In Situ and Real-Time Visualization of Mechanochemical Damage in Double-Network Hydrogels by Prefluorescent Probe via Oxygen-Relayed Radical Trapping

Abstract

Visualization of mechanochemical damages, especially for those in the molecular-scale (e.g., bond scission in polymeric materials), is of great industrial and academic significance. Herein, we report a novel strategy for in situ and real-time visualization of mechanochemical damages in hydrogels by utilizing prefluorescent probes via oxygen-relayed free-radical trapping. Double-network (DN) hydrogels that generate numerous mechanoradicals by homolytic bond scission of the brittle first network at large deformation are used as model materials. Theoretical calculation suggests that mechanoradicals generated by the damage of the first network undergo an oxygen-relayed radical-transfer process which can be detected by the prefluorescent probe through the radical−radical coupling reaction. Such an oxygen-relayed radical-trapping process of the prefluorescent probe exhibits a dramatically enhanced emission, which enables the real-time sensing and visualization of mechanochemical damages in DN hydrogels made from brittle networks of varied chemical structures. To the best of authors' knowledge, this work is the first report utilizing oxygen as a radical-relaying molecule for visualizing mechanoradical damages in polymer materials. Moreover, this new method based on the probe post-loading is simple and does not introduce any chemical structural changes in the materials, outperforming most previous methods that require chemical incorporation of mechanophores into polymer networks.

Visualization of mechanochemical polymer-chain scission in double-network elastomers using a radical-transfer-type fluorescent molecular probe

“…In the case of DN gels, it is well established that large numbers of mechanoradicals are generated only in the neck region in the constant-stress regime above the yielding point. 22,24 However, the DAAN -doped DN elastomers exhibited fluorescence over almost the entire film. This result indicates that the mechanoradicals are generated over a larger area in the DN elastomers than in the DN gels, and thus suggests a mechanoradical-generation mechanism is different from that of the DN gels.…”

“…19–21 In DN hydrogels, the mechanoradicals generated by the cleavage of the sacrificial polymer chains can be visualized by using the Fenton reaction, 22 a combination of mechanoradical-induced polymerization and an environment-responsive fluorescent probe, 23 or the use of prefluorescent probes for detecting oxygen-relay radical-trapping reactions. 24 The results of these pioneering studies have greatly advanced research into the fracture and toughening mechanisms of DN hydrogels. However, these mechanoradical-visualization methods theoretically require the presence of water and/or dissolved oxygen in water, and thus cannot be directly applied to DN elastomer systems.…”

Self‐Growing Organic Materials

“…This force-induced growth can be achieved repetitively when the samples are immersed in nutrient solutions, and the covalent bond scission is realtime visualizable. [18] The radicals generated in this case are also rare, so the self-growing demonstration was demonstrated in a glovebox filled with inert gas. Although the force-induced fracture of the first brittle network allows the system to expand without limitation in principle, the reported expansion degree of the sample is relatively low during the growth.…”

“…It is well-known that polymer chains will break to form radicals when they suffer mechanical tension. [18] Such fracture at the molecular level is typically passive and leads to unexpected decay in materials' mechanical properties. However, an interesting self-strengthening can be expected when the generated radicals are designed to induce the polymerization of externally-provided monomers to form additional interpenetrating chains in networks.…”

Fluorescence-readout as a powerful macromolecular characterisation tool

“…Zheng et al established an elegant method to detect and visualise mechanochemical damages in hydrogels by utilising a radical-trapping pro-fluorescent probe (nitroxide radical tethered to coumarin-based luminophore). 445 The mechanochemical damages often lead to the formation of short-lived free radicals that can either react directly with the probe via the radical coupling reaction or undergo an oxygen-relayed radical-transfer reaction first to increase the probability of the coupling reaction, thereby resulting in enhanced fluorescence emission and altered emission colour (Fig. 16C).…”