Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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A Tough and Stiff Hydrogel with Tunable Water Content and Mechanical Properties Based on the Synergistic Effect of Hydrogen Bonding and Hydrophobic Interaction Xin Ning Zhang,† Yan Jie Wang,† Shengtong Sun,‡ Lei Hou,‡ Peiyi Wu,‡,§ Zi Liang Wu,*,† and Qiang Zheng†
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Ministry of Education Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China ‡ Center for Advanced Low-dimension Materials & College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai 201620, China § State Key Laboratory of Molecular Engineering of Polymers, Laboratory for Advanced Materials, Department of Macromolecular Science, Fudan University, Shanghai 200433, China S Supporting Information *
ABSTRACT: Hydrogels are usually recognized as soft and weak materials, the poor mechanical properties of which greatly limit their applications as structural elements. Designing of hydrogels with high strength and high modulus has both fundamental and practical significances. Herein we report a series of tough, stiff, and transparent hydrogels facilely prepared by copolymerization of 1-vinylimidazole and methacrylic acid in dimethyl sulfoxide followed by solvent exchange to water. The equilibrated hydrogels with water content of 50−60 wt % possessed excellent mechanical properties, with tensile breaking stress, breaking strain, Young’s modulus, and tearing fracture energy of 1.3−5.4 MPa, 40−330%, 20−170 MPa, and 600−4500 J/m2, respectively. These tough hydrogels were also stable over a wide pH range (2 ≤ pH ≤ 10), resulting from the formation of dense and robust hydrogen bonds between imidazole and carboxylic acid groups. Moreover, the water content and mechanical properties of one gel can be adjusted over a wide range by controlling the dissociation and re-formation of hydrogen bonds during the solvent exchange and heating process; the treated hydrogel with specific characters was stable in water at room temperature. This is because the density of hydrogen bonds can be modulated at high temperature yet immediately fixed at room temperature due to the high stiffness and glassy state of the hydrogel. This strategy to prepare tough and stiff hydrogels should be applicable to other systems as structural materials with promising applications in diverse fields. bonds such as ionic bonds and hydrophobic interactions.24−28 The toughening is attributed to the wide distribution of strength of noncovalent bonds or their clusters; the relatively weak bonds break to dissipate energy, whereas the strong ones survive to maintain integrity of the gel.24 Among the diverse noncovalent bonds, hydrogen bonding is popular in natural systems and has been widely used to design functional materials.29−31 However, using hydrogen bonds to develop tough hydrogels is challenging because they are easily disturbed by water molecules and not stable in the aqueous environment, especially at relatively high temperature.32,33 In recent years, a few strategies have been explored to stabilize the hydrogen bonds and thus to develop mechanically robust hydrogels.34−40 For example, Meijer, Vlassak, and their co-
1. INTRODUCTION In recent years, tough hydrogels inspired by soft biotissues such as tendons and cartilages have attracted increasing attention due to their promising applications as structural biomedical and engineering materials.1−10 So far, a variety of tough hydrogels with typical network structures,11−23 including nanocomposite gels,11,12 topological slide-ring gels,15 doublenetwork gels,16−19 and dual-cross-link gels,20−23 have been designed based on different toughening mechanisms. For example, Zhou and co-workers have prepared dual-cross-link hydrogels with high strength and toughness by using Fe3+ ions to further cross-link a chemically cross-linked poly(acrylamideco-acrylic acid) network via carboxyl−Fe3+ coordination bonds.21 Under loading, the metal-coordination bonds break ahead the chemical bonds, dissipating vast energy to toughen the integrated hydrogel. Following similar principle, various tough supramolecular hydrogels have been developed, in which the polymer chains are solely cross-linked by noncovalent © XXXX American Chemical Society
Received: July 13, 2018 Revised: September 17, 2018
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DOI: 10.1021/acs.macromol.8b01496 Macromolecules XXXX, XXX, XXX−XXX
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properties of one gel can be tuned over a wide range by controlling the incubation temperature and time. The treated hydrogel was stable in water and can recover to its original state by swelling it in DMSO to dissociate the hydrogen bonds and then in pure water to re-form the hydrogen bonds. Such a tough hydrogel with high stiffness, as well as tunable properties, should find applications as structural elements in diverse areas.
workers have prepared tough supramolecular hydrogels by incorporating hydrophobic segments into the hydrophilic polymer chains to protect the quadruple hydrogen bonding between self-complementary ureidopyrimidinone (UPy) units.34,35 Although the aforementioned hydrogels have high strength and toughness, their stiffness (Young’s modulus usually less than 1 MPa) is much lower than that of some biotissues such as cartilages and skins (modulus up to 100 MPa).41−44 The tough yet soft synthetic hydrogels, even those with moderate water content, will experience large deformations under loading, limiting their applications as structural materials. To increase the stiffness of tough hydrogels, one may (i) increase the cross-linking density and/or chain entanglement and (ii) reduce the flexibility of chain segments. Sheiko and co-workers have developed tough and stiff poly(N,N-dimethylamide-comethacrylic acid) (P(DMAA-co-MAAc)) hydrogels, in which strong hydrogen bonds formed between the amide and acrylic acid groups that were stabilized by the hydrophobic methyl motifs.40 The resultant gels showed excellent mechanical properties, with tensile breaking stress σb, breaking strain εb, Young’s modulus E, and tearing fracture energy G up to 2 MPa, 800%, 28 MPa, 9.3 kJ/m2, respectively. By forming interchain hydrogen bonds, we also prepared ultrastiff and tough double-network hydrogels of poly(acrylic acid)/poly(Nisopropylacrylamide) (PAAc/PNIPAm) with σb of 4.6 MPa and E of 226 MPa.45 Besides the amide group, imidazole can also form robust hydrogen bond with the carboxylic acid group.46 In our previous work, we prepared poly(1-vinylimidazole-co-acrylic acid) (P(VI-co-AAc)) hydrogels by copolymerization of VI and AAc.47 The formation of intra- and interchain hydrogen bonds endowed the gel with prominent mechanical properties at room temperature, with σb of 0.3−1.8 MPa, εb of 920−1400%, E of 0.1−0.7 MPa, and G of 0.7−5.6 kJ/m2. Because of the robust hydrogen bonds, the gels were stable over a wide range of temperature and pH. Despite low water content (35−50 wt %), E of the gels was still not so high, when compared to aforementioned hydrogen bond-mediated tough and stiff gels. To improve the strength of hydrogen bonds and the rigidity of polymer chains, a simple hydrophobic methyl motif is introduced to acrylic acid; i.e., methacrylic acid (MAAc) is used for copolymerization with VI in this work to prepare hydrogels. We demonstrate here tough and stiff poly(1-vinylimidazoleco-methacrylic acid) (P(VI-co-MAAc)) hydrogels facilely prepared by the copolymerization of VI and MAAc with chemical cross-linker in dimethyl sulfoxide (DMSO) and subsequent solvent exchange to water, in which dense hydrogen bonds formed between the imidazole and carboxylic acid groups. The equilibrated gels with water content of 50−60 wt % showed excellent mechanical performances, with σb of 1.3−5.4 MPa, εb of 40−330%, E of 20−170 MPa, and G of 600−4500 J/m2, superior to most existing hydrogels and comparable to tough soft biotissues, especially in terms of modulus (Figure S1). These gels were also stable over a wide range of pH (2 ≤ pH ≤ 10), yet high temperature led to contraction of gel due to the lower critical solution temperature (LCST) behavior of P(VI-co-MAAc) chains. However, the contracted gel did not recover to its original state after the temperature was back to room temperature, which immediately fixed the structure by a dramatic increase in the modulus. Therefore, the water content and mechanical
2. EXPERIMENTAL SECTION 2.1. Materials. Methacrylic acid (MAAc), acrylic acid (AAc), azobis(isobutyronitrile) (AIBN), and potassium persulfate (KPS) were used as received from Aladdin Chemistry Co., Ltd.; 1vinylimidazole (VI) and N,N′-methylenebis(acrylamide) (MBAA, used as the chemical cross-linker) were purchased from SigmaAldrich. Hydrochloric acid, sodium hydroxide, and dimethyl sulfoxide (DMSO) were used as received from Sinopharm Chemical Reagent Co., Ltd. Millipore deionized water was used in all the experiments. 2.2. Synthesis of Hydrogels. P(VI-co-MAAc) hydrogels with different compositions were synthesized by free-radical copolymerization of VI and MAAc in DMSO. Prescribed amounts of VI, MAAc, MBAA, and KPS were dissolved in DMSO. The obtained homogeneous solution was poured into a reaction cell consisting of a pair of glasses separated with 0.5 mm silicone spacer, which was then kept in an oven at 70 °C for 6 h. After polymerization, the asprepared organogel was soaked in a large amount of water for solvent exchange.48 The water was exchanged every day, and the equilibrated hydrogels were obtained after 1 week. The resultant hydrogels are named as VM-Cm-f v-Cx, in which Cm is the concentration of total monomers in M, f v is the molar fraction of VI, and Cx is the concentration of MBAA in mol %. The concentration of KPS was kept as 0.5 mol % (relative to the monomer) in all the gel synthesis. P(VIco-AAc) hydrogel was synthesized in a similar way, and the resultant gel was named VA-Cm-f v-Cx.47 PVI gel was synthesized by free-radical polymerization of VI in toluene at 70 °C for 6 h. The concentrations of VI, MBAA, and AIBN in the precursor solution were 3 M, 1 mol %, and 0.5 mol % (relative to the monomer), respectively. PMAAc gel was synthesized in a similar way with water as the solvent. The concentrations of MAAc, MBAA, and KPS were 3 M, 1 mol %, and 0.5 mol % (relative to the monomer), respectively. The resultant gels were dried in an oven and then incubated under a large amount of water to achieve the equilibrium state. 2.3. Characterizations. Light transmittance (600 nm wavelength) of P(VI-co-MAAc) aqueous solution at different temperature was measured using an ultraviolet spectrophotometer (Shimadzu, UV 1800). Infrared spectra of the polymers and hydrogels were recorded by attenuated total reflectance Fourier transform infrared spectroscopy (Nicolet iS10 ATR-FTIR; Thermo Scientific, USA). All the spectra were obtained at room temperature with 32 scans and a resolution of 4 cm−1 in the range 4000−400 cm−1. Dynamic light scattering (DLS) measurements were performed on a Malvern Zetasizer Nano ZS instrument at different temperatures. At each temperature, the sample was maintained for 5 min before the measurement. Scanning electron microscopy (SEM) was performed to the gel samples by using Hitachi S4800 field emission scanning electron microscopy. The samples were prepared by freeze-drying and then cryogenically fractured in liquid nitrogen, and the fractured surface was coated with a thin layer of gold using a sputter coater. The accelerating voltage for SEM observation was 3 kV. Mechanical properties of hydrogels were measured on a commercial tensile tester (Instron 3343). For tensile tests, the samples were cut from equilibrated gel sheets into dumbbell shape with initial gauge length of 12 mm and width of 2 mm. The tensile tests were performed at room temperature with a stretch rate of 100 mm/min. The nominal engineering stress (σ) and strain (ε) were recorded, in which the stress was defined as the tensile force divided by original cross-section area of the sample, and the strain was defined as the displacement of the cross-head divided by the gauge length of B
DOI: 10.1021/acs.macromol.8b01496 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules gel sample. Young’s modulus (E) was calculated from the initial slope of the stress−strain curve with a strain below 10%. Tensile tests at different temperatures, as well as the cyclic tensile tests, were performed in a water bath with a certain temperature. For tearing tests, the hydrogel samples were cut into rectangular shape (35 mm × 8 mm) with 10 mm initial notch at the middle of the short edge. Two arms of the specimen were clamped, and the upper arm was pulled upward at a constant velocity of 100 mm/min. The tearing energy G was calculated from the averaged loading force F according to the equation of G = 2F/d, where d is the gel thickness.23 Rheological behaviors of the equilibrated hydrogels were analyzed using a discovery hybrid rheometer (TA Instruments, USA). Temperature sweep was performed from 10 to 80 °C (heating rate: 2 °C/min) at a strain amplitude of 0.03% and a frequency of 10 rad/s (in the linear region). Water content of equilibrated hydrogels, q, was calculated by q = (ws − wd)/ws, where ws and wd are the mass of gels in the swollen and dried states, respectively. The relative swelling ratio in length, S, at different pH or after being immersed in DMSO was calculated by S = L1/L0, where L1 and L0 are the length of equilibrated gel in acidic/alkaline solution or in DMSO and water mixture and the length of original hydrogel in pure water, respectively. The pH value was controlled by the addition of hydrochloric acid and sodium hydroxide.
monomers in M, f v is the molar fraction of VI in mol %, and Cx is the concentration of MBAA in mol % (relative to the total monomers). We should note that a small amount of chemical cross-linker (Cx > 1 mol %) was needed to obtain an intact gel. The formation of hydrogen bonds between the imidazole and carboxylic acid groups was confirmed by FTIR. As shown in Figure 2, the characteristic peak CN (CN (ring)
Figure 2. FTIR spectra of PVI, PMAAc, and P(VI-co-MAAc) hydrogels at room temperature.
3. RESULTS AND DISCUSSION 3.1. Gel Synthesis and Mechanical Properties. P(VIco-MAAc) hydrogels with different compositions were synthesized by copolymerization of VI and MAAc in DMSO followed by solvent exchange from DMSO to water (Figure 1a). In DMSO, the formation of hydrogen bonds between
stretching mode) shifted from 1500 cm−1 in the spectrum of PVI hydrogel to 1546 cm−1 in the spectra of P(VI-co-MAAc) hydrogel. In addition, the peak at 1685 cm−1 in the spectrum of PMAAc hydrogel ascribed to the CO stretching vibration of carboxylic acid group shifted to 1690 cm−1 in the spectrum of P(VI-co-MAAc) hydrogel. These results indicated the formation of hydrogen bonds between the carboxylic acid and imidazole groups.47 As shown in Figures 3a and 3b, the equilibrated hydrogels had excellent mechanical properties at room temperature. As the molar fraction of VI, f v, increased from 7.5 to 20 mol %, σb and E of the hydrogels increased from 1.3 and 20 MPa to 3.0 and 80 MPa, respectively, whereas εb decreased from 330% to 40%. Further increase in f v led to an extremely low εb. In addition, the gels possessed high G, ranging from 600 to 4500 J/m2 (Figure S2), higher than that of native cartilages (∼1000 J/m2). We also investigated the effect of Cm and Cx on the mechanical performances of the hydrogels. In Figures 3c and 3d, as Cm varied from 3 to 5 M, σb and E of the hydrogel increased from 1.8 and 40 MPa to 3.3 and 175 MPa, respectively, yet εb decreased from 240% to 117%. The same tendency was observed in the systems with different Cx. As Cx increased from 1 to 4 mol %, σb and E of hydrogel showed a dramatic increase from 1.8 and 40 MPa to 3.7 and 160 MPa, respectively, yet εb decreased from 220% to 53% (Figures 3e and 3f). We should note that these gels with high stiffness still had moderate water content, q, ranging from 50 to 60 wt %, which decreased with the increase in f v, Cm, or Cx. Considering the combined mechanical properties, VM-3-10-1 was selected to investigate other properties, if there are no specified instructions. According to the results of tensile tests, the P(VI-co-MAAc) hydrogels had high strength and stiffness. Young’s modulus of the gels ranged from dozens to hundreds of MPa. Typical yielding phenomenon (Figure S3) followed by strain softening was observed during the tensile tests, indicating the forced elastic deformation of these tough and stiff hydrogels, which
Figure 1. (a) Schematic synthesis of P(VI-co-MAAc) hydrogels. (b) Photos to show the appearance and stiffness of the equilibrated hydrogels with different compositions. Thickness of the hydrogel: 1 mm.
imidazole and carboxylic acid was suppressed. As a consequence, the polymer chains were flexible, and the resultant organogels were too weak to measure the mechanical properties by tensile tests. The organogels were then immersed in a large amount of water for solvent exchange and removal of residuals. The solvent exchange from DMSO to water led to the formation of dense hydrogen bonds and thus dramatic improvement of mechanical properties of the gels (Figure 1b). The equilibrated hydrogels are transparent and denoted as VM-Cm-f v-Cx, in which Cm is the concentration of total C
DOI: 10.1021/acs.macromol.8b01496 Macromolecules XXXX, XXX, XXX−XXX
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Figure 3. Tensile stress−strain curves (a, c, e) and corresponding mechanical properties (b, d, f) of the equilibrated hydrogels with different fractions of VI, f v (VM-3-f v-1) (a, b), different concentrations of monomers, Cm (VM-Cm-10-1) (c, d), and different concentrations of cross-linker, Cx (VM-3-10-Cx) (e, f). The water content of gels is also presented in the figure.
At room temperature, the gel cannot fully recover to its original state upon unloading (Figures 5a and 5b). However, the gel readily recovered after incubation in hot water (60 °C) for 3 min (Figures 5c and 5d). The fast self-recovery process was further demonstrated by the cyclic tensile tests at 60 °C. At a maximum strain of 100%, the gel showed significant hysteresis in the loading−unloading curve, indicating that huge energy was dissipated during the loading process (Figure 5c). A large residual strain (∼35%) was observed after unloading. However, after waiting for a few minutes, the residual strain reduced gradually, and the loading−unloading loop approached the first one. The waiting time (tw)-dependent residual strain εr and hysteresis ratio Rh (calculated as the area ratio of the second hysteresis to the first) are shown in Figure 5d. εr fully disappeared and Rh was 99.5% after waiting for 5 min, indicating the fast self-recovery of the gel at high temperature. The self-recovery behavior is due to the reformation of hydrogen bonds broken during stretching process, affording the gels high fatigue resistance that is important for practical applications. The formation and dissociation of hydrogen bonds can also be tuned by the pH of incubated solution. Because of the high stability of the ionic hydrogen bonds between the imidazole and carboxylic acid groups,47,51 the hydrogel (VM-3-10-1) was
was usually observed in glassy thermoplastics or semicrystalline polymers.30,49,50 3.2. Dynamic Properties and Responsiveness. Because of the dynamic nature of hydrogen bonds, the mechanical properties of the gels depended on the deformation rate and test temperature. As the stretch rate increased from 20 to 500 mm/min, σb and E of the hydrogel (VM-3-10-1) at room temperature increased from 0.9 and 32.3 MPa to 2.1 and 47.5 MPa, respectively, whereas εb decreased from 230% to 170% (Figures 4a and 4b). Even at a stretch rate of 20 mm/min, typical yielding occurred at a small stain of 4%, and the hydrogel had much higher stiffness than other tough gels.11−27 Tearing fracture energy G of the hydrogel (VM-3-10-1) also increased from 1600 to 3940 J/m2, when the stretch rate increased from 20 to 500 mm/min (Figure S2). As the temperature increased, the gel became soft and stretchable due to the decreased strength and dissociation of hydrogen bonds (Figure 4c). σb and E of the hydrogel (VM-3-10-1) decreased from 1.7 and 62 MPa to 0.5 and 0.86 MPa, respectively, whereas εb increased from 160% to 830%, when the temperature increased from 10 to 70 °C (Figure 4d). Yielding disappeared at temperature above 60 °C. Therefore, the gel experienced plastic deformation at relatively low temperature yet elastic deformation at high temperature. D
DOI: 10.1021/acs.macromol.8b01496 Macromolecules XXXX, XXX, XXX−XXX
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Figure 4. Tensile stress−strain curves (a, c) and the corresponding mechanical properties (b, d) of VM-3-10-1 hydrogel at different stretch rates (a, b) and different temperatures (c, d). The tensile tests at different stretch rates were performed at room temperature (25 °C), and the tests at different temperatures were performed at a stretch rate of 100 mm/min.
Figure 5. Cycle tensile loading−unloading curves and corresponding residual strain εr and hysteresis ratio Rh of VM-3-10-1 hydrogel at 25 °C (a, b) and 60 °C (c, d) after different waiting time, tw.
stable at room temperature over a wide range of pH (2 ≤ pH ≤ 10). However, the gel became swollen in strongly acidic or alkaline conditions (pH < 2 or pH > 10) (Figure 6). The hydrogen bonds will be destructed by the ionization of imidazole at strongly acidic condition or the ionization of carboxylic acid at strongly alkaline conditions.52 Further increase in pH when pH > 10 or decrease in pH when pH < 2 led to volume contraction of the gel. This was because that
the further increase/decrease in pH led to the increase in ionic strength and the electrostatic screening of the charged network.53 Besides the variation of swelling ratio, the mechanical properties of the gel also changed with pH. As shown in Figures 6a and 6b, the mechanical properties of the gel were well maintained over a wide range of pH (2 ≤ pH ≤ 10), yet they were drastically weakened in strongly acidic and basic conditions due to the dissociation of hydrogen bonds. E
DOI: 10.1021/acs.macromol.8b01496 Macromolecules XXXX, XXX, XXX−XXX
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Figure 6. Tensile stress−strain curves (a) and corresponding mechanical properties (b) of the hydrogel VM-3-10-1 equilibrated solutions with different pH. The swelling ratio in length of the gel at different pH was also presented in (b).
Figure 7. (a) Schematic for the solvent treatment and dimension change of VM-3-10-1 hydrogel. (b) Swelling ratios in length of the gel, S and S′, in the mixture of DMSO and water with different volume fraction of DMSO, fd, and then in pure water, respectively. Water content of the equilibrated gel in water, q, was also showed in the figure. (c, d) Tensile stress−strain curves (c) and corresponding mechanical properties (d) of equilibrated hydrogels after solvent treatment of identical VM-3-10-1 gel.
When compared to the state in pure water (pH ∼ 6), the gel slightly contracted its volume at pH = 0. The acid-treated gel was soft and stretchable (Figure 6a). After being transferred back into pure water to achieve the equilibrium state, the hydrogel became tough and stiff again. E of thus obtained hydrogel was 67 MPa, higher than that (40 MPa) of the original gel (Figure S4). Meanwhile, the water content of the resultant hydrogel was 56 wt %, lower than that (60 wt %) of the original gel. This result indicated the formation of a denser hydrogen bonds of the gel in pure water after acid treatment. According to this fixation effect, anisotropic hydrogel can be obtained by stretching the acid-treated hydrogel and then incubating it in pure water to fix the anisotropic structure (Figure S5). Considering the collapse state of the gel at pH = 0, we hypothesized that the re-formation of hydrogen bonds during the variation of condition (e.g., increase of pH to 6) was readily completed, which immediately froze the network structure in a nonequilibrium state. This hypothesis was confirmed in the following section.
3.3. Tunable Properties of the Hydrogel. The hydrogen bonds between VI and MAAc could be destroyed in the polar aprotic solvent, DMSO, which caused swelling of the gel. The swelling ratio in length, S, of the gel in the mixture of DMSO and water could be controlled by the volume fraction of DMSO, fd. As shown in Figure 7a, the equilibrated hydrogel VM-3-10-1 was swollen in the mixture solvent. As fd increased from 10 to 100 vol %, the swelling ratio in length of the gel, S, increased from 1 to 2.5 (Figure 7b). Different S of gel in the mixture solution was due to the different solvent quality for the solubility of polymer chains and different dissociation degree of the hydrogen bonds.54 Then, the swollen gel was transferred into a large amount of water, in which the gel contacted its volume to some extent and reached the equilibrium state. The volume contraction of gel was related to the re-formation of hydrogen bonds. The volume of thus obtained equilibrated hydrogel was still larger than that of the original hydrogel. The swelling ratio in length of the treated hydrogel, S′, relative to the original hydrogel, increased from 1 to 1.3 as fd increased F
DOI: 10.1021/acs.macromol.8b01496 Macromolecules XXXX, XXX, XXX−XXX
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Figure 8. (a) Schematic and photo to show the contraction of HG-80 hydrogel after heat treatment for different time, th. (b−d) Contraction ratio in length S″ and water content q (b), tensile stress−strain curves at room temperature (c), and corresponding mechanical properties (d) of the hydrogel after heat treatment for different th. (e, f) Tensile stress−strain curves (f) of the hydrogel after cyclic heat treatment at 60 °C for 30 min and sequential solvent treatment in DMSO. After each treatment, the gel was incubated in water at room temperature to achieve the equilibrium state before the tensile tests. Corresponding q of the hydrogel is also presented in (f).
the mechanical properties, which are important for practical applications. The hydrogel pretreated by DMSO had a water content of ∼80 wt %, higher than that (∼60 wt %) of the hydrogel prepared by directly swelling the as-prepared organogel in water. For convenience, the solvent-treated VM-3-10-1 hydrogel with a water content of 80 wt % was coded as HG-80, and the original hydrogel was coded as HG-60. As revealed by SEM observation, the HG-80 had a looser network structure than HG-60 (Figure S6). The hydrogel with high water content was not stable at high temperature. When incubated in a 60 °C water bath, the HG-80 gel gradually shrank its volume due to the increased mobility of chain segment and possibility to form denser hydrogen bonds. The contracted gel maintained its size after the temperature was back to room temperature. As shown in Figures 8a and 8b, the HG-80 contracted to 61% of the original length, accompanied by a decrease in water content to 40 wt %, after incubation at 60 °C for 2 h. The heat-treated gel was also stable in water at room temperature. The volume contraction of the gel was accompanied by the improvement of mechanical properties (Figures 8c and 8d). As the heat treatment time, th, increased from 0 to 2 h, E and σb of the gel increased from 4 and 0.4 MPa to 78 and 2.7 MPa, respectively.
from 10 to 100 vol % (Figure 7b). Accordingly, the water content of the treated hydrogel, q, increased from 62 to 80 wt % with the increase in fd. We should note that the treated hydrogels with different features from one identical gel were stable in water at room temperature. The specific phenomenon was due to the dramatic increase in cross-linking density of gel matrix by the formation of dense hydrogen bonds during the solvent exchange from DMSO (or mixture of DMSO and water) to pure water. Therefore, the network structure of the gel was readily frozen with balanced contraction force and rigidity of the gel matrix that depended on fd. It should be rational that the hydrogen bonds were not well-arranged with close packing due to the steric hindrance. Besides the swelling ratio, the solvent treatment also influenced the mechanical properties of the final equilibrated hydrogel. As shown in Figures 7c and 7d, the pretreated hydrogel with low fd had compact network structure and thus high strength and modulus. In contrast, the hydrogel pretreated with high fd showed reduced mechanical strength and modulus yet increased breaking strain. The yielding disappeared in the gel treated with high fd, which only had E of ∼4 MPa. These results indicated that the solvent treatment indeed altered the network structure, water content, and thus G
DOI: 10.1021/acs.macromol.8b01496 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Further extending th did not change the water content and dimensions of the hydrogel. As expected, incubation of the gel at higher temperature will speed up the volume contraction, which can also be maintained after the temperature was back to room temperature. The heat treatment can be interrupted and continued multiple times to tune the properties of hydrogel equilibrated in water at room temperature. However, this process was irreversible; the gel became more compact after the heat treatment and cannot recover to the original state. The state of the gel can only be reset by swelling it in DMSO to dissociate the dense hydrogen bonds. Transferring the equilibrated organogel to water reproduced the hydrogel with high water content of ∼80 wt %, which showed tunable properties by heat treatment. This process was fully reversible for at least three cycles, affording one hydrogel with tunable water content and mechanical properties (Figures 8e and 8f). 3.4. Reasons for Tunable Properties and High Stiffness. It is interesting that the hydrogel with a high water content contracts its volume during the incubation at relatively high temperatures (Figure S7). This behavior is different from many hydrogen-bond-reinforced hydrogels. For example, in the PAAc/PNIPAm DN hydrogels, the hydrogen bonds between the amide and carboxylic acid groups dissociate at 60 °C, leading to the inflation of the gel, which cannot recover to the original state after the temperature is back to room temperature.45 To reveal the essential reasons, we investigated the solution behavior of linear P(VI-co-MAAc) at different temperatures. The aqueous solution with a polymer concentration of 1 wt % was transparent at room temperature. However, the solution gradually became turbid as the temperature increased from 10 to 60 °C (Figure 9a).
solubility of P(VI-co-MAAc). At high temperatures, the hydrogen bonds between the polymer and water molecules dissociated, while those between the carboxylic acid and imidazole groups gradually formed, resulting in the chain collapse of polymer chains. We should note that the LCST (∼30 °C) of polymer solution is different from the plastic-to-elastic transition temperature observed in the tensile test (∼60 °C), which is related to the destruction of hydrogen bonds. The phase transition of polymer solution is fully reversible, different from the irreversible volume contraction during the heat treatment of hydrogel. Stable hydrogen bonds existed in the hydrogel with a high polymer content, indicating the increased bonding strength due to the cooperativity of hydrogen bonds.56 Upon incubation at high temperatures, the hydrogen bonds between the carboxylic acid and imidazole groups rearrange to form more developed and denser chain networks, as confirmed by the FTIR spectra of VM-3-10-1 hydrogel at 70 °C upon one heat−cooling cycle (Figure 10). The increase and decrease in the integral area of the peak of CO stretching vibration indicated the overall changes hidden in the dissociation and reformation of hydrogen bonds during the heating and cooling processes (Figure 10a−d). A transition temperature at ∼62 °C can be observed in the heating processes, in accordance with tensile tests (Figure 4c). Large hysteresis occurred during the first cooling and second heating, indicating that the dissociation and re-formation of hydrogen bonds were not fully reversible. Judging from the second order derivative curves in Figure 10e, much less hydrogen-bonding types of carboxylic acid groups (with a more symmetrical IR profile) were observed for the second heating scan than the first heating scan, indicating a more developed and denser hydrogen bonding network formed after the heating−cooling cycle.57 At high temperatures, the collapsed polymer segments became more compact and had higher chain mobility, favoring the rearrangement of the motifs to form more developed hydrogen bonds. In contrast, the polymer content in solution is low, even in the aggregates, and the intra- and interchain hydrogen bonds are not so dense and robust as those in the hydrogel. When the temperature was back to room temperature, the newly formed hydrogen bonds will dissociate, leading to the redissolution of the aggregates. Another interesting point was the distinct properties between the P(VI-co-MAAc) and P(VI-co-AAc) gels.47 Compared to P(VI-co-MAAc) hydrogels, the P(VI-co-AAc) hydrogels with similar compositions had a high polymer content yet much lower stiffness (Figure 11a). At room temperature, the P(VI-co-MAAc) gels showed plastic deformations with typical yielding phenomena, whereas the P(VI-coAAc) hydrogels only showed elastic deformations. These results indicated that the hydrophobic methyl groups of MAAc enhanced the strength of hydrogen bonds in P(VI-co-MAAc) gels, probably due to the protection of hydrogen bonds against the attack of water molecules.40,45 The pendant methyl groups should also decrease the rotation flexibility of chain segments to some extent.58 The synergistic effect of the hydrogen bonds and hydrophobic interactions reduced the segmental mobility and resulted in a compact gel matrix with mechanical properties like a glassy polymer. The different states of P(VI-co-AAc) and P(VI-co-MAAc) gels with the same composition were characterized by temperature-sweep rheological measurements (Figure 11b). The storage modulus G′ and loss modulus G″ of P(VI-co-AAc)
Figure 9. Digital photos (a) and variations of transmittance at 600 nm wavelength and number-average particle size Dn (b) of 1 wt % P(VIco-MAAc) aqueous solution during the heating process.
Transmittance measurement showed that the phase transition occurred at ∼30 °C (Figure 9b), consistent with the result of DLS measurement. Large particles with sizes up to 240 nm were detected at high temperatures, indicating the aggregation of polymer chains. This is a typical behavior of polymer with lower critical solution temperature (LCST).55 At low temperatures, the carboxylic acid and imidazole groups formed hydrogen bonds with water molecules, leading to the good H
DOI: 10.1021/acs.macromol.8b01496 Macromolecules XXXX, XXX, XXX−XXX
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Figure 10. FTIR spectra with the focus of CO stretching vibration of VM-3-10-1 hydrogel in D2O during the first heating from 29 to 70 °C (a), first cooling from 70 to 40 °C (b), second heating from 40 to 70 °C (c), and corresponding integral area as a function of temperature (d). (e) FTIR spectra of CO stretching vibration and corresponding second-order derivative curves of VM-3-10-1 hydrogel at 70 °C after the first (red) and the second (blue) heating process.
Figure 11. (a) Tensile stress−strain curves, modulus, and water content (inset) of VM-3-10-1 and VA-3-10-1 hydrogels. (b) Temperature-sweep of VM-3-10-1 and VA-3-10-1 hydrogel at a frequency of 1 Hz and a strain amplitude of 0.03%.
tensile tests and temperature-variable FTIR analysis (Figures 4c and 10d). This plastic-to-elastic transition of the hydrogel should be related to the dissociation of hydrogen bonds and improved mobility of chain segments.24 Therefore, it is rational that the P(VI-co-MAAc) gel showed a forced elastic
gel only slightly decreased with the increase in temperature from 25 to 80 °C. In contrast, G′ and G″ of P(VI-co-MAAc) gel decreased by 1 order of magnitude with an obvious rise of loss factor tan δ at about 60 °C, which was consistent with the plastic-to-elastic transition temperature Tt determined by I
DOI: 10.1021/acs.macromol.8b01496 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules deformation at room temperature, several tens of degree lower than the Tt. For P(VI-co-AAc) gel, the Tt should be lower than 10 °C, and the gel showed elastic deformation at room temperature. Compared to P(VI-co-AAc) hydrogel, P(VI-co-MAAc) hydrogel had higher water content as well as higher tensile breaking stress and Young’s modulus. This was also due to the different states of gels at room temperature. As stated before, the hydrogen bonds should immediately form during the solvent exchange process. The P(VI-co-MAAc) hydrogel readily changed from elastic to glassy, freezing the gel matrix with relatively loose structure, which can be condensed with the lowest water content of ∼40 wt % by incubating the gel at a high temperature. In contrast, the gel matrix of P(VI-co-AAc) was in an elastic state before and after the solvent exchange. The network collapsed into a compact state, resulting in relatively low water content, ∼40 wt %, which did not change after incubating the gel at a high temperature. Therefore, the incorporation of hydrophobic methyl groups had enhanced the strength of the hydrogen bonds and the rigidity of gel matrix, resulting in the glassy state of P(VI-coMAAc) hydrogel with high stiffness and toughness. The tunable properties of one hydrogel was also related to the glassy state, which can freeze the network structure after modulating the hydrogen bond density within the gel matrix during the solvent and heat treatment.
Peiyi Wu: 0000-0001-7235-210X Zi Liang Wu: 0000-0002-1824-9563 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51773179) and Thousand Young Talents Program of China.
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(1) Calvert, P. Hydrogels for soft machines. Adv. Mater. 2009, 21, 743−756. (2) Wu, Z. L.; Kurokawa, T.; Gong, J. P. Novel developed systems and techniques based on double-network principle. Bull. Chem. Soc. Jpn. 2011, 84, 1295−1311. (3) Zhao, X. Multi-scale multi-mechanism design of tough hydrogels: building dissipation into stretchy networks. Soft Matter 2014, 10, 672−687. (4) Zhang, Y. S.; Khademhosseini, A. Advances in engineering hydrogels. Science 2017, 356, eaaf3627. (5) Zhang, L.; Zhao, J.; Zhu, J. T.; He, C. C.; Wang, H. L. Anisotropic tough poly(vinylalcohol) hydrogels. Soft Matter 2012, 8, 10439−10447. (6) Li, J.; Suo, Z.; Vlassak, J. J. Stiff, strong, and tough hydrogels with good chemistry stability. J. Mater. Chem. B 2014, 2, 6708−6713. (7) Zhang, Y.; Gao, H.; Wang, H.; Xu, Z.; Chen, X.; Liu, B.; Shi, Y.; Lu, Y.; Wen, F.; Li, Y.; Li, Z.; Men, Y.; Feng, X.; Liu, W. Radiopaque highly stiff and tough shape memory hydrogel microcoils for permanent embolization of arteries. Adv. Funct. Mater. 2018, 28, 1705962. (8) Bilici, C.; Ide, S.; Okay, O. Yielding behavior of tough semicrystalline hydrogels. Macromolecules 2017, 50, 3647−3654. (9) Zhang, H. J.; Sun, T. L.; Zhang, A.; Ikura, Y.; Nakajima, T.; Nonoyama, T.; Kurokawa, T.; Ito, O.; Ishitobi, H.; Gong, J. P. Tough physical double-network hydrogels based on amphiphilic triblock copolymers. Adv. Mater. 2016, 28, 4884−4890. (10) Hao, J.; Weiss, R. A. Viscoelastic and mechanical behavior of hydrophobically modified hydrogels. Macromolecules 2011, 44, 9390− 9398. (11) Haraguchi, K.; Takehisa, T. Nanocomposite hydrogels: a unique organic-inorganic network structure with extraordinary mechanical, optical, and swelling/de-swelling properties. Adv. Mater. 2002, 14, 1120−1124. (12) Gao, G.; Du, G.; Sun, Y.; Fu, J. Self-healable, tough, and ultrastretchable nanocomposite hydrogels based on reversible polyacrylamide/montmorillonite adsorption. ACS Appl. Mater. Interfaces 2015, 7, 5029−5037. (13) Sakai, T.; Matsunaga, T.; Yamamoto, Y.; Ito, C.; Yoshida, R.; Suzuki, S.; Sasaki, N.; Shibayama, M.; Chung, U. Design and fabrication of a high-strength hydrogel with ideally homogeneous network structure from tetrahedron-like macromonomers. Macromolecules 2008, 41, 5379−5384. (14) Wang, Q.; Mynar, J. L.; Yoshida, M.; Lee, E.; Lee, M.; Okuro, K.; Kinbara, K.; Aida, T. High-water-content mouldable hydrogels by mixing clay and a dendritic molecular binder. Nature 2010, 463, 339− 343. (15) Okumura, Y.; Ito, K. The polyrotaxane gel: a topological gel by figure-of-eight cross-links. Adv. Mater. 2001, 13, 485−487. (16) Gong, J. P.; Katsuyama, Y.; Kurokawa, T.; Osada, Y. Doublenetwork hydrogels with extremely high mechanical strength. Adv. Mater. 2003, 15, 1155−1158. (17) Chen, Q.; Zhu, L.; Zhao, C.; Wang, Q.; Zheng, J. A robust, onepot synthesis of highly mechanical and recoverable double network hydrogels using thermoreversible sol-gel polysaccharide. Adv. Mater. 2013, 25, 4171−4176.
4. CONCLUSIONS In summary, we have developed tough and stiff P(VI-coMAAc) hydrogels by copolymerizing VI and MAAc with a small amount of chemical cross-linker in DMSO and subsequent solvent exchange to water. The hydrogen bonds between imidazole and carboxylic acid groups was enhanced by hydrophobic methyl groups, endowing the hydrogels with excellent mechanical properties and good stability over a very wide range of pH. The obtained tough hydrogels with moderate water content (50−60 wt %) possessed high Young’s modulus up to 170 MPa, superior to most hydrogels and soft biotissues. In addition, the dissociation and formation of hydrogen bonds can be reversibly controlled by solvent and heat treatment, affording tunable properties of one hydrogel including the water content and mechanical properties over a wide range. The glassy state of hydrogel favored the fixation of gel matrix with specific water content and mechanical properties during the solvent and heat treatment process. These P(VI-co-MAAc) hydrogels with high toughness and stiffness, along with tunable properties, should be an ideal candidate to design load-bearing artificial tissues and structural elements of soft actuators.
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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.8b01496. Figures S1−S7 (PDF)
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REFERENCES
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Shengtong Sun: 0000-0001-7471-686X J
DOI: 10.1021/acs.macromol.8b01496 Macromolecules XXXX, XXX, XXX−XXX
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(39) Mihajlovic, M.; Staropoli, M.; Appavou, M. S.; Wyss, H. M.; Pyckhout-Hintzen, W.; Sijbesma, R. P. Tough supramolecular hydrogel based on strong hydrophobic interactions in a multiblock segmented copolymer. Macromolecules 2017, 50, 3333−3346. (40) Hu, X.; Vatankhah-Varnoosfaderani, M.; Zhou, J.; Li, Q.; Sheiko, S. S. Weak hydrogen bonding enables hard, strong, tough, and elastic hydrogels. Adv. Mater. 2015, 27, 6899−6905. (41) Gartner, L. P.; Hiatt, J. L. Color Textbook of Histology, 2nd ed.; Saunders: Philadelphia, PA, 2001. (42) Wegst, U. G. K.; Ashby, M. F. The mechanical efficiency of natural materials. Philos. Mag. 2004, 84, 2167−2186. (43) Little, C. J.; Bawolin, N. K.; Chen, X. Mechanical properties of natural cartilage and tissue-engineered constructs. Tissue Eng., Part B 2011, 17, 213−227. (44) Nakayama, A.; Kakugo, A.; Gong, J. P.; Osada, Y.; Takai, M.; Erata, T.; Kawano, S. High mechanical strength double-network hydrogel with bacterial cellulose. Adv. Funct. Mater. 2004, 14, 1124− 1128. (45) Wang, Y. J.; Li, C. Y.; Wang, Z. J.; Zhao, Y.; Chen, L.; Wu, Z. L.; Zheng, Q. Hydrogen bond-reinforced double-network hydrogels with ultrahigh elastic modulus and shape memory property. J. Polym. Sci. Part B: Polym. Phys. 2018, DOI: 10.1002/polb.24620. (46) Arslan, A.; Kiralp, S.; Toppare, L.; Bozkurt, A. Novel conducting polymer electrolyte biosensor based on poly(1-vinyl imidazole) and poly(acrylic acid) networks. Langmuir 2006, 22, 2912−2915. (47) Ding, H.; Zhang, X. N.; Zheng, S. Y.; Song, Y.; Wu, Z. L.; Zheng, Q. Hydrogen bond reinforced poly(1-vinylimidazole-co-acrylic acid) hydrogels with high toughness, fast self-recovery, and dual pHresponsiveness. Polymer 2017, 131, 95−103. (48) Chen, F.; Zhou, D.; Wang, J.; Li, T.; Zhou, X.; Gan, T.; Handschuh-Wang, S.; Zhou, X. Rational fabrication of anti-freezing, non-drying tough organohydrogels by one-pot solvent displacement. Angew. Chem., Int. Ed. 2018, 57, 6568−6571. (49) Hendrich, M.; Lewerdomski, L.; Vana, P. Biomimetic triblock and multiblock copolymers containing l-phenylalanine moieties showing healing and enhanced mechanical properties. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 2809−2819. (50) Lazurkin, J. S. Cold-drawing of glass-like and crystalline polymers. J. Polym. Sci. 1958, 30, 595−604. (51) Meotner, M. The ionic hydrogen bond. Chem. Rev. 2005, 105, 213−284. (52) Das, M.; Mordoukhovski, L.; Kumacheva, E. Sequestering gold nanorods by polymer microgels. Adv. Mater. 2008, 20, 2371−2375. (53) Peppas, N. A.; Klier, J. Controlled release by using poly(methacrylic acid-g-ethylene glycol) hydrogels. J. Controlled Release 1991, 16, 203−214. (54) Mathis, L.; Chen, Y.; Shull, K. R. Tuning the viscoelasticity of hydrogen-bonded polymeric materials through solvent composition. Macromolecules 2018, 51, 3975−3982. (55) Sudre, G.; Tran, Y.; Creton, C.; Hourdet, D. pH/temperature control of interpolymer complexation between poly(acrylic acid) and weak polybases in aqueous solutions. Polymer 2012, 53, 379−385. (56) Song, G.; Zhang, L.; He, C.; Fang, D. C.; Whitten, P. G.; Wang, H. Facile fabrication of tough hydrogels physically cross-linked by strong cooperative hydrogen bonding. Macromolecules 2013, 46, 7423−7435. (57) Sun, S.; Tang, H.; Wu, P. Tracing dynamic self-disassociation behavior of pyrrole with novel T-shaped hydrogen bonding. Phys. Chem. Chem. Phys. 2009, 11, 7611−618. (58) Rubinstein, M.; Colby, R. H. Polymer Physics; Oxford University Press: New York, 2003.
(18) Sun, J. Y.; Zhao, X.; Illeperuma, W. R. K.; Chaudhuri, O.; Oh, K. H.; Mooney, D. J.; Vlassak, J. J.; Suo, Z. Highly stretchable and tough hydrogels. Nature 2012, 489, 133−136. (19) Haque, M. A.; Kurokawa, T.; Gong, J. P. Super tough double network hydrogels and their application as biomaterials. Polymer 2012, 53, 1805−1822. (20) Li, J.; Illeperuma, W. R. K.; Suo, Z.; Vlassak, J. J. Hybrid hydrogels with extremely high stiffness and toughness. ACS Macro Lett. 2014, 3, 520−523. (21) Lin, P.; Ma, S.; Wang, X.; Zhou, F. Molecularly engineered dual-crosslinked hydrogel with ultrahigh mechanical strength, toughness, and good self-recovery. Adv. Mater. 2015, 27, 2054−2059. (22) Henderson, K. J.; Shull, K. R. Effects of solvent composition on the assembly and relaxation of triblock copolymer-based polyelectrolyte gels. Macromolecules 2012, 45, 1631−1635. (23) Yang, Y.; Wang, X.; Yang, F.; Shen, H.; Wu, D. A universal soaking strategy to convert composite hydrogels into extremely tough and rapidly recoverable double-network hydrogels. Adv. Mater. 2016, 28, 7178−7184. (24) Sun, T. L.; Kurokawa, T.; Kuroda, S.; Ihsan, A. B.; Akasaki, T.; Sato, K.; Haque, M. A.; Nakajima, T.; Gong, J. P. Physical hydrogels composed of polyampholytes demonstrate high toughness and viscoelasticity. Nat. Mater. 2013, 12, 932−937. (25) Zhu, F.; Lin, X. Y.; Wu, Z. L.; Cheng, L.; Yin, J.; Song, Y.; Qian, J.; Zheng, Q. Processing tough supramolecular hydrogels with tunable strength of polyion complex. Polymer 2016, 95, 9−17. (26) Zheng, S. Y.; Ding, H.; Qian, J.; Yin, J.; Wu, Z. L.; Song, Y.; Zheng, Q. Metal-coordination complexes mediated physical hydrogels with high toughness, stick-slip tearing behavior, and good processability. Macromolecules 2016, 49, 9637−9646. (27) Xu, L.; Zhao, X.; Xu, C.; Kotov, N. A. Water-rich biomimetic composites with abiotic self-organizing nanofiber network. Adv. Mater. 2018, 30, 1703343. (28) Abdurrahmanoglu, S.; Can, V.; Okay, O. Design of hightoughness polyacrylamide hydrogels by hydrophobic modification. Polymer 2009, 50, 5449−5455. (29) Perrin, C. L.; Nielson, J. B. Strong” hydrogen bonds in chemistry and biology. Annu. Rev. Phys. Chem. 1997, 48, 511−544. (30) Yanagisawa, Y.; Nan, Y.; Okuro, K.; Aida, T. Mechanically robust, readily repairable polymers via tailored noncovalent crosslinking. Science 2018, 359, 72−76. (31) Aida, T.; Meijer, E. W.; Stupp, S. I. Functional supramolecular polymers. Science 2012, 335, 813−817. (32) Bental, N.; Sitkoff, D.; Topol, I. A.; Yang, A. S.; Burt, S. K.; Honig, B. Free energy of amide hydrogen bond formation in vacuum, in water, and in liquid alkane solution. J. Phys. Chem. B 1997, 101, 450−457. (33) Eberhardt, E. S.; Raines, R. T. Amide-amide and amide-water hydrogen bonds: implications for protein folding and stability. J. Am. Chem. Soc. 1994, 116, 2149−2150. (34) Guo, M.; Pitet, L. M.; Wyss, H. M.; Vos, M.; Dankers, P. Y. W.; Meijer, E. W. Tough stimuli-responsive supramolecular hydrogels with hydrogen-bonding network junctions. J. Am. Chem. Soc. 2014, 136, 6969−6977. (35) Jeon, I.; Cui, J.; Illeperuma, W. R. K.; Aizenberg, J.; Vlassak, J. J. Extremely stretchable and fast self-healing hydrogels. Adv. Mater. 2016, 28, 4678−4683. (36) Tang, L.; Liu, W.; Liu, G. High-strength hydrogels with integrated functions of H-bonding and thermoresponsive surfacemediated reverse transfection and cell detachment. Adv. Mater. 2010, 22, 2652−2656. (37) Dai, X.; Zhang, Y.; Gao, L.; Bai, T.; Wang, W.; Cui, Y.; Liu, W. A mechanically strong, highly stable, thermoplastic, and self-healable supramolecular polymer hydrogel. Adv. Mater. 2015, 27, 3566−3571. (38) Zhang, Y.; Li, Y.; Liu, W. Dipole-dipole and H-bonding interactions significantly enhance the multifaceted mechanical properties of thermoresponsive shape memory hydrogels. Adv. Funct. Mater. 2015, 25, 471−480. K
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