Mechanochromic Polymers Based on Microencapsulated Solvatochromic Dyes

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100

tion occurs to avoid catastrophic materials failure and to possibly redirect degradation processes into useful responses. Consequently, research efforts directed toward the investigation and development of mechanoresponsive materials has drawn ever increasing attention. [2,3,5-8] Polymers that translate mechanical stresses into a defined response are thought to be useful for many different applications, including as tamper-proof packaging materials, [9] as degradable plastics, [10,11] or for structural health monitoring. [12] One possibility to render polymers mechanoresponsive is the integration of so-called mechanophores. The latter generally feature a weak covalent bond that undergoes either homo-or heterolytic cleavage upon experiencing a force that exceeds a certain threshold. [1,5,7,13,14] These motifs are covalently incorporated into a polymer and serve as predefined weak links that preferentially break or transform in response to an applied mechanical force. [1,6,14-16] Widely investigated mechanophores include spiropyrans, 1,2-dioxetanes, and Diels-Alder adducts. [16-18] The rate constants, threshold forces, and transition state bond-lengths and energies of many mechanophores have been well-characterized via techniques such as single-molecule force spectroscopy (SMFS), [19-22] force-modified potential energy surface modeling, [23] and in situ activation using sonication. [2,22,24] The insights gained in such studies have driven advancements in the field of mechanoresponsive materials and the mechanical activation has, for example, been harnessed to promote thermodynamically unfavorable reactions, [19] affect the intra-or intermolecular transfer of protons, [20,25,26] release small molecules, [27] initiate the depolymerization of the surrounding material, [10,28] or elicit changes in the material's color or fluorescence. [16,17,29] The fact that the active bond or functional group of a mechanophore is typically cleaved irreversibly can limit its utility. Some mechanophores can be returned to their original state through an additional stimulus such as light or heat, but unless the mechanophore undergoes a nonscissile cleavage (e.g., a ring-opening reaction, such as in spiropyrans), the reversal is generally hampered by kinetic factors that leave the mechanophore in its cleaved state. [30] An irreversible response can be desirable for some

Section: Molecular Responses To Mechanical Stimulationmentioning

confidence: 99%

“…In the latter case, however, the change in fluorescence was permanent, whereas the solvent‐only capsules furnished a temporary mechanochromic response that lasted only until the released solvent had evaporated. [ 118 ]…”

Section: Molecular Responses To Mechanical Stimulationmentioning

confidence: 99%

From Molecules to Polymers—Harnessing Inter‐ and Intramolecular Interactions to Create Mechanochromic Materials

Traeger

Kiebala

Weder

et al. 2020

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100

“…Nevertheless, solvent casting processes have also been employed to prepare mechanochromic polymer coatings with thermoset matrices, such as polystyrene (PS), polyacrylic acid (PAA), SBS, and PETG. [ 42,46 ] While many investigations have studied the mechanics of single microcapsules, the role of the mechanical properties of the host matrix in the rupture of the microcapsule and in the resulting chromic change is still an area of investigation that remains underexplored. So far, most investigations have focused on the qualitative detection of damages created by scratches or impacts.…”

Section: Mechanochromic Polymers Based On Microcapsule Containersmentioning

confidence: 99%

Polymer‐Based Mechanochromic Composite Material Using Encapsulated Systems

Calvino

2020

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materials for the food, medical supply, and pharmaceutical industries). [5,6] Conformational changes of polymer chains, cleavage of labile bonds, or dispersion of dye aggregates embedded in polymer matrices are the different mechanisms that have been developed to impart mechanochromism into polymeric materials. [7] Early developments in this field exploited conjugated building blocks to induce a change of absorption and/or emission characteristics as a result of a conformational shift within the dyes' morphology. [8,9] Later strategies relied on the covalent incorporation of chemical motifs, called mechanophores that carry mechanically labile bonds able to, reversibly or irreversibly, cleave upon mechanical activation. During this process, the mechanical forces applied to the material transduce to the chemical motif and convert into chemical reactions leading to optical changes. This approach has been widely employed to access a plethora of mechanochromic systems and has been exploited for a broad range of polymeric materials. [10-12] However, the covalent attachment of chromophores to macromolecules is not a straightforward task and can require significant synthetic efforts. Furthermore, most of the reported chromic responses are not solely activated by mechanical stimulation but can also be triggered by other stimuli such as heat or light. [13,14] The mechanically induced dispersion of aggregachromic chromophores, i.e., molecules that change their optical properties through the disruption of their intermolecular aggregates within the matrix, have been exploited as an alternative to impart mechanochromic properties into materials. [15-17] However, while this strategy has the advantage of simplified synthesis and potential for scalability, its applicability is still restricted to a limited number of matrices. Note that for all the systems described above, it is in general difficult to achieve a mechanochromic response in rigid polymers due to a reduced stress transfer efficiency. Although these enumerated approaches have led to the development of numerous materials enabling a chromogenic signal upon mechanical activation, the design of an efficient mechanochromic material that can be obtained through only a few synthetic steps and is applicable to a broad range of matrices is currently an attractive target. A recent approach relies on the encapsulation of dye solutions into containers that upon mechanical stimulation rupture and release a payload within the material. During the The development of mechanochromic or self-reporting polymers that can indicate damage or fatigue of materials with an optical signal has become of paramount interest to ensure the reliability of the materials and prevent catastrophic failure. This technology can potentially find usefulness for various applications, including in situ monitoring of mechanical events and structural health monitoring systems. An emerging and versatile approach to achieve mechanochromic properties relies on the encapsulation of dye solutions that can be release...

“…Since the AIE phenomenon was first reported in 2001 by Tang's group, [ 3 ] it has been extensively studied for its distinctive property that the AIEgens overcome the drawback of aggregation‐caused quenching (ACQ) effect, which means they could exhibit strong emission in the aggregated states. [ 4–7 ] Tetraphenylethylene (TPE) is a classical AIEgen and has been applied to develop polymeric materials in many fields such as sensing, tracing, and bioimaging. [ 8–10 ] For example, Li et al.…”

Section: Figurementioning

confidence: 99%

Synthesis and Fluorescent Thermoresponsive Properties of Tetraphenylethylene‐Labeled Methylcellulose

Wang

Deng

et al. 2020

Functional polymer, especially the one based on renewable and sustainable materials, has attracted increasing attention to satisfy the growing demand for the design of stimuli‐responsive devices. Methylcellulose (MC) is a water‐soluble derivative of cellulose, which has been widely used in many fields for its biocompatibility and biological inertness. In this work, MC is labeled by tetraphenylethylene (TPE) via azide–alkyne click reaction to obtain a fluorescent cellulose‐based derivative of MC‐TPE. The degree of substitution of MC‐TPE is determined to be 0.074, which can be self‐assembled into micelles in water with the size of 42 ± 6 nm. MC‐TPE shows thermoresponsivity and thermoreversibility in size, transmittance, and fluorescence, enabling it to work as a fluorescent thermosensor. Moreover, MC‐TPE exhibits nontoxicity and biocompatibility, allowing its application in MCF‐7 cell imaging. Therefore, this newly functional natural polymer shows promising potentials in the fields of sensing and bioimaging.