Mechanochromic Polymers That Recognize the Duration of the Mechanical Stimulation via Multiple Mechanochromism.

Mechanochromic polymers can be used as stress- and damage-detecting sensors in polymeric materials, given that mechanical stimuli can be visualized by color changes. Although many types of mechanochromic polymers have been reported so far, there are only few examples on their further functionalization based on multiple color changes (multicolor mechanochromism). Herein, preliminary results are reported on the use of multicolor mechanochromism to detect the duration of the mechanical stimulation by simply mixing white powders of two mechanochromic polystyrene samples that contain a different radical-type mechanochromophore at the midpoint of each polymer chain and thus exhibit different colors in response to mechanical stimuli. The mechanosensitivity can be tuned via the polymer length and shape, and a combination of these two types of mechanochromic polymers allows detecting the duration with multicolor mechanochromism, i.e., a color change from white to blue upon short exposure to grinding and a color change from white to gray upon longer exposure to grinding. Electron paramagnetic resonance and solid-state UV-vis measurements support the mechanism proposed for this multiple mechanochromism.

Mechanochromic dendrimers: the relationship between primary structure and mechanochromic properties in the bulk.

Mechanochromic dendrimers (MDs) were developed as an ideal model for the elucidation of the relationship between the primary structure of molecules/macromolecules with a mechanophore. We discovered that the ratio of activated mechanophores, i.e., the performance parameter to quantitatively evaluate the mechanical response, drastically increases in higher-generation dendrimers.

Mechanochromic Polymers That Turn Green Upon the Dissociation of Diarylbibenzothiophenonyl: The Missing Piece toward Rainbow Mechanochromism.

Mechanochromic polymers, that is, polymers sensitive to mechanical impact, promise great potential for applications in damage sensors. In particular, radical-type mechanochromic polymers, which produce colored radical species in response to mechanical stress, may enable not only the visualization of mechanical stress, but also its quantitative evaluation by electron paramagnetic resonance analysis. Herein, a radical-type mechanochromic polymer that exhibits a color change from white to green upon dissociation of a diarylbibenzothiophenonyl moiety at the mid-point of a polystyrene chain is presented, and its mechanochromic behavior is examined. Mechanochromic materials that show a variety of colors ("rainbow colors") in response to mechanical stress were prepared by simply mixing radical-type mechanochromic polymers of primary colors.

Multicolor Mechanochromic Polymer Blends That Can Discriminate between Stretching and Grinding.

Mechanochromic polymers, which react to mechanical force by changing color, are expected to find applications in smart materials such as damage sensors. Although numerous types of mechanochromic polymers have been reported so far, developing mechanochromic polymers that can recognize different mechanical stimuli remains a formidable challenge. Materials that not only change their color in response to a mechanical stimulus but also detect its nature should be of great importance for practical applications. In this paper, we report our preliminary findings on multicolor mechanochromic polymer blends that can discriminate between two different mechanical stimuli, i.e., stretching and grinding, by simply blending two mechanochromic polymers with different architectures. The rational design and blending of two mechanochromic polymers with radical-type mechanochromophores embedded separately in positions adjacent to soft or hard domains made it possible to achieve multicolor mechanochromism in response to different stimuli. Electron paramagnetic resonance and solid-state UV-vis measurements supported the mechanism proposed for this discrimination.

Freezing-Induced Mechanoluminescence of Polymer Gels.

Mechanochromism can be triggered by different mechanical stimuli, such as tension, compression, shearing, and sonication. Freezing a polymer gel also induces mechanical stress on the polymer network. Herein, freezing-induced mechanoluminescence is demonstrated for the first time by introduction of a tetraarylsuccinonitrile moiety as a light-emitting mechanochromophore at the cross-linking points of a polymer network, in which the mechanical stress induces not only a color change but also light emission. The detailed mechanism and characteristics of this freezing-induced mechanoluminescence were quantitatively evaluated by electron paramagnetic resonance spectroscopy.