Letter pubs.acs.org/macroletters
Mechanochromic Dynamic Covalent Elastomers: Quantitative Stress Evaluation and Autonomous Recovery Keiichi Imato,† Takeshi Kanehara,‡ Tomoyuki Ohishi,§ Masamichi Nishihara,§ Hirofumi Yajima,∥ Masayoshi Ito,∥ Atsushi Takahara,‡,§ and Hideyuki Otsuka*,† †
Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan ‡ Graduate School of Engineering and §Institute for Materials Chemistry and Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan ∥ Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan S Supporting Information *
ABSTRACT: Stress evaluation in polymeric materials is important in order to not only spot danger in them before serious failure, but also precisely interpret the destructive mechanism, which can improve the lifetime and durability of polymeric materials. Here, we are able to visualize stress by color changes, as well as quantitatively estimate the stress in situ, in segmented polyurethane elastomers with diarylbibenzofuranone-based dynamic covalent mechanophores. We prepared films of the segmented polyurethanes, in which the mechanophores were incorporated in the soft segments, and efficiently activated them by mechanical force. Cleavage of the mechanophores during uniaxial elongation and their recovery after the removal of the stress were quantitatively evaluated by in situ electron paramagnetic resonance measurements, accompanied by drastic color changes.
C
Furthermore, it experiences homolytic bond cleavage in response to mechanical force, forming the stable colored radicals in air (Figure 1a).45 This system, in which the dynamic covalent mechanophore undergoes bond scission and recombination, is quantitatively evaluable in situ via electron paramagnetic resonance (EPR) spectroscopy.45 These features could help to clarify the currently debated mechanisms of failure and fatigue in polymeric materials. Here, we demonstrate the in situ quantitative evaluation of the cleavage of DABBF linkages by tensile deformation and recombination after removal of the stress in polyurethane elastomers in air at room temperature. EPR spectroscopy in combination with a tensile tester (Supporting Information, Figure S1) enables the estimation of the generated radicals (stress) during tensile deformation and under constant strain (stress relaxation). In addition to the color changes in the elastomers in response to mechanical force, the in situ quantitative assessment highlights the potential utility of the DABBF-based dynamic covalent mechanophore as a practical probe for stress evaluation in a wide variety of polymeric materials.
atastrophic damage to a material can lead to the diminution of its properties and reduce its reliability. Stress detection would enable to spot danger in the materials before serious failure, and therefore, is of significant importance, particularly in load-bearing polymeric materials. This capability would also increase the understanding of the failure, fatigue, and deterioration mechanisms of polymeric materials, and would be expected to improve their lifetimes and durability. Previously, stress visualization in polymeric materials was achieved by observing changes in the molecular aggregation states of dyes in the polymer matrices1−7 or tunable structural colors.8−13 More recently, color-changing,14−27 fluorescent,28−31 or light-emitting32−34 mechanophores have been incorporated into polymer chains, with the aim of comprehending the stress in the polymers in more detail. Although their material design was sophisticated, no reports achieved both quantitative stress evaluation at the molecular level and autonomous recovery. Therefore, a new class of mechanochromic polymers with the ability to alert us to impending and fatal stress, as well as self-heal minor damage, are required. Diarylbibenzofuranone (DABBF), a dynamic covalent bonding unit, is a candidate for the ideal mechanophore.35−37 DABBF exists in equilibrium with the corresponding radicals at room temperature without any byproducts, although the amount of formed radical species is very small. 38−44 © 2015 American Chemical Society
Received: October 7, 2015 Accepted: November 4, 2015 Published: November 9, 2015 1307
DOI: 10.1021/acsmacrolett.5b00717 ACS Macro Lett. 2015, 4, 1307−1311
Letter
ACS Macro Letters
depending on concentration and temperature, similar to supramolecular polymers.46 Therefore, the polymerization conditions were optimized through a control experiment of polyurethane 2 without the DABBF units (Figures S2 and S3). Both reaction solutions were homogeneous, and gelation was not observed over the course of either reaction. The composition ratio of obtained 1 was determined to be 1/1/ 7.1/2.7 DABBF/PPG/MDI/BDO by 1H NMR measurements, which was largely similar to the feed ratio (1/1/5.1/3.1; Figure S4). The glass transition of the soft segment and the endothermic peak originating from the melting of the hard segment were observed at −31 and 178 °C, respectively, in the differential scanning calorimetry (DSC) curve of 1, indicating that the target segmented structure was successfully obtained (Figure S5). Polyurethane 1 showed good mechanical properties and large hysteresis in the tensile tests, which is the typical behavior of segmented polyurethanes (Figures S6 and S7). The stress evaluation in 1 was demonstrated under uniaxial tensile loading. Films of 1 were prepared by the solution casting method and punched out into dumbbell-shaped specimens. An intense color change from pale yellow to blue occurred rapidly when the specimens were stretched manually (Figure 2a and Movie S1). In situ EPR spectra obtained during the uniaxial elongation are shown in Figure 2b. The g value of these spectra was that of carbon and oxygen radicals (2.003), indicating that the spectra originated from dissociated DABBF radicals.45 Small peaks due to the radicals were observed even without strain at room temperature, because the incorporated DABBF linkages were in equilibrium. The intensity of the peaks increased as the strain increased and the spectral shape was unchanged. These results indicate that the tensile deformation induced the scission of the DABBF linkages in 1 and generated the corresponding relatively stable blue radicals. Figure 2c shows part of the typical stress−strain curve of 1 and the ratio of the dissociated DABBF linkages as a function of the elongation ratio. The DABBF linkages began to be cleaved after an elongation of 50% strain and increased almost linearly with strain thereafter. We could not measure the ratio at higher strains because of slippage of the specimens out of the testing grips. Contrary to the clear color change, the dissociation ratio was surprisingly quite low, less than 0.1%. The generation of the blue radicals during uniaxial elongation was also confirmed by UV−vis absorption measurements (Figure S8). In the spectra, increases in the peaks at 550 and 650 nm (λmax) were clearly observed. To determine the necessity of incorporating the DABBF linkages into the polymer chains for stress detection, we used segmented polyurethane 2 with a similar structure to 1 but without the DABBF linkages. We also prepared control film 3, in which DABBF mechanophores (dihydroxy DABBF) were simply dispersed (Figure S2). Importantly, no color change was observed in the tensile deformation of control film 3 (Figure S11). The ratio of the dissociated mechanophores in 3 remained unchanged against the increasing strain, at less than 0.0002%. The reason why the ratio of dissociated DABBF in 1 was slightly larger than that in 3 before elongation is probably due to the thermal mobility of the polymer chains linked to the mechanophores.43,45 Therefore, we concluded that, for stress detection by DABBF mechanophores, they must be incorporated into the polymer chains. Because DABBF is in equilibrium between its dissociated and associated forms at room temperature, we evaluated the recombination behavior of the mechanically cleaved DABBF
Figure 1. (a) Mechanically triggered establishment of equilibrium between the pale yellow DABBF and blue radicals. (b) Scheme for the incorporation of DABBF mechanophores into segmented polyurethane elastomer 1. The hard segments consist of MDI and BDO, and the soft segments consist of DABBF, PPG, and MDI.
The efficient mechanochemical activation of spiropyran mechanophores, which can isomerize to the corresponding merocyanine forms with mechanical stress, has been achieved by their chemical incorporation into the soft domains of microphase-separated systems.21−25 We designed segmented polyurethane 1 containing DABBF linkages in the soft segments (Figure 1b). The hard segments were expected to aggregate and form hard domains due to strong hydrogen bonding among the urethane linkages. Polyurethane 1 was prepared via a soft prepolymer composed dihydroxy DABBF, poly(propylene glycol) (PPG; Mn = 2700), and 4,4′methylenebis(phenyl isocyanate) (MDI), and subsequent formation of the hard segment by adding the chain extender 1,4-butanediol (BDO). It is difficult to estimate the molecular weight of 1 because the DABBF linkages in 1 are in equilibrium at room temperature,41 that is, the molecular weight changes 1308
DOI: 10.1021/acsmacrolett.5b00717 ACS Macro Lett. 2015, 4, 1307−1311
Letter
ACS Macro Letters
Figure 3. (a) Optical images of dumbbell-shaped 1 before and after being manually stretched and unloaded. (b) EPR spectra of 1 under a constant strain of 50% as a function of time after being stretched once to 300% strain. (c) Typical stress relaxation of 1 and ratio of dissociated DABBF mechanophores in 1 under a constant strain of 50% after being stretched once to 300% strain.
Figure 2. (a) Optical images of manually stretched dumbbell-shaped 1. (b) EPR spectra of 1 as a function of increasing strain (0, 50, 100, 150, 200, 250, and 300%). The intensity was normalized by the volume, with the assumption that the Poisson’s ratio of 1 was 0.5. (c) Part of the typical stress−strain curve of 1 and the ratio of dissociated DABBF mechanophores in 1 vs strain. Error bars show the maximum and minimum values of five samples.
during this relaxation period were measured every 5 min under a constant strain of 50% (without slack) at room temperature, after elongation to 300% strain to activate enough DABBF mechanophores for the behavior to be clearly observed (Figure 3b). The intensity of the peaks decreased with time, indicating that the cleaved linkages spontaneously recombined, probably
linkages through EPR measurements. The blue color of the stretched polyurethane 1 gradually faded over 5 h at room temperature after removal of the stress, with nearly complete recovery to the original dimensions (Figure 3a). EPR spectra 1309
DOI: 10.1021/acsmacrolett.5b00717 ACS Macro Lett. 2015, 4, 1307−1311
ACS Macro Letters
■
due to the high thermal mobility of the linkages incorporated in the soft segments. Figure 3c shows the dissociation ratio upon relaxation as a function of time; the ratio gradually declined over 5 h. On the other hand, the stress remained constant under the constant strain of 50%, and the recombination of the dissociated mechanophores did not induce an increase in the stress. These results suggest that the incorporation of the DABBF mechanophores and the resulting stress-detection capability affected the mechanical properties to a very slight extent. Therefore, the present system could be useful for many existing polymeric materials, even those that are load bearing, because it can impart risk management capabilities without decreasing their mechanical properties. In this study, we demonstrated that DABBF mechanophores are very useful for sensitive stress evaluation in polymeric materials. Thermoplastic elastomers comprising a hard−soft microphase-separated system, in which the dynamic covalent DABBF mechanophores were chemically incorporated in the soft segments, enabled the visualization of cleavage of the mechanophores during tensile deformation, as well as recombination during stress relaxation, by color changes. The in situ quantitative estimation of the amount of dissociated DABBF mechanophores through EPR measurements is expected to lead to a more precise understanding of the failure and fatigue mechanisms in polymeric materials. The cleavage and recombination of the mechanophores did not result in any notable effects on the material’s mechanical properties, showing the potential for practical use, particularly in load-bearing materials. In addition, the spontaneous recombination of the dissociated mechanophores at room temperature, a unique property of this system, would enable an ideal material with the ability to alert us to impending and fatal damage, as well as selfheal minor internal and external damage.
■
REFERENCES
(1) Rubner, M. F. Macromolecules 1986, 19, 2129−2138. (2) Lowe, C.; Weder, C. Adv. Mater. 2002, 14, 1625−1629. (3) Crenshaw, B. R.; Weder, C. Chem. Mater. 2003, 15, 4717−4724. (4) Pucci, A.; Bertoldo, M.; Bronco, S. Macromol. Rapid Commun. 2005, 26, 1043−1048. (5) Crenshaw, B. R.; Weder, C. Macromolecules 2006, 39, 9581− 9589. (6) Crenshaw, B. R.; Burnworth, M.; Khariwala, D.; Hiltner, A.; Mather, P. T.; Simha, R.; Weder, C. Macromolecules 2007, 40, 2400− 2408. (7) Sagara, Y.; Kato, T. Nat. Chem. 2009, 1, 605−610. (8) Asher, S. A.; Holtz, J.; Liu, L.; Wu, Z. J. Am. Chem. Soc. 1994, 116, 4997−4998. (9) Sumioka, K.; Kayashima, H.; Tsutsui, T. Adv. Mater. 2002, 14, 1284−1286. (10) Haque, M. A.; Kamita, G.; Kurokawa, T.; Tsujii, K.; Gong, J. P. Adv. Mater. 2010, 22, 5110−5114. (11) Schäfer, C. G.; Gallei, M.; Zahn, J. T.; Engelhardt, J.; Hellmann, G. P.; Rehahn, M. Chem. Mater. 2013, 25, 2309−2318. (12) Wang, X.-Q.; Wang, C.-F.; Zhou, Z.-F.; Chen, S. Adv. Opt. Mater. 2014, 2, 652−662. (13) Williams, G. A.; Ishige, R.; Cromwell, O. R.; Chung, J.; Takahara, A.; Guan, Z. Adv. Mater. 2015, 27, 3934−3941. (14) Potisek, S. L.; Davis, D. A.; Sottos, N. R.; White, S. R.; Moore, J. S. J. Am. Chem. Soc. 2007, 129, 13808−13809. (15) Davis, D. A.; Hamilton, A.; Yang, J.; Cremar, L. D.; Van Gough, D.; Potisek, S. L.; Ong, M. T.; Braun, P. V.; Martínez, T. J.; White, S. R.; Moore, J. S.; Sottos, N. R. Nature 2009, 459, 68−72. (16) Lee, C. K.; Davis, D. A.; White, S. R.; Moore, J. S.; Sottos, N. R.; Braun, P. V. J. Am. Chem. Soc. 2010, 132, 16107−16111. (17) Kingsbury, C. M.; May, P. A.; Davis, D. A.; White, S. R.; Moore, J. S.; Sottos, N. R. J. Mater. Chem. 2011, 21, 8381−8388. (18) Beiermann, B. A.; Davis, D. A.; Kramer, S. L. B.; Moore, J. S.; Sottos, N. R.; White, S. R. J. Mater. Chem. 2011, 21, 8443−8445. (19) Ramachandran, D.; Liu, F.; Urban, M. W. RSC Adv. 2012, 2, 135−143. (20) Beiermann, B. A.; Kramer, S. L. B.; Moore, J. S.; White, S. R.; Sottos, N. R. ACS Macro Lett. 2012, 1, 163−166. (21) Lee, C. K.; Beiermann, B. A.; Silberstein, M. N.; Wang, J.; Moore, J. S.; Sottos, N. R.; Braun, P. V. Macromolecules 2013, 46, 3746−3752. (22) Jiang, S.; Zhang, L.; Xie, T.; Lin, Y.; Zhang, H.; Xu, Y.; Weng, W.; Dai, L. ACS Macro Lett. 2013, 2, 705−709. (23) Fang, X.; Zhang, H.; Chen, Y.; Lin, Y.; Xu, Y.; Weng, W. Macromolecules 2013, 46, 6566−6574. (24) Hong, G.; Zhang, H.; Lin, Y.; Chen, Y.; Xu, Y.; Weng, W.; Xia, H. Macromolecules 2013, 46, 8649−8656. (25) Chen, Y.; Zhang, H.; Fang, X.; Lin, Y.; Xu, Y.; Weng, W. ACS Macro Lett. 2014, 3, 141−145. (26) Gossweiler, G. R.; Hewage, G. B.; Soriano, G.; Wang, Q.; Welshofer, G. W.; Zhao, X.; Craig, S. L. ACS Macro Lett. 2014, 3, 216−219. (27) Balkenende, D. W. R.; Coulibaly, S.; Balog, S.; Simon, Y. C.; Fiore, G. L.; Weder, C. J. Am. Chem. Soc. 2014, 136, 10493−10498. (28) Cho, S.-Y.; Kim, J.-G.; Chung, C.-M. Sens. Actuators, B 2008, 134, 822−825. (29) Bruns, N.; Pustelny, K.; Bergeron, L. M.; Whitehead, T. A.; Clark, D. S. Angew. Chem., Int. Ed. 2009, 48, 5666−5669. (30) Makyła, K.; Müller, C.; Lörcher, S.; Winkler, T.; Nussbaumer, M. G.; Eder, M.; Bruns, N. Adv. Mater. 2013, 25, 2701−2706. (31) Brantley, J. N.; Bailey, C. B.; Cannon, J. R.; Clark, K. A.; Vanden Bout, D. A.; Brodbelt, J. S.; Keatinge-Clay, A. T.; Bielawski, C. W. Angew. Chem., Int. Ed. 2014, 53, 5088−5092. (32) Chen, Y.; Spiering, A. J. H.; Karthikeyan, S.; Peters, G. W. M.; Meijer, E. W.; Sijbesma, R. P. Nat. Chem. 2012, 4, 559−562. (33) Ducrot, E.; Chen, Y.; Bulters, M.; Sijbesma, R. P.; Creton, C. Science 2014, 344, 186−189. (34) Chen, Y.; Sijbesma, R. P. Macromolecules 2014, 47, 3797−3805.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.5b00717. Experimental data, including sample characterization, mechanical properties, UV−vis absorption measurements, and control experiments (PDF). Movie of stretch-induced mechanochromism (AVI).
■
Letter
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS H.O. gratefully acknowledges financial support from the Funding Program for Next Generation World-Leading Researchers (No. GR077) and JSPS KAKENHI (Nos. 26288057 and 26620175) from Japan Society of the Promotion of Science (JSPS). A part of this work was also supported by ImPACT Program of Council for Science, Technology and Innovation (Cabinet Office, Government of Japan). K.I. acknowledges financial support through JSPS Research Fellowships for Young Scientists (No. 24·7074). 1310
DOI: 10.1021/acsmacrolett.5b00717 ACS Macro Lett. 2015, 4, 1307−1311
Letter
ACS Macro Letters (35) Rowan, S. J.; Cantrill, S. J.; Cousins, G. R. L.; Sanders, J. K. M.; Stoddart, J. F. Angew. Chem., Int. Ed. 2002, 41, 898−952. (36) Maeda, T.; Otsuka, H.; Takahara, A. Prog. Polym. Sci. 2009, 34, 581−604. (37) Wojtecki, R. J.; Meador, M. A.; Rowan, S. J. Nat. Mater. 2011, 10, 14−27. (38) Imato, K.; Nishihara, M.; Kanehara, T.; Amamoto, Y.; Takahara, A.; Otsuka, H. Angew. Chem., Int. Ed. 2012, 51, 1138−1142. (39) Urban, M. W. Nat. Chem. 2012, 4, 80−82. (40) Nishihara, M.; Imato, K.; Irie, A.; Kanehara, T.; Kano, A.; Maruyama, A.; Takahara, A.; Otsuka, H. Chem. Lett. 2013, 42, 377− 379. (41) Imato, K.; Ohishi, T.; Nishihara, M.; Takahara, A.; Otsuka, H. J. Am. Chem. Soc. 2014, 136, 11839−11845. (42) Imato, K.; Nishihara, M.; Irie, A.; Takahara, A.; Otsuka, H. Gels 2015, 1, 58−68. (43) Imato, K.; Takahara, A.; Otsuka, H. Macromolecules 2015, 48, 5632−5639. (44) Frenette, M.; MacLean, P. D.; Barclay, L. R. C.; Scaiano, J. C. J. Am. Chem. Soc. 2006, 128, 16432−16433. (45) Imato, K.; Irie, A.; Kosuge, T.; Ohishi, T.; Nishihara, M.; Takahara, A.; Otsuka, H. Angew. Chem., Int. Ed. 2015, 54, 6168−6172. (46) Brunsveld, L.; Folmer, B. J. B.; Meijer, E. W.; Sijbesma, R. P. Chem. Rev. 2001, 101, 4071−4098.
1311
DOI: 10.1021/acsmacrolett.5b00717 ACS Macro Lett. 2015, 4, 1307−1311
