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Improvement of Mechanical Strength and Fatigue Resistance of Double Network Hydrogels by Ionic Coordination Interactions Qiang Chen, Xiaoqiang Yan, Lin Zhu, Hong Chen, Bing Jiang, Dandan Wei, Lina Huang, Jia Yang, Baozhong Liu, and Jie Zheng Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b01920 • Publication Date (Web): 31 Jul 2016 Downloaded from http://pubs.acs.org on August 4, 2016

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Chemistry of Materials

Improvement of Mechanical Strength and Fatigue Resistance of Double Network Hydrogels by Ionic Coordination Interactions Qiang Chen1*, Xiaoqiang Yan1, Lin Zhu1, Hong Chen2, Bing Jiang1, Dandan Wei1, Lina Huang1, Jia Yang1, Baozhong Liu1, and Jie Zheng2* 1

School of Materials Science and Engineering Henan Polytechnic University, Jiaozuo, China, 454003

2

Department of Chemical and Biomolecular Engineering The University of Akron, Akron, Ohio, USA, 44325 * Corresponding author: [email protected] and [email protected]

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Abstract Double network hydrogels (DN gels) are considered as one of the toughest soft materials. However, conventional chemically-linked DN gels is often lack of high self-recovery and fatigue resistance properties due to permanent damage of covalent bonds upon deformation. Current strategies to improve self-recovery and fatigue resistance properties of tough DN gels mainly focus on the manipulation of the first network structure. In this work, we proposed a new design strategy to synthesize a new type of Agar/PAMAAc-Fe3+ DN gels, consisting of an agar gel as the first physical network and a PAMAAc-Fe3+ gel as the second chemical-physical network. By introducing Fe3+ ions into the second network to form strong coordination interactions, at optimal conditions, Agar/PAMAAc-Fe3+ DN gels can achieve extremely high mechanical properties (σf of ~8 MPa, E of ~8.8 MPa, and W of ~16.7 MJ/m3), fast self-recovery (~50% toughness recovery after 1 min resting), and good fatigue resistance against properties cyclic loadings by simply controlling AAc content in the second network. The high toughness and fast recovery of Agar/PAMAAc-Fe3+ DN gel is mainly attributed to energy dissipation through reversible noncovalent bonds in both networks (i.e. hydrogen bonds in the agar network and Fe3+ coordination interactions in the PAMAAc network). The time-dependent recovery of Agar/PAMAAc-Fe3+ gels at room temperature and the absence of recovery in Agar/PAMAAc gels also confirm the important role of Fe3+ coordination interactions in mechanical strength, self-recovery, and fatigue resistance of DN gels. Different mechanistic models were proposed to elucidate the mechanical behaviors of different agar-based DN gels. Our results offer a new design strategy to improve strength, self-recovery, and fatigue resistance of DN gels by controlling the structures and interactions in the second network. We hope that this work will provide alterative view for the design of tough hydrogels with desirable properties.


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1. Introduction Polymer hydrogels as soft-wet materials have been widely used in biomedical, pharmaceutical, and industrial applications, including scaffolds for tissue engineering1, biomaterials for wound healing2, delivery carriers for drugs and gene3, and substitutes for soft tissues4. However, most of polymer hydrogels are weak or brittle, and they often fracture at a low compression and tensile stress of 800 g cm-2 and melting point of 85~90oC) and Fe(NO3)3.9H2O were purchased from Sigma-Aldrich Inc., 2-hydroxy-4’-(2-hydoxyethoxy)-2-methylpropiophenone (Irgacure 2959) and acrylamide (AAm, 98%) were purchased from TCI Shanghai Inc. Acrylic acid, N,N′-methylenebis(acrylamide) (MBA) and other metal ions salts were purchased from Aladdin (Shanghai) Inc. 2.2 Preparation of Agar/PAMAAc-Fe3+ DN gels Agar/PAMAAc-Fe3+ DN gels were synthesized by two step process. Firstly, Agar/PAMAAc DN gels were prepared by a one-pot method as reported in our previous work37. Briefly, for synthesis DN gels with 5 mol% AAc, agar (140 mg), AAm (1.9936 g), AAc (101 uL, density of 1.06 g/mL), MBA (68 uL, 20mg/mL aqueous solution), Irgacure 2959 (0.0662 g, 1 mol% of total monomer) and water (7 mL) were added into a reactor, and the reactor was sealed under N2 protection after three degassing cycles and then gradually heated up to 95 oC in an oil bath for about 10 min to dissolve all the reactants in the aqueous solution. The resulting solution was injected into two glass slide with 1 mm spacer, and followed by cooling the solution at 4 oC for 30 min to form an agar gel first network. The photopolymerization reaction was carried out to form an Agar/PAMAAc DN gel under UV light (λ=365 nm wavelength, intensity of 8 W) for 1 h. Secondly, Agar/PAMAAc-Fe3+ DN gel was obtained by soaking Agar/PAMAAc DN gel in a Fe3+ ions solution with proper concentration for 3 days. PAMAAc-Fe3+ gel was also prepared by the same process except for without agar added. While our one-pot method is different from the conventional two-or-multiple-step methods for DN hydrogels, the one-pot method still follows the sequential 5 ACS Paragon Plus Environment


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polymerization steps to form two networks independently, i.e. the physical agar network was formed firstly by the heating-cooling process, during which the precursor components for the second network (PAMAAc, crosslinker, and Fe ions) will be filled and be distributed into the first, agar network. Then, upon photo-polymerization, the PAMAAc network was formed. So, in terms of the order of the network formation, the agar network is considered as the first network and PAMAAc network as the second network. More importantly, our agar-based hybrid hydrogels exhibited many structural and toughening characteristics similar to DN hydrogels, including (i) the first agar network being rigid, brittle and tightly crosslinked and the second PAMAAc network being soft, ductile and loosely crosslinked and (ii) the first agar network being used as sacrificial bonds to dissipate energy and to protect the second network. 2.3 Mechanical test Tensile tests. Uniaxial tensile tests of dumbbell-shaped gels (length of 25 mm, width of 4 mm and thickness of 1 mm) were carried out using a universal tensile tester equipped with a 100 N load cell with a crosshead speed of 100 mm min-1. For hysteresis measurement, gel specimens were first stretched to a maximum extension ratio λ1 and then unloaded. After returning to the original length, the specimens were reloaded and stretched to an increased extension ratio λ2 at 100 mm/min as the first loading and unloaded again. The dissipated energy (Uhys) was estimated by area below the stress-strain curves or between the loading-unloading curves. For successive loading-unloading tests, the loading-unloading operations were repeatedly conducted on the same specimen with increased λ3, λ4, ..., λn until the specimen failed at a elongation break. λmax denotes the extension ratio where the specimen experienced. The elastic modulus (EDN) at each λmax was calculated in the range of linear relationship between stress and strain. Tearing tests. Tearing testing was performed using commercial test machine with a 100 N load cell. The gel samples were cut into a trousers shape (80 mm in length, 10mm in width, and 1 mm in thickness) with an initial notch of 20 mm. The two arms of the samples were clamped, in which the one arm was fixed, while the other one was pulled at 50 mm/min. The tearing energy (T) is defined as the work required to tear a unit area37, as estimated by T = 2 Fave w

(1)

where Fave is the average force of peak values during steady-state tear, and w is the width of the specimen. Statistical analysis was performed by the one-way analysis of variance (ANOVA) and a LSD’s multiple comparison post-test with 95% confidence interval among the applied treatments (p< 0.05). 3. Results and Discussion 3.1. Formation of Agar/PAMAAc-Fe3+ DN gels Figure 1 illustrates a two-step synthesis process for Agar/PAMAAc-Fe3+ DN gels: 6 ACS Paragon Plus Environment

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(1) Agar/PAMAAc DN gels, containing of physical linked agar gel as first network and chemical linked copolymer gel of acrylamide (AM) and acrylic acid (AAc) as second network, were firstly synthesized by the one-pot method as reported in our previous work38 and (2) Agar/PAMAAc DN gel was then immersed into Fe3+ ions solution at room temperature for 3 days, which allows Fe3+ ions to diffuse into the gel networks, to strongly interact with carboxyl groups of PAMAAc via coordination interactions, and eventually to form Agar/PAMAAc-Fe3+ DN gels. Once formed, Agar/PAMAAc-Fe3+ DN gels displayed a red color (see Figure 2), which is different from transparent Agar/PAMAAc DN gel, demonstrating successful incorporation of Fe3+ ions into Agar/PAMAAc-Fe3+ DN gels. In the absence of Fe3+, Agar/PAMAAc DN gel was similar to Agar/PAM DN gel. Differently, introduction of Fe3+ into Agar/PAMAAc DN gel allows to form additional crosslinks between Fe3+ ions and carboxyl groups of the second network. The second network of PAM in the conventional DN gels is soft and ductile, which usually can not bear large stress as the first network fractures. Thus, the Fe3+ coordination interactions with PAMAAc network will offer additional capacity for sustaining large stress and improving fatigue resistance of DN gels.

Figure 1. Synthesis scheme of Agar/PAMAAc-Fe3+ DN gels. 3.2. Mechanical properties of Agar/PAMAAc-Fe3+ DN gels As shown in Figure 2, Agar/PAMAAc-Fe3+ DN gels exhibited excellent tensile properties. The gels can be easily stretched (Fig. 2a) and twisted/stretched (Fig. 2b) to approximately 4 times its original length. When the two gel strips crossed over each other, they both can be stretched to ~5 times their original lengths without breaking (Fig. 2c). The gels can also sustain a large load of 500 g (Fig. 2d) at the stress of ~1.11 MPa. The physically linked networks could be easily extended and contributed to the 7 ACS Paragon Plus Environment


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stretchability of DN gels. Moreover, Agar/PAMAAc-Fe3+ DN gels can be readily adapted to different complex shapes with fine structures, including lobster, inkfish and conch (Fig. 2e-g), demonstrating their free-shapeable property.

Figure 2. Agar/PAMAAc-Fe3+ DN gels is highly tough and flexible to withstand different high-level deformations of (a) stretching, (b) twisted stretching, (c) cross-over stretching, (d) sustaining of a 500 g at the stress of 1.11 MPa. The gels are also free shapeable into different shapes of (e) lobster; (f) inkfish and (g) conch. All DN gels were made with 5 mol% AAc. Figure 3a shows typical stress-strain curves of Agar/PAMAAc-Fe3+ DN gel, Agar/PAMAAc DN gel, and PAMAAc-Fe3+ SN gel, prepared at 5 mol% of AAc:(AAc+AAm). Compared to PAM SN gel in our previous work (σf of 0.265 MPa, εf of 18.22 mm/mm, E of 7 kPa, and W of 1.8 MJ/m3), PAMAAc-Fe3+ SN gel exhibited much stronger mechanical properties (σf of 0.67 MPa, εf of 8.03 mm/mm, E of 37 kPa, and W of 2.02 MJ/m3), indicating that the coordination interactions between Fe3+ and carboxyl acid indeed enhance the mechanical properties of hydrogels. When agar network was introduced, Agar/PAMAAc DN gel was superior to PAMAAc-Fe3+ SN gel, and demonstrated its ultrahigh stretchability and toughness in tensile tests (σf of 0.45 MPa, εf of 21.24 mm/mm, E of 149 kPa, and W of 4.36 MJ/m3). As expected, when Fe3+ were introduced to have coordination interactions with the second network, Agar/PAMAAc-Fe3+ DN gel significantly improved its σf to 1.55 MPa, εf to 14.1 mm/mm, E to 267 kPa, and W to 9.13 MJ/m3. As compared to Agar/PAMAAc DN gel and PAMAAc-Fe3+ gel, Agar/PAMAAc-Fe3+ DN gel increased its σf by 3.4 and 2.3 times, respectively, confirming the enhanced coordination interactions on the mechanical strength of DN gels. On the other hand, in Figure 3b, tearing energies (T) 8 ACS Paragon Plus Environment

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of Agar/PAMAAc-Fe3+ DN gel, Agar/PAMAAc DN gel, and PAMAAc-Fe3+ gel were 894±102, 1359±329 and 349±34 J/m2, respectively. There was no statistical difference in toughness between Agar/PAMAAc-Fe3+ DN gel and Agar/PAMAAc DN gel. However, statistical difference was observed between Agar/PAMAAc DN gel and PAMAAc-Fe3+ gel, indicating that the formation of double networks is more effective for improving toughness than the formation of coordination bonds between PAMAAc network and Fe3+ ions. It is interesting to observe that the tearing energies of Agar/PAMAAc-Fe3+ DN gel were smaller than those of Agar/PAMAAc DN gel, which could be interpreted by two possible reasons. First, under tearing deformation, the electrostatic coordination interactions in the Agar/PAMAAc-Fe3+ networks may not function as important as reversible scarified bonds of agar and PAMAAc against crack propagation. Another possible interpretation is that after immersing Agar/PAMAAc DN gel into Fe3+ ion solution, the additional cross-linking in the PAMAAc network makes the second network be over crosslinked, leading to decrease of tearing energy. Gong and co-workers23, 39 also found that tearing energies of PAMPS/PAAm DN gel decreased sharply when the cross-linker density of the second network was higher than the optimum value of 0.01 mol%. T of 894 J/m2 for Agar/PAMAAc-Fe3+ gels was still within a range of 102-103 J/m2 for cartilage, rubber, and conventional DN gels. Furthermore, we also compared mechanical properties of three different agar-based DN gels developed in our lab. Agar/PAMAAc DN gel exhibited relatively weaker mechanical properties than Agar/PAM DN gel, this probably because AAc monomers reduce the cross-linking density of hydrogen bonds in the agar network. Both PAMAAc-Fe3+ and HPAAm gels exhibited similar toughness (T=350 J/m2), but PAMAAc-Fe3+ gel (σf=0.67 MPa) exhibited 10 times higher strength than HPAAm gel (σf=0.069 MPa), this indicates that PAMAAc-Fe3+ gel possess tough and strong characteristic as the second network, in sharp contrast to soft and ductile HPAAm network in Agar/HPAAm DN gels. 2.0

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Figure 3. (a) Tensile stress-strain curves and (b) tearing energy of Agar/PAMAAc-Fe3+ DN gel, Agar/PAMAAc DN gel, and PAMAAc-Fe3+ gel. All gels were tested at the as-prepared state without swelling equilibrium in ions or water solution. Experimental conditions: agar concentration of 20 mg/mL, total monomer concentration (Ctotal) of 9 ACS Paragon Plus Environment


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30% w/v, AAc of 5 mol%, MBA of 0.03 mol%, and UV-I2 of 1 mol% of total monomer, and Fe3+ ions of 0.02 mol/L. Figure 4a showed that the tensile properties of Agar/PAMAAc-Fe3+ DN gels were greatly influenced by AAc content. Fracture stress (σf) significantly and monotonically increased from 0.306 to 8.04 MPa as AAc content increased from 0 to 20 mol% (Figure 4b). Using σf of 1 MPa as a cutoff value for high mechanical strength, it can be seen that adding of a very small amount of 3 mol% AAc into the second network enables to achieve high tensile strength of 0.99 MPa. Elastic modulus (E) slightly increased from 168 to 429 kPa as AAc contents increased from 0 to 10 mol%. However, elastic modulus significantly increased to 1.623 MPa at 15 mol% of AAc and 8.83 MPa at 20 mol% of AAc (Figure 4c). Consistently, deformation energy (W) also significantly increased from 1.92 MJ/m3 at 0 mol% of AAc to 16.56 MJ/m3 at 20 mol% of AAc (Figure 4d). However, different from the increase trend of σf, E, and W as AAc content, fracture strain (εf) retained almost unchanged at low concentrations of AAc (i.e. εf=13~14 mm/mm at 0-5 mol%), but decreased sharply from 14.1 to 2.86 mm/mm at AAc of 5-20 mol% (Figure 4d). Overall, at optimal conditions, Agar/PAMAAc-Fe3+ DN gels can achieve extremely high mechanical properties (σf of ~8 MPa, E of ~8.8 MPa, and W of ~16.7 MJ/m3) by simply controlling AAc content in the second network. So, unless otherwise stated, we will use Agar/PAMAAc-Fe3+ DN gel prepared at agar of 20 mg/mL, Ctotal of 30% w/v, AAc of 5 mol%, MBA of 0.03 mol%, UV-I2 of 1 mol%, and Fe3+ ions of 0.02 mol/L for later discussion. At this condition, 5 mol% of AAc was selected because (a) Agar/PAMAAc-Fe3+ DN gel prepared at 5 mol% of AAc already showed high mechanical strength of 1.55 MPa, but much better extensibility than other DN gels prepared at other higher AAc contents. (b) PAMAAc-Fe3+ SN gel at 5 mol% of AAc exhibited similar tearing energy to HPAAm SN gel (T~350 J/m2), but much higher strength than HPAAm SN gel (10 times), which will help to make a fair comparison for the second network effect on the mechanical properties of DN gels. In Figure 4a, we have demonstrated that by adding agar network, Agar/PAMAAc-Fe3+ DN gels exhibit much better mechanical properties than PAMAAc-Fe3+ gels. Meanwhile, it is also interesting to compare our Agar/PAMAAc-Fe3+ DN gels with PAMAAc-Fe3+ gels independently reported by Lin et al40, both of which were prepared at similar conditions (i.e. 15 mol % AAc). PAMAAc-Fe3+ gels exhibited E of ~250 kPa, σf of ~3.75 MPa, and W of ~7.5 MJ/m3 (namely d-hydrogel-0.15, Figure S6 in their work), in comparison with our Agar/PAMAAc-Fe3+ DN gels with E of 1623 kPa, σf of 4.43 MPa, and W of 15.73 MJ/m3. This comparison further confirms that the superior mechanical properties of Agar/PAMAAc-Fe3+ DN gels is attributed to its unique double network structure, in which agar addition allows to form enormous hydrogen bonds and chain entanglement between agar and PAMAAc, leading to a continuous and strong network.


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Figure 4. (a) Tensile stress-strain, (b) fracture stress, (c) elastic modulus, and (d) fracture strain and deformation energy of Agar/PAMAAc-Fe3+ DN gels as a function of AAc content. Experimental conditions: agar concentration of 20 mg/mL, total monomer concentration (Ctotal) of 30% w/v, AAc of 5 mol%, MBA of 0.03 mol%, and UV-I2 of 1 mol% of total monomer, and Fe3+ ions of 0.02 mol/L. We further investigated the effects of Fe3+ concentrations and other cation ions on tensile properties of Agar/PAMAAc-Fe3+ DN gels (Figure S1, S2). It was found that too high or too low Fe3+ concentrations led to unbalanced mechanical properties of Agar/PAMAAc-Fe3+ DN gels, so there existed an optimal Fe3+ concentration of 0.02 mol/L for Agar/PAMAAc-Fe3+ DN gel to achieve the balanced high strength (1.55 MPa) and high extensibility (14.1 mm/mm) (Figure S1). Furthermore, other cation ions (Al3+, Ca2+, Mg2+, and Zn2+) were also used to examine whether they could realize similar cation-enhanced mechanical properties of DN gels via coordination interactions with the second network. Figure S2 showed that upon introduction of different cations (Zn2+, Mg2+, Ca2+, Al3+, and Fe3+) into Agar/PAMAAc DN gels, no obvious mechanical changes in elastic modulus, fracture strength and fracture strain were observed as compared to Agar/PAMAAc DN gels. This may suggest that these cations do not form strong coordination interactions with the second network that determines the mechanical behavior of the hydrogels. Since there are many peaks overlapped in FTIR spectroscopy between polyacrylamide (PAM) and poly(acrylic acid) (PAAc), to demonstrate the formation of PAAc-Fe3+ bonds, we first synthesized PAAc gel and then immersed the PAAc gel into Fe3+ solution to form PAAc-Fe3+ gel. Both PAAc and 11 ACS Paragon Plus Environment


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PAAc-Fe3+ gels were frozen-dried for FTIR tests. Comparison of FTIR data (Figure S3) from both gels showed that PAAc gel displayed a characteristic peak at ~1712 cm-1 corresponding to vibrations of –COO- group. Upon the association of Fe3+ with PAAc, the absorption band of –COO- group was obviously shifted to 1721 cm-1, while other peaks remained almost unchanged. This indicates the formation of coordination interactions between Fe3+ and PAAc chains. In parallel, Li et al. 41 have used FTIR to study how Fe3+ and COO- are coordinated using the oxidized konjac glucomannan (OKGM) microspheres. They found that the asymmetric vibration of COO- in the OKGM appeared at 1608.4 cm-1 in the absence of Fe3+, but after Fe3+ treatment, this peak was split into two peaks at 1737.6 and 1627.7 cm-1 indicating the formation of two modes coordination between Fe3+ and COO- . We also examined the effects of swelling on the structural and mechanical properties of Agar/PAMAAc-Fe3+ DN gels. Figure S4a showed that Agar/PAMAAc-Fe3+ DN gel (18.9 mm) had a larger size than Agar/PAMAAc DN gel (14.0 mm). After swelling in water for three days, Agar/PAMAAc-Fe3+ DN gel showed a little shrinkage of the size (17.5 mm). After immersing the Agar/PAMAAc-Fe3+ gels into water for three days, Agar/PAMAAc-Fe3+ DN gel slightly shrank its size from 18.9 nm to 17.5 mm. We also compared the tensile and tearing properties of Agar/PAMAAc-Fe3+ DN gel before and after swelling in water. After swelling in water for three days, Agar/PAMAAc-Fe3+ DN gel showed E of 77 kPa, σf of 0.80 MPa, εf of 8.42 mm/mm, and W of 2.86 MJ/m3, and all of these mechanical properties were weaker than those of Agar/PAMAAc-Fe3+ DN gel before swelling (E of 267 kPa, σf of 1.55 MPa, εf of 14.1 mm/mm, and W of 9.13 MJ/ m3). (Fig. S4b) The tearing energies of Agar/PAMAAc-Fe3+ DN gel also decreased from 894±102 to 460±27 J/m2 after swelling (Fig. S4c). Figure S5 further shows SEM images of swollen Agar/PAMAAc and Agar/PAMAAc-Fe3+ DN gels at different length scales. It can be seen that Agar/PAMAAc DN gels without Fe3+ exhibited small pores of 2~4 µm. However, Agar/PAMAAc-Fe3+ DN gels exhibited unique contrasting network structures with the co-existence of both large and small pores ranging from 300-500 nm to 3-5 µm.. 3.3. Energy dissipation of Agar/PAMAAc-Fe3+ DN gels Figure 5a showed the loading-unloading curves of Agar/PAMAAc-Fe3+ DN gel, Agar/PAMAAc DN gel, and PAMAAc-Fe3+ gel. All three gels displayed distinct hysteresis loops, indicating that energy dissipation occurs for all gels upon deformation. Among them, Agar/PAMAAc-Fe3+ DN gel showed the largest hysteresis loop and dissipated the highest energy at λ=6 (Uhys of 721.8 kJ/m3), while both Agar/PAMAAc DN gel and PAMAAc-Fe3+ gel had much lower hysteresis energies (i.e. loop area) of 252.7 kJ/m3 and 199.8 kJ/m3, respectively (Figure 5b). Furthermore, cyclic loading tests were performed on different fresh Agar/PAMAAc-Fe3+ DN gels at a series of different maximum strains (λmax). Evidently, loop areas increased with λmax (Figure 5c), i.e. hysteresis energies increased from 6.43 kJ/m3 at λmax =1.5 to 2343.5 kJ/m3 at λmax =10 (Figure 5d). Clearly, there was residual strain after unloading. The high toughness of Agar/PAMAAc-Fe3+ DN gel was attributed to large energy dissipation through a 12 ACS Paragon Plus Environment

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combination effect of (i) pulling out agar chains in the first network and (ii) unzipping of coordination interactions between Fe3+ ions and carboxyl acid groups in the second network. Instead, only one of energy dissipation mechanisms involves in Agar/PAMAAc DN gel or PAMAAc-Fe3+ gel.

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Figure 5. (a) Loading-unloading curves and (b) the corresponding dissipated energies of Agar/PAMAAc-Fe3+ DN gel, Agar/PAMAAc DN gel, and PAMAAc-Fe3+ gel. (c) Cyclic loading curves and (b) the corresponding dissipated energies of Agar/PAMAAc-Fe3+ DN gel at different λmax. Successive loading-unloading tests were also performed to investigate the energy dissipation process of Agar/PAMAAc-Fe3+ DN gels (Figure 6a). During loading-unloading tests, no resting time was given between any two consecutive loading cycle. So, the extension ratio of each cycle is calculated using the initial length of gel specimen from the first loading-unloading cycle. It can be seen that ∆Uhys increased as λmax, and the reloading curve of a DN gel was close but not completely overlapped to the previous unloading curve, indicating that while the original loading had induced network changes, the partial recovery of reversible agar and ionic crosslinks was achieved. As shown in Figure 6b, elastic modulus (E) of Agar/PAMAAc-Fe3+ DN gel decreased from 267 kPa at λmax=1 to 24 kPa at λmax=13. The E-λmax curves showed two characteristic regions: (i) a rapid decrease of E as λmax< 4 and (ii) a slow decrease of E as λmax> 4. To better quantify the internal fracture behavior of DN gels, we defined a fracture parameter of softness by the percent of the loss of initial elastic modulus ((1-Eλmax/E0)×100)), where Eλmax and E0 are elastic modulus at 13 ACS Paragon Plus Environment


λmax and initial modulus, respectively. Consistently, softness of DN gels showed a rapid increase to 74% at λmax =4, followed by a slow increase to 91% at λmax =13. The final softness value of 91% indicates that almost 91% of elastically effective chains are fractured at its failure. Similar behaviors were also observed in our Agar/PAM DN gels with ~87% softness and Agar/HPAAm DN gels with ~79% softness. 280

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Figure 6. (a) Successive loading-unloading curves of the same Agar/PAMAAc-Fe3+ DN gel at different λmax. No resting time is applied between any two consecutive loadings. (b) Elastic modulus and softness of Agar/PAMAAc-Fe3+ DN gel at different λmax. 3.4. Self-recovery and fatigue resistance of Agar/PAMAAc-Fe3+ DN gels Since both networks are the physically-linked via reversible non-covalent interactions, the broken noncovalent bonds could be recovered under appropriate conditions. Here, we used cyclic loading tests to examine the extent of self-recovery on Agar/PAMAAc-Fe3+ DN gels at different resting times. In Figure 7a, as a control, when no resting time was applied between two consecutive loadings, hysteresis energy (Uhys) was significantly lost from the first loading (721.8 kJ/m3) to the immediate second loading (148.6 kJ/m3). But, when the resting time increased, hysteresis loops became larger and energy dissipation increased as well. Quantitatively, Figure 7b shows toughness recovery rate, defined as the ratio of dissipated energy at different recovery times to that of the first loading cycle at λmax =6, as a function of resting time. Toughness recovery rates showed that Agar/PAMAAc-Fe3+ DN gel can rapidly recover its toughness to ~48% and ~69% after 1-3 min. and 5-7 min. resting, respectively. After 20 min. resting, the gels can recover ~95% of toughness. Comparison of Agar/PAMAAc-Fe3+ DN gel with Agar/HPAAm DN gels reveals that Agar/HPAAm DN gel takes much longer time (120 min.) to recover ~66% of toughness, demonstrating that Agar/PAMAAc-Fe3+ DN gel has the better self-recovery capacity than Agar/HPAAm DN gel.


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Figure 7. (a) Cyclic loading curves and (b) toughness recovery rate of Agar/PAMAAc-Fe3+ DN gel at different resting times. In parallel, since Agar/PAMAAc-Fe3+ DN gels exhibit the fast and efficient recovery of the mechanical properties, they are expected to have good fatigue resistance as well. Figure 8a shows six cyclic loadings on the same gel specimen with 5 min. resting time between cyclic loadings. It can be seen that hysteresis loops changed their sizes between cycles 1 and 2 and thereafter hysteresis loops retained almost unchanged during cycles 2–6. These data indicate that the gels experience some structural changes during the first loading, but after the second loadings the gels were able to retain approximately networks structures, and the constant high hysteresis energy upon cyclic loadings demonstrates the excellent fatigue resistance.



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Figure 8. (a) Six cyclic loading curves of Agar/PAMAAc-Fe3+ DN gel, with 5 min. resting between two consecutive loadings. (b) Dissipated energies of Agar/PAMAAc-Fe3+ DN gel for each cyclic loading, in comparison with those of Agar/PAMAAc DN gel and PAMAAc-Fe3+ gel in the first cyclic loading. Dissipated energy (Uhys) of Agar/PAMAAc-Fe3+ DN gels for each cyclic loading was shown in Figure 8b, in comparison with Uhys from the first loading of Agar/PAMAAc DN gel and PAMAAc-Fe3+ gel. For Agar/PAMAAc-Fe3+ DN gels, Uhys in cycles 2-6 was 500~660 kJ/m3, smaller than 721.8 kJ/m3 in the first loading, indicating that some rearrangement in the network structure during the first cycle could experience permanent changes, thus the network or cross-linking density can not be fully recovered at room temperature. It was also found that Agar/PAMAAc-Fe3+ DN gels can not recover immediately without the resting, resulting in very small energy dissipation in the second loading (148.6 kJ/m3). This further confirms that the recovery of the first-loading-induced network fracture is not instantaneous. More interestingly, Agar/PAMAAc DN gel without Fe3+ was found to dissipate less energy (253 kJ/m3) in the first cycle, similar to another Agar/PAM DN gels that show almost negligible recovery at the temperature below agar Tm of 90 oC. Comparison of Uhys between Agar/PAMAAc DN gel and Agar/PAMAAc-Fe3+ DN gels indicates that Fe3+ cross-links in the second network indeed contribute to fatigue resistance of Agar/PAMAAc-Fe3+ DN gels. Taken together, Agar/PAMAAc-Fe3+ DN gel exhibited 16 ACS Paragon Plus Environment

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not only highly mechanical strength and toughness, but also excellent self-recovery and fatigue resistance properties at room temperature without any external stimuli. Collective data also confirm that the high toughness of Agar/PAMAAc-Fe3+ DN gel is attributed to energy dissipation through the unloading of network strands in both physically-linked networks with a large load/unload hysteresis, and the introduction of strong ionic coordination interactions in the second network will synergistically enhance the mechanical strength and fatigue resistance of DN gels. 3.5. Mechanistic toughening models for different DN gels Fundamental understanding of energy dissipation and toughening mechanisms of different types of DN hydrogels is critical important to design next generation high strength hydrogels with desirable properties. The current knowledge of toughening mechanisms mainly stems from pure chemically linked DN gels. It is well known that the chain scission of covalent bonds in the first network is considered as a main strategy to effectively dissipate energy and to create strong and tough chemically-linked DN hydrogels. However, the fracture of the chemically-linked first network (i.e. permanent breaking of covalent bonds under loading) also causes irreversible softening phenomenon and poor fatigue resistance. To overcome these limits, different types of noncovalent bonds including ionic interactions, hydrogen bonding, or hydrophobic association are introduced into hydrogel networks, so that the broken noncovalent bonds could be reformed under appropriate conditions, leading to the recovery of mechanical properties of DN hydrogels. Different from chemically-linked DN gels, we synthesized hybrid Agar/PAM DN gels that exhibit different yielding and necking behaviors, including much lower yielding stress/strain, no flat necking platform, and simultaneous necking, suggesting a new “chain-pulling-out” fracture mechanism. During the deformation process, the agar chains in the first network progressively pull out from the aggregated agar helical bundles. Such “chain-pulling-out” and disassociation behaviors of agars neither break the first network nor change agar helical conformation, thus the agar network retains its continuous phase. This continuous facture process causes hybrid Agar/PAM gels to show velocity-dependent fracture behaviors and toughness, which is fundamentally different from the nearly velocity-independent mechanical properties in chemically-linked PAMPS/PAM gels. Similar chain pulling-out behavior was also observed in ionic/covalent Ca2+-alginate/PAM DN gels42. In these hybrid DN gels, the physically-linked first network can be reformed during a time-interval after the loading or under temperature stimuli, leading to the self-recovery of internal damage. It should be noted that for both chemically-linked PAMPS/PAM and hybrid-linked Agar/PAM gels (regardless of physically or chemically linked), large hysteresis and high toughness stems from the fracture of the first network, resulting in effective energy dissipation. Recently, we synthesized fully physically-linked Agar/HPAAm DN gel and proposed new the fracture and recovery mechanisms to describe its mechanical properties. For fully physically-linked Agar/HPAAm gels, 17 ACS Paragon Plus Environment


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upon deformation, both networks participated in energy dissipation via chain dissociations, resulting in a large hysteresis of DN gels. More importantly, the self-recovery and self-healing properties of Agar/HPAAm DN gel at room temperature origins from the second HPAAm network via reversible, noncovalent, hydrophobic interactions between SDS micelles and alkyl groups of SMA side chains. The temporarily dissociated noncovalent bonds in the second HPAAm network can be rapidly reconstructed at room temperature without any external stimuli, leading to the fast self-recovery of hydrogel’s stiffness (~60%) and toughness (40~50%) within several minutes and ~40% of healing efficiency. Different from all DN gels described above, herein we synthesized a new type of Agar/PAMAAc-Fe3+ DN gels with a unique network structure, i.e. the first agar network is still physically linked via hydrogen bonds, while the second PAMAAc-Fe3+ network is hybrid crosslinked by coordination interactions and covalent bonds. A mechanistic model was proposed to interpret the energy dissipation and mechanical recovery of Agar/PAMAAc-Fe3+ DN gels (Figure 9). Upon deformation, noncovalent bonds in both networks (i.e. hydrogen bonds in the agar network and Fe3+ coordination interactions in the PAMAAc network) are expected to be fractured and participated in energy dissipation via the dissociation and/or reorganization of physical cross-links, resulting in high toughness and large recovery of DN gels. More importantly, several lines of evidences have proved that introduction of Fe3+ ions into the second network can indeed improve both mechanical and recovery properties of Agar/PAMAAc-Fe3+. (1) Comparison of Agar/PAMAAc-Fe3+ and Agar/PAMAAc DN gels also reveals that when Fe3+ ions are introduced into Agar/PAMAAc gel, the resulting Agar/PAMAAc-Fe3+ DN gel significantly improved mechanical properties by 2-3 times, confirming the enhanced coordination interactions on the mechanical strength of DN gels (Figure 3a). (2) Agar/PAMAAc-Fe3+ DN gel exhibits much larger hysteresis loop and dissipates more energies than Agar/PAMAAc DN gel, indicating that ionic–covalent hybrid network is the main contributor to energy dissipation and toughness (Figure 5a and 5b). (3) The time-dependent recovery of Agar/PAMAAc-Fe3+ gels at room temperature and the absence of recovery in Agar/PAMAAc gels can be interpreted as the gradual reformation of Fe3+ coordination interactions for the second network (Figure 7), simply because the agar network can not be self-recovered at room temperature. It is also fundamentally important to compare Agar/PAMAAc-Fe3+ gels and Agar/HPAAm gels, both of which contain noncovalent bonds in both networks. Both gels share some common structural and mechanical characteristics. (1) Both gels show large hysteresis, excellent self-recovery, and fatigue resistance properties at room temperature; (2) Both gels possess strong physical interactions in the second network; (3) The two networks in both gels participate in energy dissipation and network reconstruction/self-recovery. More importantly, both gels also show some large differences in structural and mechanical properties. (1) Agar/PAMAAc-Fe3+ gel has hybrid networks combining the first physical network and the second hybrid 18 ACS Paragon Plus Environment

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chemical-physical network, while Agar/HPAAm DN gel has fully physical networks. (2) Agar/PAMAAc-Fe3+ DN gels (1~8 MPa) exhibit much higher tensile strength than Agar/HPAAm DN gels (0.1~1 MPa) at optimal conditions. (3) Agar/HPAAm DN gel has self-healing property, while Agar/PAMAAc-Fe3+ gel do not. These differences appear to be derived from the second network in both gels. The hybrid PAMAAc-Fe3+ network is strong and tough, while the physical HPAAm network is tough but weak. Therefore, Agar/PAMAAc-Fe3+ DN gels can bear the larger stress to have the better mechanical strength than Agar/HPAAm DN gels. The fast and significant recovery of Agar/PAMAAc-Fe3+ DN gels (50~90%) after tensile loading and unloading suggests different toughening mechanisms from Agar/HPAAm DN gels (40-60%). Such high recovery rate also indicates that the first agar network in Agar/PAMAAc-Fe3+ DN gels has a relative low degree of internal fracture during deformation. It is likely that PAMAAc-Fe3+ network is more easily fractured than agar network. In addition, from the preparation method point of view, the introduction of ionic coordination interactions in the second network is much easier than the introduction of hydrophobic associated interactions in the second network, because ionic coordination interactions can be easily realized by the copolymerization of AAc monomers with other monomers, followed by different ion treatments. This preparation method could be generally applied to different types of DN gels regardless of their network structures. It is also interesting to observe that our Agar/PAMAAc-Fe3+ gels exhibit large hysteretic loops but low tear energies. Generally speaking, tough DN hydrogels with high tearing energies (T of 102~104 J/m2) often possess effective energy dissipation. For example, PAMPS/PAAm DN gels43 (T of 102~103 J/m2) and Ca2+-Alginate/PAAm hybrid gels28 (T of ~104 J/m2) all exhibited large hysteresis loops at each loading-unloading cycle. Therefore, large hysteresis is often considered as an indicator for high toughness (or tearing energy) of hydrogels. However, tetra-PEG hydrogels, which have nearly homogeneous network structure and no hysteresis loop during loading-unloading cycle, showed high tensile strength (~27 Ma)44 but low tearing energies (

Improvement of Mechanical Strength and Fatigue ... - ACS Publications

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