Reversible Ratiometric Mechanochromic Fluorescence Switching in Highly Stretchable Polyurethane Elastomers with Ultratoughness Enhanced by Polyrotaxane

Abstract

Multistimuli-responsive polymers containing mechanophoric motifs are highlighted as promising mechano-luminescent materials upon stretching via optical fluorescence signals.Herein, we report a distinct ratiometric force-induced fluorescence changes from green-emissive napthalimide stopper (donor) embedded in polyrotaxane (PR) cross-linkers to red-emissive rhodamine mechanophore (acceptor) incorporated in polyurethane (PU) backbones. The utilization of PR (1 wt %) can significantly enhance both toughness and stretchability in our targeted PU elastomers, where the mechanical work is largely dissipated by the ring-sliding motion and pulley effects of PR crosslinkers during stretching. Therefore, the simple soft robotic behavior with ultrafast shape memory and reversible ratiometric mechanochromic fluorescence switching by heating (60 °C) in PR-based PU films provide a distinctive strategy for the construction of molecular machines in the high-performance elastomers, which also offer pathways for practical applications of stimuli-responsive polymers in highly stretchable artificial muscles featuring both outstanding pulley effects on enhanced mechanical properties and novel signal variations of mechanochromic fluorescence simultaneously.

Hygroscopic MXene/Protein Nanocomposite Fibers Enabling Highly Stretchable, Antifreezing, Repairable, and Degradable Skin-Like Wearable Electronics

“…With the gradual increase of interdisciplinary convergence and technological convergence, many new areas of development have emerged, such as implantable medical devices, soft robots, wearable devices, electronic fabrics, etc. − Particularly, the rise of personalized healthcare has spurred the development of flexible wearable and implantable electronic devices for monitoring physiological signals. Compared to conventional 3D and 2D electronic devices, fiber-based electronic devices have the advantages of high aspect ratio, lightweight, high flexibility, and weavability. − When applied to the skin, it can achieve a high degree of adaptability to the skin, thereby improving the comfort of the human body and enhancing signal fidelity during motion. , In health monitoring, fiber-based flexible sensors can monitor large-scale (such as the fingers, arms, and legs) and small-scale (such as emotional expression of face, breathing, and swallowing) human body movements to diagnose vocal cord damage, respiratory disorders, angina pectoris, etc. , Typically, the reported fiber-based sensors use stretchable elastomers, such as polydimethylsiloxane (PDMS), Ecoflex, and polyurethane (PU), as substrates with the conductive materials coated on the surface or embedded in the matrix to realize a close fit with the human body. − For instance, Seyedin et al prepared a fiber-based wearable strain sensor with Ti 3 C 2 T x MXene embedded in PU by wet spinning, which exhibited high sensitivity and could be used to monitor elbow joint movement. However, these polymers display poor air permeability and biocompatibility, which will cause skin discomfort during long-term usage.…”

Force-Induced Shuttling of Rotaxanes Controls Fluorescence Resonance Energy Transfer in Polymer Hydrogels

“…FRET effects have been widely used to develop photofunctional materials, including supramolecular polymers, − organogels, , fluorescent proteins, and fluorescent probes . Since the FRET efficiency depends on the distance between donor and acceptor fluorophores and the directions of their transition dipole moments, and because these factors can be controlled by applying mechanical forces, adequately coupled FRET pairs can display mechanochromic behavior. − Besides, combination of mechanophore and FRET has been reported to obtain mechanochromic polymer. , For our rotaxane mechanophores, bulky fluorophores are not suitable for the emitters directly incorporated into cycle structure because the association constant between the quencher and cycle becomes low, resulting in poor contrast upon stretching the films in which the mechanophores are covalently introduced. Indeed, we previously reported a red-light-emitting supramolecular rotaxane mechanophore with a bulky red fluorophore in the cyclic structure, and the polyurethane film containing the mechanophore exhibited a less pronounced fluorescence contrast .…”

“… 27 − 30 Besides, combination of mechanophore and FRET has been reported to obtain mechanochromic polymer. 31 , 32 For our rotaxane mechanophores, bulky fluorophores are not suitable for the emitters directly incorporated into cycle structure because the association constant between the quencher and cycle becomes low, resulting in poor contrast upon stretching the films in which the mechanophores are covalently introduced. Indeed, we previously reported a red-light-emitting supramolecular rotaxane mechanophore with a bulky red fluorophore in the cyclic structure, and the polyurethane film containing the mechanophore exhibited a less pronounced fluorescence contrast.…”

Programming Positive Mechanofluorescence in Liquid Crystalline Elastomers

“…Thanks to this phenomenon, carbazole-containing LSCEs can be exploited as optical mechanotransducers, i.e., materials capable of detecting mechanical stimuli and converting them into processable optical signals . As a whole, materials that modulate their emission of light upon the application of forces and other external stimuli have sparked a great interest recently because of their widespread applications. − …”