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Cite This: Chem. Mater. 2018, 30, 5013−5019

Highly Stretchable and Instantly Recoverable Slide-Ring Gels Consisting of Enzymatically Synthesized Polyrotaxane with Low Host Coverage Lan Jiang,*,† Chang Liu,† Koichi Mayumi,† Kazuaki Kato,†,‡ Hideaki Yokoyama,† and Kohzo Ito*,† †

Chem. Mater. 2018.30:5013-5019. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 10/21/18. For personal use only.

Department of Advanced Materials Science, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561, Japan ‡ Research Center for Structural Materials, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan S Supporting Information *

ABSTRACT: Gels with high mechanical performance have attracted great interest because of their potential biomedical applications. Tough gels reported thus far usually contain sacrificial species to dissipate energy, thus compromising the fatigue resistance. In this study, highly stretchable and recoverable gels can be achieved by cross-linking cyclodextrin (CD)-based polyrotaxane with a low host coverage, synthesized via a one-pot enzymatic end-capping reaction with 90% yield and ∼2% CD coverage (PR02). The low coverage allows the CD cross-links to freely slip on the axis over large distance (∼2/3 of the axis length) and thus allows the PR02 slide-ring network keep intact under large deformation via the pulley effect. The PR02-hydrogel can be stretched up to ∼1600% long, withstand ∼1 MPa stress, and fully recover instantly. PR02-DMSO gels exhibit a shape memory behavior that withstood large deformation. As the first research to control the final property of the network by precisely controlling the slide distance of the cross-links, this work not only pushes the performance envelope of soft matters but also opens new opportunities for designing tough materials.

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INTRODUCTION

high-level loading−unloading processes are still long pursued yet largely unmet. A “slide-ring gel”,15 a typical “slide-ring material”16 based on polyrotaxane (PR), can provide a peculiar mechanical property that is different from conventional chemical, physical or multicomponent gels. PR is a mechanically interlocked molecule composed of a linear polymer guest (poly(ethylene glycol) (PEG)) threaded into multiple cyclic molecule hosts (cyclodextrin (CD)). “Slide-ring materials” can be formed by using the CD hosts as cross-linking points in various structural materials such as hydrogels,17 elastomers,18 silica aerogels,19 and silicon anode binders.20 As the CD cross-links are able to slip on the polymer axis between adjacent CDs, the inhomogeneous structure can be adjusted to some extent, and the tension in the network can be equalized in a manner similar to that in the case of pulleys (Scheme 1a). Thus, the slide-ring gels seems to have potential to provide good mechanical properties without compromising recoverability. Unfortunately, the reported slidering gels can only work well in low strength (stress at break only

Gels are three-dimensional polymer networks containing a large amount of solvent, which have great potential to be utilized in tissue engineering,1 drug delivery,2 actuators,3 and so on. Practical applications of gels, however, are restricted because of their low mechanical strength, which is a result of the inhomogeneous structure created by cross-linking.4 One common strategy to improve the strength of gels is through the dissipation of energy under deformation, e.g., doublenetwork hydrogels,5,6 dual-cross-linked hydrogel,7,8 or nanocomposite gels.9,10 Although energy-dissipating gels can provide remarkable stress or stretchability, they generally cannot recover to their initial state instantly in loading−unloading processes and exhibit large hysteresis loops even after small deformations, owing to the permanent rupture of the network. Some efforts were achieved by introducing reversible sacrifice bonds11,12 or nanoparticles13,14 in the system, but the gel recovery was still time-consuming and only functioned for relatively small deformations and strains, which poses a great obstacle to applications such as artificial cartilage or force sensors that required both stretchability and recoverability. Currently, tough gels with high fatigue resistance that can withstand consecutive © 2018 American Chemical Society

Received: March 22, 2018 Revised: July 15, 2018 Published: July 17, 2018 5013

DOI: 10.1021/acs.chemmater.8b01208 Chem. Mater. 2018, 30, 5013−5019

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work not only provides a new strategy to obtain recoverable gels with good mechanical performance but also opens new opportunities for designing tough materials.

Scheme 1. (a) Pulley Effect, (b) High-Coverage Polyrotaxanes Easily Undergo Crack Propagation, and (c) Low-Coverage Polyrotaxanes Can Withstand Large Tension and Possess Good Stretchability and Recoverability Stemming from the Wide Slidable Range for Cross-Links

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EXPERIMENTAL SECTION

Materials. All reagents were used as received. The Microbial Transglutaminase was provided by Ajinomoto Co. (Tokyo, Japan). The (2-hydroxypropyl)-α-cyclodextrin (HP-α-CD, hydroxylpropyl modification degree ∼37.7%), N-α-carbobenzoxy-L-glutaminyl-glycine (ZGln-Gly), divinyl sulfone (DVS), glutaraldehyde (GA), poly(vinyl alcohol) (PVA, Mn = 31000), and N,N′-carbonyldiimidazole (CDI) were purchased from Sigma-Aldrich. The phosphate buffer solution (PBS, 0.1 mol/L, pH 8.0) was purchased from Wako. The aminoterminated PEG (PEG-NH2, Mw = 10000, 20000, and 30000) was purchased from NOF Corp. The 25% CD covered PR was purchased from Advanced Softmaterials Inc. (PEG axis Mw = 35000, CD coverage ∼24.2%, hydroxylpropyl modification degree ∼52%, denoted as PR25 (Figure S1)). Synthesis of the 2% Covered PR. The HP-α-CD (1 g) was dissolved in 3 mL of PBS (0.1 mol/L, pH 8.0), and then PEG-NH2 (0.2 g) was added and stirred to obtain a clear solution. The mixture was stood at 4 °C for 36 h, and then Z-Gln-Gly (0.08 g) and TGase (10 mg) were added into the mixture for end-capping. The reaction stirred at room temperature for 3 days. Then, the mixture was dialyzed (MWCO: 12000−14000) to remove the free CD and Z-Gln-Gly. After freezedrying, the rough product was washed by THF to remove free PEG and then dialyzed again. The dialyzed solution was filtrated via 0.45 μm membrane filter and then freeze-dried to obtain the final product. Preparation of the PR Hydrogel. PR was dissolved in 0.01 M NaOH with different weight concentrations. The DVS was added to the solution of PR in different weight concentrations to obtain pregel solutions. These pregel solutions were cross-linked at room temperature for 24 h in a 1 mm thickness Teflon mold. The obtained gels were removed from the mold, and the edges were cut off to obtain rectangular gels for the tensile measurements. The resulting PR hydrogels named “x% PR02/25−y% DVS-hydrogel” (x refers to the PR concentration, and y refers to the DVS concentration (wt %)). Preparation of the PVA Hydrogel. PVA was dissolved in water with 16 wt %. The 1 M HCl solution (3.5 wt %) and GA in different weight concentration were added to the solution to obtain the pregel solution. These pregel solutions were cross-linked at room temperature for 24 h in a 1 mm thickness Teflon mold. The obtained gels were removed from the mold, and the edges were cut off to obtain rectangular gels for the tensile measurements. The resulting PVA hydrogels named “x% PVA−y% GA-hydrogel” (x refers to the PVA concentration, and y refers to the cross-linker concentration (wt %)). Preparation of the PR DMSO Gel. PR was dissolved in anhydrous 16 wt % DMSO. The CDI was added to the solution to obtain pregel solution. The pregel solutions were cross-linked at 60 °C overnight in a 1 mm thickness Teflon mold. The obtained gels were removed from the mold, and the edges were cut off to obtain rectangular gels. The resulting DMSO gels named “x% PR02/25−y% CDI-DMSO gel” (x refers to the PR concentration, and y refers to the cross-linker concentration (wt %)). Characterization. NMR Spectroscopy. 1H NMR spectra were recorded at 400 MHz on a JEOL JNM-AL400 spectrometer under temperature of 343 K in d6-DMSO solvent. ATR−Fourier Transform Infrared Spectroscopy. ATR−Fourier transform infrared spectra were recorded in air using a NICOLETis50 spectrometer (Thermo Electron Co., Ltd.), where the samples were directly measured under a DuraSamplIR II diamond ATR accessory. Small-Angle X-ray Scattering (SAXS). SAXS for 2% covered PR solution (0.5 wt % in 0.01 M NaOH) was done at beamline BL-05SS at SPring-8 (Hyogo, Japan) for obtaining the gyration radius. The sampleto-detector distance was 3863 mm, and the 2D SAXS patterns were recorded with a PILATUS 1M system (Dectris). The exposure time was 10.0 s. In Figure S2 we present the SAXS profile of 0.5 wt % 2% covered PR solution and its fitting results using the Debye equation (eq S1). The

several tens of kPa),17,21 which is far below the level that energydissipating gels can achieve (∼MPa). We think it is probably because the slidable range of the cross-links is limited in the products reported thus far. The commonly used CD-based PR composed of PEG (Mw = 10K−40K) usually exhibit 25−30% CD coverage (percent ratio of covered length of axis polymer chain); a large number of free CDs remain on the strand after gelation, which greatly limits the slipping of the cross-links and causes the failure of the network (Scheme 1b). As the pulley effect plays a key role in slide-ring gel, making the cross-links slip over large distance may be of vital importance to develop the full potential of the pulley effect, so as to obtain gels with a high strength, toughness, and recoverability. Based on the interlocked structure of the PR, decreasing the number of cyclic hosts21 is expected to be a promising way to make CD cross-links slip over large distances on the polymer axis without obstacles (Scheme 1c). However, it has been difficult to control the host−guest stoichiometry effectively thus far. Conventional syntheses of PR always require complicated steps with a series of solvent changes22,23or strict conditions with toxic catalysts,24−27 and the yield greatly shrinks when attempting to control over the CD coverage.21,28,29 In this study, a CD-based PR with very low CD coverage (∼2%) is successfully obtained via a one-pot enzymatic end-capping reaction in very high yield (>90%) for the first time. It is found that the 2% covered PR hydrogel can be stretched ∼16 times in length with ∼1 MPa stress, which is almost the best mechanical properties for reported chemical single-network hydrogel without energy dissipation and even comparable to some energy-dissipating gels, but still instantly recoverable. As the PEG axis in the low coverage PR is almost exposed without the shelter of CDs, the corresponding DMSO gel exhibits a shape memory behavior in large deformation due to the synergy of the slide-ring effect and orientation-induced PEG crystallization. The mechanism of the improvement is also investigated in this work via comparing with the properties of the hydrogels based on 25% covered PR and poly(vinyl alcohol) (PVA). It is proved that by decreasing of the host coverage from 25% to 2%, the slide range of the CD cross-links is increased from 18% to 65% of the axis length in the corresponding gels, so the polymer strands in the network can be fully utilized under large deformation. This 5014

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Chemistry of Materials gyration radius of 2% covered PR Gaussian chain Rg was calculated as 13.3 nm. Tensile Tests. These tests were performed at 25 °C on a Shimadzu EZ-S universal tester with a 5 N load cell. For uniaxial tensile tests, 18 × 3 × 1 mm3 sized samples were stretched until failure at 60 mm/min deformation rate. For loading−unloading tests, 7 × 3 × 1 mm3 sized samples were placed into a humidity-controlled chamber (Figure S3) and loaded for 100 cycles without stop at 250 mm/min deformation rate. For stress relaxation tests, a 7 × 3 × 1 mm3 sized sample was loaded to extension ratio λ = 2, 4, and 7 in sequence. Between each step, the sample was held for 10 min. The stress was recorded as a function of time t. To minimize dehydration during the fatigue and relaxation tests, we made a simplified humidity chamber30 (Figure S3). A humidifier provided continuous water vapor into the chamber. The samples were weighed before and after tests and had less than 5% weight loss. Fracture Tests. These tests were also performed at 25 °C on a Shimadzu EZ-S universal tester with a 5 N load cell. A 2 mm length initial crack was introduced to the middle point of the vertical boundary of a 25 × 10 × 1 mm3 sized “single edge notched tensile” (SENT) specimen by a sharp blade (Figure S4a). Stress−strain curves of notched and un-notched samples were both obtained at a 60 mm/min strain rate. The strain for the initial crack to start propagation was considered to be the critical strain of fracture εc. The fracture energy Γ was calculated as Γ = 2kAc k=

3 1 + εc

Scheme 2. One-Pot Synthesis Strategy of PR02

studies of the PR were carried under different mTGase concentration, reaction time, and temperature (Table S1), and the final protocol was fixed as shown in the Experimental Section. The structure of the PR can be confirmed by 1H NMR and GPC. The 1H NMR spectrum showed clear signals of H1 protons on cyclodextrin (4.8 ppm, Figure S5) and methylene groups on PEG (3.5 ppm), which can be used to calculate the coverage of the PR. In the GPC results, the molecular weight of PR was larger than PEG axis without a CD peak (Figure S6), which means the CDs shown in 1H NMR spectra were all capped in polyrotaxane. This protocol can work well for PEG with different molecular weights (10K−30K) in very high yield (>90%, calculated based on the PEG weight), with 2−3% CD coverage. The PEG30K-based PR was used for further investigation, which was named “PR02” (the PR02-2 in Table S1). The PR02 can be easily dissolved in H2O, DMF, and DMSO. This protocol, for the first time, gave the PR with such low coverage and high yield. Additionally, the synthesis avoided the use of toxic reagents, allowing the resulting PR to be used easily not only in traditional structural materials but also in biomaterials applications. Highly Stretchable and Instantly Recoverable PR02Hydrogel. The PR02-hydrogels were prepared with different polymer and cross-linker (DVS) concentrations. The PR02hydrogels were transparent and flexible and exhibited excellent mechanical properties, which are listed in Table S2. The gels can be easily knotted and quickly stretched to more than 10 times in length with a knot (Figure 1a and Movie S1). When the DVS concentration was fixed at 3 wt %, the PR02-hydrogels could be easily extended more than 12 times in length (Figure 1b) with different PR concentrations. The 25% PR02−3% DVS-hydrogel exhibited better mechanical performance with ∼1.1 MPa stress, ∼1600% strain, and ∼157 J/m2 fracture energy and afforded ∼6.3 MJ/m3 input energy (work of extension W, estimated by calculating the area under the tensile curve) at break, which is comparable to those of other tough hydrogels with energy dissipation.33 When the PR concentration was fixed (16 wt %), the Young’s modulus increased (20−50 kPa) with increasing cross-linker concentration (Figure S7). Most traditional elastic hydrogels can only afford several tens to a hundred or so kPa stress with 2−8 extension ratio.17,34,35 Some dissipating-energy hydrogels can dissipate >50% input energy to be extended to more than 10 times length and withstand 0.1−5 MPa stress but compromise instant recoverability and fatigue resistance.12,36 PR02-hydrogels, for the first

(1)

(2)

where k is a parameter related to the critical strain, A is the strain energy density which could be deduced from the area covered by the unnotched specimen s-s curve from 0 to εc, and c is the initial crack length (Figure S4b). Wide-Angle X-ray Scattering (WAXS). Synchrotron wide-angle Xray scattering measurements were conducted at the beamline BL-05SS at SPring-8 (Hyogo, Japan). The wavelength of the incident X-ray beam was 1 Å, and the beam size was 0.1 mm (vertical) × 0.2 mm (horizontal). The sample-to-detector distance was 64 mm, and a CMOS flat-panel detector from Hamamatsu Photonics (C9728DK) was used as the 2D detector. Opaque gels are stepwisely heated from room temperature to 50 °C by a LINKAM TST350 tensile hot stage. The sample is soaked at the desired temperature for 2 min and then exposed to X-ray for 5 s to collect the scattering profile.

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RESULTS AND DISCUSSION Synthesis and Characterization of the Polyrotaxane with Low Cyclodextrin Coverage. The synthesis of CDbased PR generally has two key points: the formation of pseudopolyrotaxane (PPR) and the end-capping reaction. Because hydrogen bonds between the native CDs play the primary role in the formation of common PPRs, native CDs tend to aggregate along the main chain and form a channel-like structure,31 making it difficult to control the host−guest stoichiometry effectively. In this work, the hydroxylpropyl αCD was used as the host instead of native α-CD to avoid hydrogen bonds between adjacent CDs and minimize the CD number on the PR. For the end-capping step, an enzymatic endcapping reaction was established for the first time in this work by using microbial transglutaminase (mTGase) as the catalyst and dipeptide Z-Gln-Gly as the end group. The mTGase is stable and expresses enzymatic activity over wide pH (pH 4−9) and temperature ranges (4−50 °C)32 in aqueous solution, and thus a high-yield synthesis protocol of PR can be established. The one-pot synthesis of PR with a low host coverage included the formation of PPR and end-capping reaction in the same system (Scheme 2). To optimize the conditions for the enzymatic end-capping reaction with mTGase, the synthesis 5015

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Figure 1. (a) Highly stretchable 16% PR02−3% DVS-hydrogel with a knot. (b) Stress−extension ratio curves of PR02-hydrogels with 16%, 20%, and 25% PR concentration. (c) Loading−unloading tests of PR02-hydrogels with 16%, 20%, and 25% PR concentration. (d) Successive loading− unloading cycles of the 25% PR02−3% DVS-hydrogel with different strain without stop and the residual strain after each cycle. (e) Successive cyclic tensile tests of the 25% PR02−3% DVS-hydrogel with λ = 7 for 100 cycles. (f) Relaxation test of the 25% PR02−3% DVS-hydrogel with extension ratio λ = 2, 4, and 7 and the loading profile of the stress-relaxation test.

Figure 2. Influence of the pulley effect on the mechanical properties of polymer hydrogels. (a) Stress−extension ratio curves of PVA, PR02, and PR25 hydrogels with an ∼35 kPa Young’s modulus. (b) Relationship between fracture energy (Γ) and Young’s modulus (E) in PVA, PR25, and PR02 hydrogels. (c) Crack tip molecular behavior of PVA, PR25, and PR02 hydrogels.

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Figure 3. (a) Images of the PR02-DMSO gel showing that it could not be easily cut by a knife, and its compressed state. (b) Stress−extension ratio curves of the PR02- and PR25-DMSO gel with an ∼30 kPa Young’s modulus. (c) Elongation state and the corresponding structure of the PR02-DMSO gel. (d) Loading−unloading cyclic tensile tests of PR02-DMSO gel with different strains. (e) WAXS results of the fresh PR02-DMSO gel and the crystallized PR02-DMSO gels under different temperatures. (f) Shape memory behavior of the PR02-DMSO gel.

As a kind of chemical single-network hydrogel without energy dissipation, PR02-hydrogel provides the most competitive mechanical properties in terms of strength and extensibility, most likely due to the wide slidable range of the cross-links on axis. Based on its stable and repeatable mechanical properties in high deformation, the PR02-hydrogel has unique and great potential for applications requiring both high strength and fatigue resistance, such as fabricating cartilage, artificial skin, and force sensors. Mechanism Discussion. A fundamental understanding of toughening mechanisms of slide-ring hydrogels is critically important for designing the next generation high strength hydrogels with desirable properties. To further investigate the pulley effect in slide-ring gels, the hydrogels based on polyrotaxane with 25% CD coverage (PR25, Mw = 35000, Figure S1) and poly(vinyl alcohol) (PVA, Mw = 31000) were prepared with 16 wt % polymer concentrations and different cross-linker concentrations as references (Table S3). For hydrogels with the same Young’s modulus (∼35 kPa, Figure 2a), the maximum elongation and stress of the PR02-hydrogel (∼1250% strain and ∼125 kPa stress) were substantially greater than those of the PR25-hydrogel (∼110% strain and ∼50 kPa) or PVA-hydrogel (∼180% strain and ∼46 kPa stress). For conventional fixed cross-links chemical gels, there is a trade-off relation between fracture energy Γ and Young’s modulus E: with increasing cross-linking density, the Young’s modulus E increases, while the fracture energy Γ decreases. However, SR gels show an unusual fracture behavior without this trade-off relation.38 In Figure 2b, the fracture energy Γ plotted against the Young’s modulus for PVA, PR25, and PR02 hydrogels on a double-logarithmic scale. For fixed cross-link 16% PVA hydrogels, Γ was proportional to the inverse root square of the Young’s modulus with a −0.46 power, which means that increasing cross-linking density decreases fracture toughness. On the other hand, Γ of the PR25 and PR02 hydrogels was uncorrelated to their initial Young’s modulus as

time, emerged as a special recoverable chemical gel with high strength and extensibility. Figure 1c shows the loading− unloading curves of PR02-hydrogels with different PR concentration and 3% DVS; all three gels can completely recover to the original length instantly at room temperature after extending to 10−12 times long with 0.1−0.9 MPa stress. It is noted that the PR02-hydrogels did not display distinct hysteresis loops. The hysteresis energy of the 16% PR02−3% DVShydrogel in loading−unloading cycle with λ = 11 was only ∼14 kJ/m3, about 3% of the input energy, and in the loading− unloading cycle with λ = 13, the hysteresis energies of the 20% PR02−3% DVS-hydrogel (∼160 kJ/m3) and 25% PR02−3% DVS-hydrogel (∼314 kJ/m3) were about 7% and 10% of the input energy, respectively. It is suggested that the structure of a PR02 network can almost keep intact under large deformation with the help of the pulley effect. In successive loading−unloading cyclic tests with different extension ratios, the 25% PR02−3% DVS-hydrogel exhibited almost no hysteresis with less than 1% residual strain in the first three cycles with λ = 2, 4, and 7 and only ∼3% residual strain in the fourth cycle with λ = 13 (Figure 1d). To investigate the fatigue performance of the PR02-hydrogel, we loaded the 25% PR02−3% DVS-hydrogel for 100 cycles in λ = 7 without stopping. The stress−extension ratio curves almost overlap (Figure 1e), the residual strain after 100 cycles was less than 7%, and the maximum stress was steady after the fifth cycle with only ∼5% softening (Figure S8), which is similar to elastic chemical single-network gels.37 The stress relaxation of 25% PR02−3% DVS-hydrogel was also measured (Figure 1f) in λ = 2, 4, and 7; the PR02-hydrogel exhibited almost no relaxation in small deformation (λ = 2 and 4) and only relaxed no more than 5% stress at λ = 7. Therefore, the CD cross-links can slide on the axis with low friction, and the PR02-hydrogel can exhibit highly elastic and keep the network intact under consecutive, highly deformed loading−unloading processes to provide stable and repeatable mechanical properties. 5017

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Chemistry of Materials the cross-links can slide along the chain.38 The 16% PR02hydrogels showed constant Γ values around 55 J/m2, which was much larger than those of 16% PR25-hydrogels (6 J/m2) with a high CD coverage and the same polymer axis length. The slidable range can be estimated from the fracture energy Γ. From the obtained fracture energies, the final monomer number N of one strand between the cross-links before fracture can be calculated based on eq S239 and eq S4.38 For the same initial Young’s modulus E ≈ 40 kPa, NPVA was calculated to be 128, corresponding to ∼18% of the whole axis; NPR25 was around 108, ∼15% of the whole axis, while NPR02 was around 477, ∼65% of the whole axis. This means that compared to PVA and PR25, PR02 can provide a much larger slide range for cross-links (Figure 2c). In PR02 network, ∼2/3 length of the polymer strands can be fully utilized to equalize the tension regardless of the initial structure or Young’s modulus and thus improve the strength and toughness of the chemical single-network effectively, to construct a highly stretchable and instantly recoverable hydrogel as we expected. PR02-DMSO Gel with Shape Memory Behavior. To investigate the performance of the organic gel based on PR with a low CD coverage, PR DMSO gels were prepared by using N,N′-carbonyldiimidazole (CDI) as cross-linker with 16% PR concentration (16% PR02/25-DMSO gel). The resulting PR02DMSO gels were flexible and not easily cut with a knife (Figure 3a). With the same Young’s modulus (∼30 kPa), the 16% PR02DMSO gel can be stretched to more than 13 times its length with ∼160 kPa stress at break, while the PR25-DMSO gel can only be stretched to no larger than twice its length with ∼30 kPa stress at break (Figure 3b). The fracture energy (Table S4) of the PR02DMSO gel (∼146 J/m2) is about 20 times larger than that of the PR25-DMSO gel (∼7 J/m2). Different from the PR02-hydrogels, the PR02-DMSO gels can only recover shape without hysteresis under small deformation (Figure 3d, 100% strain) but turn to opaque (Figure 3c) and cannot fully recover at room temperature after large deformation (Figure 3d, 500% and 1000% strain). However, the opaque gel can revert to the initial shape and transparent again after heating above 50 °C. The structure of the PR02-DMSO gels was analyzed by wide-angle X-ray scattering (Figure 3e). The original transparent PR02-DMSO gel exhibited a homogeneous amorphous structure without crystallization peak, while the opaque gel showed clear peaks of PEG crystallization.40,41 The PEG crystallization peaks decreased as the opaque sample turned transparent again with the stepwise heating. This reversible orientation-induced crystallization behavior of the PEG, which had never been observed in other PR materials before, is attributed to the very low CD coverage in this work. Unlike conventional high coverage PR, the PR02 only contains a small number of CDs as cross-linkers, which can slide over large distance on the axis under stretching to relax the internal stress. The PEG chains in PR02-DMSO gels are more exposed outside of the CDs than those in PR25-DMSO ones and tend to orient and crystallize, resulting in opaque gels (Figure 3c). Based on the synergy of the slide-ring effect and the PEG crystallization, the PR02-DMSO gel can be applied to shape memory materials with excellent mechanical properties (Figure 3f and Movie S2). The shape of the PR02-DMSO gel can be fixed under large deformation and recover after heating.

cially scalable synthetic protocol was established in this work for efficiently preparing PR with a low host coverage. By introducing functionalized cyclodextrin and enzymatic end-capping reaction, PR with a CD coverage as low as ∼2% was successfully obtained in a high yield of up to 90% without the use of toxic catalysts or organic solvents. The CD cross-links can slip freely along ∼2/3 of the axis in PR02, and thus the failure of the corresponding slide-ring network can be avoided even under large deformation. As an elastic hydrogel without energy dissipation, the PR02-hydrogel provided the most competitive mechanical properties with high toughness and instant recoverability, offering great potential to be utilized in applications that require both high mechanical strength and high fatigue resistance, such as cartilage, artificial skin, and force sensors. PR02-DMSO gels exhibited shape memory behavior with large deformation. As the slidable cross-links can be introduced into various structural materials, this work not only pushes the performance envelope of soft matters but also opens new opportunities for designing tough materials.

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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.chemmater.8b01208.

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Cross-link slidable range calculation method, 1H NMR spectra and GPC curve of polyrotaxanes, mechanical property data of gels (PDF) Movie S1: tensile tests of knotted PR02-hydrogel (AVI) Movie S2: shape memory behavior of PR02-DMSO gel (AVI)

AUTHOR INFORMATION

Corresponding Authors

*(L.J.) E-mail: [email protected]. *(K.I.) E-mail: [email protected]. ORCID

Lan Jiang: 0000-0003-0805-2246 Koichi Mayumi: 0000-0002-1976-3791 Kazuaki Kato: 0000-0002-9997-8599 Hideaki Yokoyama: 0000-0002-0446-7412 Funding

This work was supported by AIST-UTokyo Advanced Operando-Measurement Technology Open Innovation Laboratory (OPERANDO-OIL) and ImPACT Program of Council for Science, Technology and Innovation (Cabinet Office, Government of Japan). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The wide-angle and small-angle X-ray scattering experiments were performed at BL05SS at Spring-8, Hyogo, Japan. The mTGase is provided by Ajinomoto Co. (Tokyo, Japan).

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ABBREVIATIONS H NMR, proton nuclear magnetic resonance; GPC, gel permeation chromatography; DMF, dimethylformamide; DMSO, dimethyl sulfoxide.

CONCLUSION In this work, we proposed a new design strategy to create highly stretchable gels with instant recoverability. A novel commer-

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DOI: 10.1021/acs.chemmater.8b01208 Chem. Mater. 2018, 30, 5013−5019