Spiropyran (SP) is an effective mechanophore because it is easy to be covalently incorporated into polymers and capable of changing color upon mechanical loading. SP motif is a model mechanophore in fundamental studies of mechanochemistry. Therefore, it is of great significance to gain a deep and comprehensive knowledge of SP mechanochemistry for the exploration of mechanochemistry in general. In the beginning of this thesis, a review of SP mechanophore was presented from an engineering perspective. A workflow for SP mechanochemistry, applications in various polymeric systems, impacting factors and characterization techniques as well as conclusions were thoroughly presented. The review aimed to offer deep insight into polymer mechanochemistry and provide approaches to study other mechanophores using the example of SP mechanochemistry in polymers. So far there have been three types of SP mechanophores (SP1, SP2 and SP3) reported in the literature. SP1 and SP2 are sensitive to both UV light and mechanical force, whereas SP3 is sensitive to mechanical force but not to UV, which is an excellent candidate for outdoor applications. Due to the unique feature of SP3, this project is mainly focused on applying SP3 mechanophore into functional and structural polymeric materials. • We designed and synthesized divinyl SP3 mechanophore cross-linker, which can be employed in chain growth polymerization, accounting for more than 80% of total polymer products. As a demonstration, SP3 was incorporated as a cross-linker in the free radical polymerization of methyl acrylate (MA). The mechanoactivation and UV activation of SP3-cross-linked PMA were investigated in details. • SP3 mechanophore cross-linker was covalently incorporated into two widely used polyolefins through facile cross-linking. It represents the first example of smart polyolefins that feel the force by color changing, opening the possibilities of applying SP mechanophore into widely used polyolefin materials, accounting for more than half of the total polymer materials. • We prepared force sensitive acrylic latex coating via covalent incorporation of SP3 mechanophore cross-linker. It is the first example of mechanochromic acrylic latexes, and it provides insight into the design of force-sensitive and self-reporting polymer coatings. • We reported the CO2-breathing induced reversible activation of SP3 mechanophore within microgels. This work provides an effective approach to study the forces inside swollen microgels. It also demonstrates the biomimetic processes with shape deformation and concomitant color/fluorescence change. / Thesis / Doctor of Philosophy (PhD) / Smart polymer has been a research focus for recent decades. One of the most critical responses is to monitor mechanical failures of structural materials, such as stress fraction, fatigue and hysteresis within the polymer by giving off early warnings to prevent the catastrophic failure from occurring. The most prevalent approaches to design a mechanoresponsive polymer is to incorporate a “mechanophore”, containing mechanically labile bonds that are subjective to change under exogenous forces. Spiropyrans (SP) are great candidates for stress/strain sensing in terms of mechanochromism. When mechanical force is applied onto Cspiro-O bond, SP undergoes reversible 6-π ring opening reaction to yield merocyanine (MC). The ring-closed form SP is colorless or yellow and nonfluorescent, whereas the ring-open form MC is purple or blue or red and fluorescent. In this project we first designed and synthesized divinyl spiropyran cross-linker, fitting for chain growth polymerization, which accounts for more than 80% of polymer products. Then the divinyl spiropyran cross-linker was covalently incorporated into polymethylacrylate, polyolefins, acrylic latex coating and CO2-breathing microgels, aiming to broaden the potential applications of mechanophore into various polymers. We also summarized the recent development and studies of spiropyran mechanophore into a comprehensive review from an engineering prospective to provide insights into polymer mechanochemistry and study approaches for other mechanophores.

京都大学 / 新制・課程博士 / 博士(理学) / 甲第23025号 / 理博第4702号 / 新制||理||1674(附属図書館) / 京都大学大学院理学研究科化学専攻 / (主査)准教授 齊藤 尚平, 教授 依光 英樹, 教授 時任 宣博 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

<p>The coupling of mechanical stress fields in polymers to covalent chemistry (polymer mechanochemistry) has provided access to previously unattainable chemical reactions and polymer transformations. In the bulk, mechanochemical activation has been used as the basis for new classes of stress-responsive polymers that demonstrate stress/strain sensing, shear-induced intermolecular reactivity for molecular level remodeling and self-strengthening, and the release of acids and other small molecules that are potentially capable of triggering further chemical response. The potential utility of polymer mechanochemistry in functional materials is limited, however, by the fact that to date, all reported covalent activation in the bulk occurs in concert with plastic yield and deformation, so that the structure of the activated object is vastly different from its nascent form. Mechanochemically activated materials have thus been limited to “single use” demonstrations, rather than as multi-functional materials for structural and/or device applications. Here, we report that filled polydimethylsiloxane (PDMS) elastomers provide a robust elastic substrate into which mechanophores can be embedded and activated under conditions from which the sample regains its original shape and properties. Fabrication is straightforward and easily accessible, providing access for the first time to objects and devices that either release or reversibly activate chemical functionality over hundreds of loading cycles. </p><p>While the mechanically accelerated ring-opening reaction of spiropyran to merocyanine and associated color change provides a useful method by which to image the molecular scale stress/strain distribution within a polymer, the magnitude of the forces necessary for activation had yet to be quantified. Here, we report single molecule force spectroscopy studies of two spiropyran isomers. Ring opening on the timescale of tens of milliseconds is found to require forces of ~240 pN, well below that of previously characterized covalent mechanophores. The lower threshold force is a combination of a low force-free activation energy and the fact that the change in rate with force (activation length) of each isomer is greater than that inferred in other systems. Importantly, quantifying the magnitude of forces required to activate individual spiropyran-based force-probes enables the probe behave as a “scout” of molecular forces in materials; the observed behavior of which can be extrapolated to predict the reactivity of potential mechanophores within a given material and deformation.</p><p>We subsequently translated the design platform to existing dynamic soft technologies to fabricate the first mechanochemically responsive devices; first, by remotely inducing dielectric patterning of an elastic substrate to produce assorted fluorescent patterns in concert with topological changes; and second, by adopting a soft robotic platform to produce a color change from the strains inherent to pneumatically actuated robotic motion. Shown herein, covalent polymer mechanochemistry provides a viable mechanism to convert the same mechanical potential energy used for actuation into value-added, constructive covalent chemical responses. The color change associated with actuation suggests opportunities for not only new color changing or camouflaging strategies, but also the possibility for simultaneous activation of latent chemistry (e.g., release of small molecules, change in mechanical properties, activation of catalysts, etc.) in soft robots. In addition, mechanochromic stress mapping in a functional actuating device might provide a useful design and optimization tool, revealing spatial and temporal force evolution within the actuator in a way that might also be coupled to feedback loops that allow autonomous, self-regulation of activity. </p><p>In the future, both the specific material and the general approach should be useful in enriching the responsive functionality of soft elastomeric materials and devices. We anticipate the development of new mechanophores that, like the materials, are reversibly and repeatedly activated, expanding the capabilities of soft, active devices and further permitting dynamic control over chemical reactivity that is otherwise inaccessible, each in response to a single remote signal.</p> / Dissertation