Multicolor Mechanochromic Polymer Blends That Can Discriminate

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Letter Cite This: ACS Macro Lett. 2018, 7, 556−560

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Multicolor Mechanochromic Polymer Blends That Can Discriminate between Stretching and Grinding Kuniaki Ishizuki, Daisuke Aoki, Raita Goseki, and Hideyuki Otsuka* Department of Chemical Science and Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan S Supporting Information *

ABSTRACT: 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.

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between two colors, the mixture provides access to a desirably wide range of colors like mixing paint colors. For example, orange mechanochromism can be realized from a mixture of green and pink radicals. In this paper, we report our preliminarily findings on multicolor mechanochromic polymer blends that are able to discriminate between different mechanical stimuli, i.e., stretching and grinding, by simply blending two different mechanochromic polymers with different architectures and molecular weight. The key point of this research is the introduction of two types of mechanochromophores into different environments, i.e., soft and hard domains in a polymer blend and using appropriate molecular weight, as the mechanochromic properties of radical-type mechanochromic polymers are strongly affected by the properties of the polymers that are connected to the mechanochromophores, particularly their chain mobility, which in turn depends on the glass-transition temperature (Tg).22,26 In bulk materials that consist of both soft and hard domains, the soft domains are preferentially deformed when a low shear rate is applied (e.g., stretching); i.e., the mechanical stress accumulates in the soft domains. On the other hand, when a high shear rate is applied (e.g., grinding), both soft and hard domains experience mechanical stress. The separate introduction of two mechanochromophores into the soft and

echanochromic polymers, i.e., functional polymeric materials that change color in response to mechanical stimuli such as stretching and grinding, have attracted much attention in recent years.1−16 Such mechanochromic polymers are expected to find applications in damage sensors, which would allow replacing or repairing damaged materials before serious failure occurs. One of the main advantages of mechanochromic polymers is the diversity of their polymer chains, which allows changing the mechano-responsiveness5−11,13,14,16 of mechanochromophores and introducing specific properties that originate from the polymeric nature of these materials. Therefore, various types of mechanochromic polymers that contain mechanochromophores in their polymer chains have been developed since Sottos et al. have reported mechanochromic polymers with a spiropyran-based mechanochromophore in the center of the polymer chain.5 However, most mechanochromic polymers display a simple change between two colors upon exposure to the mechanical stimulus. In addition to such simple color-changing systems, there is also a limited number of reports17−19 on systems with multicolor mechanochromism. However, the rational molecular design of such multicolor mechanochromic polymers is still unknown, predominantly due to a lack of information on the relationship between their structures and the mechanochromic behavior. Recently, we have reported the combinatorial generation of color in mechanochromic polymers by mixing three different mechanochromic polystyrenes that generate differently colored radicals13,20−26 upon grinding.26 Although each mechanochromic polystyrene displays a simple change © XXXX American Chemical Society

Received: March 24, 2018 Accepted: April 16, 2018

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DOI: 10.1021/acsmacrolett.8b00224 ACS Macro Lett. 2018, 7, 556−560

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ACS Macro Letters hard domains in a polymer blend should result in different colors in response to different mechanical stimuli, given that the change in color depends on the individual responsiveness of the mechanochromophores embedded in the soft and hard domains. In addition, the mechanochromophore in the soft domains is activated preferentially over the mechanochromophore in the hard domain by increasing the molecular weight used in the soft domain because the mechanoresponsiveness is improved as the molecular weight of the polymer is increased.23 The purpose of this work is to exhibit the difference in (i) colors generated from two mechanophores and (ii) their mechanoresponsiveness, in a polymer system, which leads to achieve multicolor mechanochromism in response to different stimuli. For that purpose, we designed and synthesized two mechanochromic polymers that contain different radical-type mechanochromophores based on tetraarylsuccinonitrile (TASN)24 and diarylbibenzothiophenonyl (DABBT).26 The TASN and DABBT units afford the corresponding diarylacetonitrile (DAAN) and arylbenzothiophenonyl (ABT) radicals, which are pink and green, respectively, by homolytic cleavage of their central carbon−carbon bonds in response to mechanical stress. It should be noted that these two radical species do not combine, which suppresses exchange reactions between the mechanochromophores embedded in the soft and hard domains (Figures S18 and S19). As for the polymer design, polystyrene with a relatively high Tg was used for the hard domains, while poly(methyl acrylate) with a Tg lower than rt was used for the soft domains. Based on this principal molecular design, we designed two mechanochromic polymers, i.e., a linear polystyrene with a TASN moiety at the center of the PS chain (PS-TASN-PS; Figure 1a) and a star-shaped polymer with a DABBT moiety at the center and four polystyrene-b-poly(methyl acrylate) arms ((PS-b-PMA)4-DABBT; Figure 1b). The star-shaped polymer structure was employed in order to amplify the mechanical stress on DABBT as much as possible.23 PS-TASN-PS was synthesized via a click reaction according to a previously reported method.24 (PS-b-PMA)4-DABBT was synthesized via a click reaction between the corresponding azide-terminated polystyrene-b-poly(methyl acrylate) block copolymer and a DABBT derivative with four ethynyl groups. In addition, a control sample was synthesized, which consisted of a fourarmed star polystyrene-b-poly(methyl acrylate) with a bisphenol A (BPA) moiety at the center of the polymer structure ((PS-b-PMA)4-BPA; Figure 1c). The characterization of the obtained polymers was summarized (Table S1), which clearly shows the successful synthesis of these polymers with a relatively narrow Mw/Mn. It is very important for such mechanochromic polymers that the mechanochromophore is located at the center of the polymer chains.5 To confirm the structure of the obtained (PS-bPMA)4-DABBT sample, we carried out a selective scission of the DABBT mechanochromophore moiety via a treatment with a radical generator. After the scission reaction using 2,2′azobis(4-methoxy-2,4-dimethylvaleronitrile) (V-70), the GPC trace of the obtained polymer was shifted in the direction of the lower-molecular-weight region (Figure S16), which demonstrates that the mechanochromophore was successfully introduced at the center of the polymer chain. PS-TASN-PS, (PS-b-PMA)4-DABBT, and (PS-b-PMA)4BPA were blended in appropriate weight ratios in order to ensure that all samples exhibit a comparable composition

Figure 1. Chemical structures, molecular weights (Mn), and polydispersity indices (Mw/Mn) of (a) PS-TASN-PS, (b) (PS-bPMA)4-DABBT, and (c) (PS-b-PMA)4-BPA (red areas: hard domains; blue areas: soft domains).

(Table 1), before dumbbell-shaped, colorless films were prepared using a solvent-casting method. As all films are Table 1. Experimental Conditions for the Preparation of Films 1−4 film

mixing ratio (w/w)

1 2 3 4

(PS-b-PMA)4-DABBT PS-TASN-PS/(PS-b-PMA)4-BPA = 1/9 PS-TASN-PS/(PS-b-PMA)4-DABBT = 1/9 (PS-b-PMA)4-BPA

composed of four-armed star polystyrene-b-poly(methyl acrylate) as the main polymer part, a microphase separation between the hard polystyrene domains and the soft poly(methyl acrylate) domains should occur. This notion was supported by the results of transmission electron microscopy (TEM) measurements, small-angle X-ray scattering (SAXS) analyses, and the transparency of the films (Figures S24−S28). Figure 2 shows a schematic representation of the polymer chains in the samples shown in Table 1. Film 1 contains the DABBT moieties in the soft domains, while Film 2 is a blend that contains the TASN moieties in the hard domains. Film 3 is a blend that contains both the DABBT and the TASN moieties in the soft and the hard domains, respectively. Film 4 is a model polymer that exhibits almost the same composition as Film 1, albeit without any mechanochromophores. In addition, the polymer which contains DABBT in the soft domain ((PS-bPMA)4-DABBT) has a larger molecular weight than the 557

DOI: 10.1021/acsmacrolett.8b00224 ACS Macro Lett. 2018, 7, 556−560

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Figure 3. (a) Photographs of Films 1−4 after tensile tests. (b) EPR spectra of Films 1−4 during tensile tests (just before the breaking point).

increased during the stretching (Figure 3b). On the other hand, the EPR spectrum of Film 2, which contains the TASN moieties in the hard domains, exhibited almost the same EPR spectrum as Film 4, which does not contain any mechanochromophores (Figure 3b). This result confirms that the TASN moieties in the hard domains remained intact during the tensile stress tests, as only insufficient strain was transmitted onto the TASN units. Therefore, Film 3, which contains two types of mechanochromophores, i.e., DABBT moieties in the soft domains and TASN moieties in the hard domains, showed the same color change to green as Film 1 upon stretching, which arises from the homolytic cleavage of the DABBT moieties in the soft domains. Subsequently, we carried out grinding tests on Films 1−4, which represent an exposure to a mechanical stress that is different from that of stretching (Figure 4a). Color changes to green were observed upon grinding a sample of Film 1, while a color change to pink was observed upon grinding a sample of Film 2, which results from the formation of green DABBT radicals in the soft domains and pink TASN radicals in the hard domains, respectively. Furthermore, Film 3 changed to a yellow/green color that is close to orange,26 which suggests that both TASN moieties in the hard domains and DABBT moieties in the soft domains were cleaved. Similar to the tensile stress tests, EPR measurements were carried out on ground samples of Films 1−4 in order to clarify the origins of these color changes. After grinding the films using a ball mill (30 Hz, 10 min), EPR measurements were carried out to evaluate the generation of radicals (Figure 4b). All films that contained mechanochromophores (Films 1−3) showed increased signal intensities after grinding. These results clearly demonstrate that the color change to yellow/green in Film 3 upon grinding is due to the dissociation of DABBT and TASN moieties. The ratio of DAAN and ABT radicals of ground Film 3 was

Figure 2. Schematic representation of the polymer chains in (a) Film 1, (b) Film 2, (c) Film 3, and (d) Film 4 (red areas: hard domains; blue areas: soft domains).

polymer which contains TASN in the hard domain (PS-TASNPS). To investigate the mechanochromic properties of these films, tensile stress tests were carried out (Figure 3a). Film 1 and Film 3, which contain the DABBT units in the soft domains, exhibited a color change to green upon stretching. In contrast, Film 2 showed no significant color change, despite the presence of the TASN moieties in the hard domains. We assume that virtually all TASN moieties remained intact under these tensile stress test conditions, probably due to the insufficient deformation of the hard domains upon stretching. On the other hand, the DABBT moieties in the soft domains were cleaved, as the soft domains easily deformed during the tensile test. Apart from the qualitative visualization of the mechanical stress, another, arguably even more important, advantage of radical-based mechanochromic polymers is the possibility to quantitatively evaluate the mechanical stress by electron paramagnetic resonance (EPR) spectroscopy in solution and in the solid state. To gain further insight into the quantitative analysis of the mechanical stress, in situ EPR measurements were performed during the tensile stress tests. In Film 1 and Film 3, the strength of the radical signals clearly 558

DOI: 10.1021/acsmacrolett.8b00224 ACS Macro Lett. 2018, 7, 556−560

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radicals after stretching Film 3. On the other hand, both ABT and DAAN radicals were observed after grinding Film 3 (Figure 5b). In other words, Film 3 represents a multicolor mechanochromic polymer that is capable of discriminating between the mechanical stress arising from stretching or grinding. We have developed a mechanochromic polymer system that can discriminate between stretching and grinding. The design crucially relies on the introduction of two mechanochromophores, i.e., DABBT and TASN moieties, which exhibit different color changes in response to mechanical stimuli. These mechanochromophores were introduced into different domains in a polymer blend; i.e., the DABBT moieties were connected to the soft poly(methyl acrylate) blocks in a fourarmed star of polystyrene-b-poly(methyl acrylate). The formation of ABT radicals was observed during tensile stress and grinding tests. On the other hand, TASN moieties were connected to hard polystyrene, and these domains were insensitive to tensile stress tests, but sensitive to grinding. These responses, which are based on the mixing of colors,26 make it possible to qualitatively recognize mechanical stimuli based on their color changes from transparent to yellow/green (grinding) or green (stretching). Although the present results remain preliminary, the concept shown here provides fundamental guidelines for the next generation of mechanochromic polymers. As the options for the selection of the polymers is almost infinite, it should be possible to generate materials with different physical properties by judiciously choosing appropriate polymer compositions. Furthermore, in this system, the macroscopic color change indicates where the applied force is concentrated on the microscale, which may lead to an elucidation of the details of the mechanisms underlying the destruction of such polymeric materials. The results of this study should thus represent a substantial step forward toward the molecular design of advanced smart materials.

Figure 4. (a) Photographs and (b) EPR spectra of Films 1−4 after grinding tests using a ball mill (30 Hz, 10 min).

calculated from the dissociation ratio of the ground Film 2 giving only DAAN radicals. As a result, DAAN radicals accounted for 5.8%, and ABT radicals accounted for 94.2% in ground Film 3. Therefore, the color change due to grinding stands in stark contrast to that by stretching (Figure 5a). Solidstate UV−vis measurements revealed only the presence of ABT

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.8b00224. Experimental section and 1H NMR spectra, IR spectra, GPC profiles, DSC charts, HPLC chromatograms, SAXS profiles, TEM images, stress−strain curves, and solidstate UV−vis spectra for the synthesized compounds and polymers (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hideyuki Otsuka: 0000-0002-1512-671X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by KAKENHI grants (26288057 and 17H01205 to H.O.; 15K17907 to R.G.) from the Japan Society for the Promotion of Science (JSPS) and the ImPACT Program of the Council for Science, Technology and Innovation (Cabinet Office, Government of Japan). Moreover,

Figure 5. (a) Photographs of Film 3 and schematic illustration of the origin of the different colors upon exposure to stretching and grinding as different types of mechanical stimuli. (b) Solid-state UV−vis spectra of Film 3 before and after stretching and grinding. 559

DOI: 10.1021/acsmacrolett.8b00224 ACS Macro Lett. 2018, 7, 556−560

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(18) Ma, Z.; Wang, Z.; Teng, M.; Xu, Z.; Jia, X. Mechanically Induced Multicolor Change of Luminescent Materials. ChemPhysChem 2015, 16, 1811−1828. (19) Wang, T.; Zhang, N.; Dai, J.; Li, Z.; Bai, W.; Bai, R. Novel Reversible Mechanochromic Elastomer with High Sensitivity: Bond Scission and Bending-Induced Multicolor Switching. ACS Appl. Mater. Interfaces 2017, 9, 11874−11881. (20) Imato, K.; Irie, A.; Kosuge, T.; Ohishi, T.; Nishihara, M.; Takahara, A.; Otsuka, H. Mechanophores with a Reversible Radical System and Freezing-Induced Mechanochemistry in Polymer Solutions and Gels. Angew. Chem., Int. Ed. 2015, 54, 6168−6172. (21) Imato, K.; Kanehara, T.; Nojima, S.; Ohishi, T.; Higaki, Y.; Takahara, A.; Otsuka, H. Repeatable Mechanochemical Activation of Dynamic Covalent Bonds in Thermoplastic Elastomers. Chem. Commun. 2016, 52, 10482−10485. (22) Kosuge, T.; Imato, K.; Goseki, R.; Otsuka, H. Polymer− Inorganic Composites with Dynamic Covalent Mechanochromophore. Macromolecules 2016, 49, 5903−5911. (23) Oka, H.; Imato, K.; Sato, T.; Ohishi, T.; Goseki, R.; Otsuka, H. Enhancing Mechanochemical Activation in the Bulk State by Designing Polymer Architectures. ACS Macro Lett. 2016, 5, 1124− 1127. (24) Sumi, T.; Goseki, R.; Otsuka, H. Tetraarylsuccinonitriles as Mechanochromophores to Generate Highly Stable Luminescent Carbon-Centered Radicals. Chem. Commun. 2017, 53, 11885−11888. (25) Imato, K.; Natterodt, J. C.; Sapkota, J.; Goseki, R.; Weder, C.; Takahara, A.; Otsuka, H. Dynamic Covalent DiarylbibenzofuranoneModified Nanocellulose: Mechanochromic Behaviour and Application in Self-Healing Polymer Composites. Polym. Chem. 2017, 8, 2115− 2122. (26) Ishizuki, K.; Oka, H.; Aoki, D.; Goseki, R.; Otsuka, H. Mechanochromic Polymers that Turn Green upon the Dissociation of Diarylbibenzothiophenonyl: The Missing Piece toward Rainbow Mechanochromism. Chem. - Eur. J. 2018, 24, 3170−3173.

R.G. gratefully acknowledges funding from the Japan Prize Foundation. We would like to thank Prof. Teruaki Hayakawa (Tokyo Tech.) for carrying out SAXS measurements. We would also like to thank Ryohei Kikuchi (National University Corporation, Tokyo Tech. Center for Ascended Materials Analysis) for carrying out TEM measurements.

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DOI: 10.1021/acsmacrolett.8b00224 ACS Macro Lett. 2018, 7, 556−560