Article pubs.acs.org/Macromolecules
Dual Physically Cross-Linked Hydrogels with High Stretchability, Toughness, and Good Self-Recoverability Yang Hu,†,‡ Zhengshan Du,† Xiaolan Deng,† Tao Wang,† Zhuohong Yang,‡ Wuyi Zhou,*,‡ and Chaoyang Wang*,† †
Research Institute of Materials Science, South China University of Technology, Guangzhou 510640, China Institute of Biomaterials, College of Materials and Energy, South China Agricultural University, Guangzhou 510642, China
‡
S Supporting Information *
ABSTRACT: The applications of hydrogels are severely limited by their weak mechanical properties. Despite recent significant progress in fabricating tough hydrogels, it is still a challenge to realize high stretchability, toughness, and recoverability at the same time in a hydrogel. Herein, we develop a novel class of dual physically cross-linked (DPC) hydrogels, which are triggered by clay nanosheets and iron ions (Fe3+) as cross-linkers. First, clay nanosheets induce the formation of the first cross-linking points through the interaction of hydrogen bonds with poly(acrylamide-co-acrylic acid) (PAm-co-Ac) chains. Then the secondary cross-linking points are introduced by ionic coordinates between Fe3+ and −COO− groups of PAm-co-Ac polymer chains. The mechanical properties of DPC hydrogels can be tuned readily by varying preparation parameters such as clay concentration, Fe3+ concentration, and molar ratio of Ac/Am. More importantly, the optimal DPC hydrogels possess high tensile strength (ca. 3.5 MPa), large elongation (ca. 21 times), remarkable toughness (ca. 49 MJ m−3), and good self-recoverability (ca. 65% toughness recovery within 4 h without any external stimuli). Thus, this work provides a promising strategy for the fabrication of novel tough hydrogel containing a dual physical cross-linked network.
1. INTRODUCTION Polymer hydrogels as “soft−wet” materials, which contain a large amount of water in their three-dimensional networks, have achieved considerable attention in a variety of applications, such as superabsorbent agents,1 sensors,2−4 actuators,5,6 tissue engineering scaffolds,7−9 and drug delivery carriers.10,11 However, most of conventional synthetic hydrogels are generally mechanically weak and brittle12 with bad mechanical strength, low stretchability, poor toughness, and/or low recoverability because of their intrinsic structural heterogeneity and/or lacking effective energy dissipation mechanisms,13,14 which severely limit their extensive uses for the applications requiring highly mechanical properties. To overcome the above issues and extend the practical applications of hydrogels, significant efforts have been contributed to prepare mechanically strong hydrogels with toughening mechanisms and novel microstructures, such as double-network (DN) hydrogels,15−18 nanocomposite (NC) hydrogels,19−21 microgel-reinforced hydrogels,22 slide-ring hydrogels,23 polyampholyte hydrogels,24 tetra-arm poly(ethylene glycol) hydrogels,25 and others.26,27 Among them, DN hydrogels are composed of a stiff and brittle network of energy-dissipating sacrificial linkages with a soft and ductile network of extensible linkages.28 When the DN hydrogel is deformed under stress, the stiff and brittle network acting as “sacrificial bond” ruptures to efficiently dissipate energy, protect the soft and ductile network from crack propagation, maintain stress, store energy, and in turn improve the mechanical properties of the hydrogels. Conventional DN © XXXX American Chemical Society
hydrogels possess two chemical networks formed by the crosslinking of covalent bonds,29,30 which have displayed the excellent mechanical properties of strength (tensile stress at break 1−10 MPa), stretchability (tensile strain at break 1000− 2000%), and toughness (fracture energy 100−1000 J m−2).31 The toughness of the conventional DN hydrogels is comparable to that of rubber and cartilage. However, the conventional DN hydrogels often suffer from permanent damage caused by the irreversible bond rupture of the stiff and brittle network during loading cycles, leading to poor recoverability and fatigue resistance.32,33 To address the above problem, a promising strategy is the introduction of reversible noncovalent bonds instead of sacrificial covalent bonds.34−39 Upon deformation, noncovalent bonds in the stiff and brittle network crack and dissipate energy, but these bonds can reform during unloading process, resulting in the recovery of DN hydrogels from damages. Suo et al.40 designed a DN hydrogels consisting of Ca2+ cross-linked alginate (Ca2+-Alg) network and covalently cross-linked polyacrylamide (PAAm) network. The Ca2+-Alg/PAAm hydrogel could recover to ca. 74% of the first loading after storing at 80 °C for 1 day, but its tensile strength was only ca.160 kPa. Chen et al.36 developed an agar/ polyacrylamide (Agar/PAAm) DN hydrogel, in which the agar network was cross-linked by hydrogen bonds and the PAAm network was cross-linked by covalent bonds. The toughness of Received: March 22, 2016 Revised: July 6, 2016
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Figure 1. Preparation of DPC hydrogels: in situ polymerization of Am and Ac in the presence of clay to form physically cross-linked network; the interaction of −COO− groups of polymer chains with Fe3+ and then reorganization of Fe3+ coordinates to introduce Fe3+ coordination physically cross-linked points.
Agar/PAAm DN hydrogels could recover ca. 65% of the first loading after placing 10 min at 100 °C, but the recovery effect was hardly observed at temperatures below the melting point of agar gel. Besides, Chen et al.13 also produced a fully physically cross-linked agar/hydrophobically associated polyacrylamide (HPAAm) DN hydrogel that could recover ca. 40% of the toughness within 2 min at room temperature, but it was soft with fracture stress ca. 0.267 MPa. Recently, Zhou et al.41 reported a tough and strong dual cross-linked hydrogel containing covalent cross-linking and ionic coordinate interactions in poly(acrylamide-co-acrylic acid) p(Am-co-Ac) polymer chains; its hysteresis loop could be recovered ca. 87.6% after 4 h of the first stretch cycle at room temperature, but its elongation was smaller than 8 times. To this end, despite recent important progress in fabricating tough hydrogels, it is still a big challenge to develop a novel tough hydrogel possessing high stretchability, mechanical toughness, and good self-recoverability at room temperature. Inorganic nanoparticles such as clay, graphene oxide, and carbon dot are usually used as effective physical cross-linkers to prepare NC hydrogels,42−48 due to their high specific surface areas and multifunctional groups on surfaces. In NC hydrogels, the strong hybrid polymer network is formed by physical association between flexible polymer chains and nanoparticles.49,50 The nanoparticles served as both the physical cross-linkers and reinforcing fillers can relax the applied stress and defer the complete fracture of hydrogels. Thus, the stretchability of NC hydrogels is generally larger than that of the chemically cross-linked hydrogels, and changing the contents of nanoparticles or the monomers can facilely control the number of cross-link points and the flexibility of polymer chains between two nanoparticles, which provides a great diversity of approaches to improve the mechanical properties of hydrogels.51 In this study, we introduced clay nanosheets (Laponite XLG) and Fe ions acted as the physical cross-linkers to prepare a new type of tough dual physically cross-linked (DPC) hydrogel. To be specific, acrylamide (Am) and acrylic acid (Ac) were physically cross-linked by clay nanosheets to form poly(acrylamide-co-acrylic acid) (PAm-co-Ac)/clay hydrogels with homogeneous network structures. Then Fe3+ induced the formation of secondary cross-linking points through ionic coordination interactions with −COO− of PAm-co-Ac polymer side chains.52 Afterward, the ionic coordinates were reorganized
and formed more tridentate coordinates between Fe3+ and −COO− groups,41 and the optimal DPC hydrogels were obtained that possessed high tensile strength (ca. 3.52 MPa), large elongation (ca. 21.13 times), remarkable toughness (ca. 49.11 MJ m−3), and good self-recovery property (ca. 65% toughness recovery at room temperature within 4 h, at 800% strain). This work provides a new approach to prepare the fully physically linked hydrogel with superior mechanical properties and can be generally used to develop a new generation of tough DPC hydrogels with desirable properties.
2. EXPERIMENTAL SECTION 2.1. Materials. Clay, Laponite XLG ([Mg5.34Li0.66Si8O20(OH)4]Na0.66) was obtained from Byk Additives & Instruments. Acrylamide (Am) and ammonium persulfate (APS) were purchased from Shanghai Rich Joint Chemical Reagent Co. Ltd., China. Acrylic acid (Ac) and iron(III) chloride hexahydrate (FeCl3·6H2O) were provided by Tianjin Fuchen Chemical Factory, China. N,N,N′,N′-Tetramethylethylenediamine (TEMED) was bought from J&K Chemical Ltd., China. All the chemicals and solvents were used as received without further purification. The water (resistivity more than 18.2 MΩ cm) used in all the experiments was produced by deionization and filtration with a Millipore purification apparatus (USA). 2.2. Fabrication of Dual Cross-Linked Hydrogels. First, clay was added into water and mechanically stirred for 2 h to form a uniform clay aqueous suspension (0.5, 1.0, 1.5, or 2.0 wt %, weight ratio of clay/water). Next, the monomers Am (1.5, 3.0, 4.5, or 6.0 mol L−1, with respect to the volume of water) and Ac (10, 15, 20, or 25%, molar ratio of Ac/Am) were added into the suspension, and then the suspension system was stirred for 30 min in an ice−water bath. After the suspension system was degassed with nitrogen gas for 10 min, the initiator APS (0.1 wt %, with respect to the water weight) and the catalyst TEMED (20 uL) were added under ultrasonication at the ice− water temperature. Afterward, the resulting suspension was transferred into cylindrical glass tubes, which were sealed with plastic wraps and then placed at room temperature for 16 h to form physically monocross-linked hydrogels (named as m-hydrogel, where m referred to only mono-cross-linking). Furthermore, the m-hydrogels were soaked into FeCl3 solution (0.01, 0.06, 0.12, or 0.30 mol L−1) for 3 h to introduce the second cross-linked points by ionic coordinates between Fe3+ and −COO− groups and produce the transition state dual-crosslinked hydrogels (coded as d-hydrogel, d presented dual-cross-linking at transition state). Finally, the d-hydrogels were soaked into water for 48 h with the removal of superfluous Fe3+ to obtain the reorganization state dual-cross-linked hydrogels (denoted as D-hydrogel, D stood for dual-cross-linking at reorganization state). Unless specifically noted, B
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Macromolecules the molar ratio of Ac/Am was 15%, and the concentrations of clay, monomer Am, and FeCl3 solution were 1.0 wt %, 3.0 mol L−1, and 0.06 mol L−1, respectively. The general schematic illustration of the fabrication of DPC hydrogels is shown in Figure 1. 2.3. Mechanical Measurements. Uniaxial mechanical property tests of the m-hydrogels, d-hydrogels, and D-hydrogels were performed by a universal material testing machine (SLBL-1KN, Shimadzu) at room temperature. For the tensile tests and tensile loading−unloading tests, the crosshead speed used in the experiments was 100 mm min−1. The initial length of the hydrogel samples for the tensile test between the two clamps was 20 mm, and the diameter of the m-hydrogel samples was 3.2 mm. For the compression test, the crosshead speed used in the experiments was 1 mm min−1. The hydrogel samples for the compression test were 12 mm in height, and the diameter of the m-hydrogel samples was 8 mm. As for the successive loading−unloading tests, the silicone oil was coated on the surface of the hydrogel samples before the tests. Once the test finished, the tested hydrogel sample was conserved by being coated with a silicone oil layer and then being encased with a plastic wrap to avoid the water evaporation. At least three duplicates were tested for each hydrogel, and the mean value was calculated. The strain of the hydrogel sample was regarded as the length change related to the initial length of the sample (between the two clamps for the tensile tests), and the stress was assessed by the force divided by the initial cross-sectional area of the hydrogel sample. The elastic modulus was determined from the slope of the initial linear region of the stress− strain curve, while the toughness was estimated by the area below the stress−strain curve. The dissipated energy was calculated from the area between the loading−unloading profiles. The recovery values of elastic modulus and the dissipated energy of the hydrogel sample were obtained from the cyclic loading−unloading tests.
Figure 2. D-hydrogel shows outstanding mechanical and tough properties: D-hydrogel can be (a) bended, (b) twisted into doublehelix structure, and (c) knotted. (d) D-hydrogel with the diameter of 3.4 mm lifts up dumbbell of ca. 3 kg in weight. (e) High strain is applied to the D-hydrogel. (f) D-hydrogel can be strongly compressed and quickly recovered its shape upon the removal of compression force.
toughness and stretchability of D-hydrogel. Furthermore, the D-hydrogel was not any obviously damaged by compressing with an extremely high strain. Upon removal of the compression force, D-hydrogel quickly restored its original shape (see Figure 2f), indicating its good shape-recovery property. 3.2. Mechanical Properties of DPC Hydrogels. The quantitatively examination of the mechanical properties of DPC hydrogels was performed using a Shimadzu universal material testing machine with a 1 kN load cell at room temperature. Figure 3 and Figure S1 show the typical stress−strain profiles, elastic modulus, and toughness of m-, d-, and D-hydrogels. It was observed that the tensile strength, fracture strain, elastic modulus, and toughness of m-hydrogel were only ca. 62 kPa, 1406%, 0.025 MPa, and 0.51 MJ m3, respectively. In contrast, the tensile strength, fracture strain, elastic modulus, and toughness of D-hydrogel were ca. 3.52 MPa, 2113%, 0.637 MPa, and 49.11 MJ m3, respectively, which were greatly larger than those related values of m-hydrogel. The above results showed that the introduction of the ionic coordinate interactions could significantly improve the mechanical strength, extensibility, stiffness, and toughness. Furthermore, the tensile mechanical property of the PAm-co-Ac hydrogels cross-linked only with ferric ions by ionic coordinate interactions (coded as Fe3+-hydrogels) have been tested, and the stress−strain curves of Fe3+-hydrogels are shown in Figure S2. The tensile strength and fracture strain of the optimal Fe3+hydrogel were ca. 21 kPa and 610%, respectively. Both mhydrogel and Fe3+-hydrogel were very ductile, and their strengths were very weak. However, when the ionic coordinates were introduced into m-hydrogel by the interactions between Fe3+ and −COO− groups of polymer chains, the obtained DPC hydrogels displayed the much higher tensile mechanical property than the m-hydrogel and Fe3+-hydrogel. This indicated that the dual cross-linked effects clearly assisted in enhancing the mechanical property of hydrogels. In addition, it was also observed from Figure 3 that the tensile strength, elastic modulus, and toughness of D-hydrogel were about 1.5, 2.34, and 1.83 times larger than those of d-hydrogel. It was found
3. RESULTS AND DISCUSSION 3.1. Fabrication of DPC Hydrogels. The DPC hydrogels were fabricated using a “two-step” method (see Figure 1), which involves first preparing physically mono-cross-linked hydrogels and then forming physically dual cross-linked hydrogels. To be specific, clay was suspended in water, and the monomers Am and Ac were added to form a uniform suspension that was degassed with nitrogen gas. Afterward, the initiator ammonium APS and the catalyst TEMED were added to the suspension, then transferred into cylindrical glass tubes, and polymerized to prepare physically mono-cross-linked hydrogels (m-hydrogels). Furthermore, the m-hydrogels were soaked into FeCl3 solution to obtain the transition state DPC hydrogels (d-hydrogels) by ionic coordinations between Fe3+ and −COO− groups of polymer side chains from the mhydrogels. As shown in Figure 1, because of superfluous Fe3+ in the network, the Fe3+ could produce a mixture of mono-, bi-, or tridentates with −COO− of polymer side chains. For the formation of more tridentates which was supposed to assist in the improved mechanical properties, an immersing process was used for the removal of excess Fe3+ and in turn reorganizing the ionic coordinates to obtain the reorganization state DPC hydrogels (D-hydrogels). The detailed fabrication procedure is shown in the Experimental Section. The prepared D-hydrogel displayed a high performance in ductility and mechanical properties, as shown in Figure 2. It was observed from Figure 2a−c that the D-hydrogel could be bended, twisted into double-helix structure, and even knotted, which indicated that the D-hydrogel was very ductile. And Figure 2d demonstrates that a stick of D-hydrogel with the diameter of 3.4 mm could lift up a dumbbell with the weight of ca. 3 kg, showing the incredible mechanical strength of Dhydrogel. As shown in Figure 2e, the D-hydrogel could be stretched a greatly large strain, displaying the excellent C
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Figure 3. (a) Typical tensile stress−strain profiles and (b) elastic modulus and toughness of m-, d-, and D-hydrogels.
Figure 4. Stress−strain profiles of (a) d-hydrogels and (b) D-hydrogels prepared at different clay concentrations. (c) Elastic modulus and (d) toughness of d-hydrogels and D-hydrogels prepared at different clay concentrations.
that when immersing in water, the DPC hydrogel faded from dark brown red color to red (Figure S3). This indicated that excess Fe3+ diffused into hydrogel, leading to form the mixture of mono-, bi-, and tridentates. When the d-hydrogel was immersed in water, excess Fe3+ could diffuse out of hydrogel and the ionic coordinates could be reorganized to form more tridentates in hydrogel. Compared with the monodentate and bidentate coordinations, the tridentate coordinates are supposed to be more favorable for enhancing the mechanical property of hydrogels. Thus, the d-hydrogels were immersed water to remove superfluous Fe3+ and formed more tridentate coordinates in the hydrogel network, which was beneficial for the enhancement of the mechanical properties of D-hydrogel.41 Moreover, it was found that DPC hydrogel slightly shrank after being soaking water for 48 h (Figure S5). The shrinkage behavior resulted in the increase in the cross-linking density of hydrogel network, which also assisted in the enhancement of the mechanical properties of D-hydrogels. As the same two reasons, the compression modulus and stress at 80% stain of Dhydrogel were also clearly higher than that of d-hydrogel (see Figure S4). The mechanical properties were crucial to influence the practical applications of hydrogels. Many factors could affect the mechanical properties of hydrogels. Herein, we quantita-
tively characterized the mechanical properties of DPC hydrogels in response to the changes in the clay concentration, molar ratio of Ac/Am, Am concentration, the concentration of ferric solution, and the immerse time in ferric solution. Figure 4 shows the influence of the clay concentration on the tensile mechanical properties of d-hydrogels and D-hydrogels. It could be obviously seen that with increasing the clay concentration from 0.5 to 1 wt %, the tensile strength, related ruptured strain, and toughness of DPC hydrogels considerably increased. However, if clay concentration increased further (1.5 to 2 wt %), the tensile strength, related ruptured strain, and toughness obviously decreased on the contrary. Additionally, the elastic modulus had an increase trend with the increase in the clay concentration. The above results should be ascribed to the fact that clay nanosheets could be acted as physical crosslinkers to prepare m-hydrogel. At low content, the clay nanosheets could be uniformly dispersed in the polymer matrix, and increasing clay concentration caused more crosslinkers interacted with polymer chains through hydrogen bonds, resulting in the formation of more robust and flexible network structure, in turn showing the outstanding increase of the strength, stretchability, and toughness of hydrogels. However, the excess clay nanosheets would aggregate53−55 in the polymer matrix and shorten the polymer chains between D
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Figure 5. Stress−strain profiles of (a) d-hydrogels and (b) D-hydrogels prepared at different Ac/Am molar ratios. (c) Elastic modulus and (d) toughness of d-hydrogels and D-hydrogels prepared at different Ac/Am molar ratios.
Figure 6. Stress−strain profiles of (a) d-hydrogels and (b) D-hydrogels prepared at different Fe3+ concentrations. (c) Elastic modulus and (d) toughness of d-hydrogels and D-hydrogels prepared at different Fe3+ concentrations.
Figure 5 shows the influence of the Ac/Am molar ratio on the tensile mechanical properties of d-hydrogels and Dhydrogels. It was found that the tensile mechanical properties of d-hydrogel and D-hydrogel had greatly dependence on the Ac/Am molar ratios. With the increase in Ac/Am molar ratio from 10% to 25%, the elastic modulus gradually increased. With the increase of Ac/Am molar ratio from 10% to 15%, the tensile strength, related ruptured strain, and toughness strongly increased. However, increasing Ac/Am molar ratio to 20% and 25% led to the distinct decrease of tensile strength, related ruptured strain, and toughness. The above results should be ascribed to the fact that with the increase of Ac/Am molar ratio from 10% to 15% more −COO− groups of polymer side chains in hydrogels could interact with more Fe3+ (see Figure S6) to form more ionic coordinations, which resulted in the increase
clay nanosheets, which decreased the structural homogeneity and increased the cross-linking density of hydrogels and thus led to reduce the strength, stretchability, and toughness of hydrogels. Furthermore, it was found that the D-hydrogels prepared at different clay concentrations slightly shrank with different degrees (Figure S5), which also led to the enhancement of the cross-linking density of hydrogels. Therefore, a moderate clay concentration (1.0 wt %) could be used to prepare DPC hydrogels with outstanding tensile mechanical properties. Furthermore, it was also observed that the mechanical properties of D-hydrogels were obviously higher than that of d-hydrogels, indicating that the treatment of dhydrogels in water had an important effect on their performance. E
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Figure 7. (a) Loading−unloading tests of D-hydrogel under different strains. (b) The total and dissipated toughness of calculated from (a). (c) Loading−unloading tests of m-hydrogel, d-hydrogel, and D-hydrogel under 800% strain. (d) The total and dissipated toughness calculated from (c).
formed between Fe3+ and −COO− groups. After treatment with high ferric solution, the ionic coordinates existed between Fe3+ and −COO− groups were more likely as bidentate coordinates and/or even monodentate coordinates, resulting in lower secondary cross-linking degree of the DPC hydrogels. Thus, the tensile mechanical properties became worse. Furthermore, it was also observed that when the m-hydrogel was treated in the low ferric solution (0.01 mol L−1), the tensile mechanical properties of D-hydrogel were clearly lower than that of the related d-hydrogel. This should be due to the fact that a few ionic coordinates were formed between Fe3+ and −COO− groups after treatment with low ferric solution (0.01 mol L−1), and the related d-hydrogel was swollen in water (see Figure S8), which resulted in the decrease in the tensile mechanical properties of D-hydrogel. However, when mhydrogels were treated with high ferric solution (0.12 or 0.3 mol L−1), the tensile strength and elastic modulus of Dhydrogel were higher than that of the related d-hydrogel, while the ruptured strain and toughness of D-hydrogel were lower than that of the related d-hydrogel. This should be attributed to the shrinkage of hydrogels (see Figure S8) and formation of more tridentate coordinates after treating with water, which led to the increase of the secondary cross-linking degree, in turn greatly enhanced the tensile strength and elastic modulus, but exerted serious adverse effects on the ruptured strain and toughness of D-hydrogels. As discussed above, only a moderate Fe3+ concentration (0.06 mol L−1) enabled an appropriate secondary cross-linking degree, which translated into highly stretchable and supertough DPC hydrogels. Additionally, the change of immersing time in ferric solution could also adjust the secondary cross-linking degree of the DPC hydrogels, thus the immersing time in ferric solution had also vital influence on the tensile mechanical properties of DPC hydrogels (see Figure S9). 3.3. Hysteresis and Self-Recovery of DPC Hydrogels. The hysteresis behavior of hydrogels during the loading− unloading cycle could be evaluated by stretching samples and recovering at the same speed. The test could reflect the energy
of the cross-linking density in the second network and, in turn, enhanced the strength, stretchability, and toughness of DPC hydrogels. However, further increasing Ac/Am molar ratio up to 20% and 25% led to obtain DPC hydrogels with highly dense cross-linked network, and the obtained DPC hydrogels became brittle and easily ruptured at the relatively low strains, which exerted obviously adverse effects on the strength, stretchability, and toughness of the DPC hydrogels. As discussed above, only a moderate Ac/Am molar ratio (15%) enabled an appropriate cross-linking density, which translated into supertough and highly stretchable DPC hydrogels. Furthermore, it could also tune the tensile mechanical properties of the d-hydrogel and Dhydrogel by varying the polymer content (in terms of Am concentrations from 1.5 to 6 mol L−1, while maintaining monomer ratio constant) in the preparation of m-hydrogel, and the results are shown in Figure S7. When the polymer content increased from 1.5 to 3 mol L−1, the tensile mechanical properties increased because of the enhancement of the absolute number of ionic coordinates between −COO− groups of polymer side chains and Fe3+. But once the polymer content further increased from 3 to 6 mol L−1, the tensile mechanical properties decreased on the contrary. After treatment with water, the tensile strength enhanced, especially for samples of 3 mol L−1. Thus, moderate polymer content (Am concentration of 3 mol L−1) could use to fabricate the DPC hydrogels possessed excellent comprehensive properties. Besides the preparation parameters of the m-hydrogels, the Fe3+ concentration also had an important influence on the tensile mechanical properties of DPC hydrogels. Figure 6 shows the influence of the Fe3+ concentration on the tensile mechanical properties of d-hydrogels and D-hydrogels. It was seen that the tensile strength, related ruptured strain, elastic modulus, and toughness of the DPC hydrogels increase with the increase of Fe3+ concentration at the initial stage, while decreased when the Fe3+ concentration was excessive. When the Fe3+ concentration was above 0.06 mol L−1, the degraded mechanical properties could be obviously seen. The result should be ascribed to the different forms of ionic coordinates F
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Figure 8. Self-recovery and successive loading−unloading cycling test of D-hydrogel. (a) good self-recovery of the as-prepared samples; (b) the timedependent recovery of elastic modulus and hysteresis loop; ten successive loading−unloading cycles of the (c) as-prepared sample and (d) recovered sample (24 h at room temperature).
dissipation of hydrogel sample in a cycle, which was an indicator to assess the toughness of the sample. Figure 7a displays the loading−unloading profiles at varying strains of Dhydrogel. It was observed that all the loading−unloading profiles of D-hydrogel demonstrated apparent hysteresis loops, indicating that the D-hydrogel could dissipate energy effectively. The quantified results are shown in Figure 7b. It was clearly shown that the as-prepared D-hydrogel could dissipate energy effectively as much as 9.26 MJ m−3 at the strain of 800%. Furthermore, the comparison of stress−strain profiles and energy dissipation among m-hydrogel, d-hydrogel, and Dhydrogel at the same strain (800%) was performed and displayed in Figure 7c,d. As observed, both d-hydrogel and Dhydrogel showed obviously large hysteresis loops, while weak m-hydrogel displayed almost invisible hysteresis loop. Consistently, the energy dissipations of d-hydrogel (4.09 MJ m−3) and D-hydrogel (9.26 MJ m−3) were much higher than that of m-hydrogel (0.12 MJ m−3), indicating that the introduction of ionic coordination interactions could make the DPC hydrogels dissipate energy more efficiently than m-hydrogel, thus leading to high toughness. This result should be ascribed to the reason as follows. When the external loading was applied on the DPC hydrogels, the strong ionic coordination bonds served as the reversible sacrificial bonds and cracked to effectively dissipate energies, while the clay nanosheets cross-linked to the polymer chains could keep the configuration of the DPC hydrogels and act as stress transfer centers to transfer stress from the polymer chains to the clay nanosheets. Hence, the introduction of ionic coordination interactions could obviously enhance the toughness of hydrogels. In addition, the D-hydrogel had larger hysteresis loop and energy dissipation than those of d-hydrogel, suggesting D-hydrogel had much more effective energy dissipation pathway, where tridentate interaction played a pivotal role. Considering that the network in D-hydrogels were physically linked using noncovalent interactions, we further carried out the loading−unloading tests at the strain of 800% to evaluate
the self-recoverability of D-hydrogels at room temperature without any external stimuli (see Figure 8a,b). It was observed that the D-hydrogel had good self-recovery, and the recovery rates of elastic modulus and dissipated energy of D-hydrogels increased with the increase in the resting time. When the second test was conducted immediately, the elastic modulus recovered to 24% of its original value, and the dissipated energy recovered to 25% of its original value. When the second test was conducted after 4 h, the recovery ratios of elastic modulus and dissipated energy were 58% and 65%, respectively. The internal network damage could be better recovered with prolonging the resting time before reloading. The good selfrecoverability of D-hydrogel might also indicate favorable fatigue resistance of the D-hydrogel at the same conditions. Furthermore, ten successive loading−unloading cycles were used on the D-hydrogel, which was aimed to allow the hydrogel to undergo enough fatigue. Figure 8c,d shows the ten successive loading−unloading cycles of the as-prepared D-hydrogel sample and recovered D-hydrogel sample. It was clearly seen that after the first ten successive stretching loading−unloading cycles the tensile strength and hysteresis loop could recover effectively and demonstrated only a little worse performance for the second ten successive loading−unloading cycles after 24 h at room temperature, which also indicated the good selfrecoverability and fatigue resistance of D-hydrogel. The above results should be ascribed to the fact that the ionic coordinates could serve as special dynamic junction points; when the external loading was applied, the strong ionic coordination bonds acted as the reversible sacrificial bonds and cracked to dissipate energies. When the external loading was removed, the temporarily dissociated ionic coordinate bonds could be rapidly reconstructed without any external stimuli at room temperature.
4. CONCLUSION In this study, we developed a novel design strategy to prepare a new type of dual physically linked DPC hydrogel. The G
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Macromolecules
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engineered hydrogels had acrylic components into a clay-crosslinked physical network with a homogeneous structure, on which the secondary cross-link points were formed by ionic coordination interactions between Fe3+ and −COO− groups of polymer side chains. The introduction of the ionic coordination interactions could not only effectively dissipate energy and hence obviously increase the mechanical properties but also endow good self-recoverability via reversible network reconstruction. The mechanical property of DPC hydrogel could be readily tuned in an extensive range for different applications by changing the clay concentration, molar ratio of Ac/Am, Am concentration, the concentration of ferric solution, and the immerse time in ferric solution. At the optimal formulation, the DPC hydrogel possessed high tensile strength, large elongation, and remarkable toughness. Furthermore, because of its unique physically reversible network structures, the DPC hydrogel could sufficiently reconstruct its network structures, resulting in good self-recovery from damages without any external stimuli. This work opened up a new method for development of softand-tough materials with highly integrated mechanical properties and could be effectively used to generate different tough DPC hydrogels by replacing clay with other nanoparticle crosslinkers such as graphene oxide, carbon dot, and cellulose nanocrystal.
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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.6b00584. Characterization of m-hydrogels, d-hydrogels, and Dhydrogels (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (W.Z.). *E-mail:
[email protected] (C.W.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge financial support from NSFC (21474032), 973 Program (2012CB821500), the China Postdoctoral Science Foundation (2015M580720), and the Fundamental Research Funds for the Central Universities (2015ZM158).
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DOI: 10.1021/acs.macromol.6b00584 Macromolecules XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.macromol.6b00584 Macromolecules XXXX, XXX, XXX−XXX
