Super Tough, Ultrastretchable Hydrogel with Multistimuli

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Functional Nanostructured Materials (including low-D carbon)

Super Tough, Ultra-Stretchable Hydrogel with Multi-Stimuli Responsiveness Meng-Meng Song, Ya-Min Wang, Bing Wang, XiangYong Liang, Zhi-Yi Chang, Bang-Jing Li, and Sheng Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01410 • Publication Date (Web): 13 Apr 2018 Downloaded from http://pubs.acs.org on April 15, 2018

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ACS Applied Materials & Interfaces

Super Tough, Ultra-Stretchable Hydrogel with Multi-Stimuli Responsiveness Meng-Meng Song a, Ya-Min Wang a, Bing Wang a, Xiang-Yong Liang b, Zhi-Yi Chang b, Bang-Jing Li*b, Sheng Zhang*a a

State Key Laboratory of Polymer Materials Engineering (Sichuan University), Polymer Research Institute of Sichuan University, Chengdu 610065, China

b

Key Laboratory of Mountain Ecological Restoration and Bioresource Utilization, Chengdu Institute of Biology, Chinese Academy of Sciences, Chengdu 610041, China

Corresponding authors: [email protected], [email protected]

KEYWORDS: High functionality cross-linkers, energy-dissipating, multi-stimuli responsiveness, host-guest inclusion, hydrophobic aggregation, hydrogel

ABSTRACT

The research of hydrogel has been increasingly focused on designing effective energy dissipation structure in recent years. Here, we report a kind of novel supramolecular crosslinkers, which formed by self-assembling amphiphilic block copolymers with guest groups at the end and vinyl functionalized cyclodextrin (CD) through host guest 1 ACS Paragon Plus Environment

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interaction. These crosslinkers could dissipate energy effectively since they combined multiple sacrificial mechanisms across multi-scales through physical interactions. The resulted hydrogel shows distinguishing mechanical properties (fracture toughness of 2.68 ±0.69 MJ/m3, tension strength of up to 475 kPa, uniaxial stretch over 2100 %), remarkable fatigue resistance and thermal- and light-responsive behaviors.

1. INTRODUCTION

Hydrogel, consisting of a cross-linked polymer network and high content of water, are abundant in nature, with examples ranging from xylem and phloem of plants to muscles, tissues, and cartilages of animals. Inspired by nature, extensive hydrogel have been synthesized and widely used in diverse applications, such as tissue engineering, drug and protein delivery, wound dressings, cosmetics, absorbents in waste managements, actuator and sensor1-7. Many applications of hydrogel require them to support huge mechanical loads and/or accommodate significant deformation, therefore, intensive efforts have been devoted to improve the mechanical strength of hydrogel in the last few decades8-14. A general principle for designing tough hydrogel is to implement mechanisms into hydrogel to dissipate mechanical energy and maintain high elasticity15-17. Hydrogel having high-functionality crosslinkers are representative examples, in which multiple polymer chains connected by two adjacent crosslinkers, and these chains usually have non-uniform lengths. As the polymer networks are deformed, relatively short chains are 2 ACS Paragon Plus Environment

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ruptured for the energy dissipation, but the long chains still maintain high elasticity of hydrogel18-19. However, little consideration was involved in introducing functionality such as responsive properties to these hydrogel20. Introducing physical crosslinkers in polymer matrix is a good way to dissipate mechanical energy in tough hydrogel. Since the physical interactions can be usually recovered after being detached, it is possible to endow anti-fatigue property to hydrogel21. Micelles are a kind of assemblies formed by hydrophobic

interactions

between

amphiphilc

chains.

Using

micelles

as

macro-crosslinkers to design tough hydrogel, the internal rearrangements of hydrophobic association in micelles offer additional energetic dissipation to hydrogel, and the synergistic deformation of micelles enable hydrogel to maintain high elasticity22. Different from the other physical cross-linked hydrogel, the hydrogel cross-linked by micelles exhibit ultra-stretchability, since the soft micelles may allow chain slippage and disentanglements under loading. Recently, Fu group further developed a kind of responsive and tough hydrogel by combining micelle enhancement mechanism with intrinsic responsive hydrogel matrix23. The limitation of these hydrogel is that the tensile strength of them is relatively low (less than 300 KPa) compared to other tough hydrogel, and this data is far from practical application. In addition, the stimulus of these hydrogel is mainly pH or temperature, since the choice of responsive polymer matrix is limited. It is still a challenge to design a responsive hydrogel with high toughness and ultra-stretchability.

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In this paper, we report the design and preparation of a temperature- and light-responsive hydrogel with the characteristic of high toughness, ultra-stretchability and anti-fatigue. The strategy for design is to implement multiple mechanisms for enhancement across multiple length scales into nano- and micro-structures of hydrogel. The key to this strategy is the supramolecular crosslinkers with high-functionality, which formed by self-assembling amphiphilic block copolymers with guest groups at the end and vinyl functionalized cyclodextrin (CD) through host guest interactions. Unlike the traditional micelles, these assemblies formed micelle-like structure but having host-guest “switches” in the hydrophilic shell. In this study, we selected Pluronic F127 (PEO99-PPO65-PEO99) as amphiphilic copolymer, azo groups as guest groups and β-CD as host groups. As a result, the hydrogel developed in this study showed the characteristic of supramolecular hydrogel, but having improved mechanical properties. The reversible switches and hydrophobic association in micelles acted as nano- and micro-scale energy dissipating structures respectively, enabling the resulting hydrogel showed fracture toughness of 2.68 ±0.69 MJ/m3 and tension strength of up to 475 kPa. The high-functionality crosslinkers and the synergistic deformation of micelles maintain high elasticity of hydrogel, which showed uniaxial stretch over 2100 %. The hydrogel can bear 100 % compressive strain without evident rupture, and exhibit excellent anti-fatigue properties. Furthermore, the sensitive host-guest switches endowed the hydrogel temperature- and light-sensitive properties. For all we know, this is the first time that introducing physical interactions to


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link a micellar crosslinker to hydrogel matrix. We hope that this research may provide a new strategy to design robust and smart hydrogel.

2.

Materials and Methods

2.1. Materials

4-(Phenylazo) benzoic acid was provided by J & K Scientific Ltd. Pluronic F127 (PEO99-PO65-PEO99), 2-methylacrylic acid anhydride (MCD), dicyclohexylcarbodiimide (DCC), 4-dimethylaminopyridine (DMAP), acrylamide (AAM), acryloyl chloride, potassium peroxydisulfate (KPS) and (N,N,N',N')-tetramethylethylenediamine (TEMDA) were received from Aladdin Reagent Database Inc. β-Cyclodextrin (β-CD) and the other solvents and reagents, purchased by Chengdu Kelong Chemical Co., Ltd., were analytical grade. All drugs and reagents were used as received without special instructions. Mono-6-(p-tolylsulfonyl)-β-cyclodextrin

(β-CD-OTs)

and

mono-6-deoxy-6-ethylenediamine-β-CD (EDA-β-CD) were synthesized according to our previous methods24-25. The F127 diacrylate (F127DA) was prepared according to methods reporting in the literature22.

2.2. Preparation of (1-methacrylamidoethyl) amino-6-deoxy-β-cyclodextrin (β-CD-MCD) and phenylazo terminated pluronic F127 (F127AZO).

Synthesis and characterization of β-CD-MCD：



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β-CD-MCD was prepared with slight modification on ETHEM KAYA’s method26. Briefly, EDA-β-CD (4 g, 3.398 mmol) in 30 ml anhydrous DMF was added dropwise into 5 mL DMF of MCD (0.628 g, 4.073 mmol) under nitrogen atmosphere. After stirring for 12 hours, the mixture was poured into excess acetone. The obtained white power was washed several times and dried under reduced pressure to get β-CD-MCD (3.14g, 74.2 %). β-CD-MCD: 1H NMR (400 MHz, D2O, ppm): δ = 5.70 (s, 1H, vinyl), 5.43 (s, 1H, vinyl), 5.08 (s, 7H, C1H of β-CD), 3.86 (m, 20H, C3,5,6H of β-CD), 3.59 (m, 14H, C2, 4H of β-CD), 3.41 (m, 2H, Ca H of β-CD), 2.84 (m, 4H, Cb&cH), 1.94 (s, 3H, methyl). ESI-MS: m/z 1246 [M+H]+. Synthesis and characterization of F127AZO：

F127AZO was prepared by esterification between Pluronic F127 and 4-(phenylazo) benzoic acid. Pluronic F127 was dried for 4 hours in 110 ℃ vacuum before reaction. Briefly, Pluronic F127 (4 g, 0.317 mmol), 4-(phenylazo) benzoic acid (0.215 g, 0.951mmol) and DMAP (0.058 g, 0.475 mmol) were first dissolved in 500 mL anhydrous dichloromethane at 0 ℃. Then, DCC (0.196 g, 0.951 mmol) was added into the mixture and the temperature of the mixture was allowed to room temperature. After stirring for 72 hours at room temperature, the mixture was condensed to 40 mL and filtered to remove dicyclohexylurea (DCU). Subsequently, the filtrate was condensed and poured into 500 mL anhydrous ether. Finally, the precipitate collected from filtration was washed with anhydrous ether and dried under the reduced pressure to obtain F127AZO (3.1 g, 75.0 %). 6 ACS Paragon Plus Environment

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F127AZO: 1H NMR (400 MHz, D2O, ppm): δ = 8.28 (m, 4H, phenyl), 7.96 (m, 8H, phenyl), 7.64 (m, 6H, phenyl), 4.57 (m, 4H, CH2OCO), 3.20-3.98 (m, CH2 of F127), 1.13 (s, 195H, CH3 of F127).

2.3. Self- asembly of F127AZO and β-CD-MCD.

25 mg β-CD-MCD and 130 mg F127AZO were mixed in 2 mL deionized water. Then after stirring for 24 hours at room temperature, the mixture was freeze-dried to obtain F127AZO@β-CD macromolecular crosslinkers.

2.4. Characterization of F127AZO@β-CD macromolecular crosslinkers.

The nanostructure size of F127AZO@β-CD macromolecular crosslinkers in water (0.001 mol/L) was get using DLS via a coherent innove 304 laser with a wavelength of 532 nm and a scattering angle of 90 ° at 25 °C. And the measure condition of F127DA crosslinkers is same with the F127AZO@β-CD macromolecular crosslinkers.

The nanostructure morphology of F127AZO@β-CD macromolecular crosslinkers was inspected by using TEM. The solution concentration of sample used in TEM was 0.001 mol/L. It is worthy noted that ultrasound (about 2~3 hours) is needed before the TEM test is needed to prevent aggregation of particles.

2.5. Preparation of F127AZO@β-CD hydrogel.

Briefly, AAM (5M), KPS (0.1mol% with respect to the AAM monomer), 7 ACS Paragon Plus Environment


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F127AZO@β-CD (0.02 mol% with respect to the AAM monomer) were dissolved in deionized water. After bubbling under Ar atmosphere for 10 min, TMEDA (0.27 mol% with respect to the AAM monomer) was added into the mixture. Then the mixture was transferred to molds with different shapes. Glasses were put onto the molds to prevent evaporation of water and keep air out. Finally, F127AZO@β-CD hydrogel (as referred to F127AZO@β-CD-PAAM hydrogel) were obtained by polymerization at room temperature for 12 hours. Hydrogel with the same concent of crosslinkers of MBAA and F127DA (as referred to F127AZO@β-CD-PAAM hydrogel and F127DA-PAAM hydrogel respectively) were prepared according to the methods above. As-prepared samples were used in subsequent tests.

2.5. Characteration. NMR A Bruker Avance-Ⅲ400 MHz spectrometer was used to record 1H NMR spectra of samples in D2O or DMSO-d6. 2D nuclear overhauser enhancement spectroscopy (2D NOESY) 1H NMR spectra was recorded with a Bruker Avance-Ⅲ600 MHz spectrometer in D2O. TMS was used as the internal reference. ESI-MS A Finnigan LCQDECA ESI-MS spectrometer (San Jose, CA, USA) was used for the mass spectrometry analysis.

Morphology The nanostructure morphology of F127AZO@β-CD crosslinker was getted by TecnaiG2-20 microscope (TEM) and dynamic light scattering (DLS) . The dynamic 8 ACS Paragon Plus Environment

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light scattering (DLS) results were recorded on BI-9000AT, BI-200SM, Brookhaven Instruments Co., USA.

Tensile and Compressive strength measurements Samples used in tensile strength measurements were rectanglar shapes with size of about 40 × 8 × 3 mm with a tensile speed of 80 mm × min-1. Samples used in compressive strength measurements were cylinders with a diameter of 20 mm and a heigth of 15 mm. The compressive crosshead speed here is 10 % strain per min. Tensile and compressive strength were both tested using Universal Test Machine AG-10TA at room temperature (25 °C). To prevent evaporation of water, samples used in tests were covered with a layer of silicone oil.

XRD The XRD analyses were obtained on an X-ray diffractometer (XRD-6000, Shimadzu, Japan).

ESR The swelling experiments were performed by immersing the hydrogel in a large excess of water at room temperature until swelling equilibrium.

The equilibrium swelling ratio (ESR) was calculated by the following equation, ESR= (Ws −Wd )/Wd , where Ws and Wd are the mass of the swollen and corresponding dried hydrogel, respectively.

Rheological tests Rheological tests were performed using an HAAKE MARS rheometer (modular advanced rheometer system, Rheometric Scientific Inc.) at 20 ℃ . The disc-shaped samples were prepared with thicknesses of ~2 mm and diameters of 20 mm. 9 ACS Paragon Plus Environment


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3. RESULTS AND DISCUSSION

3.1. Preparation of F127AZO@β-CD-PAAM hydrogel

Firstly, guest group (azo) was linked at the end of pluronic F127 and vinyl was functionalized to β-CD respectively. The synthetic route of β-CD-MCD and F127AZO was shown in Figure 1. β-CD was functionalized with vinyl groups by esterification reaction between EDA-β-CD and 2-methylacrylic acid anhydride (MCD). According to mass spectra of β-CD-MCD (Figure S1), most methylacrylic group was connected to β-CD with ratio of 1:1. The grafting ratio was 86 % (calculated based on 1H NMR spectra of β-CD-MCD, Figure S2). The F127AZO was prepared through one-step esterification reaction between F127 and 4-(phenylazo) benzoic acid. The signal in 4.57 ppm owned by ester group of F127AZO suggested that the azo groups were successfully connected to F127. And the grafting ratio was calculated to 1.96 through the area of relative signal in 1

H NMR spectra (Figure S3).


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Figure 1. Preparation of a) β-CD-MCD and b) F127AZO.

The β-CD-MCD and F127AZO molecules easily self-assembled in water through the host-guest interactions between azo and β-CD27. Figure 2 showed the (2D NOESY) 1H NMR spectra of the F127AZO@β-CD. It can be seen that the signal of β-CD (3-, and 5-H protons at 3.30-3.76 ppm)

28

correlated with azo moieties (7.0-7.9 ppm), indicating that

azo moieties have embedded into the cavities of β-CD to form F127AZO@β-CD assemblies.



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Figure 2. 2D NOESY NMR spectrogram of the F127AZO@β-CD macromolecular cross-linkers (solvent: D2O). It is known that amphiphilic F127 can form micelles in aqueous solution24. The morphology of F127AZO@β-CD was investigated by DLS and TEM. As shown in Figure 3A, in the TEM images, the F127AZO@β-CD assemblies were spherical particles with size about 180 nm. The black core of the particle is supposed to be the hydrophobic aggregations of F127, which is denser than the hydrophilic shells. DLS results showed that the hydration radius of F127AZO@β-CD particles was about 780 nm. It should be noticed that the hydration radius of F127AZO@β-CD was larger than that of pure F127 micelles (around 30 nm, Figure 3B). This result may be since that the introduction of β-CD-MCD increased the hydrophobic ratio of the system. It has been reported that the 12 ACS Paragon Plus Environment

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increasing the hydrophobic ratio in copolymers will cause larger aggregation29. The micelle-like F127AZO@β-CD had a lot of vinyl groups on the shell, so they can act as high-functionality crosslinkers to prepare hydrogel. A series of hydrogel were prepared by free radical polymerization of AAm in the presence of F127AZO@β-CD. The preparation and structure of F127AZO@β-CD-PAAM hydrogel were shown in Scheme 1.

Figure 3. A) TEM images of the F127AZO@β-CD macromolecular cross-linkers (scale bar: 200nm). B) DLS results for (a) F127AZO@β-CD and (b) F127.

Figure 4 showed the dynamic mechanical properties of F127AZO@β-CD-PAAM hydrogel (the concentration of F127AZO@β-CD was 0.02 mol%). It can be seen that the storage modulus (G') was much larger than the loss modulus (G'') over a wide frequency range, which manifested the formation of a stable 3D network. The storage modulus of F127AZO@β-CD-PAAM hydrogel increased with the increase of shear frequency, which was a consequence of the viscoelastic nature of the hydrogel30. G'' also shows a slight 13 ACS Paragon Plus Environment


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increase with the increase of frequency. Figure 4b showed the G' and G'' curves as a function of strain-dependent behaviors. It can be seen that F127AZO@β-CD-PAAM hydrogel exhibited nonlinear viscoelastic behavior under high shear strains. When γ increases from 10 % to 100 %, the value of G' decreased sharply and the value of G'' showed a maximum. A reverse can be seen between G' and G'' at high γ, indicating that the gels transform from a solid to a liquid state. It has been demonstrated that apparent strain dependent viscoelastic response is a characteristic of the supramolecular hydrogel30. In case of F127AZO@β-CD-PAAM hydrogel, the high-functionality crosslinkers (F127AZO@β-CD) will dissociate under high shear strain. Thus, G’ decreased obviously. The equilibrium swelling ratio of F127AZO@β-CD-PAAM hydrogel was much higher than that of PAAM hydrogel crosslinked by chemical crosslinker (Table S1), suggesting that the micellar-like corsslinkers were also swollen.

Figure 4. a) Storage modulus (G') and loss modulus (G'') as a function of frequency (f) for F127AZO@β-CD-PAAM hydrogel (γ = 0.005). b) G' and G'' as a function of Strain 14 ACS Paragon Plus Environment

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(γ) for F127AZO@β-CD-PAAM hydrogel (ω = 1 Hz).

Scheme 1. A) Schematic illustration of the preparation of F127AZO@β-CD HFCs. B) a. In

F127AZO@β-CD-PAAM

hydrogel,

the

F127AZO@β-CD

HFCs

and

the

entanglements of the polymer chains serve cooperatively as connection points of the whole network; b. The polymer chains extend gradually under the external force; c. After further stretching, the hydrophobic cores of the F127AZO@β-CD HFCs disassemble to dissipate energy; d. The host guest switches unlock upon subsequent stretching, which gives additional dissipating energy.

3.2. Mechanical properties of F127AZO@β-CD-PAAM hydrogel. 15 ACS Paragon Plus Environment


We

prepared

a

series

of

F127AZO@β-CD-PAAM

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hydrogel

with

different

F127AZO@β-CD concentrations and tested their tensile properties. The results showed that the highest tensile strength of F127AZO@β-CD-PAAM hydrogel would be obtained when the concentration of F127AZO@β-CD was 0.02 mol% (compared to AAM) (Figure S4). As also shown in Figure 5a, this sample exhibit excellent ductility and can be stretched to more 20 times than its own length. Even the fully swollen hydrogel sample can be stretched over 15 times than the original length. Thus we select the sample with 0.02 mol% F127AZO@β-CD to do the later experiments. In order to investigate the influence of crosslinkers on the mechanical properties of PAAM hydrogel, we prepared two control samples. One was PAAM hydrogel crosslinked by F127 diacrylate (F127DA-PAAM), micelles without host-guest switchers. Another was PAAM hydrogel crosslinked by MBAA (MBAA-PAAM), a typical chemical crosslinker. Figure 5b showed the stress-strain curves of three kinds of PAAM hydrogel. It can be seen that using F127 micelles as crosslinkers improved the stretchability of hydrogel as reported before. More importantly, after introducing β-CD-AZO switches to the micelles, both stretchability and tensile strength of hydrogel were increased significantly. The tensile strength of F127AZO@β-CD-PAAM hydrogel was nearly 2 times higher than that of MBAA-PAAM and F127DA-PAAM hydrogel. And the corresponding ultimate elongation was 2108 %, which was not only much higher that the hydrogel with chemical crosslinkers (about 3 times higher than that of MBAA-PAAM), but also higher than that


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of

hydrogel

crosslinked

by

F127

micelles.

In

Figure

5c,

we

can

see

F127AZO@β-CD-PAAM hydrogel with 0.02 mol % cross-linkers even showed better toughness than the two control hydrogel with much higher crosslinkers content. The good toughness of F127AZO@β-CD-PAAM hydrogel resulted from the introduction of multiple dissipative mechanisms. As shown in Scheme 1B, during process of stretching, different dissipative units across multiple length scales from nano- to micro-structure gradually disassociated to absorb energy.

Cyclic tensile loading-unloading tests are usually used to illustrate the toughening mechanism of sacrificial bonds15, and the hysteresis indicates energy dissipation. As shown in Figure 5d, it is clear that F127AZO@β-CD-PAAM hydrogel showed larger hysteresis in loading-unloading cycle compared to MBAA-PAAM and F127DA-PAAM hydrogel, suggesting that F127AZO@β-CD-PAAM hydrogel can dissipate the applied energy more effectively during the deformation. This more effective energy dissipation is attributed to combination of reversible host-guest interaction and synergistic deformation of micellar-like high-functionality crosslinkers (Scheme 1B). Figure 5e showed stress-strain

curves

of

F127AZO@β-CD-PAAM

hydrogel

after

consecutive

loading-unloading cycles. The hysteresis became much smaller if the next cycle was conducted immediately after the previous. As increase of the cyclic number, the loops became less and less pronounced, and the maximum stress declines slightly. These results are attributed that the rupture of the physical crosslinkers in the system cannot recover in 17 ACS Paragon Plus Environment


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time. It is astonishing that the hydrogel showed even better mechanical properties after recovery at 25 ℃ for 30 min. The most likely reason is that a small amount of water was lost during the process of stretching and recovery, though we smeared a layer of Vaseline on hydrogel surface to prevent water loss. We also did XRD measurement after the sample recovered. It can be seen that the diffraction peak of the gel after self-recovery became stronger and appeared red-shift (Figure 5f), suggesting the decrease of the spacing between the lamellar layers31. Therefore, denser network of the recovered sample may also contribute the increase of mechanical properties.

Figure 5. a) Visual photos used to show the excellent ductility. b) Tensile stress-strain curves of MBAA-PAAM, F127DA-PAAM and F127AZO@β-CD-PAAM hydrogel. c) Fracture toughness of F127AZO@β-CD-PAAM, MBAA-PAAM (0.1mol%) and F127DA-PAAM

(0.1

mol%) hydrogel.

d)

Tensile loading-unloading test

of

MBAA-PAAM, F127DA-PAAM and F127AZO@β-CD-PAAM hydrogel at strain of 18 ACS Paragon Plus Environment

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400 %. e) Cyclic loading-unloading tensile curves of the F127AZO@β-CD-PAAM hydrogel at 1000 % strain for four cycles and after subsequent recovery at 25 °C for 30 min. f) The XRD patterns of the F127AZO@β-CD-PAAM hydrogel before and after self-recovery.

F127AZO@β-CD-PAAM hydrogel also showed favorable compressive properties. As depicted in Figure 6a, a cylindrical-shape sample (diameter of 20 mm, height of ~7 mm) was flattened to 100 % strain using a HP-63 thermal press (pressure: 10 MPa, holding time: 3 min, temperature: room temperature). When the pressure was removed, the F127AZO@β-CD-PAAM hydrogel still maintain structural integrity, while the MBAA-PAAM hydrogel were almost destroyed. Furthermore, F127AZO@β-CD-PAAM hydrogel still showed good elasticity after being compressed (Figure 6b). These results indicated that F127AZO@β-CD-PAAM hydrogel could maintain structural integrity well and were also tough under large compressive pressure. Figure 6c showed compressive strengths

at

85

%

strain

of

F127AZO@β-CD-PAAM,

MBAA-PAAM

and

F127DA-PAAM hydrogel. It can be seen from Figure 6c that the compressive strength of F127AZO@β-CD-PAAM hydrogel increased obviously compared to the other two kinds of hydrogel. The good compressive properties of F127AZO@β-CD-PAAM hydrogel may be since that the stereoscopic globular micelle could dissipate energy in compressive deformation.

Cyclic compressive loading unloading tests were conducted to illustrate fatigue resistance 19 ACS Paragon Plus Environment


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of F127AZO@β-CD-PAAM hydrogel22. As shown in Figure 6d, a slight decrease of hysteresis after the first cycle, but the hysteresis loops remains constant in the second and third cycles and even after recovery. These results signified a good fatigue resistance. To give a detailed description of the fatigue resistance properties, we compared the area of hysteresis loops at each cycle and at room temperature after the recovery of 30 min (Figure S5). It was found that the area of the hysteresis loops remains constant in the second and third cycles and even increases a little after recovery. The decrease of the area of hysteresis loops may attribute to the broken of some short polymer chains.

Figure 6. a) The F127AZO@β-CD-PAAM hydrogel exhibit super high compressive toughness under 100% strain, compared with the MBAA-PAAM hydrogel. b) The F127AZO@β-CD-PAAM hydrogel recovered after compression in Figure 6a for 30 min can still be stretched to 600 % of the initial length. c) Compressive stress−strain curves of 20 ACS Paragon Plus Environment

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the MBAA-PAAM, F127DA-PAAM and F127AZO@β-CD-PAAM hydrogel. d) Cyclic compressive curves of F127AZO@β-CD-PAAM hydrogel at 85 % strain for three cycles (no waiting time between cycles) and after subsequent recovery at 25 °C for 30 min.

3.3. Light- and thermal-responsive behaviors.

It is known that the complexion between β-CD and azo groups are sensitive to temperature and light32-34. The β-CD-AZO complexes dissociate at high temperature or under UV-light (360 nm) irradiation and re-associate at relatively low temperature or under visible-light irradiation (480 nm). Therefore, the β-CD-AZO could serve as switches to endow light- and thermal- sensitive properties to F127AZO@β-CD-PAAM hydrogel. As shown in Figure 7a, the mechanical properties of F127AZO@β-CD-PAAM hydrogel were different before and after UV irradiation. After being irradiated by UV light for 3 h, both strength and strain of hydrogel reduced obviously, suggesting that β-CD-AZO complexes played a very important role to dissipate the energy. The mechanical properties of hydrogel in responding to temperature are hard to measure accurately since the water lost during the heating process. But the thermal induced shape memory

behavior

could

reflect

the

thermal-sensitive

properties

of

F127AZO@β-CD-PAAM hydrogel clearly. As shown in Figure 7b, a straight strip sample of F127AZO@β-CD-PAAM hydrogel was easily deformed to “S” shape under external stress at 90 °C, and this deformation was fixed by cooling the sample rapidly to room temperature. When the deformed film was heated above 90 °C again, it recovered its 21 ACS Paragon Plus Environment


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original shape. The ratio of recovery could reach up to 100 %. This thermal-induced shape memory behavior was caused by the fact that the crosslinking density decreased sharply due to the dissociation of β-CD-AZO.

Figure 7. a) Tensile stress-strain curves about light-responsive behavior of F127AZO@β-CD-PAAM hydrogel. b) Visual illustration of thermal-responsive behavior of F127AZO@β-CD-PAAM hydrogel (process 1: deforming at 90 ℃and cooling rapidly to room temperature , process 2: recover at 90 ℃).

4. CONCIUSIONS

In summary, we developed a novel strategy to construct responsive and tough hydrogel. Key structural feature of the hydrogel is the supramolecular crosslinkers, which show micellar structure and have host-guest “switches”. Since having multi-scale and multiple sacrificial units, these crosslinkers could dissipate energy progressively and effectively to enhance the mechanical properties of the resulted hydrogel. The hydrogel also exhibited


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excellent fatigue resistance during cyclic mechanical tests, which result from the absence of the chemical cross-linkers and result in improvement of the life scan during practical application. Moreover, due to the introduction of host guest switches, the hydrogel also showed light- and thermal-responsive behavior. Combination of multiple sacrificial mechanisms across multi-scales through physical interactions may provide a new way to develop tough hydrogel with responsive properties.

ASSOCIATED CONTENT

Supporting Information The supporting information is available free of charge on ACS Publications website. 1H NMR spectra, mass spectra, tensile strength and chart of area of hysteresis loop. (PDF)

AUTHOR INFORMATION

Corresponding Authors

*S. Zhang. Email: [email protected].

*B.-J. Li. Email: [email protected].

ORCID

Bang-Jing Li: 0000-0002-0405-6883

Author Contributions 23 ACS Paragon Plus Environment


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Meng-Meng Song designed experiments, performed, analyzed the results, and drafted the manuscript. Ya-Min Wang, Bing Wang, Xiang-Yong Liang and Zhi-Yi Chang gave assistants on experiments. Professor Sheng Zhang and Bang-Jing Li supervised the project, helped to design the experiments and revise the manuscript. All authors contributed to the analysis of the manuscript.

ACKNOWLEDGMENTS

This work was funded by the National Natural Science Foundation of China (Grant Nos. 51573187, 51373174), supported by the Project of State Key Laboratory of Polymer Materials Engineering (Sichuan University).

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Table of Contents (TOC): In this work, a novel high functionality cross-linker integrating host-guest inclusion and hydrophobic aggregations together has been introduced to hydrogel matrix. Multiple mechanisms across multiple length scales into nano- and micro-structures of hydrogel have also been implemented at the same time. Due to the extra energy dissipation, hydrogel exhibit excellent toughness and ductility. Meanwhile, host-guest interaction endows light- and thermal- sensitive properties to hydrogel. Through our research, we hope to provide a new strategy to design robust and smart hydrogel.


Super Tough, Ultrastretchable Hydrogel with Multistimuli

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