infrared spectroscopy and in situ wide angle x-ray scattering revealed a strain-induced intersegmental structural change during stretching. At the onset of strain hardening, the intermolecular hydrogen bonding was found to adopt different bonding modes and the interchain distance displayed an increase. The hydrogen bonding in the structure strengthens the supramolecular packing. Once disrupted, the reconfigured bonding structure results in an incomplete strain recovery upon heating.