Mechanochromic Dynamic Covalent Elastomers: Quantitative Stress Evaluation and Autonomous Recovery

Self-Amplified HF Release and Polymer Deconstruction Cascades Triggered by Mechanical Force

Hydrogen fluoride (HF) is a versatile reagent for material transformation, with applications in self-immolative polymers, remodeled siloxanes, and degradable polymers. The responsive in situ generation of HF in materials therefore holds promise for new classes of adaptive material systems. Here, we report the mechanochemically coupled generation of HF from alkoxy-gem-difluorocyclopropane (gDFC) mechanophores derived from the addition of difluorocarbene to enol ethers. Production of HF involves an initial mechanochemically assisted rearrangement of gDFC mechanophore to α-fluoro allyl ether whose regiochemistry involves preferential migration of fluoride to the alkoxy-substituted carbon, and ab initio steered molecular dynamics simulations reproduce the observed selectivity and offer insights into the mechanism. When the alkoxy gDFC mechanophore is derived from poly(dihydrofuran), the α-fluoro allyl ether undergoes subsequent hydrolysis to generate 1 equiv of HF and cleave the polymer chain. The hydrolysis is accelerated via acid catalysis, leading to self-amplifying HF generation and concomitant polymer degradation. The mechanically generated HF can be used in combination with fluoride indicators to generate an optical response and to degrade polybutadiene with embedded HF-cleavable silyl ethers (11 mol %). The alkoxy-gDFC mechanophore thus provides a mechanically coupled mechanism of releasing HF for polymer remodeling pathways that complements previous thermally driven mechanisms.

Solvent Polarity Effects on the Mechanochemistry of Spiropyran Ring Opening

The spiropyran mechanophore (SP) is employed as a reporter of molecular tension in a wide range of polymer matrices, but the influence of surrounding environment on the force-coupled kinetics of its ring opening has not been quantified. Here, we report single-molecule force spectroscopy studies of SP ring opening in five solvents that span normalized Reichardt solvent polarity factors (ETN) of 0.1-0.59. Individual multimechanophore polymers were activated under increasing tension at constant 300 nm s-1 displacement in an atomic force microscope. The extension results in a plateau in the force-extension curve, whose midpoint occurs at a transition force f* that corresponds to the force required to increase the rate constant of SP activation to approximately 30 s-1. More polar solvents lead to mechanochemical reactions that are easier to trigger; f* decreases across the series of solvents, from a high of 415 ± 13 pN in toluene to a low of 234 ± 9 pN in n-butanol. The trend in mechanochemical reactivity is consistent with the developing zwitterionic character on going from SP to the ring-opened merocyanine product. The force dependence of the rate constant (Δx‡) was calculated for all solvent cases and found to increase with ETN, which is interpreted to reflect a shift in the transition state to a later and more productlike position. The inferred shift in the transition state position is consistent with a double-well (two-step) reaction potential energy surface, in which the second step is rate determining, and the intermediate is more polar than the product.

Rational Design of Stimuli-Responsive Inorganic 2D Materials via Molecular Engineering: Toward Molecule-Programmable Nanoelectronics

The ability of electronic devices to act as switches makes digital information processing possible. Succeeding graphene, emerging inorganic 2D materials (i2DMs) have been identified as alternative 2D materials to harbor a variety of active molecular components to move the current silicon-based semiconductor technology forward to a post-Moore era focused on molecule-based information processing components. In this regard, i2DMs benefits are not only for their prominent physiochemical properties (e.g., the existence of bandgap), but also for their high surface-to-volume ratio rich in reactive sites. Nonetheless, since this field is still in an early stage, having knowledge of both i) the different strategies for molecularly functionalizing the current library of i2DMs, and ii) the different types of active molecular components is a sine qua non condition for a rational design of stimuli-responsive i2DMs capable of performing logical operations at the molecular level. Consequently, this Review provides a comprehensive tutorial for covalently anchoring ad hoc molecular components-as active units triggered by different external inputs-onto pivotal i2DMs to assess their role in the expanding field of molecule-programmable nanoelectronics for electrically monitoring bistable molecular switches. Limitations, challenges, and future perspectives of this emerging field which crosses materials chemistry with computation are critically discussed.

Deep-red photoluminescent mechanoresponsive polymers with dynamic CuI-arylamide mechanophores

We demonstrate the use of copper arylamide complexes as efficient photoluminescent mechanophores to design deep-red/near-IR emissive polymers showing reversible changes in photoluminescence intensity in the red/near-IR region in response to mechanical stretching. The mechanoresponse was repeatable over 30 cycles, showing a measurable increase of photoluminescence intensity even at a small applied stress of ca. 0.01 MPa. We demonstrate the potential of using conformationally dynamic copper amide complexes as sensitive and reversible mechanophores for near-IR imaging; systematic control over the emission range was achieved using amide modification.

Dual Ratiometric Fluorescence Monitoring of Mechanical Polymer Chain Stretching and Subsequent Strain-Induced Crystallization

Tracking the behavior of mechanochromic molecules provides valuable insights into force transmission and associated microstructural changes in soft materials under load. Herein, we report a dual ratiometric fluorescence (FL) analysis for monitoring both mechanical polymer chain stretching and strain-induced crystallization (SIC) of polymers. SIC has recently attracted renewed attention as an effective mechanism for improving the mechanical properties of polymers. A polyurethane (PU) film incorporating a trace of a dual-emissive flapping force probe (N-FLAP, 0.008 wt %) exhibited a blue-to-green FL spectral change in a low-stress region (<20 MPa), resulting from conformational planarization of the probe in mechanically stretched polymer chains. More importantly, at higher probe concentrations (∼0.65 wt %), the PU film showed a second spectral change from green to yellow during the SIC growth (20-65 MPa) due to self-absorption of scattered FL in a short wavelength region. The reversibility of these spectral changes was demonstrated by load-unload cycles. With these results in hand, the degrees of the polymer chain stretching and the SIC were quantitatively mapped and monitored by dual ratiometric imaging based on different FL ratios (I525/I470 and I525/I600). Simultaneous analysis of these two mappings revealed a spatiotemporal gap in the distribution of the polymer chain stretching and the SIC. The combinational use of the dual-emissive force probe and the ratiometric FL imaging is a universal approach for the development of soft matter physics.

Fluorescence-readout as a powerful macromolecular characterisation tool

The last few decades have witnessed significant progress in synthetic macromolecular chemistry, which can provide access to diverse macromolecules with varying structural complexities, topology and functionalities, bringing us closer to the aim of controlling soft matter material properties with molecular precision. To reach this goal, the development of advanced analytical techniques, allowing for micro-, molecular level and real-time investigation, is essential. Due to their appealing features, including high sensitivity, large contrast, fast and real-time response, as well as non-invasive characteristics, fluorescence-based techniques have emerged as a powerful tool for macromolecular characterisation to provide detailed information and give new and deep insights beyond those offered by commonly applied analytical methods. Herein, we critically examine how fluorescence phenomena, principles and techniques can be effectively exploited to characterise macromolecules and soft matter materials and to further unravel their constitution, by highlighting representative examples of recent advances across major areas of polymer and materials science, ranging from polymer molecular weight and conversion, architecture, conformation to polymer self-assembly to surfaces, gels and 3D printing. Finally, we discuss the opportunities for fluorescence-readout to further advance the development of macromolecules, leading to the design of polymers and soft matter materials with pre-determined and adaptable properties.

Imaging of Accumulated Mechanical Stresses Using Self-Assembled Layered Conjugated Polymer

When mechanical stresses, such as tensile, compressive, and frictional stresses, are applied to objects by various motions, they are accumulated in materials. Conventional mechanoresponsive materials and sensors detect one-time applied stress. However, the accumulated stresses are not visualized or measured in previous works. The present study demonstrated imaging and sensing of not only one-time but also accumulated tensile, compressive, and frictional stresses. Polyurethane (PU) film was combined with 2D layered polydiacetylene (PDA), a stimuli-responsive color-changing polymer. PDA generally exhibits no color changes with the application of tensile and compression stresses because the molecular motion leading to the color change is not induced by such mechanical stresses. Here the versatile mechanoresponsiveness was achieved using a block copolymer guest partially intercalated in the layered PDA. As the interlayer and outerlayer segments interact with PDA and PU, respectively, the applied stresses to the film are transferred from PU to PDA via the block copolymer guest. The color changes of the film imaged and quantified the accumulated work depending on the number and strength of the applied multiple stresses such as tensile, compressive, and frictional stresses. The design strategy of materials and methodology of sensing can be applied to the development of new sensors for accumulated mechanical stresses in a wide range of length and strength scales.

Highly Stretchable Stress-Strain Sensor from Elastomer Nanocomposites with Movable Cross-links and Ketjenblack

Practical applications like very thin stress-strain sensors require high strength, stretchability, and conductivity, simultaneously. One of the approaches is improving the toughness of the stress-strain sensing materials. Polymeric materials with movable cross-links in which the polymer chain penetrates the cavity of cyclodextrin (CD) demonstrate enhanced strength and stretchability, simultaneously. We designed two approaches that utilize elastomer nanocomposites with movable cross-links and carbon filler (ketjenblack, KB). One approach is mixing SC (a single movable cross-network material), a linear polymer (poly(ethyl acrylate), PEA), and KB to obtain their composite. The electrical resistance increases proportionally with tensile strain, leading to the application of this composite as a stress-strain sensor. The responses of this material are stable for over 100 loading and unloading cycles. The other approach is a composite made with KB and a movable cross-network elastomer for knitting dissimilar polymers (KP), where movable cross-links connect the CD-modified polystyrene (PSCD) and PEA. The obtained composite acts as a highly sensitive stress-strain sensor that exhibits an exponential increase in resistance with increasing tensile strain due to the polymer dethreading from the CD rings. The designed preparations of highly repeatable or highly responsive stress-strain sensors with good mechanical properties can help broaden their application in electrical devices.

Anomalous photochromism and mechanochromism of a linear naphthopyran enabled by a polarizing dialkylamine substituent

In contrast to common angular naphthopyrans that exhibit strong photochromic and mechanochromic behavior, constitutionally isomeric linear naphthopyrans are typically not photochromic, due to the putative instability of the completely dearomatized merocyanine product. The photochemistry of linear naphthopyrans is thus relatively understudied compared to angular naphthopyrans, while the mechanochromism of linear naphthopyrans remains completely unexplored. Here we demonstrate that the incorporation of a polarizing dialkylamine substituent enables photochromic and mechanochromic behavior from polymers containing a novel linear naphthopyran motif. In solution phase experiments, a Lewis acid trap was necessary to observe accumulation of the merocyanine product upon photochemical and ultrasound-induced mechanochemical activation. However, the same linear naphthopyran molecule incorporated as a crosslinker in polydimethylsiloxane elastomers renders the materials photochromic and mechanochromic without the addition of any trapping agent. This study provides insights into the photochromic and mechanochromic reactivity of linear naphthopyrans that have conventionally been considered functionally inert, adding a new class of naphthopyran molecular switches to the repertoire of stimuli-responsive polymers.

Conformer Ring Flip Enhances Mechanochromic Performance of ansa-Donor-Acceptor-Donor Mechanochromic Torsional Springs

Mechanochromophores based on conformational changes of donor-acceptor-donor (DAD) springs allow sensing of forces acting on polymer chains by monotonic changes of absorbance or photoluminescence (PL) wavelength. Here, we identify a series of thiophene (D)-flanked quinoxalines (A) as molecular torsional springs for force sensing in bulk polymers at room temperature. The mode of DAD linkage to the polymer matrix and linker rigidity are key parameters that influence the efficacy of force transduction to the DAD spring and thus mechanochromic response, as probed by in situ PL spectroscopy of bulk films during stress-strain experiments. The largest shift of the PL maximum, and thus the highest sensitivity, is obtained from an ansa-DAD spring exhibiting bridged D units and a stiff A linker. Using detailed spectroscopy and density functional theory calculations, we reveal conformer redistribution in the form of a thiophene ring flip as the major part of the overall mechanochromic response. At forces as low as 27 pN at early stages of deformation, the ring flip precedes mechanically induced planarization of the ansa-DAD spring, the latter process producing a PL shift of 21 nm nN^-1. Within the stress-strain diagram, the thiophene ring flip and DAD planarization are thus two separated processes that also cause irreversible and reversible mechanochromic responses, respectively, upon sample failure. As the thiophene ring flip requires much smaller forces than planarization of the DAD spring, such micromechanical motion gives access to sensing of tiny forces and expands both sensitivity and the force range of conformational mechanochromophores.

Competitive Activation Experiments Reveal Significantly Different Mechanochemical Reactivity of Furan–Maleimide and Anthracene–Maleimide Mechanophores

During the past two decades, our understanding of mechanochemical reactivity has advanced considerably. Nevertheless, an incomplete knowledge of structure-activity relationships and the principles that govern mechanochemical transformations limits molecular design. The experimental development of mechanophores has thus benefited from simple computational tools like CoGEF, from which quantitative metrics like rupture force can be extracted to estimate reactivity. Furan-maleimide (FM) and anthracene-maleimide (AM) Diels-Alder adducts are widely studied mechanophores that undergo retro-Diels-Alder reactions upon mechanical activation in polymers. Despite possessing significantly different thermal stability, similar rupture forces predicted by CoGEF calculations suggest that these compounds exhibit similar mechanochemical reactivity. Here, we directly probe the relative mechanochemical reactivity of FM and AM adducts through competitive activation experiments. Ultrasound-induced mechanochemical activation of bis-adduct mechanophores comprising covalently tethered FM and AM subunits reveals pronounced selectivity-as high as ∼13:1-for reaction of the FM adduct compared to the AM adduct. Computational models provide insight into the greater reactivity of the FM mechanophore, indicating a more efficient mechanochemical coupling for the FM adduct compared to the AM adduct. The methodology employed here to directly interrogate the relative reactivity of two different mechanophores using a tethered bis-adduct configuration may be useful for other systems where more common sonication-based approaches are limited by poor sensitivity.

Determining Entanglement Molar Mass of Glassy Polyphenylenes Using Mechanochromic Molecular Springs

Molecular force transduction in tough and glassy poly(meta,meta,para-phenylene) (PmmpP) was investigated as a function of M_n using covalently incorporated mechanochromic donor-acceptor torsional springs based on an ortho-substituted diphenyldiketopyrrolopyrrole (oDPP). Blending oDPP-PmmpP probe chains with long PmmpP matrix chains allowed us to investigate molar-mass-dependent mechanochromic properties for a series of specimens having mechanically identical properties. In the strain-hardening regime, the mechanochromic response (Δλ_max,em) was found to be a linear function of the acting stress and fully reversible, making oDPP-PmmpP a real-time and quantitative stress sensor. For entangled and nonentangled probe chains, distinctly different values of Δλ_max,em were observed, yielding a critical molar mass of M_c ≈ 11 kg mol^-1 for PmmpP. Once physical cross-linking of oDPP in the network of PmmpP was ensured, Δλ_max,em was found to be independent of M_n. The resulting value of M_c is in very good agreement with results from rheology.

Mechanochromophore-linked Polymeric Materials with Visible Color Changes

Mechanical force as a type of stimuli for smart materials has obtained much attention in the past decade. Color-changing materials in response to mechanical stimuli have shown great potential in the applications such as sensors and displays. Mechanochromophore-linked polymeric materials, which are a growing sub-class of these materials, are discussed in detail in this review. Two main types of mechanochromophores which exhibit visible color change, summarized herein, involve either isomerization or radical generation mechanisms. This review focuses on their synthesis and incorporation into polymer matrices, the type of mechanical force used, factors affecting the mechanochromic properties, and their applications. This article is protected by copyright. All rights reserved.

Mechanochromic cyclodextrins

Mechanochromic cyclodextrins (MCDs) that can generate blue radical species, which are exceptionally stable toward atmospheric oxygen and can thus be quantitatively characterized via electron paramagnetic resonance (EPR) spectroscopy, were synthesized. MCDs have a defined structure that consists of a diarylbibenzofuranone skeleton mechanophore sandwiched between two CDs. Grinding tests and EPR measurements of the MCDs revealed their high mechanoresponsiveness, reflecting the inherent rigidity of the CDs and the formation of a supramolecular structure in the bulk.

Ratiometric Flapping Force Probe That Works in Polymer Gels

Polymer gels have recently attracted attention for their application in flexible devices, where mechanically robust gels are required. While there are many strategies to produce tough gels by suppressing nanoscale stress concentration on specific polymer chains, it is still challenging to directly verify the toughening mechanism at the molecular level. To solve this problem, the use of the flapping molecular force probe (FLAP) is promising because it can evaluate the nanoscale forces transmitted in the polymer chain network by ratiometric analysis of a stress-dependent dual fluorescence. A flexible conformational change of FLAP enables real-time and reversible responses to the nanoscale forces at the low force threshold, which is suitable for quantifying the percentage of the stressed polymer chains before structural damage. However, the previously reported FLAP only showed a negligible response in solvated environments because undesirable spontaneous planarization occurs in the excited state, even without mechanical force. Here, we have developed a new ratiometric force probe that functions in common organogels. Replacement of the anthraceneimide units in the flapping wings with pyreneimide units largely suppresses the excited-state planarization, leading to the force probe function under wet conditions. The FLAP-doped polyurethane organogel reversibly shows a dual-fluorescence response under sub-MPa compression. Moreover, the structurally modified FLAP is also advantageous in the wide dynamic range of its fluorescence response in solvent-free elastomers, enabling clearer ratiometric fluorescence imaging of the molecular-level stress concentration during crack growth in a stretched polyurethane film.

Bridging pico-to-nanonewtons with a ratiometric force probe for monitoring nanoscale polymer physics before damage

Understanding the transmission of nanoscale forces in the pico-to-nanonewton range is important in polymer physics. While physical approaches have limitations in analyzing the local force distribution in condensed environments, chemical analysis using force probes is promising. However, there are stringent requirements for probing the local forces generated before structural damage. The magnitude of those forces corresponds to the range below covalent bond scission (from 200 pN to several nN) and above thermal fluctuation (several pN). Here, we report a conformationally flexible dual-fluorescence force probe with a theoretically estimated threshold of approximately 100 pN. This probe enables ratiometric analysis of the distribution of local forces in a stretched polymer chain network. Without changing the intrinsic properties of the polymer, the force distribution was reversibly monitored in real time. Chemical control of the probe location demonstrated that the local stress concentration is twice as biased at crosslinkers than at main chains, particularly in a strain-hardening region. Due to the high sensitivity, the percentage of the stressed force probes was estimated to be more than 1000 times higher than the activation rate of a conventional mechanophore.

Dynamic covalent polymer networks with mechanical and mechanoresponsive properties reinforced by strong hydrogen bonding

Dynamic polymer materials with superior mechanical properties and mechanochromism are of great importance to a vast variety of applications including stress sensing, damage detecting, soft robot. Herein, mechanoresponsive dynamic covalent...

Strain-correlated mechanochromism in different polyurethanes featuring a supramolecular mechanophore

A previously reported, supramolecular, loop-forming mechanophore comprised of two covalently connected perylene diimide (PDI) dyes was equipped with hydroxy groups and covalently incorporated into different polyurethanes (PUs). Four PUs with...

A modular approach to mechanically gated photoswitching with color-tunable molecular force probes

Molecular force probes conveniently report on mechanical stress and/or strain in polymers through straightforward visual cues. Unlike conventional mechanochromic mechanophores, the mechanically gated photoswitching strategy decouples mechanochemical activation from the ultimate chromogenic response, enabling the mechanical history of a material to be recorded and read on-demand using light. Here we report a completely redesigned, highly modular mechanophore platform for mechanically gated photoswitching that offers a robust, accessible synthesis and late stage diversification through Pd-catalyzed cross-coupling reactions to precisely tune the photophysical properties of the masked diarylethene (DAE) photoswitch. Using solution-phase ultrasonication, the reactivity of a small library of functionally diverse mechanophores is demonstrated to be exceptionally selective, producing a chromogenic response under UV irradiation only after mechanochemical activation, revealing colored DAE isomers with absorption spectra that span the visible region of the electromagnetic spectrum. Notably, mechanically gated photoswitching is successfully translated to solid polymeric materials for the first time, demonstrating the potential of the masked diarylethene mechanophore for a variety of applications in force-responsive polymeric materials.

Mechano-Responsive Hydrogels Driven by the Dissociation of a Host-Guest Complex

We developed a mechano-responsive hydrogel that is driven by the dissociation of a host-guest complex. The hydrogel comprised a thermoresponsive linear polymer with adamantane as a guest molecule in its side chain and a nonthermoresponsive network structure with β-cyclodextrin as a host molecule. Immobilization of the thermoresponsive polymer in the hydrogel via host-guest interaction resulted in a partial restriction of its phase transition, even above its lower critical solution temperature (LCST). The hydrogel demonstrated a decrease in transmittance when mechanical stress was applied at a temperature above its LCST, indicating that the phase transition of the thermoresponsive polymer was induced by the dissociation of the host-guest complex under mechanical stress. Moreover, this mechano-responsive behavior was repeatable by cooling the hydrogel to redissolve the thermoresponsive polymer. The strategy of the mechano-responsive phase transition will be useful for various applications that demand the control of desired functions by applied stress.

A mechanochromic donor-acceptor torsional spring

Mechanochromic polymers are intriguing materials that allow to sense force of specimens under load. Most mechanochromic systems rely on covalent bond scission and hence are two-state systems with optically distinct "on" and "off" states where correlating force with wavelength is usually not possible. Translating force of different magnitude with gradually different wavelength of absorption or emission would open up new possibilities to map and understand force distributions in polymeric materials. Here, we present a mechanochromic donor-acceptor (DA) torsional spring that undergoes force-induced planarization during uniaxial elongation leading to red-shifted absorption and emission spectra. The DA spring is based on ortho-substituted diketopyrrolopyrrole (o-DPP). Covalent incorporation of o-DPP into a rigid yet ductile polyphenylene matrix allows to transduce sufficiently large stress to the DA spring. The mechanically induced deflection from equilibrium geometry of the DA spring is theoretically predicted, in agreement with experiments, and is fully reversible upon stress release.

Rotaxane-Based Dual Function Mechanophores Exhibiting Reversible and Irreversible Responses

Mechanochromic mechanophores permit the design of polymers that indicate mechanical events through optical signals. Here we report rotaxane-based supramolecular mechanophores that display both reversible and irreversible fluorescence changes. These responses are triggered by different forces and are achieved by exploiting the molecular shuttling function and force-induced dethreading of rotaxanes. The new rotaxane mechanophores are composed of a ring featuring a luminophore, which is threaded onto an axle with a matching quencher and two stoppers. In the stress-free state, the luminophore is preferentially located in the proximity of the quencher, and the emission is quenched. The luminophore slides away from the quencher when a force is applied and the fluorescence is switched on. This effect is reversible, unless the force is so high that the luminophore-carrying ring slips past the stopper and dethreading occurs. We show that the combination of judiciously selected ring and stopper moieties is crucial to attain interlocked structures that display such a dual response. PU elastomers that contain such doubly responsive rotaxanes exhibit reversible fluorescence changes over multiple loading-unloading cycles due to the shuttling function, whereas permanent changes are observed upon repeated deformations to high strains due to breakage of the mechanical bond upon dethreading of the ring from the axle. This response allows one, at least conceptually, to monitor the actual deformation of polymer materials and examine mechanical damage that was inflicted in the past on the basis of an optical signal.

Rhodamine-Installed Polynorbornenes: Molecular Design, Structure, and Stimuli-Responsive Properties

The synthesis of a number of tailored architectures of rhodamine dye-norbornene conjugate monomers and corresponding homopolymers derived from them is described. The impact of the monomer architecture on the mechanochromic, photochromic, and thermochromic properties of rhodamine-modified polynorbornenes is reported. Color changes were caused by the reversible interconversion between the "open" and "closed" spirolactam form of the covalently attached dye. Monomers were synthesized in two principle architectures that varied on: (1) the number of polymerizable norbornene groups tethered to a bifunctional rhodamine dye; (2) the presence of flexible methylene spacers between the dye and the polymerizable norbornene groups. Introduction of norbornene groups on each of the two hydroxy groups of a bifunctional rhodamine resulted in a cross-linked polymer that exhibited better mechanochromic, photochromic, and thermochromic properties compared to the corresponding polymer without cross-links, derived from the derivatization of bifunctional rhodamine with only one norbornene. The introduction of flexible methylene spacers between the two polymerizable norbornenes and the dye molecule resulted in a polymeric framework with rapidly reversible color-changing properties upon mechanical or photostimulation. The ideal monomer molecular structure, whereby (1) attaching norbornene on both sides of the rhodamine dye and (2) methylene spacers between the dye and norbornenes on both sides afforded the nonpareil polymer structure that was capable of thermoreversible mechanochromic and photochromic features, and irreversible thermochromic features. These new materials may find utility as multi-stimuli-responsive soft materials.

Colorimetric Ionic Organohydrogels Mimicking Human Skin for Mechanical Stimuli Sensing and Injury Visualization

Artificial skins with sensing ability have great potential in applications of wearable devices and soft robotics. Inspired by the functions of human skins including sensing stimuli via electrical signal and bruising for injury indication, an ionic conductive and mechanochromic organohydrogel is synthesized and demonstrated as ionic skin (I-skin). The gel consisting of mechanochromophore cross-linked micelles is mechanically robust, stretchable, and deformation durable with minor hysteresis, and it also displays good solvent retention. The change of relative resistance during elongation and compression suggests a high sensitivity. An optical change from pale yellow to bruise-like blue-purple color is observed under a large deformation. The ionic conductive organohydrogel as I-skin is attached to different parts of the human body with movements mimicking various body-bruising scenarios, demonstrating successful perception and visualization of mechanical stimuli. The work vividly presents a strain sensor with the functions of injury visualization and damage warning for mechanical impacts. The I-skin can be potentially used in the applications including prosthetic devices, wearable electronics, and intelligent robots.

Self-Strengthening of Cross-Linked Elastomers via the Use of Dynamic Covalent Macrocyclic Mechanophores

The creation of polymeric materials that self-strengthen in response to a mechanical force is an important objective in the field of polymer chemistry. Here, the mechanochemical strengthening of cross-linked elastomers using macrocyclic mechanophores that contain a dynamic covalent disulfide bond is reported. Cross-linked poly(hexyl methacrylate) (CPHMA) polymers with macrocyclic mechanophores inserted at the cross-linking points were synthesized via free radical polymerization. Tensile and swelling tests showed that the addition of the macrocyclic mechanophores to the CPHMA polymers successfully impart them with self-strengthening functionality following compression, without the need for any additives such as monomers and modifiers, or any other stimuli.

Segmented Polyurethane Elastomers with Mechanochromic and Self-Strengthening Functions

Mechanochromic elastomers that exhibit force-induced cross-linking reactions in the bulk state are introduced. The synthesis of segmented polyurethanes (SPUs) that contain difluorenylsuccinonitrile (DFSN) moieties in the main chain and methacryloyl groups in the side chains was carried out. DFSN was selected as the mechanophore because it dissociates under mechanical stimuli to form pink cyanofluorene (CF) radicals, which can also initiate the radical polymerization of methacrylate monomers. The obtained elastomers generated CF radicals and changed color by compression or extension; they also became insoluble due to the mechanically induced cross-linking reactions. Additionally, an SPU containing diphenylmethane units also exhibited highly sensitive mechanofluorescence. To the best of our knowledge, this is the first report to demonstrate damage detection ability and changes in the mechanical properties of bulk elastomers induced by simple compression or extension.

Ultrahigh-Sensitive Compression-Stress Sensor Using Integrated Stimuli-Responsive Materials

Measurement of mechanical stresses, such as compression, shear, and tensile stresses, contributes toward achieving a safer and healthier life. In particular, the detection of weak compression stresses is required for healthcare monitoring and biomedical applications. Compression stresses in the order of 10⁶ -10¹⁰  Pa have been visualized and/or quantified using mechano-responsive materials in previous works. However, in general, it is not easy to detect compression stresses weaker than 10³  Pa using conventional mechano-responsive materials because the dynamic motion of the rigid mechano-responsive molecules is not induced by such a weak stress. In the present work, weak compression stresses in the order of 10⁰ -10³  Pa are visualized and measured via the integration of stimuli-responsive materials, such as layered polydiacetylene (PDA) and dry liquid (DL), through response cascades. DLs consisting of liquid droplets covered by solid particles release the interior liquid and collapse with application of a weak compression stress. The color of the layered PDA is changed by the spilled liquid as a chemical stress. A variety of weak compression stresses, such as expiratory pressure, are visualized and colorimetrically measured using the paper-based device of the integrated stimuli-responsive materials. Diverse mechano-sensing devices can be designed via the integration of stimuli-responsive materials.

Recent Advances in Stimuli-Responsive Commodity Polymers

Known for their adaptability to surroundings, capability of transport control of molecules, or the ability of converting one type of energy to another as a result of external or internal stimuli, responsive polymers play a significant role in advancing scientific discoveries that may lead to an array of diverge applications. This review outlines recent advances in the developments of selected commodity polymers equipped with stimuli-responsiveness to temperature, pH, ionic strength, enzyme or glucose levels, carbon dioxide, water, redox agents, electromagnetic radiation, or electric and magnetic fields. Utilized diverse applications ranging from drug delivery to biosensing, dynamic structural components to color-changing coatings, this review focuses on commodity acrylics, epoxies, esters, carbonates, urethanes, and siloxane-based polymers containing responsive elements built into their architecture. In the context of stimuli-responsive chemistries, current technological advances as well as a critical outline of future opportunities and applications are also tackled.

Modeling ultrasound-induced molecular weight decrease of polymers with multiple scissile azo-mechanophores

Selective and non-selective chain scission compete upon ultrasonic treatment of polymers with randomly distributed azo units.

Mechanochemical tools for polymer materials

This review aims to provide a field guide for the implementation of mechanochemistry in synthetic polymers by summarizing the molecules, materials, and methods that have been developed in this field.

Fast reversible isomerization of merocyanine as a tool to quantify stress history in elastomers

A mechanochemistry based approach is proposed to detect and map stress history during dynamic processes. Spiropyran (SP), a force sensitive molecular probe, was incorporated as a crosslinker into multiple network elastomers (MNE). When these mechanochromic MNEs are loaded, SP undergoes a well-known force-activated reaction to merocyanine (MC) changing its absorption in the visible range (visible blue color). This SP to MC transition is not reversible within the time frame of the experiment and the color change reports the concentration of activated molecules. During subsequent loading–unloading cycles the MC undergoes a fast and reversible isomerization resulting in a slight shift of absorption spectrum and results in a second color change (blue to purple color corresponding to the loading–unloading cycles). Quantification of the color changes by using chromaticity shows that the exact color observed upon unloading is characteristic not only of the current stress (reported by the shift in color due to MC isomerization), but of the maximum stress that the material has seen during the loading cycle (reported by the shift in color due to the change in MC concentration). We show that these two color changes can be separated unambiguously and we use them to map the stress history in the loading and unloading process occurring as a crack opens up and propagates, breaking the material. Color maps on fractured samples are compared with finite element simulations and the agreement is excellent.

A mechanochemistry based approach is proposed to detect and map stress history during dynamic processes.

Onset of Mechanochromic Response in the High Strain Rate Uniaxial Compression of Spiropyran Embedded Silicone Elastomers

The molecular processes that accompany dynamic mechanical response to large deformations at high strain rate (≈1000 s^-1 or higher) underlie the early stages of damage in materials, but understanding of material response in this regime is typically limited to macroscopic constitutive equations. Here, spiropyran mechanophores are embedded in very short, stress-bearing strands in silicone elastomers, and their mechanochromic response to uniaxial compression is explored in a Split Hopkinson Pressure (or Kolsky) Bar. At strain rates of 1000 s^-1 , the onset of mechanochromism occurs at lower strains, but higher stresses, than in the same materials under quasi-static loading. Similar to quasi-static loading, however, a negligible effect of mechanophore structure on the critical strain for colorimetric onset is observed. The results suggest that nonequilibrium, inhomogeneous local tension distributions in the elastomers lead to greater stress in individual strands than at the same strains under equilibrium loading, but that within the regions of force concentration, mechanochromic onset is determined primarily by a limiting local strain threshold.

Mechanochromic Polymers That Recognize the Duration of the Mechanical Stimulation via Multiple Mechanochromism

Mechanochromic polymers can be used as stress- and damage-detecting sensors in polymeric materials, given that mechanical stimuli can be visualized by color changes. Although many types of mechanochromic polymers have been reported so far, there are only few examples on their further functionalization based on multiple color changes (multicolor mechanochromism). Herein, preliminary results are reported on the use of multicolor mechanochromism to detect the duration of the mechanical stimulation by simply mixing white powders of two mechanochromic polystyrene samples that contain a different radical-type mechanochromophore at the midpoint of each polymer chain and thus exhibit different colors in response to mechanical stimuli. The mechanosensitivity can be tuned via the polymer length and shape, and a combination of these two types of mechanochromic polymers allows detecting the duration with multicolor mechanochromism, i.e., a color change from white to blue upon short exposure to grinding and a color change from white to gray upon longer exposure to grinding. Electron paramagnetic resonance and solid-state UV-vis measurements support the mechanism proposed for this multiple mechanochromism.

An engineer's introduction to mechanophores

Mechanophores (MPs) are a class of stimuli-responsive materials that are of increasing interest to engineers due to their potential applications as stress sensors. These mechanically responsive molecules change color or become fluorescent upon application of a mechanical stimulus as they undergo a chemical reaction when a load is applied. By incorporating MPs such as spirolactam, spiropyran, or dianthracene into a material system, the real-time stress distribution of the matrix can be directly observed through a visual response, ideal for damage and failure sensing applications. A wide array of applications that require continuous structural health monitoring could benefit from MPs including flexible electronics, protective coatings, and polymer matrix composites. However, there are significant technical challenges preventing MP implementation in industry. Effective strategies to quantitatively calibrate the photo response of the MP with applied stress magnitudes must be developed. Additionally, environmental conditions, including temperature, humidity, and ultraviolet light exposure can potentially impact the performance of MPs. By addressing these limitations, engineers can work to move MPs from the synthetic chemistry bench to the field. This review aims to highlight recent progress in MP research, discuss barriers to implementation, and provide an outlook on the future of MPs, specifically focused on polymeric material systems. Although the focus is on engineering MPs for bulk materials, a brief overview of mechanochemistry will be discussed followed by methods for activation and quantification of MP photo response (concentrating specifically on fluorescently active species). Finally, current challenges and future directions in MP research will be addressed.

From force-responsive molecules to quantifying and mapping stresses in soft materials

Directly quantifying a spatially varying stress in soft materials is currently a great challenge. We propose a method to do that by detecting a change in visible light absorption. We incorporate a spiropyran (SP) force-activated mechanophore cross-linker in multiple-network elastomers. The random nature of the network structure of the polymer causes a progressive activation of the SP force probe with load, detectable by the change in color of the material. We first calibrate precisely the chromatic change in uniaxial tension. We then demonstrate that the nominal stress around a loaded crack can be detected for each pixel and that the measured values match quantitatively finite element simulations. This direct method to quantify stresses in soft materials with an internal force probe is an innovative tool that holds great potential to compare quantitatively stresses in different materials with simple optical observations.