Spiropyran Mechanophore Research Articles

Versatile Mechanochemical Reactions via Tailored Force Transmission in Mechanophores.

In polymer mechanochemistry, regulating the intrinsic mechanical reactivity of a mechanophore offers extensive opportunities in material science, enabling the development of hierarchical and multifunctional polymer-based materials. Recent advances have focused on innovating various types of mechanophores with inherent reactivity (e.g. regioselectivity and stereoselectivity). However, little attention has been given to modulating their reactivity by tailoring force transmission within mechanophores. Here, we introduce a novel approach through the implementation of a cyclic pulling geometry into an anthracene-maleimide (AM) mechanophore. This approach manipulates force transmission within the mechanophore and effectively regulates its reactivity from 0.0160 min-1 to 0.00133 min-1, achieving up to a 12-fold change. Mechanochemical coupling analysis suggests that the split force transmission within the ring may suppress the sequential bond rupture mechanism in AM mechanophores under applied forces. By leveraging the distinct force transmission pathways within cyclic and linear AM mechanophores, we covalently integrate them with a spiropyran mechanophore to design tandem mechanophore systems for hierarchical mechanochemical activation. These findings highlight the efficacy and versatility of the cyclic pulling strategy in modulating mechanophore reactivity, providing valuable insights for the design of tunable multifunctional polymer-based materials.

Versatile Mechanochemical Reactions via Tailored Force Transmission in Mechanophores

In polymer mechanochemistry, regulating the intrinsic mechanical reactivity of a mechanophore offers extensive opportunities in material science, enabling the development of hierarchical and multifunctional polymer‐based materials. Recent advances have focused on innovating various types of mechanophores with inherent reactivity (e.g. regioselectivity and stereoselectivity). However, little attention has been given to modulating their reactivity by tailoring force transmission within mechanophores. Here, we introduce a novel approach through the implementation of a cyclic pulling geometry into an anthracene‐maleimide (AM) mechanophore. This approach manipulates force transmission within the mechanophore and effectively regulates its reactivity from 0.0160 min‐1 to 0.00133 min‐1, achieving up to a 12‐fold change. Mechanochemical coupling analysis suggests that the split force transmission within the ring may suppress the sequential bond rupture mechanism in AM mechanophores under applied forces. By leveraging the distinct force transmission pathways within cyclic and linear AM mechanophores, we covalently integrate them with a spiropyran mechanophore to design tandem mechanophore systems for hierarchical mechanochemical activation. These findings highlight the efficacy and versatility of the cyclic pulling strategy in modulating mechanophore reactivity, providing valuable insights for the design of tunable multifunctional polymer‐based materials.

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.

Preferential Mechanochemical Activation of Short Chains in Bidisperse Triblock Elastomers.

Polymer mechanochemistry offers attractive opportunities for using macroscopic forces to drive molecular-scale chemical transformations, but achieving efficient activation in bulk polymeric materials has remained challenging. Understanding how the structure and topology of polymer networks impact molecular-scale force distributions is critical for addressing this problem. Here we show that in block copolymer elastomers the molecular-scale force distributions and mechanochemical activation yields are strongly impacted by the molecular weight distribution of the polymers. We prepare bidisperse triblock copolymer elastomers with spiropyran mechanophores placed in either the short chains, the long chains, or both and show that the overall mechanochemical activation of the materials is dominated by the short chains. Molecular dynamics simulations reveal that this preferential activation occurs because pinning of the ends of the elastically effective midblocks to the glassy/rubbery interface forces early extension of the short chains. These results suggest that microphase segregation and network strand dispersity play a critical role in determining molecular-scale force distributions and suggest that selective placement of mechanophores in microphase-segregated polymers is a promising design strategy for efficient mechanochemical activation in bulk materials.

Carbon dot crosslinking towards mechanochemically and photochemically induced fluorescence resonance energy transfer

Mechanochromism in polymers has attracted great interests in recent years. Herein, we develop a mechanochromic elastomer carrying spiropyran mechanophores in the chain and mechanoresponsiveness of elastomer is enhanced by embedding luminescent carbon dot ( CD ) nanoparticles as crosslinkers and energy donor in fluorescence resonance energy transfer (FRET). Compared with the control with physically blended CD , CD crosslinked elastomer exhibit excellent daylight and fluorescent colors change under stretching. The mechanochemically induced fluorescence change is a result of FRET between CD nanoparticles and the activated merocyanine. The control with physically blended CD nanoparticles shows low FRET in stretching due to lower fraction of spiropyran activation as a result of lower mechanical properties. Furthermore, the CD crosslinked elastomer has already been applied in rewritable mechanochemical and photochemical recording or printing. Our demonstration shows that interfacial chemical bonding plays a critical role in the reinforcement of mechanical properties for polymer composites and may exhibit great potential in the study of the acceleration of mechanoresponsiveness of polymer and multi-responsiveness of polymer nanocomposites. • Mechanochromic elastomers were prepared with SP being in chain and crosslinked by CD. • CD as nanofiller and energy donor improves mechanochromism of nanocomposites by FRET. • Elastomers enable use in rewritable mechano- and photochemical recording/printing.

Intrinsic healable mechanochromic materials via incorporation of spiropyran mechanophore into polymer main chain

The development of multifunctional self-healing materials is highly demanding. Herein, an intrinsic healable mechanochromic (i-HMC) material was synthesized via the incorporation of spiropyran mechanophore into the polymer main chain. The as-synthesized i-HMC material exhibited mechanically induced color change as well as great self-healing performance. The mechanical properties of the damaged sample can be completely restored with a high healing efficiency of 98.3% and great toughness of 447 MJ/m 3 . The mobility behaviors of the polymer chain and hydrogen bonding interaction were analyzed through rheological measurements and in-situ FT-IR. The as-synthesized intrinsic materials are capable of dynamic, yet tough healable mechanochromic functions. In contrast to the composites, the intrinsic healable mechanochromic materials contain no foreign additives, which are highly potential for future applications in structural health detectors, load-bearing elements, and wearable devices. • The intrinsic materials exhibited tunable mechanochromic and self-healing bifunctions. • High healing efficiency was achieved for the as-synthesized materials with a large toughness of 447 MJ/m 3 . • Rheology measurements and in-situ FT-IR were utilized to study the healing mechanism.

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.

Highly Stretchable Tough Elastomers Crosslinked by Spiropyran Mechanophores for Strain‐Induced Colorimetric Sensing

AbstractColorimetric spiropyran as a promising mechanophore has proven to be useful in visualizing stress/strain sensing and damage warning in polymeric materials. However, it remains challenging to prepare spiropyran‐containing mechanochromic materials with excellent mechanical properties. Herein, highly stretchable and tough double‐network elastomers crosslinked by spiropyran mechanophores are reported. The spiropyran‐crosslinked elastomer features both force and UV light‐induced color changes originating from the structural transformation from ring‐close spiropyran form to ring‐open merocyanine form of the mechanophores, and excellent mechanical properties (favorable elasticity of 480 kPa, large stretchability of 3400% strain, and high toughness of 8.4 kJ m−1) benefiting from the energy dissipation by breakage of sacrificial bonds. In particular, it is demonstrated that the multi‐mechanoresponsive and mechanically strong elastomer can be served as a conceptual sensor to quantify mechanical stretchability by a visible color change. This work shows promising potential for mechanochemical sensing applications.

A robust mechanochromic self-healing poly(dimethylsiloxane) elastomer

Simultaneously introducing mechanochromic and self-healing properties into polymers is almost a field unexplored, and the integration of these capabilities in one material has important theoretical and substantial significance. In this paper, a mechanochromic poly(dimethylsiloxane) (PDMS) elastomer with self-healing ability and superior mechanical properties is first reported. Spiropyran mechanophore and reversible hydrogen bonds are incorporated into the system to realize multi-stimuliinduced color change and self-healing ability, respectively. Upon uniaxial stretching, heating or UV irradiation, the elastomer exhibits a reversible color variation from yellow to purple, which can recover rapidly by white light illumination. Its excellent tensile strength (10.5 MPa) and elongation at break (785%) are distinctive among PDMS elastomers with no fillers. After heat treatment at 60°C for 24 h, the self-healing efficiency of strength can achieve 92.1%. This novel robust PDMS elastomer holds great promise for applications in visualized stress/strain sensing, self-healing biomaterials and wearable devices. mechanochromic, self-healing, poly(dimethylsiloxane) elastomer

Mechanical Reactivity of Two Different Spiropyran Mechanophores in Polydimethylsiloxane

We investigate the effect of pulling point location on the mechanochemical activation of two isomers of spiropyran in cross-linked polymeric materials through computational calculations and in situ fluorescence measurements. The threshold stress and strain required to activate the spiropyran mechanophores under tensile load are characterized. For both spiropyran isomers, applied stress favors the activated merocyanine states; however, despite differences in mechanochemical behavior predicted by quantum chemical calculations and previous single molecule experiments, both spiropyran isomers exhibit similar mechanochemical reactivity in bulk polymeric materials. The kinetics of the spiropyran–merocyanine transition under different tensile stresses are also examined. Overall, we find that varying the pulling geometry on the spiropyran mechanophore has only a minimal effect on the mechanical activation in bulk polymeric materials due to the complex nature of the macroscopic system.

Substituent Effects and Mechanism in a Mechanochemical Reaction.

We report the effect of substituents on the force-induced reactivity of a spiropyran mechanophore. Using single molecule force spectroscopy, force-rate behavior was determined for a series of spiropyran derivatives substituted with H, Br, or NO2 para to the breaking spirocyclic C-O bond. The force required to achieve the rate constants of ∼10 s-1 necessary to observe transitions in the force spectroscopy experiments depends on the substituent, with the more electron withdrawing substituent requiring less force. Rate constants at 375 pN were determined for all three derivatives, and the force-coupled rate dependence on substituent identity is well explained by a Hammett linear free energy relationship with a value of ρ = 2.9, consistent with a highly polar transition state with heterolytic, dissociative character. The methodology paves the way for further application of linear free energy relationships and physical organic methodologies to mechanochemical reactions, and the characterization of new force probes should enable additional, quantitative studies of force-coupled molecular behavior in polymeric materials.

Mechanochromic Stretchable Electronics.

Soft and stretchable electronics are promising for a variety of applications such as wearable electronics, human-machine interfaces, and soft robotics. These devices, which are often encased in elastomeric materials, maintain or adjust their functionality during deformation, but can fail catastrophically if extended too far. Here, we report new functional composites in which stretchable electronic properties are coupled to molecular mechanochromic function, enabling at-a-glance visual cues that inform user control. These properties are realized by covalently incorporating a spiropyran mechanophore within poly(dimethylsiloxane) to indicate with a visible color change that a strain threshold has been reached. The resulting colorimetric elastomers can be molded and patterned so that, for example, the word "STOP" appears when a critical strain is reached, indicating to the user that further strain risks device failure. We also show that the strain at color onset can be controlled by layering silicones with different moduli into a composite. As a demonstration, we show how color onset can be tailored to indicate a when a specified frequency of a stretchable liquid metal antenna has been reached. The multiscale combination of mechanochromism and soft electronics offers a new avenue to empower user control of strain-dependent properties for future stretchable devices.

Micellar-incorporated hydrogels with highly tough, mechanoresponsive, and self-recovery properties for strain-induced color sensors

A new strategy is developed to incorporate hydrophobic spiropyran mechanophores into the gel network for fabricating tough and mechanoresponsive hydrogels used for strain-induced color sensor.

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. Tg 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.

Photo-inactive divinyl spiropyran mechanophore cross-linker for real-time stress sensing

In this work, we study the mechanical activation of a new type of spiropyran mechanophore cross-linker, (1′-(2-(methacryloyloxy)ethyl)-3′,3′-dimethylspiro[chromene-2,2′-indolin]-6-yl)methyl methacrylate (MA-SP3-MA) in poly methyl acrylate (PMA), prepared via free radical polymerization. Compared with other SPs, SP3 mechanophore shows two unique features. First, the films exhibit good sensitivity towards mechanical force but not to UV irradiation. Second, the merocyanine (MC)-to-SP reversion in PMA films after relaxation was very fast. Further, the mechanical activation can be repeated for three times but with diminishing of peak color, caused by the damage of overstrained polymer chains. We also observe that increasing SP content leads to stiffer and more rigid elastomer as well as earlier threshold strain for activation point of SP to MC but has little effect on threshold stress for SP activation. This work offers inspirations towards mechanophore design for polymeric materials to broaden the scope of potential applications of mechanophores.

Is Molecular Weight or Degree of Polymerization a Better Descriptor of Ultrasound-Induced Mechanochemical Transduction?

A detailed understanding of the fundamental processes that govern mechanical transduction in covalent polymer mechanochemistry is essential to advance innovation in this field. In contrast to progress in the development of new mechanophores, the influence of polymer structure and composition on mechanochemical activity has received relatively little attention. In order to address this gap in knowledge, a continuous flow system with synchronous UV-vis absorption capabilities was designed to quantify the ultrasound-induced mechanical activation of a spiropyran mechanophore in real-time. Measurements of reaction kinetics with polymer tethers of varying repeating unit structure demonstrate that degree of polymerization is the key descriptor of mechanochemical activity, independent of molecular weight and pendant group constitution. These results have important implications for the rationalization of mechanochemical properties and the design of new mechanochemically active polymer systems.

High strain‐rate response of spiropyran mechanophores in PMMA

ABSTRACTWe report the high strain‐rate response of a spiropyran (SP) mechanophore in poly(methylmethacrylate). Previous work on this system has demonstrated a reversible bond scission in the SP under local tensile force, converting it to a fluorescent merocyanine form. A Hopkinson bar was used to apply fast compressive loads at rates from 102 to 104 s−1, resulting in significant activation of the SP near fracture surfaces. However, comparison with a similar thermochromic SP reveals that much of the observed activation likely arises from thermal effects during high‐rate fracture. These results show the importance of a thermally active control system in distinguishing mechanochromic response during high‐rate loading. Microscale fluorescence mapping of the fracture surfaces using a confocal Raman microspectrometer suggests that some distinct mechanical activation may be occurring in craze‐like regions during fibril rupture. The thermal response of the SP is useful in its own right for characterizing plastic heating regions during dynamic fracture. Published 2014. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 1347–1356

Solvent Swelling Activation of a Mechanophore in a Polymer Network

A spiropyran mechanophore (force-sensitive molecule), which is fluorescent in its mechanically activated state, is used to investigate swelling-induced mechanochemistry in cross-linked poly(methyl methacrylate). The spiropyran is incorporated as a cross-linker. A correlation is observed between solvent-induced polymer swelling and fluorescence intensity, suggesting that the forces during swelling are sufficient to drive the electrocyclic ring-opening of spiropyran to its colored and fluorescent merocyanine form. Control experiments and solvatochromic studies validate that activation is indeed due to swelling-induced mechanical forces and not solvent effects. Systematic studies varying solvents and cross-linking densities provide insight into how altering swelling parameters influences the mechanophore response at the molecular level.

Shear activation of mechanophore-crosslinked polymers

A new strategy is employed to impart productive mechanochemical response to crosslinked polymers. Force-sensitive molecules, termed mechanophores, are successfully incorporated as crosslinkers into poly(methyl methacrylate) through a free radical polymerization initiated with benzoyl peroxide and N,N-dimethylaniline. Evidence of a shear activated local chemical reaction (an electrocyclic ring opening) is provided by a color- and fluorescence-generating spiropyran mechanophore. Bulk polymer samples and controls were studied under shear loading. In situ full field fluorescence imaging is used to determine the threshold stress and strain required for activation as a function of shear rate and polymer architecture, both of which have a significant effect on mechanochemical activity in the bulk polymer. Increasing the shear rate leads to an increase in activation stress, similar to bulk polymer yielding. Increasing the length of the primary crosslinker with respect to the spiropyran leads to a decrease in activation stress, while the activation strain becomes more shear rate dependent with longer primary crosslinkers. These findings show that the molecular details of the network architecture can be altered to tune the mechanochemical response.