Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
A Photoresponsive Polymeric Actuator Topologically Cross-Linked by Movable Units Based on a [2]Rotaxane Yoshinori Takashima,† Yuki Hayashi,† Motofumi Osaki,† Fumitoshi Kaneko,† Hiroyasu Yamaguchi,† and Akira Harada*,†,‡ †
Department of Macromolecular Science, Graduate School of Science, Osaka University, Osaka 560-0043, Japan JST-ImPACT, Tokyo 102-0076, Japan
‡
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S Supporting Information *
ABSTRACT: A photoresponsive polymeric material, in which [2]rotaxane units with α-cyclodextrin threading onto an azobenzene derivative is used as a topological cross-link for the main chains of the polymeric material in aqueous media, was achieved. [2]Rotaxane structures were found to act as movable links in the polymer network, and the mechanical properties of the material were enhanced to show a rupture strain of 2800%. The materials were reversibly deformed by irradiation with UV or visible light in aqueous media, which caused photoisomerization of the azobenzene moiety and changed the structure of the [2]rotaxane linker, leading to deformation of the polymer network. Surprisingly, the dry materials, which had been uniaxially extended in air, showed a faster response than the hydrogel. The orientation of the polymeric network in the materials enables the efficient response. This dry material (5.6 mg) performed 5.6 μJ of mechanical work within 10 s, which is approximately 50 times higher than that achieved in our previous work.
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INTRODUCTION Biological muscles consume chemical energy through ATP dephosphorylation to perform macroscopic mechanical work.1 Myosin and actin filaments in muscle cells show sliding motions leading to contraction and expansion.2−4 This sliding motion, which is reminiscent of a contracting muscle fibril, has inspired the realization of artificial linear motors using supramolecular complexes. Polyrotaxane and [c2]daisy chains (doubly threaded rotaxane dimers) are representative stimuliresponsive molecules and components that are useful for creating functional materials based on sliding motion.5−15 A specific molecular recognition event between cyclic molecules and axis molecules can lead to sliding motion and functional properties.16−18 The introduction of rotaxane structures into polymeric materials is an effective method in the molecular design of entities with both mechanically functional and responsive properties. Figure 1 shows polymeric materials based on rotaxane molecules. Movable (topological) crosslinkers in polymeric materials indirectly connect polymers with mechanically interlocked rotaxane architectures. The movable cross-linking points along the polymer chain enable unique mechanical properties of materials, such as high flexibility and entropic elasticity, by the stress-dispersive deformation process (Figure 1a).19−35 Sauvage and colleagues observed ∼2 nm contractile coconformational changes induced by transition metal ion exchange in a [c2]daisy chain.36−39 Stoddart40−42 and Grubbs43 prepared stimuli-responsive poly([c2]daisy chain)s with crown ethers, which exhibited expansion and contraction in response © XXXX American Chemical Society
Figure 1. Schematic diagrams of three types of polymeric materials with rotaxane molecules: (a) polyrotaxane materials with cross-links between cyclic molecules; (b) [c2]daisy chain materials with polymer chains cross-linked by [c2]daisy chain molecules; (c) [2]rotaxane materials with cross-links between cyclic molecules and axis molecules.
Received: May 2, 2018 Revised: May 28, 2018
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DOI: 10.1021/acs.macromol.8b00939 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules to external stimuli. Giuseppone and colleagues prepared pHtriggered muscle-like [c2]daisy-chain materials.44,45 They also showed light-driven contraction of a macroscopic gel as another type of rotary molecular machine.46 [c2]Daisy chain molecules with cyclodextrin (CD) also demonstrated contraction and expansion properties in aqueous solutions.16−18,47,48 Previously, we prepared a topological gel having cross-links comprising a [c2]daisy chain molecule based on a αCD and azobenzene (Azo) derivative (Figure 1b). Control of the distance between the terminals of the [c2]daisy chain molecule by photostimuli led to the deformation of the [c2]daisy chain material.49 However, preparation of the [c2]daisy chain material requires multiple intricate steps. Development of efficient synthetic method for artificial actuators is still in high demand. Here, we designed a new type of photoresponsive polymeric actuator based on a [2]rotaxane (Figure 1c), which can be obtained by a single polycondensation reaction between multifunctional pseudo-rotaxane and poly(ethylene glycol) (PEG). We hypothesized that cross-linking a polymer network with movable links should result in a material with efficient responsive properties due to its flexible property. As described above, polymer networks cross-linked by [2]rotaxane give tough materials, so that the [2]rotaxane-type actuator would be able to endure high levels of stress from heavy loads. These properties were not observed in the covalently cross-linked materials or the [c2]daisy chain materials. We chose PEG and Azo as axis molecules topologically cross-linked by movable links with a rotaxane structure. The obtained topologically cross-linked gels (αCD-Azo gels) showed photoresponsive deformation behavior even in the dry state.
Figure 2. Schematic illustration of the preparation of the αCD-Azo hydrogel as a topological material (a) and the Azo hydrogel as a chemically cross-linked material (b).
Preparation of a Topological Hydrogel (αCD-Azo Hydrogel). On the basis of the structural study of the pseudo[2]rotaxane structure Lys-αCD/Am2Azo, we investigated the preparation of a poly([2]rotaxane) gel (αCD-Azo gel) cross-linked with the succinimidyl-modified PEG (Su2PEG) (MW of 3.4K or 10K g/mol). Figure 2a shows the preparation of the αCD-Azo hydrogel via a polycondensation reaction. The obtained Lys-αCD/Am2Azo inclusion complex was reacted with Su2PEG by using 2,6-lutidine as a base catalyst. We chose PEG derivatives as the axis molecule because it is highly watersoluble and lacks photoabsorption bands in the ultraviolet (UV) and visible (vis) light region. The interaction between the PEG unit and the Azo unit is negligible. The Ka of αCD with Azo derivatives is higher than that with PEG derivatives. In addition, because the glass transition temperature (Tg) of PEG derivatives is around −20 °C, the flexible PEG-based materials can exhibit photoresponsive properties in the dry state. After the polycondensation reaction, the reaction solution became a self-standing hydrogel (αCD-Azo hydrogel). For the gelation, the concentration of the solution was set at 0.1 M, as the polycondensation reaction between bifunctional amine and succinimidyl ester empirically requires high concentration (≥0.1 M) to obtain percolated gel network. The complexation ratio of Lys-αCD/Am2Azo in the solution (0.1 M) was calculated to be 89−91% by using a log K value of 2.93 ± 0.08 (see Figure S19). In the formation of the polyrotaxane structure, we did not use a bulky end-capping reagent, which is often employed for rotaxane preparations to prevent a ring molecule dethreading from an axis molecule. In this case, the end-capping reagent is unnecessary because the Lys-αCD connected to the PEG chains acts as not only the threading rings but also the end-capping moieties. To elucidate the effect of the topological structure of the αCD-Azo hydrogel, a covalently cross-linked Azo hydrogel was prepared as a reference by the polycondensation reaction among a bis-lysine derivative (BisLys), Am2Azo, and Su2PEG. Although the reaction of the Lys-αCD/Am2Azo complex and Su2PEG in
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RESULTS AND DISCUSSION Complex Formation of pseudo[2]Rotaxane (Lys-αCD/ Am2Azo). We utilized azobenzene (Azo), a guest molecule that shows specific complex formation with αCD in response to photostimuli, to prepare photostimuli-responsive poly([2]rotaxane) materials. The association constant of αCD with trans-Azo (2000 M−1) is much higher than that with cis-Azo (35 M−1).50,51 The difference in the association constant between the two photoisomers is quite important to realize photoresponsive deformation. Prior to preparing the topological hydrogel with a [2]rotaxane, we initially investigated the supramolecular structures of a pseudo[2]rotaxane, Lys-αCD/ Am2Azo, based on lysine-modified αCD (Lys-αCD) and diamino oligo(ethylene glycol)-modified Azo (Am2Azo) (Figure 2). The 1H NMR spectrum of Lys-αCD/Am2Azo in D2O shows that there is a shielding effect of the Azo group and that the peaks corresponding to the inner protons of αCD were split. The signals of the Azo group were separated into those of association and dissociation modes, indicating that Lys-αCD/ Am2Azo spontaneously forms a pseudo[2]rotaxane structure in aqueous solution (Figure S17). By using the integral value of each splitting peak at various concentrations (Table S1), the association constant (Ka) of Lys-αCD with Am2Azo was determined as Ka = (8.7 ± 1.7) × 102 M−1. The 2D ROESY NMR of Lys-αCD in an aqueous solution showed that the protons (a−d) of the Azo group in Am2Azo were correlated to the inner protons (C(3)-H and C(5)-H) of the αCD unit (Figure S18a) in D2O but not in DMSO-d6 (Figure S18b). These results confirm that the Azo unit is included in the cavity of αCD, forming a pseudo[2]rotaxane complex in aqueous solution. B
DOI: 10.1021/acs.macromol.8b00939 Macromolecules XXXX, XXX, XXX−XXX
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Figure 3. Stress−strain curves for the αCD-Azo hydrogel(3.4K), the αCD-Azo hydrogel(10K), and the Azo hydrogel(3.4K) (a). Proposed scheme of the stretching property for the αCD-Azo hydrogels during the tensile tests (b). Compression experimental setup of the αCD-Azo hydrogel(3.4K) and the αCD-Azo xerogel(3.4K) irradiated with UV and vis light (c). The repeated testing of the Young’s modulus of the αCD-Azo hydrogel(3.4K) with exposure to UV and vis light for 60 s (d). The repeated testing of the Young’s modulus of the αCD-Azo xerogel(3.4K) with exposure to UV and vis light for 15 s (e).
DMSO did not yield the αCD-Azo organogel, the reaction of BisLys, Am2Azo, and Su2PEG in DMSO gives the Azo organogel. As described above, the Lys-αCD/Am2Azo complex dissociates in DMSO. These results indicate that the Lys-αCD/ Am2Azo complex in water plays an important role as a movable cross-linking point to form αCD-Azo gel. Mechanical and Photoresponsive Properties of the αCD-Azo Hydrogel. To clarify the effect of the movable cross-linker using [2]rotaxane, the mechanical properties of the αCD-Azo hydrogels and the Azo hydrogel were evaluated by tensile measurements (Figure 3a). Although the rupture strain of the Azo hydrogel(3.4K) cross-linked with Su2PEG (Mw = 3.4K g/mol) was 350%, that of the αCD-Azo hydrogel(3.4K) cross-linked with Su2PEG(3.4K) (Mw = 3.4K g/mol) was 4 times higher (1350%). Su2PEG(10K) (Mw = 10K g/mol) was chosen to increase the range of motion by increasing the chain length of the axis molecules. The rupture strain of the αCDAzo hydrogel(10K) cross-linked with Su2PEG(10K) was increased to 2800%. Similarly, the rupture energy of the αCD-Azo hydrogels also increased with an increase in the molecular weight of PEG. The rupture energy of the αCD-Azo hydrogel(10K) was 2.8 times higher than that of the αCD-Azo hydrogel(3.4K), indicating that sliding motion of Lys-αCD along the PEG chains resulted in stress distribution within the polymer network, allowing it to endure the external stress (Figure 3b). Repeated tensile measurements revealed that the αCD-Azo hydrogel(3.4K) showed slight hysteresis loss of 16.4% when the sample was uniaxially stretched up to 500%, whereas the Azo hydrogel(3.4K) did not show any hysteresis under the same conditions (Figure S26). These results also support the sliding motion of αCD along the PEG chains and the stress distribution by the movable cross-linking point.19−35 UV−vis spectroscopy was used to monitor the photoisomerization property of the Azo unit of the αCD-Azo
hydrogel(3.4K) (Figure S24) to rule out the potential for the suppression of the photoisomerization by complex formation between the dye molecule and CD. An LED light source and a 300 W xenon lamp with a mirror module and bandpass filters were used in order to irradiate the αCD-Azo hydrogel with UV (λ = 365 nm) or vis light (λ = 430 nm), respectively. When αCD-Azo(3.4K) was irradiated with UV light, within a few minutes, the π−π* transition absorption band derived from the trans-Azo isomer decreased, and the n−π* transition absorption band at 430 nm corresponding to cis-Azo isomer increased. Conversely, irradiating with vis light restored the initial intensity of the π−π* transition absorption band and caused the n−π* transition absorption band to disappear. These results indicate that the isomerization of the Azo unit in the αCD-Azo hydrogel(3.4K) is triggered by photoirradiation and that these behaviors are reversibly controllable by using UV and vis light. UV and Vis Light-Responsive Property of the αCDAzo Hydrogel(3.4K) and the αCD-Azo Xerogel. Figure 3c−e shows the change in the cross-link density in the αCDAzo hydrogel(3.4K) and the αCD-Azo xerogel(3.4K) depending on UV and vis light irradiation. In general, cross-link density is correlated to the Young’s modulus of materials. The hydrogel was immersed in water, and the rod lens was located on the upper side of the gels (Figure 3c). The Young’s modulus of the αCD-Azo hydrogel(3.4K) was determined by a compression test. After UV light irradiation for 1 min, the Young’s modulus decreased. In addition, continuous irradiation with vis light resulted in recovery of the initial Young’s modulus (Figure 3d). After UV light irradiation, the cis-Azo unit is supposed to dethread from the αCD cavity, which increases the sliding mobility of the Lys-αCD unit and leads to a decreased Young’s modulus. This behavior is equivalent to decreasing the crosslink density because the Lys-αCD unit functions as a movable C
DOI: 10.1021/acs.macromol.8b00939 Macromolecules XXXX, XXX, XXX−XXX
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Figure 4. Experimental equipment and irradiation experimental setup of the αCD-Azo xerogel(3.4K) in air (a). Lateral views of the αCD-Azo xerogel(3.4K) hung with a clip. Flexion angle (θ) is defined here (b). Light irradiation from the right side of the αCD-Azo xerogel(3.4K) for 3 s. After UV irradiation, the αCD-Azo xerogel(3.4K) bends to the right side. Subsequent irradiation with UV light from the left side restores the initial form (c). Plots of the flexion angle (θ) as a function of the irradiation time of the αCD-Azo xerogel(3.4K). The purple and white areas denote UV irradiation and no irradiation under dry conditions, respectively. The flexion angle (θ) is measured using snapshots from Movie S2 (d). The dependency of flexion speed on the strains applied to the αCD-Azo hydrogel(3.4K) before drying process (e). The equipment of energy conversion test of αCD-Azo xerogel(3.4K) (f). The energy conversion from light to mechanical work is evaluated by using the values and equation shown in (g). Lateral views of the αCD-Azo xerogel(3.4K) lifting up the weight (h).
obtain a fiber-like αCD-Azo xerogel(3.4K). Figure S30 shows an X-ray diffraction pattern of the stretched αCD-Azo xerogel(3.4K). The stretched αCD-Azo xerogel(3.4K) showed anisotropic diffraction patterns, which are characteristic of an anisotropic structure. On the other hand, the unstretched αCDAzo xerogel(3.4K) showed a Debye−Scherrer ring, which is characteristic of an isotropic structure. These results indicate that the application of strain to the αCD-Azo hydrogel(3.4K) and the αCD-Azo xerogel(3.4K) caused orientation of the polymer chains and the Lys-αCD/Am2Azo unit. Figure 4 shows the results of deformation testing of the stretched αCD-Azo xerogel in response to photostimuli. The stretched αCD-Azo xerogel(3.4K) quickly bent toward the light source side within 3 s upon irradiation with UV light (Figure 4a). Here, the flexion angle θ is defined as shown in Figure 4b. At least 10 times deformations were observed by using a single sample. The flexion angle of the covalently cross-linked Azo xerogel was not changed by photoirradiation. In contrast, the topologically cross-linked αCD-Azo xerogel(3.4K) and αCDAzo xerogel(10K) bent 20° upon UV light irradiation only for 3 s (Figure 4c,d and Movie S1), indicating that the rotaxane cross-linking enabled fast photoresponsive bending in the dry state. The αCD-Azo xerogel(3.4K) showed slightly faster deformation and a larger amount of displacement. The αCD-
cross-linker, removing the fixed cross-linking point on the Azo unit. Similarly, the αCD-Azo xerogel(3.4K) also demonstrated a reversible Young’s modulus change even in the dry state (Figure 3e). At least five repeating cycles were performed by using a single sample in the light-responsive tests without showing any degradation. Figure S28 shows macroscopic deformation of the αCD-Azo hydrogel(3.4K) upon irradiation with UV and vis light. When the hydrogel was irradiated with UV light for 3 h, the gel slightly bent toward the light source side. In contrast, the gel deformed to its initial shape when irradiated with visible light for 3 h. The αCD-Azo hydrogel(3.4K) thus showed reversible deformation. UV and Vis Light-Responsive Actuation of the αCDAzo Xerogel. To achieve larger deformation by photostimuli, we proposed that orientational control of the Lys-αCD/ Am2Azo unit in the αCD-Azo hydrogel(3.4K) should amplify the extent of deformation. Though the covalently cross-linked Azo hydrogel(3.4K) was ruptured by only 350% strain, the αCD-Azo hydrogels(3.4K and 10K) can be extended to 1350% and 2800%, respectively (Figure 3a). Therefore, the orientational control by external force is applicable to the αCD-Azo hydrogels. To align the Lys-αCD/Am2Azo unit, the αCD-Azo hydrogel(3.4K) was uniaxially elongated and dried in vacuo to D
DOI: 10.1021/acs.macromol.8b00939 Macromolecules XXXX, XXX, XXX−XXX
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Azo hydrogel(3.4K) has higher density of the cross-linking point than the αCD-Azo hydrogel(10K), and the higher crosslinking density is supposed to result in the relatively faster deformation. In addition, the change in the flexion angle per second was found to depend on the strain applied to the αCDAzo hydrogel(3.4K) before drying (Figure 4e), suggesting that the applied strain contributed to the efficient response and that the anisotropic rotaxane network of the actuator showed an efficient response. In the uniaxially stretched αCD-Azo xerogels, both the molecular machine (αCD-Azo [2]rotaxane) and the rails along which αCD can slide (PEG chain) are supposed to be oriented and extended. Therefore, the effect of the sliding movement of αCD and the motion of PEG chain in thermodynamics might be especially extended along the stretched direction to show the macroscopic fast actuation. Figure 4f−h shows energy conversion from light to mechanical work. We used 5.6 mg of the αCD-Azo xerogel(3.4K) to perform the energy conversion, as the αCD-Azo xerogel(3.4K) showed both the fastest and largest deformation. When a αCD-Azo xerogel(3.4K) from which a weight of 174 mg was hung was irradiated with UV light, the αCD-Azo xerogel(3.4K) contracted and lifted the weight vertically upward by 3.3 mm within 10 s (Movie S2). The mechanical work (W) produced by the αCD-Azo xerogel(3.4K), which is determined by W = mgx (m: mass of the weight; g: acceleration of gravity; x: length of the weight that is lifted), was 5.6 μJ.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (A.H.). ORCID
Hiroyasu Yamaguchi: 0000-0002-4801-5071 Akira Harada: 0000-0002-9309-5939 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
This research was funded by the ImPACT Program of the Council for Science, Technology and Innovation (Cabinet Office, Government of Japan), a Grant-in-Aid for Scientific Research (B) (No. 26288062) from MEXT of Japan, and a Grant-in-Aid from Iketani Science and Technology Foundation.
(1) Alberts, B.; Johnson, A.; Lewis, J.; Raff, M.; Roberts, K.; Walter, P. Molecular Biology of the Cell, 5th ed.; Garland Science: New York, 2008. (2) Yin, H.; Wang, M. D.; Svoboda, K.; Landick, R.; Block, S. M.; Gelles, J. Transcription against an applied force. Science 1995, 270, 1653−1657. (3) Hirokawa, N. Kinesin and dynein superfamily proteins and the mechanism of organelle transport. Science 1998, 279, 519−526. (4) Vale, R. D.; Milligan, R. A. The way things move: looking under the hood of molecular motor proteins. Science 2000, 288, 88−95. (5) Schill, G. Catenanes, Rotaxanes, and Knots; Academic Press: New York, 1971. (6) Dietrich-Buchecker, C. O.; Sauvage, J. P. Interlocking of Molecular Threads: from the Statistical Approach to the Templated Synthesis of Catenands. Chem. Rev. 1987, 87, 795−810. (7) Amabilino, D. B.; Stoddart, J. F. Interlocked and Intertwined Structures and Superstructures. Chem. Rev. 1995, 95, 2725−2828. (8) Niess, F.; Duplan, V.; Sauvage, J.-P. Molecular Muscles: From Species in Solution to Materials and Devices. Chem. Lett. 2014, 43, 964−974. (9) Ashton, P. R.; Baxter, I.; Cantrill, S. J.; Fyfe, M. C. T.; Glink, P. T.; Stoddart, J. F.; White, A. J. P.; Williams, D. J. Supramolecular Daisy Chains. Angew. Chem., Int. Ed. 1998, 37, 1294−1297. (10) Kay, E. R.; Leigh, D. A.; Zerbetto, F. Synthetic Molecular Motors and Mechanical Machines. Angew. Chem., Int. Ed. 2007, 46, 72−191. (11) Coskun, A.; Banaszak, M.; Astumian, R. D.; Stoddart, J. F.; Grzybowski, B. A. Great expectations: can artificial molecular machines deliver on their promise? Chem. Soc. Rev. 2012, 41, 19−30. (12) Rotzler, J.; Mayor, M. Molecular daisy chains. Chem. Soc. Rev. 2013, 42, 44−62. (13) Bruns, C. J.; Stoddart, J. F. Rotaxane-Based Molecular Muscles. Acc. Chem. Res. 2014, 47, 2186−2199. (14) Erbas-Cakmak, S.; Leigh, D. A.; McTernan, C. T.; Nussbaumer, A. L. Artificial Molecular Machines. Chem. Rev. 2015, 115, 10081− 10206. (15) Kudernac, T.; Katsonis, N.; Browne, W. R.; Feringa, B. L. Nanoelectronic switches: Light-induced switching of the conductance of molecular systems. J. Mater. Chem. 2009, 19, 7168−7177. (16) Dawson, R. E.; Lincoln, S. F.; Easton, C. J. The foundation of a light driven molecular muscle based on stilbene and α-cyclodextrin. Chem. Commun. 2008, 3980−3982. (17) Yamauchi, K.; Takashima, Y.; Hashidzume, A.; Yamaguchi, H.; Harada, A. Switching between Supramolecular Dimer and Nonthreaded Supramolecular Self-Assembly of Stilbene Amide-α-Cyclodextrin by Photoirradiation. J. Am. Chem. Soc. 2008, 130, 5024−5025. (18) Li, S.; Taura, D.; Hashidzume, A.; Harada, A. Light-Switchable Janus [2]Rotaxanes Based on α-Cyclodextrin Derivatives Bearing Two
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CONCLUSION We have achieved a tough and photoresponsive polymeric material with [2]rotaxane structures as topological cross-links in the polymer network. The mechanical properties of the material were enhanced to show a rupture strain of 2800%. Furthermore, the rupture energy of this material was 50−150 times higher than that of a covalently cross-linked gel. The stress distribution property based on the sliding motion of the topological cross-links contributed to this excellent extensibility. In addition, the material can be reversibly deformed by repeated irradiation with UV or visible light in aqueous media. Importantly, the dried material uniaxially extended during the drying process showed a faster response than the hydrogel. The stretched xerogel quickly bent toward the light source side within 3 s upon irradiation with UV light. Moreover, 5.6 mg of the xerogel was found to convert energy from light to 5.6 μJ of mechanical work within 10 s. The orientation of the rotaxane network within the material might enable this efficient response. We believe that this dry-type actuator will contribute to both supramolecular chemistry and materials science.
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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/acs.macromol.8b00939. Experimental details, preparation of the monomers, and preparation and characterization of the gels (PDF) Movie S1 showing the αCD-Azo xerogel(10K) bending upon irradiation of UV light (AVI) Movie S2 showing the αCD-Azo xerogel(3.4K) lifting weight vertically upward upon irradiation of UV light (AVI) E
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Macromolecules Recognition Sites Linked with Oligo(ethylene glycol). Chem. - Asian J. 2010, 5, 2281−2289. (19) Okumura, Y.; Ito, K. The Polyrotaxane Gel: A Topological Gel by Figure-of-Eight Cross-links. Adv. Mater. 2001, 13 (7), 485−487. (20) Shinohara, Y.; Kayashima, K.; Okumura, Y.; Zhao, C.; Ito, K.; Amemiya, Y. Small-Angle X-ray Scattering Study of the Pulley Effect of Slide-Ring Gels. Macromolecules 2006, 39, 7386−7391. (21) Karino, T.; Shibayama, M.; Okumura, Y.; Ito, K. SANS study on pulley effect of slide-ring gel. Phys. B 2006, 385-386, 807−809. (22) Arai, T.; Jang, K.; Koyama, Y.; Asai, S.; Takata, T. Versatile Supramolecular Cross-Linker: A Rotaxane Cross-Linker That Directly Endows Vinyl Polymers with Movable Cross-Links. Chem. - Eur. J. 2013, 19 (3), 5917−5923. (23) Iijima, K.; Kohsaka, Y.; Koyama, Y.; Nakazono, K.; Uchida, S.; Asai, S.; Takata, T. Stimuli-degradable cross-linked polymers synthesized by radical polymerization using a size-complementary [3]rotaxane cross-linker. Polym. J. 2014, 46, 67−72. (24) Sawada, J.; Aoki, D.; Uchida, S.; Otsuka, H.; Takata, T. Synthesis of Vinylic Macromolecular Rotaxane Cross-Linkers Endowing Network Polymers with Toughness. ACS Macro Lett. 2015, 4, 598−601. (25) Iijima, K.; Aoki, D.; Sogawa, H.; Asai, S.; Takata, T. Synthesis and characterization of supramolecular cross-linkers containing cyclodextrin dimer and trimer. Polym. Chem. 2016, 7, 3492−3495. (26) Tan, S.; Blencowe, A.; Ladewig, K.; Qiao, G. G. A novel one-pot approach towards dynamically cross-linked hydrogels. Soft Matter 2013, 9, 5239−5250. (27) Murakami, T.; Schmidt, B. V. K. J.; Brown, H. R.; Hawker, C. J. One-Pot “Click” Fabrication of Slide-Ring Gels. Macromolecules 2015, 48, 7774−7781. (28) Yao, F.; Xu, L. Q.; Fu, G.-D.; Lin, B. P. Sliding-Graft Interpenetrating Polymer Networks from Simultaneous “Click Chemistry” and Atom Transfer Radical Polymerization. Macromolecules 2010, 43, 9761−9770. (29) Zhou, Y. X.; Fan, X. D.; Xue, D.; Xing, J. W.; Kong, J. Thermosensitive supramolecular hydrogels from atom transfer radical polymerization of polypseudorotaxanes self-assembled by triblock copolymer and α-cyclodextrins. React. Funct. Polym. 2013, 73, 508− 517. (30) Bin, I. A.; Seki, T.; Kataoka, T.; Kidowaki, M.; Ito, K.; Takeoka, Y. Fabrication of mechanically improved hydrogels using a movable cross-linker based on vinyl modified polyrotaxane. Chem. Commun. 2008, 41, 5227−5229. (31) Bin, I. A.; Seki, T.; Ito, K.; Takeoka, Y. Poly(Nisopropylacrylamide) Gel Prepared Using a Hydrophilic Polyrotaxane-Based Movable Cross-Linker. Macromolecules 2010, 43, 1975− 1980. (32) Araki, J.; Ito, K. Polyrotaxane derivatives. I. Preparation of modified polyrotaxanes with nonionic functional groups and their solubility in organic solvents. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 6312−6323. (33) Bin, I. A.; Gotoh, G.; Seki, T.; Ito, K.; Sakai, Y.; Takeoka, Y. Extremely stretchable thermosensitive hydrogels by introducing slidering polyrotaxane cross-linkers and ionic groups into the polymer network. Nat. Commun. 2014, 5, 5124. (34) Ohmori, K.; Bin, I. A.; Seki, T.; Liu, C.; Mayumi, K.; Ito, K.; Takeoka, Y. Molecular weight dependency of polyrotaxane-crosslinked polymer gel extensibility. Chem. Commun. 2016, 52, 13757− 13759. (35) Li, X.; Kang, H.; Shen, J.; Zhang, L.; Nishi, T.; Ito, K.; Zhao, C.; Coates, P. Highly toughened polylactide with novel sliding graft copolymer by in situ reactive compatibilization, crosslinking and chain extension. Polymer 2014, 55, 4313−4323. (36) Jiménez, M. C.; Dietrich-Buchecker, C.; Sauvage, J.-P. Towards Synthetic Molecular Muscles: Contraction and Stretching of a Linear Rotaxane Dimer. Angew. Chem., Int. Ed. 2000, 39, 3284−3287. (37) Jimenez-Molero, M. C.; Dietrich-Buchecker, C.; Sauvage, J.-P. Chemically Induced Contraction and Stretching of a Linear Rotaxane Dimer. Chem. - Eur. J. 2002, 8, 1456−1466.
(38) Jimenez-Molero, M. C.; Dietrich-Buchecker, C.; Sauvage, J.-P. Towards artificial muscles at the nanometric level. Chem. Commun. 2003, 1613−1616. (39) Collin, J.-P.; Dietrich-Buchecker, C.; Gaviña, P.; JimenezMolero, M. C.; Sauvage, J.-P. Shuttles and Muscles: Linear Molecular Machines Based on Transition Metals. Acc. Chem. Res. 2001, 34, 477− 487. (40) Fang, L.; Hmadeh, M.; Wu, J.; Olson, M. A.; Spruell, J. M.; Trabolsi, A.; Yang, Y.-W.; Elhabiri, M.; Albrecht-Gary, A.-M.; Stoddart, J. F. Acid−Base Actuation of [c2]Daisy Chains. J. Am. Chem. Soc. 2009, 131, 7126−7134. (41) Hmadeh, M.; Fang, L.; Trabolsi, A.; Elhabiri, M.; Albrecht-Gary, A.-M.; Stoddart, J. F. On the thermodynamic and kinetic investigations of a [c2]daisy chain polymer. J. Mater. Chem. 2010, 20, 3422−3430. (42) Bruns, C. J.; Stoddart, J. F. Supramolecular polymers: Molecular machines muscle up. Nat. Nanotechnol. 2013, 8, 9−10. (43) Clark, P. G.; Day, M. W.; Grubbs, R. H. Switching and Extension of a [c2]Daisy-Chain Dimer Polymer. J. Am. Chem. Soc. 2009, 131, 13631−13633. (44) Du, G.; Moulin, E.; Jouault, N.; Buhler, E.; Giuseppone, N. Muscle-like Supramolecular Polymers: Integrated Motion from Thousands of Molecular Machines. Angew. Chem., Int. Ed. 2012, 51, 12504−12508. (45) Goujon, A.; Lang, T.; Mariani, G.; Moulin, E.; Fuks, G.; Raya, J.; Buhler, E.; Giuseppone, N. Bistable [c2] Daisy Chain Rotaxanes as Reversible Muscle-like Actuators in Mechanically Active Gels. J. Am. Chem. Soc. 2017, 139, 14825−14828. (46) Li, Q.; Fuks, G.; Moulin, E.; Maaloum, M.; Rawiso, M.; Kulic, I.; Foy, J. T.; Giuseppone, N. Macroscopic contraction of a gel induced by the integrated motion of light-driven molecular motors. Nat. Nanotechnol. 2015, 10, 161−165. (47) Tsukagoshi, S.; Miyawaki, A.; Takashima, Y.; Yamaguchi, H.; Harada, A. Contraction of Supramolecular Double-Threaded Dimer Formed by α-Cyclodextrin with a Long Alkyl Chain. Org. Lett. 2007, 9, 1053−1055. (48) Liu, Y.; Flood, A. H.; Bonvallet, P. A.; Vignon, S. A.; Northrop, B. H.; Tseng, H.-R.; Jeppesen, J. O.; Huang, T. J.; Brough, B.; Baller, M.; Magonov, S.; Solares, S. D.; Goddard, W. A.; Ho, C.-M.; Stoddart, J. F. Linear Artificial Molecular Muscles. J. Am. Chem. Soc. 2005, 127, 9745−9759. (49) Iwaso, K.; Takashima, Y.; Harada, A. Fast response dry-type artificial molecular muscles with [c2]daisy chains. Nat. Chem. 2016, 8, 625−632. (50) Tomatsu, I.; Hashidzume, A.; Harada, A. Contrast Viscosity Changes upon Photoirradiation for Mixtures of Poly(acrylic acid)Based α-Cyclodextrin and Azobenzene Polymers. J. Am. Chem. Soc. 2006, 128, 2226−2227. (51) Tamesue, S.; Takashima, Y.; Yamaguchi, H.; Shinkai, S.; Harada, A. Photoswitchable Supramolecular Hydrogels Formed by Cyclodextrins and Azobenzene Polymers. Angew. Chem., Int. Ed. 2010, 49, 7461−7464.
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DOI: 10.1021/acs.macromol.8b00939 Macromolecules XXXX, XXX, XXX−XXX
