Shockwave loading of mechanochemically active polymer coatings

Multiple Stimuli-Responsive Color-Changing Polymer Materials for Reversible Writing and Anti-Counterfeiting

Polymer materials with multiple stimuli-responsive properties have demonstrated many potential and practical applications. By covalently introducing spiropyran (SP1) and spirothiopyran (STP) into the polyurethane backbone, photochromic, mechanochromic, and thermally discolored polymer materials have been prepared. In this work, we report for the first time that white light (violet, blue, and green light) above a certain intensity can activate STP to green color. Based on the above discovery, the polyurethane with SP1 and STP can exhibit reversible three-color changes (brown, green, and purple) in response to four stimuli: ultraviolet irradiation, white light irradiation, mechanical stress, and heat. The color-changing polymer materials have high color contrast and excellent reversibility, and can be used for reversible writing, anticounterfeiting and information encryption, etc.

A microcapsule-based reusable self-reporting system using a donor-acceptor Stenhouse adduct

Self-reporting systems automatically indicate damaged or corroded surfaces via color changes or fluorescence. In this study, a novel reusable self-reporting system is developed by exploiting the reversibility of a donor-acceptor Stenhouse adduct (DASA). The synthesized DASA precursor exhibits a color change when damaged upon reaction with diethylamine, and returns to its colorless form upon irradiation with visible light. Microcapsules are synthesized with a core comprising styrene and the DASA precursor, along with a shell formed of urea and formaldehyde. The optimal particle size and shell thickness of the microcapsules are 225 μm and 0.17 μm, respectively. The DASA precursor-containing microcapsules are embedded in a PEG gel matrix with secondary amine groups. This coating system, initially colorless, exhibits a color change, becoming pink after being damaged by scratching due to the reaction between the DASA precursor released from ruptured microcapsules with the secondary amine groups of the PEG gel, thus demonstrating self-reporting characteristics. Furthermore, the colored surface is restored to its initial colorless state by irradiation with visible light for 1.5 hours, demonstrating the reusability of the self-reporting system.

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.

Sucrose-mediated formation and adhesion strength of Streptococcus mutans biofilms on titanium

Biofilms consist of bacterial cells surrounded by a matrix of extracellular polymeric substance (EPS), which protects the colony from many countermeasures, including antibiotic treatments. Growth and formation of bacterial biofilms are affected by nutrients available in the environment. In the oral cavity, the presence of sucrose affects the growth of Streptococcus mutans that produce acids that erode enamel and form dental caries. Biofilm formation on dental implants commonly leads to severe infections and can restrict osseointegration necessary for the implant to be successful. This work determines the effect of sucrose concentration on biofilm EPS formation and adhesion of Streptococcus mutans, a common oral colonizer, to titanium substrates simulating common dental implants. Biofilm formation and profiles are visualized at high magnification with scanning electron microscopy (SEM). Large mounds and complex structures consisting of bacterial cells and EPS can be seen in biofilms at sucrose concentrations that are favorable for biofilm growth. The laser spallation technique is used to apply stress wave loading to the biofilm, causing the biofilm to delaminate at a critical tensile stress threshold. The critical tensile stress threshold is the adhesion strength. Because laser spallation applies the stress loading to the rear of the substrate, bulk adhesion properties of the biofilm can be determined despite the heterogenous composition and low cohesion strength of the biofilm. Statistical analysis reveals that adhesion strength of biofilms initially increase with increasing sucrose concentration and then decrease as sucrose concentration continues to increase. The adhesion strength of bacterial biofilms to the substrate in this study is compared to the adhesion of osteoblast-like cells to the same substrates published previously. When sucrose is present in the biofilm growth environment, S. mutans adhesion is higher than that of the osteoblast-like cells. Results of this study suggest sucrose-mediated S. mutans biofilms may outcompete osteoblasts in terms of adhesion during osseointegration, which could explain higher rates of peri-implant disease associated with high sugar diets. Further studies demonstrating adhesion differentials between biofilms and cells including co-cultures are needed and motivated by the present work.

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.

Force-mediated molecule release from double network hydrogels

The incorporation of mechanosensitive linkages into polymers has led to materials with dynamic force responsivity. Here we report oxanorbornadiene cross-linked double network hydrogels that release molecules through a force-mediated retro Diels-Alder reaction. The molecular design and tough double network of polyacrylamide and alginate promote significantly higher activation at substantially less force than pure polymer systems. Activation at physiologically relevant forces provides scope for instilling dynamic mechanochemical behavior in soft biological materials.

Mechanochemistry unveils stress transfer during sacrificial bond fracture of tough multiple network elastomers

The molecular level transfer of stress from a stiff percolating filler to a stretchable matrix is a crucial and generic mechanism of toughening in soft materials. Yet the molecular details of how this transfer occurs have so far been experimentally unreachable. Model multiple network elastomers containing spiropyran (SP) force sensors incorporated into the stiff filler network or into the stretchable matrix network are used here to detect and investigate the mechanism of stress transfer between distinct populations of polymer strands. We find that as the filler network progressively breaks by random bond scission, there is a critical stress where cooperative bond scission occurs and the macroscopic stretch increases discontinuously by necking. Surprisingly, SP molecules reveal that even in the necked region both filler and matrix chains share the load, with roughly 90% of the SPs force-activated in the filler chains before necking still being loaded in the necked region where significant activation of the SP incorporated into the matrix chains occurs. This result, where both networks remain loaded upon necking, is qualitatively consistent with the model proposed by Brown, where holes or microcracks are formed in the stiff regions and are bridged by stretched matrix chains. Detection of merocyanine (i.e. activated SP) fluorescence by confocal microscopy shows that such microcrack formation is also active at the crack tip even for materials that do not exhibit macroscopic necking. Additionally, we demonstrate that when the ethyl acrylate monomer is replaced by hexyl methacrylate in the first network, preventing molecular connections between the two networks, the stress transmission is less efficient. This study outlines the different roles played by these multiple networks in the onset of fracture and provides molecular insights for the construction of molecular models of fracture of elastomers.

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.

Evolution of the Laser-Induced Spallation Technique in Film Adhesion Measurement

Laser-induced spallation is a process in which a stress wave generated from a rapid, high-energy laser pulse initiates the ejection of surface material opposite the surface of laser impingement. Through knowledge of the stress-wave amplitude that causes film separation, the adhesion and interfacial properties of a film-on-substrate system are determined. Some advantages of the laser spallation technique are the noncontact loading, development of large stresses (on the order of GPa), and high strain rates, up to 10⁸/s. The applicability to both relatively thick films, tens of microns, and thin films, tens of nm, make it a unique technique for a wide range of materials and applications. This review combines the available knowledge and experience in laser spallation, as a state-of-the-art measurement tool, in a comprehensive pedagogical publication for the first time. An historical review of adhesion measurement by the laser-induced spallation technique, from its inception in the 1970s through the present day, is provided. An overview of the technique together with the physics governing the laser-induced spallation process, including functions of the absorbing and confining materials, are also discussed. Special attention is given to applications of laser spallation as an adhesion quantification technique in metals, polymers, composites, ceramics, and biological films. A compendium of available experimental parameters is provided that summarizes key laser spallation experiments across these thin-film materials. This review concludes with a future outlook for the laser spallation technique, which approaches its semicentennial anniversary.

3D fluorescent mapping of invisible molecular damage after cavitation in hydrogen exposed elastomers

Elastomers saturated with gas at high pressure suffer from cavity nucleation, inflation, and deflation upon rapid or explosive decompression. Although this process often results in undetectable changes in appearance, it causes internal damage, hampers functionality (e.g., permeability), and shortens lifetime. Here, we tag a model poly(ethyl acrylate) elastomer with π-extended anthracene-maleimide adducts that fluoresce upon network chain scission, and map in 3D the internal damage present after a cycle of gas saturation and rapid decompression. Interestingly, we observe that each cavity observable during decompression results in a damaged region, the shape of which reveals a fracture locus of randomly oriented penny-shape cracks (i.e., with a flower-like morphology) that contain crack arrest lines. Thus, cavity growth likely proceeds discontinuously (i.e., non-steadily) through the stable and unstable fracture of numerous 2D crack planes. This non-destructive methodology to visualize in 3D molecular damage in polymer networks is novel and serves to understand how fracture occurs under complex 3D loads, predict mechanical aging of pristine looking elastomers, and holds potential to optimize cavitation-resistance in soft materials.

Substituent Effects in Mechanochemical Allowed and Forbidden Cyclobutene Ring-Opening Reactions

Woodward and Hoffman once jested that a very powerful Maxwell demon could seize a molecule of cyclobutene at its methylene groups and tear it open in a disrotatory fashion to obtain butadiene (Woodward, R. B.; Hoffmann, R. The Conservation of Orbital Symmetry. Angew. Chem., Int. Ed. 1969, 8, 781-853). Nearly 40 years later, that demon was discovered, and the field of covalent polymer mechanochemistry was born. In the decade since our demon was befriended, many fundamental investigations have been undertaken to build up our understanding of force-modified pathways for electrocyclic ring-opening reactions. Here, we seek to extend that fundamental understanding by exploring substituent effects in allowed and forbidden ring-opening reactions of cyclobutene (CBE) and benzocyclobutene (BCB) using a combination of single-molecule force spectroscopy (SMFS) and computation. We show that, while the forbidden ring-opening of cis-BCB occurs at a lower force than the allowed ring-opening of trans-BCB on the time scale of the SMFS experiment, the opposite is true for cis- and trans-CBE. Such a reactivity flip is explained through computational analysis and discussion of the so-called allowed/forbidden gap.

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.

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.

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.

Localization of Spiropyran Activation

Functionalization of planar and curved glass surfaces with spiropyran (SP) molecules and localized UV-induced activation of the mechanophore are demonstrated. Fluorescence spectra of UV-irradiated SP-functionalized surfaces reveal that increases in surface roughness or curvature produce more efficient conversion of the mechanophore to the open merocyanine (MC) form. Further, force-induced activation of the mechanophore is achieved at curved glass-polymer interfaces and not planar interfaces. Minimal fluorescence signal from UV-irradiated SP-functionalized planar glass surfaces precluded mechanical activation testing. Curved glass-polymer interfaces are prepared by SP functionalization of E-glass fibers, which are subsequently embedded in a poly(methyl methacrylate) (PMMA) matrix. Mechanical activation is induced through shear loading by a single fiber microbond testing protocol. In situ detection of SP activation at the interface is monitored by fluorescence spectroscopy. The fluorescence increase during interfacial testing suggests that attachment of the interfacial SP molecule to both fiber surface and polymer matrix is present and able to achieve significant activation of SP at the fiber-polymer matrix interface. Unlike previous studies for bulk polymers, SP activation is detected at relatively low levels of applied shear stress. By linking SP at the glass-polymer interface and transferring load directly to that interface, a more efficient mechanism for eliciting the SP response is achieved.

Research on the Wear Behavior of the Fixed Cone Liner of a Cone Crusher Based on the Discrete Element Method

Taking reducing the wear of the fixed cone liner of a cone crusher as the starting point, the movement and geometry parameters of the cone crusher are studied using the discrete element method. To improve the service life and working efficiency of the whole cone crusher. The UG model and discrete element Yade model of the cone crusher are established, and the different shapes of the tin ore are represented using Yade's preprocessor through eight different ways of particle combination and superposition. The static friction coefficient between the manganese ore and the cone crusher is studied and calibrated using the slope method. The relative error between the Yade and test results is 1.58%, and the calibration result is 0.44. The repose angle of the manganese ore is studied using the collapse method. The repose angle increases with the increase of the static friction coefficient and the dynamic friction coefficient, but the change trend is different. The effect of the dynamic friction coefficient on the repose angle is obviously greater than that of the static friction coefficient. The dynamic friction coefficient obtained by Yade is 0.042. Taking the swing distance, rotating speed, and bottom angle of the fixed cone as the orthogonal experimental factors of Yade, the regression equation of the fixed cone liner was obtained through the nonlinear processing of SPSS 25.0. According to Matlab R2017b, the influences of the swinging distance, rotating speed, and bottom angle of the fixed cone on the fixed cone liner are obtained. According to Yade's research results, the order of the influence degree of liner wear is: the rotating speed of moving cone, the swinging distance of the fixed cone, and the base angle of fixed rotation. When the swinging distance of the fixed cone is 146 mm, the rotating speed of the moving cone is 198 rpm, the fixed rotation bottom angle is 28°, and the minimum value of the liner wear is 23 mm. Yade's results are consistent with the change trend of the wear amount of the bushing obtained from the test. The research results show the correctness of using the Yade method to study the wear of the fixed cone liner of a cone crusher, which provides a theoretical basis for reducing the wear of the fixed cone liner of a cone crusher, and puts forward a new method to study the wear of relevant parts of a fixed cone crusher. At the same time, the research results are of great significance for achieving energy-saving in mining enterprises.

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.

Interfacial Force-Focusing Effect in Mechanophore-Linked Nanocomposites

Enhanced force transmission to mechanophores is demonstrated in polymer nanocomposite materials. Spiropyran (SP) mechanophores that change color and fluorescence under mechanical stimuli are functionalized at the interface between SiO2 nanoparticles and polymers. Successful mechanical activation of SP at the interface is confirmed in both solution and solid states. Compared with SP-linked in bulk polymers, interfacial activation induces greater conversion of SP to its colored merocyanine form and also significantly decreases the activation threshold under tension. Experimental observations are supported by finite element simulation of the interfacial stress state. The interfacial force-focusing strategy opens a new way to control the reactivity of mechanophores and also potentially indicates interfacial damage in composite materials.

Polymer mechanochemistry-enabled pericyclic reactions

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

Phase transformation induced mechanochromism in a platinum salt: a tale of two polymorphs

Red crystals of [Pt(tpy)Cl]NO₃·HNO₃ turn yellow when subjected to a mechanical stimulus. Here we report the structural details of mechanochromic system.

Adhesion of biofilms on titanium measured by laser-induced spallation

Eradication of established implant-associated and bacterial biofilm-forming infections remains difficult in part because these biofilms remain well-adhered to the implant surface. Few experimental techniques are available to measure macro-scale strength of bacterial biofilm-implant adhesion. We have adapted the laser spallation technique to compare the macro-scale adhesion strength of biofilms formed on titanium. By using a rapid pressure wave (35 ns) to load the interface, we prevent disturbance of the biofilm surface prior to measurement, and preclude the time necessary for the biofilm to respond to and adapt under loading. Biofilms of Streptococcus mutans, a Gram-positive bacterium associated with human dental caries (cavities) were cultured directly on commercially pure titanium within our custom substrate assembly. Growth conditions were varied by adding sucrose to the Todd Hewitt Yeast (THY) broth: THY control, 37.5 mM, 75 mM, 375 mM, and 750 mM sucrose. Multiple locations on each biofilm were loaded using the laser spallation technique. Loading pressure wave amplitude was controlled by adjusting laser fluence, energy per area. Initially, addition of sucrose to the media increased biofilm adhesion to titanium. However, once a saturation concentration of 75 mM sucrose was reached, increasing the sucrose concentration further resulted in a decrease in biofilm adhesion. This study is the first demonstration of the adaptation of the laser spallation technique to measure bacterial biofilm adhesion. Establishment of this macro-scale biofilm adhesion measurement technique opens the door for many biofilm-surface adhesion studies. We anticipate further work in this area towards understanding the complex relationships among bacteria species, environmental factors, surface characteristics, and biofilm adhesion strength.

Color-Switchable Polar Polymeric Materials

Spiropyran is an important mechanophore, which has rarely been incorporated as a cross-linker in polar polymer matrices, limiting its applications in innovative mechanochromic devices. Here, three spiropyrans with two- or three-attachment positions were synthesized and covalently bonded in polar poly(hydroxyethyl acrylate) (PHEA), to achieve color-switchable materials, triggered by light and when swollen in water. The negative photochromism in the dark and mechanical activation by swelling in water were investigated. Measurements of negative photochromism were conducted in solution and cross-linked PHEA bulk polymers, with both showing color reversibility when stored in the dark or on exposure to visible light. The force of swelling in water was sufficient to induce the ring-opening reaction of spiropyran. It was found that tri-substituted spiropyran (SP3) was less influenced by the polar matrix but showed the fastest color activation during swelling. SP3 also showed accelerated ring opening to the colored state during the swelling process. Bleaching rates and color switchability were investigated under swollen and dehydrated conditions. The effect of cross-link density on the swelling activation was explored to better understand the interaction between the mechanophore and the polar environment. The results demonstrated that influences from both the polar environment and the mechanochromic nature of spiropyran had an impact on the absorption intensity, rate of change, and the decoloration rate of the materials. This study provides the opportunity to manipulate the properties of spiropyrans to afford materials with a range of color-switching properties under different stimuli.

High-intensity focused ultrasound-induced mechanochemical transduction in synthetic elastomers

While study in the field of polymer mechanochemistry has yielded mechanophores that perform various chemical reactions in response to mechanical stimuli, there is not yet a triggering method compatible with biological systems. Applications such as using mechanoluminescence to generate localized photon flux in vivo for optogenetics would greatly benefit from such an approach. Here we introduce a method of triggering mechanophores by using high-intensity focused ultrasound (HIFU) as a remote energy source to drive the spatially and temporally resolved mechanical-to-chemical transduction of mechanoresponsive polymers. A HIFU setup capable of controlling the excitation pressure, spatial location, and duration of exposure is employed to activate mechanochemical reactions in a cross-linked elastomeric polymer in a noninvasive fashion. One reaction is the chromogenic isomerization of a naphthopyran mechanophore embedded in a polydimethylsiloxane (PDMS) network. Under HIFU irradiation evidence of the mechanochemical transduction is the observation of a reversible color change as expected for the isomerization. The elastomer exhibits this distinguishable color change at the focal spot, depending on ultrasonic exposure conditions. A second reaction is the demonstration that HIFU irradiation successfully triggers a luminescent dioxetane, resulting in localized generation of visible blue light at the focal spot. In contrast to conventional stimuli such as UV light, heat, and uniaxial compression/tension testing, HIFU irradiation provides spatiotemporal control of the mechanochemical activation through targeted but noninvasive ultrasonic energy deposition. Targeted, remote light generation is potentially useful in biomedical applications such as optogenetics where a light source is used to trigger a cellular response.

Insights into the mechanochromism of spiropyran elastomers

Colourless polymeric samples comprising mechanochromic spiropyrans (SPs) rapidly appear coloured under external pressure, due to their transition from ring closed SP to ring-opened merocyanine (MC).

Highly Sensitive, Anisotropic, and Reversible Stress/Strain-Sensors from Mechanochromic Nanofiber Composites

Mechanochromic polymeric systems are intensively investigated for real-time stress detection applications. However, an effective stress-sensing material must respond to low deformation with a detectable color change that should be quickly reversible upon force unloading. In this work, mechanochromic nanofibers made by electrospinning are used to produce mechanochromic nanofiber/poly(dimethylsiloxane) (PDMS) composites with isotropic and anisoptropic response. Due to chain alignment of spiropyran copolymer chains within the nanofibers, only very small strains are required to yield a mechanochromic response. Composites with aligned and isotropic nanofibers show anisotropic and isotropic mechanochromic behavior, respectively. Due to the special substitution pattern of spiropyran in the copolymer, the mechanochromic response of these nanofiber/PDMS composites shows fast reversibility upon force unloading. The outstanding benefit of using highly sensitive mechanochromic nanofibers as filler in composite materials allows the detection of directional stress and strain, and it is a step forward in the development of smart, mechanically responsive materials.

Self-Reporting Fiber-Reinforced Composites That Mimic the Ability of Biological Materials to Sense and Report Damage

Sensing of damage, deformation, and mechanical forces is of vital importance in many applications of fiber-reinforced polymer composites, as it allows the structural health and integrity of composite components to be monitored and microdamage to be detected before it leads to catastrophic material failure. Bioinspired and biomimetic approaches to self-sensing and self-reporting materials are reviewed. Examples include bruising coatings and bleeding composites based on dye-filled microcapsules, hollow fibers, and vascular networks. Force-induced changes in color, fluorescence, or luminescence are achieved by mechanochromic epoxy resins, or by mechanophores and force-responsive proteins located at the interface of glass/carbon fibers and polymers. Composites can also feel strain, stress, and damage through embedded optical and electrical sensors, such as fiber Bragg grating sensors, or by resistance measurements of dispersed carbon fibers and carbon nanotubes. Bioinspired composites with the ability to show autonomously if and where they have been damaged lead to a multitude of opportunities for aerospace, automotive, civil engineering, and wind-turbine applications. They range from safety features for the detection of barely visible impact damage, to the real-time monitoring of deformation of load-bearing components.

Mechanical Force Sensitive Acrylic Latex Coating

We prepared force sensitive acrylic latex coatings by covalently incorporating spiropyran mechanophore. The acrylic latexes were obtained through emulsion copolymerization of butyl acrylate (BA), methyl methacrylate (MMA) with vinyltriethoxysilane (VTES) as interparticle cross-linker, and (1'-(2-(methacryloyloxy)ethyl)-3',3'-dimethylspiro[chromene-2,2'-indolin]-6-yl)methyl methacrylate) (SP) as intraparticle cross-linker. The latexes of P(BA-co-MMA-co-SP-co-VTES) were subsequently cast onto Teflon-coated surface to form latex coatings. The condensation of hydrolyzed VTES provided interparticle cross-linking and improved mechanical properties of the formed thin films. Intraparticle cross-linker SP endowed the coatings with mechanoreponsiveness. The mechanoactivation of SP-containing latex films was demonstrated. Increasing the content of intra-cross-linker SP resulted in higher stress sensitivity and lower critical stress required for mechanoactivation. Increasing the content of interparticle cross-linker VTES resulted in higher critical stress for SP mechanoactivation but had little effect on the stress sensitivity. T_g and operation temperature also showed significant effect on mechanoactivation. Slower strain rate allowed for higher SP-to-MC conversion. This work represents the first example of mechanochromic acrylic latexes and provides insight into the design of force sensitive and self-reporting polymer coatings.

Experimental Polymer Mechanochemistry and its Interpretational Frameworks

Polymer mechanochemistry is an emerging field at the interface of chemistry, materials science, physics and engineering. It aims at understanding and exploiting unique reactivities of polymer chains confined to highly non-equilibrium stretched geometries by interactions with their surroundings. Macromolecular chains or their segments become stretched in bulk polymers under mechanical loads or when polymer solutions are sonicated or flow rapidly through abrupt contractions. An increasing amount of empirical data suggests that mechanochemical phenomena are widespread wherever polymers are used. In the past decade, empirical mechanochemistry has progressed enormously, from studying fragmentations of commodity polymers by simple backbone homolysis to demonstrations of self-strengthening and stress-reporting materials and mechanochemical cascades using purposefully designed monomers. This progress has not yet been matched by the development of conceptual frameworks within which to rationalize, systematize and generalize empirical mechanochemical observations. As a result, mechanistic and/or quantitative understanding of mechanochemical phenomena remains, with few exceptions, tentative. In this review we aim at systematizing reported macroscopic manifestations of polymer mechanochemistry, and critically assessing the interpretational framework that underlies their molecular rationalizations from a physical chemist's perspective. We propose a hierarchy of mechanochemical phenomena which may guide the development of multiscale models of mechanochemical reactivity to match the breadth and utility of the Eyring equation of chemical kinetics. We discuss the limitations of the approaches to quantifying and validating mechanochemical reactivity, with particular focus on sonicated polymer solutions, in order to identify outstanding questions that need to be solved for polymer mechanochemistry to become a rigorous, quantitative field. We conclude by proposing 7 problems whose solution may have a disproportionate impact on the development of polymer mechanochemistry.

Light and force dual-responsive waterborne polyurethane in multiple states

Novel SP-WPU presents UV light and force dual-responsive properties in both dispersed state and solid state. The environmental friendly material provides an alternative to extend the application for smart coatings and textiles in industry.

Self-reporting Polymeric Materials with Mechanochromic Properties

The mechanical transduction of force onto molecules is an essential feature of many biological processes that results in the senses of touch and hearing, gives important cues for cellular interactions and can lead to optically detectable signals, such as a change in colour, fluorescence or chemoluminescence. Polymeric materials that are able to visually indicate deformation, stress, strain or the occurrence of microdamage draw inspiration from these biological events. The field of self-reporting (or self-assessing) materials is reviewed. First, mechanochromic events in nature are discussed, such as the formation of bruises on skin, the bleeding of a wound, or marine glow caused by dinoflagellates. Then, materials based on force-responsive mechanophores, such as spiropyrans, cyclobutanes, cyclooctanes, Diels–Alder adducts, diarylbibenzofuranone and bis(adamantyl)-1,2-dioxetane are reviewed, followed by mechanochromic blends, chromophores stabilised by hydrogen bonds, and pressure sensors based on ionic interactions between fluorescent dyes and polyelectrolyte brushes. Mechanobiochemistry is introduced as an important tool to create self-reporting hybrid materials that combine polymers with the force-responsive properties of fluorescent proteins, protein FRET pairs, and other biomacromolecules. Finally, dye-filled microcapsules, microvascular networks, and hollow fibres are demonstrated to be important technologies to create damage-indicating coatings, self-reporting fibre-reinforced composites and self-healing materials.

Shock-Induced Ordering in a Nano-segregated Network-Forming Ionic Liquid

Understanding shockwave-induced physical and chemical changes of impact-absorbing materials is an important step toward the rational design of materials that mitigate the damage. In this work, we report a series of network-forming ionic liquids (NILs) that possess an intriguing shockwave absorption property upon laser-induced shockwave. Microstructure analysis by X-ray scattering suggests nano-segregation of alkyl side chains and charged head groups in NILs. Further post-shock observations indicate changes in the low-Q region, implying that the soft alkyl domain in NILs plays an important role in absorbing shockwaves. Interestingly, we observe a shock-induced ordering in the NIL with the longest (hexyl) side chain, indicating that both nano-segregated structure and shock-induced ordering contribute to NIL's shockwave absorption performance.

Polymer mechanochemistry: from destructive to productive

When one brings "polymeric materials" and "mechanical action" into the same conversation, the topic of this discussion might naturally focus on everyday circumstances such as failure of fibers, fatigue of composites, abrasion of coatings, etc. This intuitive viewpoint reflects the historic consensus in both academia and industry that mechanically induced chemical changes are destructive, leading to polymer degradation that limits materials lifetime on both macroscopic and molecular levels. In the 1930s, Staudinger observed mechanical degradation of polymers, and Melville later discovered that polymer chain scission caused the degradation. Inspired by these historical observations, we sought to redirect the destructive mechanical energy to a productive form that enables mechanoresponsive functions. In this Account, we provide a personal perspective on the origin, barriers, developments, and key advancements of polymer mechanochemistry. We revisit the seminal events that offered molecular-level insights into the mechanochemical behavior of polymers and influenced our thinking. We also highlight the milestones achieved by our group along with the contributions from key comrades at the frontier of this field. We present a workflow for the design, evaluation, and development of new "mechanophores", a term that has come to mean a molecular unit that chemically responds in a selective manner to a mechanical perturbation. We discuss the significance of computation in identifying pairs of points on the mechanophore that promote stretch-induced activation. Attaching polymer chains to the mechanophore at the most sensitive pair and locating the mechanophore near the center of a linear polymer are thought to maximize the efficiency of mechanical-to-chemical energy transduction. We also emphasize the importance of control experiments to validate mechanochemical transformations, both in solution and in the solid state, to differentiate "mechanical" from "thermal" activation. This Account offers our first-hand perspective of the change-in-thinking in polymer mechanochemistry from "destructive" to "productive" and looks at future advances that will stimulate this growing field.

3D-printed mechanochromic materials

We describe the preparation and characterization of photo- and mechanochromic 3D-printed structures using a commercial fused filament fabrication printer. Three spiropyran-containing poly(ε-caprolactone) (PCL) polymers were each filamentized and used to print single- and multicomponent tensile testing specimens that would be difficult, if not impossible, to prepare using traditional manufacturing techniques. It was determined that the filament production and printing process did not degrade the spiropyran units or polymer chains and that the mechanical properties of the specimens prepared with the custom filament were in good agreement with those from commercial PCL filament. In addition to printing photochromic and dual photo- and mechanochromic PCL materials, we also prepare PCL containing a spiropyran unit that is selectively activated by mechanical impetus. Multicomponent specimens containing two different responsive spiropyrans enabled selective activation of different regions within the specimen depending on the stimulus applied to the material. By taking advantage of the unique capabilities of 3D printing, we also demonstrate rapid modification of a prototype force sensor that enables the assessment of peak load by simple visual assessment of mechanochromism.

Dynamic behavior of molecular switches in crystal under pressure and its reflection on tactile sensing

Molecular switches have attracted increasing interest in the past decades, due to their broad applications in data storage, optical gating, smart windows, and so on. However, up till now, most of the molecular switches are operated in solutions or polymer blends with the stimuli of light, heat, and electric fields. Herein, we demonstrate the first pressure-controllable molecular switch of a benzo[1,3]oxazine OX-1 in crystal. Distinct from the light-triggered tautomerization between two optical states, applying hydrostatic pressure on the OX-1 crystal results in large-scale and continuous states across the whole visible light range (from ∼430 to ∼700 nm), which has not been achieved with other stimuli. Based on detailed and systematic control experiments and theoretical calculation, the preliminary requirements and mechanism of pressure-dependent tautomerization are fully discussed. The contributions of molecular tautomerization to the large-scale optical modulation are also stressed. Finally, the importance of studying pressure-responsive materials on understanding tactile sensing is also discussed and a possible mechanotransduction mode is proposed.