Nonlinear Mechano-Optical Behavior and Strain-Induced Structural Changes of l -Valine-Based Poly(ester urea)s
Copyright © 2018 American Chemical Society. The uniaxial mechano-optical behavior of a series of amorphous l-valine-based poly(ester urea) (VAL-PEU) with varying diol lengths was studied to elucidate the molecular mechanism associated with their thermal shape memory properties. A custom, real-time measurement system was used to capture the true stress, true strain, and birefringence during the temporary shape programming at stretching temperatures above the glass transition temperature (Tg). The mechano-optical response of VAL-PEUs exhibits an initial photoelastic behavior related to enhanced segmental correlation at low temperatures above the Tg. A characteristic temperature, defined as the liquid-liquid (Tll) transition (rubbery-viscous transition), was found at about 1.05 Tg (K) (at Tg + 15 °C) at strain rate of 0.017 s-1, above which the segmental contacts largely "melt" and the initial slope of the stress-optical curves becomes temperature independent. This temperature corresponds to the temperature where mean relaxation time for the polymer is maximized. Real-time infrared spectroscopy (IR) and in situ wide-angle X-ray scattering (WAXS) revealed a strain-induced intersegmental structural change during stretching. The intermolecular hydrogen bonding between the urea-urea and/or urea-carbonyl groups was found to adopt different bonding modes at the onset of strain hardening with a concurrent increased separation distance between adjacent segments. The hydrogen bonding strengthened supramolecular packing. The compromised shape recovery is a result of the disruption of the rigid segmental correlation, induced either by large extension at T < Tll or by progressive thermal effects at T > Tll, both of which may be minimized at Tll.
Chen, K; Dreger, NZ; Peng, F; Vogt, BD; Becker, ML; Cakmak, M
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