Highly stretchable, tough, healable and mechanoresponsive polyurethane elastomers for flexible capacitor applications

Visualizing color changes upon mechanical force in elastomers is attractive to understand stress transfer and failure mechanisms, which is valuable for applications ranging from tamper-proof packaging to structural integrity monitoring. Herein, we synthesize mechanochromic polyurethane films with an optimum stretchability (5500% strain) and a strongest toughness (324 MJ m(-3)) by using the components of bis-functionalized rhodamine, Diels-Alder (DA) adduct, 2-hydroxyethyl disulfide (HEDS), polyethylene glycol, triethanolamine and hexamethylene diisocyanate via condensation polymerization. The additions of HEDS and DA moieties enhance the healing efficiency up to 87% after 24 h of curing induced by disulfide bond exchange and Diels-Alder and retro-Diels-Alder reactions. The polyurethane film exhibits a green emission (450-500 nm) before stretching, but it shows an orange-red emission (588 nm) by force stretching because of the open form of the rhodamine moieties. A high shape recovery of 90.9% and reversible mechano-fluorescence switching by heating (immersed in 60 degrees C hot water) after structural deformations make it useful for shape memory and soft robotic applications. Finally, a high dielectric permittivity (160 at 1 Hz) and a good capacitance value (0.28 nF cm(-2) at 1 Hz) of the film exhibit its potential applications as a flexible capacitor for energy storage in modern flexible devices.