Mechanochromic Polymers Based on Mechanophores

Abstract

In recent years, diverse stimuli‐responsive allochroic materials have rapidly been developed, and smart materials with mechanochromic properties in particular have received increasing attention. This is because force fields have the advantage of being large and controllable compared to other stimulation modalities. Mechanochromic polymers mainly convert mechanical force signals into optical signals, which makes them suitable for applications in bionic actuators, encryption, and signal sensing. In this review, we summarize recent research progress in the design and development of mechanochromic polymers that are classified into two categories. The first category comprises those based on mechanophores that are physically dispersed in polymer matrices in the form of supramolecular aggregates. The second category comprises those based on mechanophores that are covalently linked to polymer networks. We focus on the working mechanisms of the mechanophores and their potential applications, which include damage monitoring and signal sensing.

Mechanochromic polymer blends made with an excimer-forming telechelic sensor molecule

“…The ability to monitor mechanical stresses and strains in polymeric materials via an easily detectable optical signal enables the investigation of deformation processes and is technologically useful, for example, in pressure-sensing films, tamper-evident packaging solutions, and damage sensing. [1][2][3] The most widely employed approach to impart polymers with mechanochromic characteristics is the covalent incorporation of so-called mechanophores, 4 i.e., molecular motifs that undergo mechanically triggered chemical reactions, [5][6][7][8][9] conformational changes, 10,11 rearrangements of interacting optically active moieties, [12][13][14][15] or other effects that cause reversible or irreversible changes to the motif's optical absorption and/or photoluminescence. 14,[16][17][18][19] This general approach has the fundamental disadvantage that chemical modifications are required to render a polymer of interest mechanochromic.…”

“…28 However, a significant limitation of using nanoscale aggregates of small-molecule dyes is that it requires that the mechanochromic additive forms nanoscale aggregates in the polymer matrix and that such aggregates can be mechanically dispersed. 2,20 In many polymers, these conditions are difficult or impossible to satisfy. While the approach generally works well in semi-crystalline polymers, where nucleation effects can promote the formation of small dye aggregates, it is difficult to realize in amorphous materials, where the aggregate formation is hard to control and, if the glass transition temperature is low, the mechanical forces that the aggregates experience may be too small to cause their breakup.…”

Solvent Polarity Effects on the Mechanochemistry of Spiropyran Ring Opening

“…SP probes have been explored in a range of polymer matrices, including polyacrylates, polycaprolactone, , polycarbonate, poly­(ionic liquid), acrylic latex, polystyrene, polyurethanes, polyphenylenes, and hydrogels. − SP mechanochromism has revealed contributions of mobility, relaxation, − and alignment − of polymer chains to molecular-level force transduction. Underlying these responses is the actual force required for SP activation, which is sometimes considered explicitly. ,, Quantitative interpretation is facilitated by a handful of experimental , and computational − studies of force-coupled SP reaction kinetics.…”

Polymer Mechanochemistry of Component Parts for Artificial Molecular Machines

“…The chemistry of dissociative mechanophores such as cargoes, cyclic molecules, metal complexes, and dynamic covalent units, which are generally not used in molecular machines, and their applications are beyond the scope of this article but well summarized in some recent excellent reviews. [25,[31][32][33][34][35][36][37][38][39][40]…”