Article pubs.acs.org/Macromolecules
Polymer−Inorganic Composites with Dynamic Covalent Mechanochromophore Takahiro Kosuge,†,‡ Keiichi Imato,† Raita Goseki,†,‡ and Hideyuki Otsuka*,†,‡ †
Department of Organic and Polymeric Materials and ‡Department of Chemical Science and Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan S Supporting Information *
ABSTRACT: Polymer−inorganic composites with diarylbibenzofuranone (DABBF) moieties, dynamic covalent mechanochromophores, were prepared, and their mechanochromic behavior was systematically investigated. The central C−C bonds in DABBF moieties can be cleaved by mechanical force to form the corresponding stable blue radicals, which can be quantitatively evaluated by electron paramagnetic resonance (EPR) spectroscopy. One controversial issue but attractive property in the DABBF system is the equilibrium between the activated and deactivated states. Although the deactivation process decreases the sensitivity of some equilibrium mechanophores, the equilibrium has rarely been considered when establishing molecular and/or material design of these systems. Herein, a rational macromolecular design to suppress the deactivation of activated dynamic mechanophores and improve sensitivity by limiting their molecular motion is proposed. Polymer−inorganic composite materials with rigid networks prepared from DABBF alkoxysilane derivatives exhibited significant activation of the incorporated DABBF linkages by grinding, with sensitivities more than 50 times as high as that of DABBF monomers. The increased sensitivity is due to the effective transmission of mechanical force to the DABBF moieties in the network structures and suppression of the recombination of the generated radicals by the rigid frameworks. Furthermore, when the rigid frameworks were incorporated into elastomers as inorganic hard domains, the DABBF mechanophores at the interface between the organic and inorganic domains were preferentially activated by elongation.
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INTRODUCTION Mechanochemistry is a growing field concerned with chemical reactions under the action of mechanical force that has attracted much attention because mechanical energy enables chemical reactions to proceed without solvent in the solid state and via different pathways than those induced by thermal or light energy.1 In the field of polymer mechanochemistry, it is known that applied mechanical force is transferred along polymer chains and that mechanoresponsive molecules, mechanophores, can be activated when incorporated into polymer chains.2 Such polymers with covalently linked mechanophores exhibit fascinating functionalities in response to mechanical stimulation, including color changes, 3−9 chemiluminescence, 10 and generation of reactive moieties,8,11−20 acids,21 or catalysts.22,23 Under external mechanical force, spiropyran, the most studied mechanophore, undergoes isomerization to the colored merocyanine form via an electrocyclic ring-opening reaction. As merocyanine reverts to the colorless spiropyran form upon exposure to visible light, this system has been used for the development of photoreversible and stress-detecting materials.4,24−36 In the case of equilibrium spiropyran mechanophore systems, it is important to focus on the thermal isomerization of merocyanine to spiropyran not only for designing repeatable mechanochromic materials but also for improving the © XXXX American Chemical Society
mechanoresponsiveness. In some cases, indeed, it has been reported that merocyanine is converted to spiropyran spontaneously at room temperature,24,37 that a smaller amount of spiropyran in polymers is activated by mechanical force when compared with UV irradiation,24 and that the mechanically induced chemical reaction of spiropyran embedded in polymers strongly depends on polymer chain mobility.4,25 These findings suggest that propagation efficiency of applied stress depends on the chain mobility, and they also suggested that the equilibrium and reversion cause the observed lower mechanical reactivity. Besides spiropyran, molecules with equilibria between dissociated and recombined forms based on dynamic covalent chemistry38,39 can be considered to be potential mechanophores, i.e., dynamic covalent mechanophores. Therefore, polymer design to control the static reaction, i.e., to interfere with the reversion, is of significant importance to improve the mechanoresponsiveness of such systems. Herein, we propose a universal and versatile approach for designing highly mechanoresponsive polymeric materials with dynamic covalent mechanophores by limiting their mobility. We selected diarylbibenzofuranone (DABBF) as a dynamic Received: June 21, 2016 Revised: July 26, 2016
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DOI: 10.1021/acs.macromol.6b01333 Macromolecules XXXX, XXX, XXX−XXX
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Figure 1. (a) Equilibrium between diarylbibenzofuranone (DABBF) and arylbenzofuranone (ABF) radicals. Schematic illustrations of a mobilitylimitation approach: (b) incorporation of DABBF mechanophores into rigid silica networks; (c) incorporation of DABBF mechanophores in rigid networks into elastomers.
Scheme 1. Synthesis of (a) Rigid Cross-Linked Polymer Si-DABBF 1 and (b) Rigid Cross-Linked Polymer Si-DABBF 2
covalent mechanophore, which is in equilibrium with an extremely small amount of dissociated radicals at room temperature.40−43 The central C−C bond in DABBF can be cleaved mechanically, generating blue and oxygen-tolerant arylbenzofuranone (ABF) radicals (Figure 1a).5,44−46 Therefore, the cleavage and reversion of DABBF can be evaluated quantitatively by electron paramagnetic resonance (EPR) measurements. In this paper, DABBF mechanochromophores were incorporated into highly cross-linked, rigid silica networks through a sol−gel method. The rigid network is expected to interfere with the recombination of the dissociated ABF radicals by limiting their mobility (Figure 1b). This hypothesis was validated by evaluation of the mechanoresponsiveness of the hard DABBF-containing organic−inorganic hybrid materials (Si-DABBF 1 and 2) through grinding tests with a milling
machine and EPR measurements. The mechanoresponsive rigid silica networks were further embedded in elastomers of crosslinked poly(butyl acrylate) to expand the availability of the present approach (Figure 1c).
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RESULTS AND DISCUSSION Synthesis of DABBF-Alkoxysilanes and Rigid Silica Networks. DABBF-containing rigid silica networks, Si-DABBF 1 and 2, were prepared from the corresponding DABBFalkoxysilane derivatives by means of a sol−gel method. The precursor compound of Si-DABBF 1, DABBF-bis(trimethoxysilane), was synthesized by reacting dihydroxyDABBF (DABBF-diol) with a 2.5-fold excess of 3-isocyanatopropyltrimethoxysilane (IPTMS) in the presence of di-nbutyltin dilaurate (DBTDL) as the catalyst at 35 °C for 2 h B
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Figure 2. Photographs of (a) Si-DABBF 1, (b) DABBF-diol, and (c) Si-DABBF 2 before and after grinding in air at rt. (d) EPR spectra of ground DABBF-diol, Si-DABBF 1, and Si-DABBF 2 (the spectra were normalized by the amount of DABBF mechanophores contained in the sample). (e) Recombination behavior of mechanically generated ABF radicals in DABBF-diol, Si-DABBF 1, and Si-DABBF 2.
(Scheme 1a).47 The 1H NMR spectrum of the reaction mixture exhibited a new signal, which was assigned to the −CO−NH− CH2− proton, and peak shifts of the methine protons in DABBF-diol and IPTMS (Figure S1 in the Supporting Information). The integral ratio of these peaks was in good agreement with the feed ratio. In addition, the GPC curve of the reaction mixture showed a monomodal distribution that was shifted to the higher molecular weight region (Figure S2). These observations indicate that DABBF-bis(trimethoxysilane) was mainly formed with few byproducts, even though it is known that DBTDL catalyzes the condensation of alkoxysilyl groups as well as urethane formation. The suppression of byproduct formation was a result of the steric effect of the alkyl chains in DBTDL, which significantly decreased the catalytic activity for condensation. 48 Then, di-n-butyltin bis(acetylacetonate) (Sn(acac)Bu2) was added to the reaction mixture, which was not subjected to any purification to avoid the hydrolysis of the methoxysilyl groups during the purification process (Scheme 1a). The mixture was cast in a poly(tetrafluoroethylene-co-perfluoropropyl vinyl ether) (PFA) Petri dish and cured for a week under air at room temperature, resulting in pale-yellow solid Si-DABBF 1. Si-DABBF 2 was synthesized by a similar procedure using tetrahydroxy-DABBF (DABBF-tetraol) (Scheme 1b). Mechanoresponsiveness of Rigid Organic−Inorganic Composites. To investigate the mechanoresponsiveness of SiDABBF 1, the obtained pale-yellow solid was ground using a milling machine. As expected, the color changed rapidly to brilliant blue (Figure 2a). The g value was estimated from EPR measurements to be 2.003, suggesting the generation of ABF radicals derived from cleaved DABBF units (Figure 2d).5,43 The ratio of dissociated DABBF was calculated to be 7.6% using 4hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPOL) as a standard. An interesting comparison can be made between DABBF-diol and Si-DABBF 1. Although these samples were measured under similar conditions, the coloration of ground SiDABBF 1 was more vivid than that of ground DABBF-diol, and the dissociation ratio of Si-DABBF 1 was more than 50 times higher that of DABBF-diol (0.14%) (Figure 2b,d). This
increased dissociation ratio is probably due to the rigid covalent polymer network in Si-DABBF 1 that effectively transfers mechanical force to the incorporated DABBF mechanophores without relief of applied mechanical force caused by molecular motion. This consideration was supported by a similar experiment with Si-DABBF 2, which was prepared from DABBF-tetrakis(trimethoxysilane) with more cross-linking points. In Si-DABBF 2, 14% of DABBF mechanophores were activated upon grinding to show a brilliant blue color (Figure 2c,d). The highly cross-linked, rigid structures are expected to play a significant role in suppressing the recombination of the generated ABF radicals by limiting the DABBF mobility, resulting in the high activity of the DABBF mechanophores. Indeed, the rates of recombination between dissociated ABF radicals to form stable dimers in Si-DABBF 1 and 2 were slightly slower than that in solid state DABBF-diol, as shown in Figure 2e. Ground Si-DABBF 1 and 2 retained the blue color for over a week in air, whereas we have reported that the mechanical activation of high-mobility DABBF units in polymers could not be detected, probably because of quick recombination,5,49 and that the color of activated DABBF mechanophores in segmented polyurethane elastomers containing no rigid silica networks faded in several hours.50,51 On the other hand, the color of activated Si-DABBF 1 and 2 immediately disappeared upon adding a portion of CH2Cl2 (Movie S1), indicating that CH2Cl2 gives the ABF radicals enough mobility to recombine. Considering these results, we can conclude that the rigid, highly cross-linked silica networks allow significant mechanically activation of the dynamic covalent mechanophores as a result of efficiently transferring the mechanical force, limiting the mobility of DABBF mechanophores and mechanically activated ABF radicals, and preventing the radicals from recombining. Synthesis, Mechanical Properties, and Mechanoresponsiveness of Elastomers. As mentioned above, we have successfully improved the mechanoresponsiveness of DABBF by using a simple sol−gel method. To expand the availability of this approach for interfering with the recombination of dynamic covalent mechanochromophores, we synthesized elastomeric C
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Macromolecules Scheme 2. Synthesis of Si-PBA/DABBF-0, -5, -16, -25, -35, and -65
polymer−inorganic composites, in which rigid silica networks composed of DABBF-bis(trimethoxysilane) were introduced as hard domains, by a sol−gel method. Commercially available poly(butyl acrylate) with dimethoxymethylsilyl functionalities at both chain ends (Si-PBA) was employed as the precursor for the soft domains, and a series of organic−inorganic hybrid elastomers with different contents of DABBF-bis(trimethoxysilane) (Si-PBA/DABBF-0, -5, -16, -25, -35, and -65: the numbers indicate the weight content (wt %) of DABBF-bis(trimethoxysilane), which equals the feed ratios (wt %) of DABBF-diol and IPTMS) were prepared (Scheme 2 and Table S1). Si-PBA/DABBF-65, which is the hybrid with the highest content of DABBF mechanophores, was very sensitive to mechanical stress. This sample was turned blue by grinding, scratching, and compressing, though it was brittle and inflexible. On the other hand, the other samples with lower DABBF contents (Si-PBA/DABBF-0, -5, -16, -25, and -35) were flexible. Tensile tests were performed to investigate the mechanical properties of the obtained elastic films (Si-PBA/ DABBF-0, -5, -16, -25, and -35) using dumbbell-shaped specimens. The stress−strain curves of typical Si-PBA/ DABBF films are shown in Figure 3, and the basic mechanical characteristics, i.e., Young’s modulus, fracture stress, fracture strain, and fracture energy, are listed in Table S2. The films became tougher with increasing DABBF alkoxysilane content. In general, such a reinforcement effect is frequently observed in polymer−inorganic materials prepared by sol−gel methods, with the mechanical properties strongly dependent on the nanostructures of the inorganic domains (shape, size, and dispersion degree).52 In fact, transmission electron microscopic (TEM) images of the four elastic samples (Si-PBA/DABBF-5, -16, -25, and -35) showed discontinuous spherelike morphologies, with the average sphere size increasing with the weight ratio of DABBF-bis(trimethoxysilane) (Figure 4). All the
Figure 3. Typical stress−strain curves for Si-PBA/DABBF-0, -5, -16, -25, and -35 (Si-PBA/DABBF-65 was too brittle to measure the tensile strength).
samples were imaged without staining, and therefore DABBFcontaining silica domains appear as darker regions and the poly(butyl acrylate) matrix appears as bright regions. From these results, it was revealed that the hard silica domains, which were mainly constructed from DABBF-bis(trimethoxysilane), efficiently reinforced the elastomers (Figure S8). The mechanoresponsiveness of the Si-PBA/DABBF elastomer films to tensile elongation was first examined by visual inspection. Si-PBA/DABBF-5, with the lowest content of DABBF alkoxysilanes, exhibited no color change, even at the breaking points (Figure 4b). Meanwhile, color changes in response to stretching were observed in the Si-PBA/DABBF16, -25, and -35 films. The strain and engineering stress related to the onset of the color changes in Si-PBA/DABBF-16, -25, D
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Figure 4. Photographs of broken dumbbell specimens and TEM images of (a) Si-PBA/DABBF-0, (b) -5, (c) -16, (d) -25, and (e) -35.
Figure 5. EPR spectra of (a) Si-PBA/DABBF-16 and (b) -35 during stretching. The spectra were normalized by the amount of DABBF mechanophores contained in each sample.
DABBF-35 started dissociating after 10% strain, with a near linear increase of the ratio. In other words, Si-PBA/DABBF-35, with higher DABBF alkoxysilane content, was more sensitive to stretching than Si-PBA/DABBF-16. The attenuation (recombination) behavior of mechanically activated ABF radicals in Si-PBA/DABBF-35 was monitored by EPR after stretching to 60% and then compressing to 9% strain (Figure 6b). Unfortunately, the Si-PBA/DABBF-16 film was not mechanically tough enough to investigate the attenuation behavior. The ABF radicals generated by stretching recombined rapidly compared with those in Si-DABBF 1 and 2. Moreover, the blue color of the dumbbell-shaped specimens of Si-PBA/ DABBF-16, -25, and -35 faded over tens of hours, even though ground Si-DABBF 1 and 2 retained their color for over a week. The attenuation (recombination) behavior of thermally activated ABF radicals in Si-PBA/DABBF-16 and -35 was also monitored by EPR after heating to 100 °C, maintaining at 100 °C until the equilibrium state was achieved, and then cooling to rt (Figure 6c,d). The DABBF units in Si-PBA/ DABBF-16 reached the equilibrium state in 10 min, while those
and -35 were about 500%, 0.80 MPa; 400%, 2.3 MPa; and 150%, 2.0 MPa, respectively. These values for Si-PBA/DABBF16 and -25 were almost the same as the breaking points of the films, and the colors were slightly changed to blue (Figure 4c,d). In contrast, the Si-PBA/DABBF-35 film showed an obvious color change when stretched (Figure 4e and Movie S2). Therefore, it was found that the mechanoresponsiveness strongly depends on the content of DABBF alkoxysilanes, phase separation structures, and mechanical properties. To obtain further insight into the mechanoresponsive behavior, tensile-EPR and variable-temperature EPR measurements were performed for Si-PBA/DABBF-16 and -35. Rectangular specimens were used for the tensile-EPR measurements. EPR signals were clearly observed during stretching, and the g values of the spectra were determined to be 2.003 (Figure 5), indicating that the detected signals were derived from ABF radicals. As shown in Figure 6a, the dissociation ratio of DABBF in the Si-PBA/DABBF-16 film was almost constant until 60% strain and then gradually increased at higher strain. On the other hand, the DABBF mechanophores in Si-PBA/ E
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Figure 6. (a) Dissociation behavior of DABBF mechanophores in Si-PBA/DABBF-16 and -35 and their typical stress−strain curves. Attenuation behavior of ABF radicals in (b) stretched Si-PBA/DABBF-35, (c) heated Si-PBA/DABBF-16, and (d) heated Si-PBA/DABBF-35.
Figure 7. Activation and recombination mechanisms of DABBF mechanophores in Si-PBA/DABBF elastomers by (a) heating and (b) stretching.
heating to 100 °C (Figure 7a). In addition, because recombination of activated ABF radicals with low mobility in hard domains is insignificant, as discussed above, we concluded that the radicals remaining after cooling were located in the hard domains. Therefore, the dissociation ratio after cooling for Si-PBA/DABBF-35 with more low-mobility units was higher than that of Si-PBA/DABBF-16. In contrast, stretching of SiPBA/DABBF-16 and -35 might be considered to dominantly activate DABBF mechanophores at the interfaces, similar to mechanophore activation at heterointerfaces by sonication (Figure 7b),53 as recombination of high-mobility ABF radicals
in Si-PBA/DABBF-35 did not reach equilibrium, even after 4 h. This is probably because Si-PBA/DABBF-35 contained more low-mobility DABBF units compared with Si-PBA/DABBF-16 owing to the higher silica content, which resulted in slower equilibration of the mechanophores. In both samples, the dissociation ratio increased with heating and remained higher than the initial value, even after cooling to rt. We assumed that the DABBF mechanophores were located in the silica hard domains, in the polymer soft domains, and at the interfaces between the two domains, and that unlike with elongation, cleavage of each mechanophore was equally possible upon F
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in the soft domains was too quick for the activation to be observed5,49 and the recombination of stretching-activated radicals was extremely fast compared with that in the silica hard domains (Figure 6b−d). We assumed that the interfaces were not well-defined, but similar to broad interlayers, in which the DABBF mechanophores have enough mobility to be activated by stretching and the cleaved radicals recombine slowly enough to be detected. A plausible mechanism for the stretching-triggered activation of DABBF mechanophores at the interfaces in Si-PBA/DABBF elastomers is shown in Figure S9. In the case of samples with low DABBF alkoxysilane content and small silica hard domains, the mechanical energy is first expended to extend the poly(butyl acrylate) chains in the direction of the force due to long distances between the silica hard domains and finally transferred to the DABBF mechanophores at the interfaces. In addition, the elastomers have fewer DABBF mechanophores at the interfaces and thus form small volume interfaces. These factors cause low mechanoresponsiveness (Figure S9a). Meanwhile, in the case of samples with high DABBF alkoxysilane content and large silica domains, the distances between the silica hard domains are shorter than those in the low DABBF alkoxysilane content samples, which restricts motion of the polymer chains in the soft domains.54 The polymer chains were easily elongated to the fully stretched state, resulting in effective propagation of mechanical force to the DABBF mechanophores at the interfaces. Additionally, the elastomers have many DABBF mechanophores at the interface due to the high DABBF alkoxysilane content and thus form large volume interfaces. These factors cause high mechanoresponsiveness (Figure S9b). Although further fundamental study on the mechanism of mechanophore activation in polymer−inorganic composites is still indispensable, the present study proposed strong possibility of interfacial mechanophore activation.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (H.O.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Mr. Ryohei Kikuchi, National University Corporation Tokyo Institute of Technology Center for Ascended Materials Analysis, for the TEM measurements, and Kaneka Corporation for the donation of the telechelic polymer sample. This work was supported by ImPACT Program of Council for Science, Technology and Innovation (Cabinet Office, Government of Japan), KAKENHI (No. 26288057), and a Grant-in-Aid for JSPS Fellows to T.K. (No. 16J07264).
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REFERENCES
(1) James, S. L.; Adams, C. J.; Bolm, C.; Braga, D.; Collier, P.; Friscic, T.; Grepioni, F.; Harris, K. D.; Hyett, G.; Jones, W.; Krebs, A.; Mack, J.; Maini, L.; Orpen, A. G.; Parkin, I. P.; Shearouse, W. C.; Steed, J. W.; Waddell, D. C. Mechanochemistry: opportunities for new and cleaner synthesis. Chem. Soc. Rev. 2012, 41, 413−447. (2) Caruso, M. M.; Davis, D. A.; Shen, Q.; Odom, S. A.; Sottos, N. R.; White, S. R.; Moore, J. S. Mechanically-induced chemical changes in polymeric materials. Chem. Rev. 2009, 109, 5755−5798. (3) Cho, S.-Y.; Kim, J.-G.; Chung, C.-M. A fluorescent crack sensor based on cyclobutane-containing crosslinked polymers of tricinnamates. Sens. Actuators, B 2008, 134, 822−825. (4) Davis, D. A.; Hamilton, A.; Yang, J.; Cremar, L. D.; Van Gough, D.; Potisek, S. L.; Ong, M. T.; Braun, P. V.; Martinez, T. J.; White, S. R.; Moore, J. S.; Sottos, N. R. Force-induced activation of covalent bonds in mechanoresponsive polymeric materials. Nature 2009, 459, 68−72. (5) 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. (6) Wang, Z.; Ma, Z.; Wang, Y.; Xu, Z.; Luo, Y.; Wei, Y.; Jia, X. A Novel Mechanochromic and Photochromic Polymer Film: When Rhodamine Joins Polyurethane. Adv. Mater. 2015, 27, 6469−6474. (7) Göstl, R.; Sijbesma, R. P. π-extended anthracenes as sensitive probes for mechanical stress. Chem. Sci. 2016, 7, 370−375. (8) Zhang, H.; Gao, F.; Cao, X.; Li, Y.; Xu, Y.; Weng, W.; Boulatov, R. Mechanochromism and Mechanical-Force-Triggered Cross-Linking from a Single Reactive Moiety Incorporated into Polymer Chains. Angew. Chem., Int. Ed. 2016, 55, 3040−3044. (9) Li, Z.; Toivola, R.; Ding, F.; Yang, J.; Lai, P. N.; Howie, T.; Georgeson, G.; Jang, S. H.; Li, X.; Flinn, B. D.; Jen, A. K. Highly Sensitive Built-In Strain Sensors for Polymer Composites: Fluorescence Turn-On Response through Mechanochemical Activation. Adv. Mater. 2016, DOI: 10.1002/adma.201600589. (10) Chen, Y.; Spiering, A. J.; Karthikeyan, S.; Peters, G. W.; Meijer, E. W.; Sijbesma, R. P. Mechanically induced chemiluminescence from polymers incorporating a 1,2-dioxetane unit in the main chain. Nat. Chem. 2012, 4, 559−562. (11) Ramirez, A. L.; Kean, Z. S.; Orlicki, J. A.; Champhekar, M.; Elsakr, S. M.; Krause, W. E.; Craig, S. L. Mechanochemical
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CONCLUSION We have demonstrated a novel macromolecular design, in which dynamic covalent mechanochromophores are incorporated into rigid silica networks, which can improve the mechanoresponsiveness of the bulk material. The silica networks were prepared from DABBF alkoxysilane derivatives through a simple sol−gel method. The high sensitivity originated from the rigid networks that allowed effective propagation of mechanical stress and suppression of the recombination of activated radicals to form thermodynamically stable DABBF. The present macromolecular design should also be useful for other mechanophores that include dynamic covalent bonding units. In addition, we have revealed that DABBF mechanophores at the interfaces between inorganic hard domains and organic soft domains are efficiently activated when incorporated into hybrid elastomers as the hard domains. This approach is expected to be promising for stress-sensing materials. The results of this study are helpful for the application of polymer mechanochemistry and dynamic covalent chemistry in various areas.
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H NMR, FT-IR, GPC, TG analysis, synthetic procedures (PDF) Movie S1: demonstration of adding a portion of CH2Cl2 to ground Si-DABBF 1 (AVI) Movie S2: demonstration of stretching of the Si-PBA/ DABBF-35 film (AVI)
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01333. G
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Article
Macromolecules strengthening of a synthetic polymer in response to typically destructive shear forces. Nat. Chem. 2013, 5, 757−761. (12) Kean, Z. S.; Niu, Z.; Hewage, G. B.; Rheingold, A. L.; Craig, S. L. Stress-responsive polymers containing cyclobutane core mechanophores: reactivity and mechanistic insights. J. Am. Chem. Soc. 2013, 135, 13598−13604. (13) Hickenboth, C. R.; Moore, J. S.; White, S. R.; Sottos, N. R.; Baudry, J.; Wilson, S. R. Biasing reaction pathways with mechanical force. Nature 2007, 446, 423−427. (14) Kryger, M. J.; Ong, M. T.; Odom, S. A.; Sottos, N. R.; White, S. R.; Martinez, T. J.; Moore, J. S. Masked cyanoacrylates unveiled by mechanical force. J. Am. Chem. Soc. 2010, 132, 4558−4559. (15) Lenhardt, J. M.; Ong, M. T.; Choe, R.; Evenhuis, C. R.; Martinez, T. J.; Craig, S. L. Trapping a diradical transition state by mechanochemical polymer extension. Science 2010, 329, 1057−1060. (16) Klukovich, H. M.; Kean, Z. S.; Iacono, S. T.; Craig, S. L. Mechanically induced scission and subsequent thermal remending of perfluorocyclobutane polymers. J. Am. Chem. Soc. 2011, 133, 17882− 17888. (17) Klukovich, H. M.; Kean, Z. S.; Black Ramirez, A. L.; Lenhardt, J. M.; Lin, J.; Hu, X.; Craig, S. L. Tension trapping of carbonyl ylides facilitated by a change in polymer backbone. J. Am. Chem. Soc. 2012, 134, 9577−9580. (18) Diesendruck, C. E.; Steinberg, B. D.; Sugai, N.; Silberstein, M. N.; Sottos, N. R.; White, S. R.; Braun, P. V.; Moore, J. S. Protoncoupled mechanochemical transduction: a mechanogenerated acid. J. Am. Chem. Soc. 2012, 134, 12446−12449. (19) Larsen, M. B.; Boydston, A. J. “Flex-activated” mechanophores: using polymer mechanochemistry to direct bond bending activation. J. Am. Chem. Soc. 2013, 135, 8189−8192. (20) Robb, M. J.; Moore, J. S. A Retro-Staudinger Cycloaddition: Mechanochemical Cycloelimination of a beta-Lactam Mechanophore. J. Am. Chem. Soc. 2015, 137, 10946−10949. (21) Nagamani, C.; Liu, H.; Moore, J. S. Mechanogeneration of Acid from Oxime Sulfonates. J. Am. Chem. Soc. 2016, 138, 2540−2543. (22) Piermattei, A.; Karthikeyan, S.; Sijbesma, R. P. Activating catalysts with mechanical force. Nat. Chem. 2009, 1, 133−137. (23) Michael, P.; Binder, W. H. A Mechanochemically Triggered “Click” Catalyst. Angew. Chem., Int. Ed. 2015, 54, 13918−13922. (24) Lee, C. K.; Davis, D. A.; White, S. R.; Moore, J. S.; Sottos, N. R.; Braun, P. V. Force-induced redistribution of a chemical equilibrium. J. Am. Chem. Soc. 2010, 132, 16107−16111. (25) Beiermann, B. A.; Davis, D. A.; Kramer, S. L. B.; Moore, J. S.; Sottos, N. R.; White, S. R. Environmental effects on mechanochemical activation of spiropyran in linear PMMA. J. Mater. Chem. 2011, 21, 8443−8447. (26) Kingsbury, C. M.; May, P. A.; Davis, D. A.; White, S. R.; Moore, J. S.; Sottos, N. R. Shear activation of mechanophore-crosslinked polymers. J. Mater. Chem. 2011, 21, 8381−8388. (27) Lee, C. K.; Beiermann, B. A.; Silberstein, M. N.; Wang, J.; Moore, J. S.; Sottos, N. R.; Braun, P. V. Exploiting Force Sensitive Spiropyrans as Molecular Level Probes. Macromolecules 2013, 46, 3746−3752. (28) Lee, C. K.; Diesendruck, C. E.; Lu, E. J.; Pickett, A. N.; May, P. A.; Moore, J. S.; Braun, P. V. Solvent Swelling Activation of a Mechanophore in a Polymer Network. Macromolecules 2014, 47, 2690−2694. (29) Kim, J. W.; Jung, Y.; Coates, G. W.; Silberstein, M. N. Mechanoactivation of Spiropyran Covalently Linked PMMA: Effect of Temperature, Strain Rate, and Deformation Mode. Macromolecules 2015, 48, 1335−1342. (30) Gossweiler, G. R.; Hewage, G. B.; Soriano, G.; Wang, Q. M.; Welshofer, G. W.; Zhao, X. H.; Craig, S. L. Mechanochemical Activation of Covalent Bonds in Polymers with Full and Repeatable Macroscopic Shape Recovery. ACS Macro Lett. 2014, 3, 216−219. (31) Zhang, H.; Chen, Y. J.; Lin, Y. J.; Fang, X. L.; Xu, Y. Z.; Ruan, Y. H.; Weng, W. G. Spiropyran as a Mechanochromic Probe in Dual Cross-Linked Elastomers. Macromolecules 2014, 47, 6783−6790.
(32) Chen, Y. J.; Zhang, H.; Fang, X. L.; Lin, Y. J.; Xu, Y. Z.; Weng, W. G. Mechanical Activation of Mechanophore Enhanced by Strong Hydrogen Bonding Interactions. ACS Macro Lett. 2014, 3, 141−145. (33) Degen, C. M.; May, P. A.; Moore, J. S.; White, S. R.; Sottos, N. R. Time-Dependent Mechanochemical Response of SP-Cross-Linked PMMA. Macromolecules 2013, 46, 8917−8921. (34) Hong, G. N.; Zhang, H.; Lin, Y. J.; Chen, Y. J.; Xu, Y. Z.; Weng, W. G.; Xia, H. P. Mechanoresponsive Healable Metallosupramolecular Polymers. Macromolecules 2013, 46, 8649−8656. (35) Fang, X. L.; Zhang, H.; Chen, Y. J.; Lin, Y. J.; Xu, Y. Z.; Weng, W. G. Biomimetic Modular Polymer with Tough and Stress Sensing Properties. Macromolecules 2013, 46, 6566−6574. (36) Jiang, S. C.; Zhang, L. X.; Xie, T. W.; Lin, Y. J.; Zhang, H.; Xu, Y. Z.; Weng, W. G.; Dai, L. Z. Mechanoresponsive PS-PnBA-PS Triblock Copolymers via Covalently Embedding Mechanophore. ACS Macro Lett. 2013, 2, 705−709. (37) Minkin, V. I. Photo-, thermo-, solvato-, and electrochromic spiroheterocyclic compounds. Chem. Rev. 2004, 104, 2751−2776. (38) Rowan, S. J.; Cantrill, S. J.; Cousins, G. R.; Sanders, J. K.; Stoddart, J. F. Dynamic covalent chemistry. Angew. Chem., Int. Ed. 2002, 41, 898−952. (39) Maeda, T.; Otsuka, H.; Takahara, A. Dynamic covalent polymers: Reorganizable polymers with dynamic covalent bonds. Prog. Polym. Sci. 2009, 34, 581−604. (40) Frenette, M.; Aliaga, C.; Font-Sanchis, E.; Scaiano, J. C. Bond dissociation energies for radical dimers derived from highly stabilized carbon-centered radicals. Org. Lett. 2004, 6, 2579−82. (41) Imato, K.; Nishihara, M.; Kanehara, T.; Amamoto, Y.; Takahara, A.; Otsuka, H. Self-healing of chemical gels cross-linked by diarylbibenzofuranone-based trigger-free dynamic covalent bonds at room temperature. Angew. Chem., Int. Ed. 2012, 51, 1138−1142. (42) Nishihara, M.; Imato, K.; Irie, A.; Kanehara, T.; Kano, A. Reversibly Crosslinked Polymeric Micelles Formed by Autonomously Exchangeable Dynamic Covalent Bonds. Chem. Lett. 2013, 42, 377− 379. (43) Imato, K.; Ohishi, T.; Nishihara, M.; Takahara, A.; Otsuka, H. Network reorganization of dynamic covalent polymer gels with exchangeable diarylbibenzofuranone at ambient temperature. J. Am. Chem. Soc. 2014, 136, 11839−11845. (44) Scaiano, J. C.; Martin, A.; Yap, G. P.; Ingold, K. U. A carboncentered radical unreactive toward oxygen: unusual radical stabilization by a lactone ring. Org. Lett. 2000, 2, 899−901. (45) Bejan, E. V.; Font-Sanchis, E.; Scaiano, J. C. Lactone-derived carbon-centered radicals: Formation and reactivity with oxygen. Org. Lett. 2001, 3, 4059−4062. (46) Frenette, M.; MacLean, P. D.; Barclay, L. R. C.; Scaiano, J. C. Radically different antioxidants: Thermally generated carbon-centered radicals as chain-breaking antioxidants. J. Am. Chem. Soc. 2006, 128, 16432−16433. (47) Yoneyama, R.; Sato, T.; Imato, K.; Kosuge, T.; Ohishi, T.; Higaki, Y.; Takahara, A.; Otsuka, H. Autonomously Substitutable Organosilane Thin Films Based on Dynamic Covalent Diarylbibenzofuranone Units. Chem. Lett. 2016, 45, 36−38. (48) Shah, G. B. Effect of length of ligand in organotin compounds on their catalytic activity for the polycondensation of silicone. J. Appl. Polym. Sci. 1998, 70, 2235−2239. (49) Imato, K.; Takahara, A.; Otsuka, H. Self-Healing of a CrossLinked Polymer with Dynamic Covalent Linkages at Mild Temperature and Evaluation at Macroscopic and Molecular Levels. Macromolecules 2015, 48, 5632−5639. (50) Imato, K.; Kanehara, T.; Ohishi, T.; Nishihara, M.; Yajima, H.; Ito, M.; Takahara, A.; Otsuka, H. Mechanochromic Dynamic Covalent Elastomers: Quantitative Stress Evaluation and Autonomous Recovery. ACS Macro Lett. 2015, 4, 1307−1311. (51) 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, DOI: 10.1039/c6cc04767j. H
DOI: 10.1021/acs.macromol.6b01333 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules (52) Mammeri, F.; Bourhis, E. L.; Rozes, L.; Sanchez, C. Mechanical properties of hybrid organic−inorganic materials. J. Mater. Chem. 2005, 15, 3787−3811. (53) Li, J.; Shiraki, T.; Hu, B.; Wright, R. A.; Zhao, B.; Moore, J. S. Mechanophore activation at heterointerfaces. J. Am. Chem. Soc. 2014, 136, 15925−15928. (54) Yahyaei, H.; Mohseni, M. Mechanically controlled, morphologically determined sol−gel derived UV curable hybrid nanocomposites: SAXS and DMTA studies. J. Sol-Gel Sci. Technol. 2013, 66, 187−192.
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DOI: 10.1021/acs.macromol.6b01333 Macromolecules XXXX, XXX, XXX−XXX
