Dioxetanes as Mechanoluminescent Probes in Thermoplastic Elastomers

Scale-Bridging Mechanics Transfer Enables Ultrabright Mechanoluminescent Fiber Electronics

Mechanoluminescent (ML) fibers and textiles enable stress visualization without auxiliary power, showing great potential in wearable electronics, machine vision, and human-computer interaction. However, traditional ML devices suffer from inefficient stress transfer in soft-rigid material systems, leading to low luminescence brightness and short cycle life. Here, we propose a tendon-inspired scale-bridging mechanics transfer mechanism for ML composites, which employs molecular-scale copolymerized cross-linking and nanoscale inorganic nanoparticles as hierarchical stress transfer sites. This strategy effectively reduces the dissipation of stress in molecular chain segments and alleviates local stress concentration, increases luminescence by 9 times, and extends cycle life to more than 10,000 times. Furthermore, a scalable (kilometer-scale) anti-Plateau-Rayleigh instability manufacturing technology is developed for thermoset ML fibers, compatible with various existing textile techniques. We also demonstrate its system-level applications in motion capture, underwater interaction, etc., providing a feasible strategy for the next generation of smart visual textiles.

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.

Thermally Stable Photomechanical Molecular Hinge: Sterically Hindered Stiff-Stilbene Photoswitch Mechanically Isomerizes

Molecular photoswitches are extensively used as molecular machines because of the small structures, simple motions, and advantages of light including high spatiotemporal resolution. Applications of photoswitches depend on the mechanical responses, in other words, whether they can generate motions against mechanical forces as actuators or can be activated and controlled by mechanical forces as mechanophores. Sterically hindered stiff stilbene (HSS) is a promising photoswitch offering large hinge-like motions in the E/Z isomerization, high thermal stability of the Z isomer, which is relatively unstable compared to the E isomer, with a half-life of ca. 1000 years at room temperature, and near-quantitative two-way photoisomerization. However, its mechanical response is entirely unexplored. Here, we elucidate the mechanochemical reactivity of HSS by incorporating one Z or E isomer into the center of polymer chains, ultrasonicating the polymer solutions, and stretching the polymer films to apply elongational forces to the embedded HSS. The present study demonstrated that HSS mechanically isomerizes only in the Z to E direction and reversibly isomerizes in combination with UV light, i.e., works as a photomechanical hinge. The photomechanically inducible but thermally irreversible hinge-like motions render HSS unique and promise unconventional applications differently from existing photoswitches, mechanophores, and hinges.

UiO-66 framework with an encapsulated spin probe: synthesis and exceptional sensitivity to mechanical pressure

Probes sensitive to mechanical stress are in demand for the analysis of pressure distribution in materials, and the design of pressure sensors based on metal-organic frameworks (MOFs) is highly promising due to their structural tunability. We report a new pressure-sensing material, which is based on the UiO-66 framework with trace amounts of a spin probe (0.03 wt%) encapsulated in cavities. To obtain this material, we developed an approach for encapsulation of stable nitroxide radical TEMPO ((2,2,6,6-tetramethylpiperidin-1-yl)oxyl) into the micropores of UiO-66 during its solvothermal synthesis. Pressure read-out using electron paramagnetic resonance (EPR) spectroscopy allows monitoring the degradation of the defected MOF structure upon pressurization, where full collapse of pores occurs at as low a pressure as 0.13 GPa. The developed methodology can be used in and ex situ and provides sensitive tools for non-destructive mapping of pressure effects in various materials.

Hierarchically Structured Deformation-Sensing Mechanochromic Pigments

Mechanochromic materials alter their color in response to mechanical force and are useful for both fundamental studies and practical applications. Several approaches are used to render polymers mechanochromic, but they generally suffer from limitations in sensing range, capacity to provide quantitative information, and their capability to enable broad and simple implementation. Here, is it reported that these problems can be overcome by combining photonic structures, which alter their reflection upon deformation, with covalent mechanophores, whose spectral properties change upon mechanically induced bond scission, in hierarchically structured mechanochromic pigments. This is achieved by synthesizing microspheres consisting of an elastic polymer with spiropyran-based cross-links and non-close-packed silica nanoparticles. A strain of less than 1% can be detected in a shift of the reflection band from the photonic structure, while the onset strain for the conversion of the spiropyran into fluorescent merocyanine ranges from 30% to 70%, creating a broad strain detection range. The two responses are tailorable and synergistic, permitting the activation strain for the mechanophore response to be tuned. The mechano-sensing photonic pigments are demonstrated to be readily incorporated into different polymeric materials of interest and quantitatively probe spatially heterogeneous deformations over a large strain range.

Inherent Flame-Retardant, Humid Environment Stable and Blue Luminescent Polyamide Elastomer Regulated by Siloxane Moiety

The rapid development of the polymeric materials market has created an urgent demand for the thermoplastic polyamide elastomer (TPAE) owing to its greater functionality, and ability to be synthesized via a facile and industrial route. In this work, a series of novel silicone-containing polyamides (PA1212/Si12) were successfully synthesized from 1,12-dodecarboxylic acid (LA), 1,12-dodecarbondiamine (DMDA), and 1,3-bis (amino-propyl) tetramethyldisiloxane (BATS), via a one-pot melt polycondensation method in the absence of a catalyst. FTIR, ¹H-NMR, GPC and inherent viscosity results cohesively prove that the polymerization of monomers was well conducted, and the chemical structure was in high accordance with the design. As expected, the Si12 unit-content of the copolymers regulate the properties of the series. As the feeding ratio of BATS in the diamines increases from 5 mol% to 40 mol%, the thermal transition temperatures, T_g and T_m, decline steadily before finally stabilizing at ~6 °C and 160 °C, respectively, indicating that the co-polyamides possess improved chain flexibility but restricted crystallization ability. The conspicuous evolution in crystalline morphology of the series was observed by XRD and AFM. The increased PA Si12 phase induces the crystallized PA 1212 phase to transit from a thermally-favorable large and rigid crystal structure (α phase) to a kinetically-favorable small and ductile crystal structure (γ phase). Reflected in their stress–strain behavior, PA1212/Si12 copolymers are successfully tailored from rigid plastic to ductile elastomer. The tensile strength mildly drops from above 40 MPa to ~30 MPa while the reversible elongation increases from ~50% to approximately 350%. Accordingly, the moderate surface tension differences in the monomers facilitate the efficient conduction of the co-polymerization process, and the distributed short siloxane unit in the backbone fulfills the copolymer with desirable elasticity. Interestingly, the novel silicone-containing polyamides also display Si12 unit-content dependent flame retardancy, humidity stability, and unconventional solid-state fluorescence properties. The elastomers exhibit a low bibulous rate and anti-fouling characteristics to dye droplets and mud contamination, pass the V–1 rating (UL 94) with a constantly declining PHRR value, and emit blue luminescence under a 365 nm light source. Herein, we propose a new facile strategy for developing a high-performance and multifunctional silicone-modified polyamide, which bears promising industrialization potential. In addition, this first reported silicone-containing thermoplastic polyamide elastomer, which is self-extinguishing, anti-fouling and blue-luminescent, will further broaden the application potential of thermoplastic polyamide elastomers.

Rationally Regulating the Mechanical Performance of Porous PDMS Coatings for the Enhanced Icephobicity toward Large-Scale Ice

Ice accumulation on various surfaces in low-temperature and high-humidity environments is still a major challenge for several engineering applications. Herein, we fabricated a kind of PDMS coating with the introduction of porous structures under the surface by a two-step curing and phase separation method. The coatings with no further surface modification showed good hydrophobicity and icephobicity, and the typical ice adhesion strength was down to 40 kPa with a water contact angle of 116.5°. More than that, the porous PDMS coatings showed extraordinary icephobicity, especially toward large-scale ice (>10 cm²). In this case, the large-scale ice layer can be rapidly removed under a small external deicing force in a form of interface crack propagation rather than whole direct fracture. It was confirmed that by regulating the pore size and porosity of PDMS coatings properly, the stiffness mismatch between coatings and ice can be controlled to induce the initiation of interfacial cracks. On this basis, under the condition of a large-scale icing area, a small external deicing force can cause an increased surface stress concentration, and the formed interface cracks can propagate quickly, resulting in the ice layer falling off easily. In addition, under the influence of the size effect, ice can be removed without an additional force, and the minimum external force (per unit width) can be only 60 N/cm. This paper proposes that prefabricating a large number of microcracks at the interface can significantly weaken the bonding between ice and coatings, that is, reduce the fracture toughness. The new coatings have a remarkable effect toward large-scale icing.

Single-Molecule Force Spectroscopy of a Tetraaryl Succinonitrile Mechanophore

Fluorescent damage reporters that use mechanochemical activation of a covalent bond to elicit an optical signal are emerging tools in material mechanics as a means to access the nanoscale distribution of forces inside materials under stress. A promising class of damage reporters are tetraaryl succinonitriles (TASN), whose mechanical activation results in stable fluorescent radical species. However, in-depth insights into the molecular mechanics of TASN activation are absent, precluding their use as quantitative mechanoprobes. Here we perform single-molecule force spectroscopy experiments to provide these insights. We use a bridged version of the TASN unit, embedded in multi-mechanophore polymer, to enable multiplexed mechanochemical measurements at the single-molecule level. Our experiments reveal that TASN activates at surprisingly low forces and short time scales compared to other covalent mechanophores. These results establish TASN as a promising candidate for reporting the lower end of relevant forces in material mechanics.

Crystalline-state chemiluminescence reactions of two-fluorophore-linked adamantylideneadamantane 1,2-dioxetane isomers accompanied by solid-to-solid phase transitions

Structural isomers (cis-syn, cis-anti and trans isomers) of an adamantylideneadamantane 1,2-dioxetane having two fluorophore side chains were prepared and investigated their chemiluminescence (CL) properties in the crystalline state. Real-time monitoring...

Research progress of smart response composite hydrogels based on nanocellulose

In recent years, smart-responsive nanocellulose composite hydrogels have attracted extensive attention due to their unique porous substrate, hydrophilic properties, biocompatibility and stimulus responsiveness. At present, the research on smart response nanocellulose composite hydrogel mainly focuses on the selection of composite materials and the construction of internal chemical bonds. The common composite materials and connection methods used for preparation of smart response nanocellulose composite hydrogels are compared according to the different types of response sources such as temperature, pH and so on. The response mechanisms and the application prospects of different response types of nanocellulose composite hydrogels are summarized, and the transformation of internal ions, functional groups and chemical bonds, as well as the changes in mechanical properties such as modulus and strength are discussed. Finally, the shortcomings and application prospects of nanocellulose smart response composite hydrogels are summarized and prospected.

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...

Designing Force Probes Based on Reversible 6π-Electrocyclizations in Polyenes Using Quantum Chemical Calculations

The conjugated π-system in polyenes can be interrupted by electrocyclic ring-closure reactions. In this work, this 6π-electrocylization is shown by means of density functional calculations to be reversible by the application of an external mechanical pulling force at the terminal ends of the interrupted polyene chain. The test systems were constrained in a fused ring system, thus locking the orientation of three π-bonds and generally promoting 6π-electrocyclic ring-closure reactions. For several systems, the forward reaction is exergonic and the corresponding reaction barrier is comparable to those reported in the literature. The reverse reaction is triggered by an external pulling force of 2 nN (nano-Newton) or less and also becomes exergonic in all investigated polyenes under these force conditions. Moreover, it proceeds via a low reaction barrier when a pulling force of 2 nN is active, indicating that the mechanical force is an efficient stimulus for triggering ring-opening reactions. Analysis of the strain energy induced by this mechanical force confirms an optimal activation of the corresponding C-C σ-bond that breaks upon ring opening when the pulling positions are located on the polyene chain.

Radical-initiated P,P-metathesis reactions of diphosphanes: evidence from experimental and computational studies

By combining the diphosphanes Ar2P-PAr2, where Ar = C6H5, 4-C6H4Me, 4-C6H4OMe, 3,5-C6H3(CF3)2, it has been shown that P,P-metathesis generally occurs rapidly under ambient conditions. DFT calculations have shown that the stability of unsymmetrical diphosphanes Z2P-PZ'2 is a function of the difference between the Z and Z' substituents in terms of size and electronegativity. Of the mechanisms that were calculated for the P,P-metathesis, the most likely was considered to be one involving Ar2P˙ radicals. The observations that photolysis increases the rate of the P,P-metatheses and TEMPO inhibits it, are consistent with a radical chain process. The P,P-metathesis reactions that involve (o-Tol)2P-P(o-Tol)2 are anomalously slow and, in the absence of photolysis, were only observed to take place in CHCl3 and CH2Cl2. The role of the chlorinated solvent is ascribed to the formation of Ar2PCl which catalyses the P,P-metathesis. The slow kinetics observed with (o-Tol)2P-P(o-Tol)2 is tentatively attributed to the o-CH3 groups quenching the (o-Tol)2P˙ radicals or inhibiting the metathesis reaction sterically.

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.

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.

Mechanically Induced Bright Luminescence from 1,2-Dioxetane Containing PDMS Boosted by Fluoroboron Complex as an In-Chain Fluorophore

Improving mechanochemiluminescent (MCL) sensitivity of 1,2-dioxetane containing polymers is important for the applications of stress-reporting soft materials. Herein, a series of MCL poly(dimethylsiloxane) (PDMS) have been synthesized by simultaneously incorporating difluoroboron β-diketonate dye and 1,2-dioxetane as the co-crosslinkers to tune the energy transfer process across polymer chains. By covalently linked fluoroboron complex in PDMS network, the aggregation of the complex is overcome. Owing to its excellent opto-physical properties, this fluoroboron complex is shown to be an effective in-chain fluorophore to effectively enhance the chemiluminescence from polymeric 1,2-dioxetane that is broken either thermally or mechanically. Studies on the optomechanical properties of these PDMS show that MCL intensity is increased with the concentration of fluoroboron complex and the wavelength of the emission is shifted. The results of the present study appear to be broadly useful for designing elastomeric networks with chemiluminescent property not only attractive for optical technology, but also useful for damage self-reporting.

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.

Empowering self-reporting polymer blends with orthogonal optical properties responsive in a broader force range

Self-reporting polymers, which can indicate damage with perceptible optical signals in a tailored force range, are useful as stress-sensitive sensors. We demonstrate a simple approach to realize this function by embedding two distinct mechanophores - rhodamine (Rh) and bis(adamantyl)-1,2-dioxetane (Ad), in polyurethane/polylactic acid blends. The deformed blends generate red coloration and red chemiluminescence. Such a unique dual-responsive behavior was evaluated by solid-state UV-vis spectroscopy, macroscopic tensile tests with in situ RGB and light intensity analyses, which supported a stress-correlated occurrence of the ring-opening of Rh, the scission of Ad and the fluorescence resonance energy transfer process between the respective mechanochemical species. Complementarity stemming from the difference in properties and manifestations of the two mechanophores is essential. That is, the more labile Rh allows shifting the appreciable optical changes to a much lower force threshold; the transient nature and high dynamic range of mechanochemiluminescence from Ad map in real time where and when many of the covalently incorporated dioxetane bonds break; besides, the disrupted yet non-scissile structure of Rh acts as a fluorescent acceptor to effectively harvest chemiluminescence from ruptured Ad. The current strategy is thus empowering multi-functional mechano-responsive polymers with greatly improved sensitivity and resolution for multimodal stress reporting.

Low Ice Adhesion Surfaces Based on Flexible Fluorinated Polymers with a Polynorbornene Backbone

Realizing icephobic surfaces with low ice adhesion and durability continues to be fascinating as well as challenging. Herein, a norbornene-based fluorinated polymer (NFP) with high flexibility and high tensile strength is designed and fabricated using a fluorinated side chain and a norbornene backbone, displaying low ice shear strength less than 20 kPa and excellent durability. Experimental and theoretical analyses show that the flexibility of the polymer chains and the synergistic macromolecular aggregation of the fluorinated side groups and the norbornene backbone play key roles in the excellent surface icephobic properties of the NFP films. Moreover, we also develop a facile approach to the design of durable icephobic slippery surfaces, which possess remarkable icephobic performance. This study not only sheds light on the relationship between the polymer molecular structure and surface icephobic properties but also provides a new avenue to conveniently realize anti-icing coatings.

Semi-IPNs Reinforced with Silica Janus Nanoparticles and Their Stress Sensing with Mechanoluminescent Probe

A series of nanocomposite elastomers are prepared by dispersing surface-modified silica Janus nanoparticles into semi-interpenetrating network (Semi-IPN) of polyurethane/polyethyl methacrylate. Benefiting from the hierarchically crosslinked structures that consist of physical interlocking mediated by hydrogen-bond-rich silica Janus nanoparticles and permanent crosslinking by Semi-IPN, these elastomers exhibit excellent mechanical properties. Moreover, the Janus nanosheet is found more effective in strengthening and toughening the Semi-IPN, in comparison to Janus hollow sphere. Since 1,2-dioxetane is covalently embedded in these elastomers as a mechanoluminescent stress probe, stress transfer between the polymer and Janus nanoparticles and the toughening mechanism can be illuminated, which offer exciting opportunities to study the failure process of complex polymer nanocomposites with high spatial and temporal resolution.

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.

Fluorescent Metallosupramolecular Elastomers for Fast and Ultrasensitive Humidity Sensing

Fluorescent supramolecular polymers that can respond to subtle external stimuli to generate luminescence signals are promising in a wide range of applications, including probes, anti-counterfeiting materials, and sensors. However, complicated preparative procedures, limited responsive speed, and relatively low sensitivity still limit their practical sensing applications. Herein, we report europium-containing metallosupramolecular (PU-Eu) elastomers for fast and ultrasensitive humidity sensing by employing hygroscopic polyurethane (PU), whose urethane groups can coordinate with europium ions (Eu³⁺), emitting a strong luminescent signal by ligand-to-metal energy transfer. The variant of the coordination bond strength triggered by external humidity imparts the PU-Eu elastomer with a fast (∼1.1 s) and ultrasensitive response to the humid condition, where the external humidity increases by ∼1% and the corresponding fluorescence intensity will drop by ∼421.98 a.u. By a dip-coating process, PU-Eu elastomers can be conveniently coated on a hydrophilic and porous cellulose acetate nanofiber membrane, and the resulting composite membrane can achieve real-time and reversible monitoring of environmental humidity and human respiration. Given the versatility of PU-Eu elastomers, this study provides a low-cost and facile route of obtaining fluorescent metallosupramolecular polymers for fast and ultrasensitive humidity sensing.

Visualized Bond Scission in Mechanochemiluminescent Polymethyl Acrylate/Cellulose Nanocrystals Composites

A series of nanocomposites based on cellulose nanocrystals (CNCs) and polymethyl acrylate (PMA) with covalently incorporated 1,2-dioxetane as a luminescent mechanophore were prepared. Through surface-initiated single-electron transfer radical polymerization, the CNCs nanofiller offers good compatibility with polymer matrix. As a consequence, all the composite materials exhibit reinforced mechanical properties with increased stiffness and strength. Most importantly, 1,2-dioxetane is demonstrated as a sensitive platform to characterize the dissipation pathway of fracture energy, as well as the polymer chain scission in the Mullins effect within these polymer nanocomposites. The combined use of mechanical macroscopic testing and molecular bond scission data herein provides detailed information on how force distributes and failure occurs in complex soft materials.

Isomeric difference in the crystalline-state chemiluminescence property of an adamantylideneadamantane 1,2-dioxetane with a phthalimide chromophore

Syn- and anti-isomers of an adamantylideneadamantane 1,2-dioxetane having a phthalimide side chain were prepared and investigated their crystalline-state chemiluminescence (CL) properties. The isomers showed contrastive CL properties depending on their crystal-structural characteristics, indicating that CL provides an attractive target for real-time monitoring of a chemical reaction in the crystal.

Detection of stress in polymers: mechanochemical activation of CuAAC click reactions in poly(urethane) networks

We report on copper(i)-bis(N-heterocyclic carbene)s (NHC) for quantitative stress-sensing, embedded within polyurethane networks, triggering a fluorogenic copper(i) azide alkyne cycloaddition (CuAAC) of 8-azido-2-naphtol and 3-hydroxy phenylacetylene. A completely transparent, force responsive poly(urethane) material is generated, allowing a quantification of the applied stress.

Methods for Exerting and Sensing Force in Polymer Materials Using Mechanophores

In recent years, polymer mechanochemistry has evolved as a methodology to provide insights into the action-reaction relationships of polymers and polymer-based materials and composites in terms of macroscopic force application (stress) and subsequent deformation (strain) through a mechanophore-assisted coupling of mechanical and chemical phenomena. The perplexity of the process, however, from the viewpoint of mechanophore activation via a molecular-scaled disruption of the structure that yields a macroscopically detectable optical signal, renders this otherwise rapidly evolving field challenging. Motivated by this, we highlight here recent advancements of polymer mechanochemistry with particular focus on the establishment of methodologies for the efficient activation and quantification of mechanophores and anticipate to aptly pinpoint unresolved matters and limitations of the respective approaches, thus highlighting possible developments.

Polymer mechanochemistry-enabled pericyclic reactions

Polymer mechanochemical pericyclic reactions are reviewed with regard to their structural features and substitution prerequisites to the polymer framework.

Fractography of poly(N-isopropylacrylamide) hydrogel networks crosslinked with mechanofluorophores using confocal laser scanning microscopy

Employing mechanofluorophores in polymer fractography to obtain new information on force-induced events when analyzed by confocal laser scanning microscopy.

Reduced strain mechanochemical activation onset in microstructured materials

In this study, we show that mechanochemical activation in responsive materials with designed, periodic microstructures can be achieved at lower applied strains than their bulk counterparts.

Synthesis of well-defined mechanochromic polymers based on a radical-type mechanochromophore by RAFT polymerization: living radical polymerization from a polymerization inhibitor

Synthesis of mechanochromic polymers based on a radical-type mechanochromophore by RAFT polymerization: living radical polymerization from a polymerization inhibitor.

An optomechanical study of mechanoluminescent elastomeric polyurethanes with different hard segments

An optomechanical study of a series of mechanochemiluminescent polyurethanes with different hard segments provides a detailed picture of microphase separation dependent force transduction during their failure process.

Highly sensitive mechano-controlled luminescence in polymer films modified by dynamic CuI-based cross-linkers

Dynamic Cu^I-based mechanophores used as cross-linkers in polybutylacrylates enable highly sensitive detection of mechanical stress even at small strain (<50%) and stress (<0.1 MPa) values via reversible changes in luminescence intensity. Such sensitivity is superior to previously reported systems based on classical organic mechanophores and it allows for direct visualization of mechanical stress by imaging methods.

Strain-Dependent Kinetics in the Cis-to-Trans Isomerization of Azobenzene in Bulk Elastomers

The cis-to-trans isomerization of azobenzene is accelerated in a bulk PDMS elastomer under uniaxial tension. The kinetics are cleanly described by a single-exponential first-order process (k = 2.7 × 10^-5 s^-1) in the absence of tension but become multiexponential under constant strains of 40-90%. The complex kinetics can be reasonably modeled as a two-component process. The majority (∼92%) process is slower and occurs with a rate constant that is similar to that of the unstrained system (k = 2.3-2.7 × 10^-5 s^-1), whereas the rate constant of the minority (∼8%) process increases from k = 10.1 × 10^-5 s^-1 at 40% strain to k = 21.3 × 10^-5 s^-1 at 90% strain. Simple models of expected force-rate relationships suggest that the average force of tension per strand in the minority component ranges from 28 to 44 pN across strains of 40-90%.

Diselenide-Containing Polymeric Vesicles with Osmotic Pressure Response

Mechanophore is a kind of functional group that can undergo chemical reactions when given mechanical force stimuli. In this paper, osmotic pressure was used as an external force to trigger a diselenide exchange reaction. A diselenide bond containing block polymer capable of self-assembling to a vesicle structure and an ester bond containing a counterpart were synthesized in this study. When NaCl was added into the solution to generate the osmotic pressure difference inside and outside vesicles, diselenide containing vesicles were ruptured, while the ester bond counterpart stayed still. Further investigation into the chemical composition of both vesicles indicated the occurrence of the diselenide exchange reaction. The osmotic pressure response of the diselenide bond enriched the diselenide dynamic covalent chemistry and offers a potential application in a controlled release system.

Thermochemiluminescent peroxide crystals

Chemiluminescence, a process of transduction of energy stored within chemical bonds of ground-state reactants into light via high-energy excited intermediates, is known in solution, but has remained undetected in macroscopic crystalline solids. By detecting thermally induced chemiluminescence from centimeter-size crystals of an organic peroxide here we demonstrate direct transduction of heat into light by thermochemiluminescence of bulk crystals. Heating of crystals of lophine hydroperoxide to ~115 °C results in detectable emission of blue-green light with maximum at 530 nm with low chemiluminescent quantum yield [(2.1 ± 0.1) × 10^‒7 E mol^‒1]. Spectral comparison of the thermochemiluminescence in the solid state and in solution revealed that the solid-state thermochemiluminescence of lophine peroxide is due to emission from deprotonated lophine. With selected 1,2-dioxetane, endoperoxide and aroyl peroxide we also establish that the thermochemiluminescence is common for crystalline peroxides, with the color of the emitted light varying from blue to green to red.

Chemiluminescence is known in solution, but has remained undetected in macroscopic crystalline solids so far. Here the authors demonstrate direct transduction of heat into light by thermochemiluminescence in a centimeter-size lophine hydroperoxide crystal.

Mechanochromic dendrimers: the relationship between primary structure and mechanochromic properties in the bulk

Radical-type mechanochromic dendrimers were developed and their mechano-responsivity drastically increased in higher-generation dendrimers.

Enhanced optomechanical properties of mechanochemiluminescent poly(methyl acrylate) composites with granulated fluorescent conjugated microporous polymer fillers

With the combination of mechanochemiluminescence from 1,2-dioxetane coupled polymers and conjugated microporous polymer nanosheets, a new kind of filling-type mechanolumninescent polymer composite was developed.

With the combination of mechanochemiluminescence from 1,2-dioxetane coupled polymers and granulated conjugated microporous polymer (CMP) nanosheets, a new kind of filling-type mechanolumninescent polymer composite was developed. Herein, polymeric 1,2-dioxetane performed as an autoluminescent probe of chain scission. Besides benefiting from their excellent optical properties and good interfacial compatibility with poly(methyl acrylate) (PMA) media, two stable and fluorescent CMP nanosheets were prepared and dispersed in crosslinked PMA, which can serve as effective energy acceptors and reinforcing nano-fillers. These polymer nanocomposites present both reinforced mechanical strength and mechanochemiluminescence, and offer exciting opportunities to study the failure process of polymer nanocomposites with unprecedented temporal and spatial resolution.