Stress-Dependent Multicolor Mechanochromism in Epoxy Thermosets Based on Rhodamine and Diaminodiphenylmethane Mechanophores

Abstract

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.

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

“…As shown in Figure 2c, the intensity of the Rh absorption band became more pronounced ratiometrically with increasing compressive stress, which is consistent with a previous study. 12 Alternatively, the amount of force-activated DS moiety increased with stress rapidly and plateaued at ~1.5 GPa for both EP-DS 3.5 and EP-DS 0.35 . Otsuka et al also reported a similar plateau in the dissociation ratio of diarylbibenzofuranone (DABBF) due to the high mechanosensitivity of DABBF in star-shaped polystyrene in contrast to DABBT in linear polystyrene upon grinding.…”

“…Epoxy thermosets with covalently linked Rh and DS mechanochromophores were prepared by a facile strategy, as shown in Figure 1. The Rh-functionalized epoxy prepolymer DGEBA-Rh was prepared via the condensation reaction of Rh-NH 2 (4.5 mol%) with excess DGEBA at 150 C for 3 h. 12 The mixtures of DGEBA-Rh, curing agents DS-NH 2 and 2,2 0 -(ethylenedioxy)bis(ethylamine) (EDR-148), in a stoichiometric epoxide-amine ratio, were fully cured by sequential curing procedures (80 C for 8 h and 150 C for 4 h). A series of epoxy thermosets with varying Rh:DS molar ratios were obtained (Table S1); the samples are referred to as EP-Rh 4.5 -DS y , where y represents the molar percentage ratio of the DS moiety relative to the epoxy resin.…”

“…Epoxy thermosets with covalently linked Rh and DS mechanochromophores were prepared by a facile strategy, as shown in Figure 1. The Rh‐functionalized epoxy prepolymer DGEBA‐Rh was prepared via the condensation reaction of Rh‐NH 2 (4.5 mol%) with excess DGEBA at 150°C for 3 h 12 . The mixtures of DGEBA‐Rh, curing agents DS‐NH 2 and 2,2′‐(ethylenedioxy)bis(ethylamine) (EDR‐148), in a stoichiometric epoxide–amine ratio, were fully cured by sequential curing procedures (80°C for 8 h and 150°C for 4 h).…”

“…Recently, we incorporated Rh into a commercial epoxy resin containing a 4,4 0 -diaminodiphenylmethane (DDM) moiety, which could serve as a second mechanochromophore, and saw a multicolor switch under compression. 12 Such multicolor switching in structural polymers is in critical demand for practical engineering applications as epoxy thermosets are widely employed as structural adhesives, coating and matrices of composites. 13 However, this method is not universal across conventional epoxy samples, and the stress required was extremely high (~6 GPa).…”

“…Distinguishing between different magnitudes of tensile or compression stresses in the bulk state directly through visible color changes remains a challenge, especially for a single polymer. Recently, we incorporated Rh into a commercial epoxy resin containing a 4,4′‐diaminodiphenylmethane (DDM) moiety, which could serve as a second mechanochromophore, and saw a multicolor switch under compression 12 . Such multicolor switching in structural polymers is in critical demand for practical engineering applications as epoxy thermosets are widely employed as structural adhesives, coating and matrices of composites 13 .…”