Epoxy thermosets with a multicolor switching during both compression and recovery processes

Multicolor mechanochromic polymers that can distinguish between different levels of stress indicated by color changes remain difficult to develop. This work reports a versatile strategy to construct mechanochromic epoxy thermosets that exhibit multicolor switching via the incorporation of two distinct mechanochromophores, rhodamine (Rh) and disulfide (DS), at finely tuned ratios. Owing to the different mechanosensitivities of the two components, the relative populations between the activated Rh and DS changed with compressive stress, as demonstrated by solid UV-vis spectroscopy, which revealed a color switch from pale yellow to green at 0.5 GPa, then to red at 2.0 GPa. The samples compressed at low stresses also showed a multicolor switching during the recovery process owing to the activated DS having faster recombination rates than the Rh. The facile preparation and ability to observe the stress intensity and stress history with the naked eye make these thermosets useful in practical applications.