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 (T-g). The mechanooptical response of VAL-PEUs exhibits an initial photoelastic behavior related to enhanced segmental correlation at low temperatures above the T-g. A characteristic temperature, defined as the liquid-liquid (T-ll) transition (rubbery-viscous transition), was found at about 1.05 T-g (K) (at T-g + 15 degrees 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 < T-ll or by progressive thermal effects at T > T-ll, both of which may be minimized at T-ll.