Mechanochromic
polymers exhibit potential applications in damage
reporting and stress sensing. Despite the rapid progress achieved
in recent years, mechanochromic structural polymers, such as epoxy
thermosets, remain difficult to develop, let alone structural materials
that can distinguish different levels of stress by a visual color
change. This work presents a new class of multicolor mechanochromic
epoxy thermosets that can discriminate between low and high compressive
stresses via the incorporation of two distinct mechanophores. Amino-functionalized
rhodamine (Rh) moieties serve as efficient curing agents and ratiometric
stress sensors for epoxy thermosets. The Rh mechanophore in the epoxy
network can be activated either by scratch or uniaxial compression,
showing a reversible color change and a red fluorescence turn-on response.
A stress-dependent multicolor response under uniaxial compression
and hydrostatic pressure is achieved by the combination of Rh mechanophores
with a commercial epoxy resin containing a 4,4′-diaminodiphenylmethane
(DDM) framework. The Rh zwitterion formed at a low compressive stress
turns the sample red, while a high compressive stress turns the sample
green via the formation of the quinoidal methine form of DDM. Differential
activation under varying degrees of compressive stress is demonstrated
by UV–vis spectroscopy. The facile preparation and ability
to recognize stress intensity by the naked eye make this strategy
suitable for practical applications.
Aggregation-induced emission (AIE) materials are attracting increasing research interests due to their broad application potential. The major design strategy for AIE luminogens (AIEgens) is incorporating molecular rotors, such as tetraphenylethylene...
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