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Cite This: Chem. Mater. 2018, 30, 1743−1754
General Strategy To Fabricate Strong and Tough Low-MolecularWeight Gelator-Based Supramolecular Hydrogels with Double Network Structure Feng Chen, Qiang Chen,*,† Lin Zhu,† Ziqing Tang,† Qingfeng Li,‡ Gang Qin,† Jia Yang,† Yanxian Zhang,§ Baiping Ren,§ and Jie Zheng*,§ †
School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo 454003, China The Key Laboratory of Rare Earth Functional Materials and Applications, Zhoukou Normal University, Zhoukou 466001, China § Department of Chemical and Biomolecular Engineering, The University of Akron, Akron, Ohio 44325, United States ‡
S Supporting Information *
ABSTRACT: Low-molecular-weight gelator (LMWG)-based supramolecular hydrogels, self-assembled by small molecules via noncovalent interactions, have recently attracted great attention due to their unique structure−property relationship and potential applications spanning from functional materials to biomedical devices. Unfortunately, many LMWG-based supramolecular hydrogels are mechanically weak and can not even be handled by conventional tensile and tearing tests. Here, we propose several design principles to fabricate new LMWG-based hydrogels with a true double-network structure (G4· K+/PDMAAm DN gels), consisting of the supramolecular self-assembly of guanosine, B(OH)3 and KOH as the first, physical G4·K+ network and the covalently cross-linked poly(N,N′-dimethyacrylamide) (PDMAAm) as the second, chemical network. Different from those LMWG-based supramolecular hydrogels, G4·K+/PDMAAm DN gels exhibit high tensile properties (elastic modulus = 0.307 MPa, tensile stress = 0.273 MPa, tensile strain = 17.62 mm/mm, and work of extension = 3.23 MJ/m3) and high toughness (tearing energies = 1640 J/m2). Meanwhile, the dynamic, noncovalent bonds in the G4·K+ network can reorganize and reform after being broken, resulting in rapid self-recovery property and excellent fatigue resistance. The stiffness/ toughness of G4·K+/PDMAAm DN gels can be recovered by 65%/58% with 1 min resting at room temperature, and the recovery rates are further improved with the increase of temperatures and resting times. Interestingly, G4·K+/PDMAAm DN gels also exhibit UV-triggered luminescence due to the unique G4-quartet structure in the G4·K+ supramolecular first network. A new toughening mechanism is proposed to interpret the high strength and toughness of G4·K+/PDMAAm DN gels. We believe that our design principles, along with new G4·K+/PDMAAm DN gel system, will provide a new viewpoint for realizing the tough and strong LMWG-based gels. linked hydrogels,11−14 hydrogen bonding or dipole−dipole enhanced hydrogels,15,16 macromolecular microparticle composite hydrogels,17 tetra-PEG gels,18 and double-network hydrogels.19 Among them, double network hydrogels (DN gels) have exhibited excellent mechanical properties of strength (tensile
1. INTRODUCTION Development of mechanically strong and multifunctional hydrogels is critical for a variety of applications, including tissue scaffolds,1,2 superabsorbents,3 drug delivery carriers,4 and agriculture and food chemistry.5−7 However, conventional hydrogels are often very weak or brittle, and they usually break at a tensile stress of 30% for GG/PAAm DN gel. Beyond RT, G4·K+/PDMAAm DN gel achieves 67% and 69% of toughness recovery after 5 min resting at 80 and 100 °C, respectively (Figure 9d), as compared to 74% recovery for Ca2+-Alg/PAAm gel at 80 °C for 1 day resting and 40% recovery for agar/PAAm DN gel at 100 °C for 5 min resting. Taken together, our G4·K+/PDMAAm DN gels are superior to LMWG-based hybrid DN gels in terms of mechanical properties and comparable to other typical hybrid DN gels in terms of both mechanical and self-recovery properties.
PDMAAm DN gels were not further improved even after 60 min resting at room temperature. This suggests that supramolecular self-assembly is a very fast and efficient process for the self-recovery of the G4·K+ network, and within a very short time, the network is repaired to a saturate level. Due to the reversible sol−gel nature of G4·K+ supramolecular gel (Figure 1c), we tested the temperaturedependent self-recovery of G4·K+/PDMAAm DN gels at two higher temperatures of 80 and 100 °C. As shown in Figure 9c,d, both stiffness and toughness recovery increased as resting times and temperatures. Consistently, a two-stage recovery was observed at both elevated temperatures, i.e., the gels underwent a rapid mechanical recovery before 10 min, followed by a slow recovery after that. Within 10 min, stiffness/toughness can be recovered to 72%/68% and 98%/76% at 80 and 100 °C, respectively. Upon resting for 60 min at 100 °C, the stiffness/ toughness recovery can finally reach 100% and 85%, respectively. Moreover, G4·K+/PDMAAm DN gels also showed the λmax-dependent self-recovery property, i.e., both stiffness and toughness recovery decreased as λmax increased (Tables S9 and S10). Specifically, at λmax = 2, after 5 min recovery at room temperature the stiffness/toughness recovery was 65%/85%, respectively, while at λmax = 10, both values were reduced to 51% and 42%, respectively. To test the fatigue resistance of G4·K+/PDMAAm DN gels at room temperature, Figure 10a shows 11 cyclic loadings on the same gel specimen with 5 min recovery between two consecutive cyclic loadings. G4·K+/PDMAAm DN gels exhibited a large hysteresis loop at the first loading cycle and a smaller hysteresis loop at the second loading cycle. However, after second loading cycles, all hysteresis loops obtained from the third to 11th loading cycles remained almost the same sizes as the second loop. As shown in Figure 10b and Table S11, at the first loading cycle, E and Uhys were 307 kPa and 256 kJ/m,3 respectively. Nevertheless, during the second to 11th loading cycles, E and Uhys remained almost unchanged at ∼166 kPa and ∼113 kJ/m3, which were much larger than E of 29 kPa and Uhys of 70 kJ/m3 at the immediate second loading without any resting. The cyclic loading results indicate that our G4·K+/ PDMAAm DN gels not only exhibit high mechanical strength and toughness, but also demonstrate rapid self-recovery and excellent fatigue resistance properties. 2.5. Toughening Mechanism of G4·K+/PDMAAm DN Gels. It is important to understand the toughening mechanism of G4·K+/PDMAAm DN gels. G4·K+/PDMAAm DN gels belong to the LMWG-based gel family, in which guanosine (G) assisted by B(OH)3 and KOH can self-assemble into G4·K+ supramolecular structure as the first physical network. Herein, we performed a systematic comparison between our G4·K+/ 1751
DOI: 10.1021/acs.chemmater.8b00063 Chem. Mater. 2018, 30, 1743−1754
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Chemistry of Materials The high strength and toughness of G4·K+/PDMAAm DN gels are likely attributed to the sequential fracture of the first G4·K+ physical network and the second PDMAAm chemical network, during which the first G4·K+ physical network serves as “sacrificial bonds” to effectively dissipate energies and to protect the second PDMAAm network from crack propagation. This hypothesis can be demonstrated by a significant difference in hysteresis loop between G4·K+/PDMAAm DN gel (Uhys of 256 kJ/m3) and PDMAAm SN gel (Uhys of 68 kJ/m3) (Figure 7a,b), indicating that the first G4·K+ supramolecule network indeed serves as reversible “sacrificial bonds” to effectively dissipate energies during deformation. The G4·K+ supramolecule network is formed by the hierarchical self-assembly of G, B(OH)3, and KOH, where the G4-quartets, formed by four GB diesters, are stacked on top of each other to generate extended G4-fibers, which ultimately entangle and interact with each other via hydrogen bonding interactions to form a gel network. So, it is interesting to find that the G4·K + supramolecule network is similar to the agar network, both of which are formed by aggregated fibrils. Based on this network structural similarity, here we propose a “chain-pulling-out” hypothesis to describe the fracture process of G4·K+ supramolecule network in G4·K + /PDMAAm DN gels. To demonstrate this hypothesis, G4-fibers must maintain their stability during deformation. XRD data in Figure 11 showed
G4·K+/PDMAAm DN gels; (b) physical snapping of the stacked G4-quartets from G4-fibers by breaking π−π interactions and hydrogen bonding interactions. Both processes contribute to energy dissipation, however, the former one can be rapidly recovered due to the dynamic nature of a supramolecule network, while the latter one is permanently damaged and can not be recovered. The snapping of G4-fibers may not dissipate as much energy as the chain pulling-out mode because the stiffness and toughness recovery of our G4· K+/PDMAAm DN gels at λ = 6 can reach to 100 and 85% at 100 °C for 60 min, respectively. Also, the snapping of G4-fibers may increase as strain. As shown in Table S10, at RT, the toughness recovery decreases from 85 to 42% as λmax increased from 2 to 10, consistent with the XRD results (Figure 11).
3. CONCLUSIONS While use of supramolecular assembly to fabricate hydrogels is a great promising idea, almost all LMWG-based supramolecular hydrogels are extremely mechanically weak, can not bear compressive, tensile, or tearing tests, and only survive under rheological tests with storage modulus (G′) of 102−105 Pa. To overcome these limits, we propose a new fabrication strategy to prepare novel LMWG-based hybrid gels with a true DN structure, consisting of a self-assembly supramolecular network (G4·K+ gel) as the first physical network and a chemically cross-linked PDMAAm as the second network. G4·K+/ PDMAAm DN gel possesses excellent mechanical properties (σf of 0.273 MPa, εf of 17.62 mm/mm, E of 0.307 MPa, W of 3.234 MJ/m3, and tearing energies of 1640 J/m2), which are much stronger than those of G4·K+ SN gel (too weak to be tested by tensile and tearing experiments) and PDMAAm SN gel (σcom of 6.7 MPa, σf of 0.091 MPa, εf of 9.29 mm/mm, E of 0.053 MPa, and W of 0.54 MJ/m3). G4·K+/PDMAAm DN gels also exhibit fast self-recovery properties. E and Uhys can be rapidly recovered by 65% and 58% after 1 min resting at room temperature, and self-recovery rates increase with resting time and elevated temperature, in which E and Uhys are restored to 100% and 85% after 60 min resting at 100 °C, respectively. Moreover, E and Uhys remain almost unchanged during ten successive loading−unloading cycles with 5 min resting between cycles, demonstrating excellent fatigue resistance for G4·K+/PDMAAm DN gels. Interestingly, G4·K+/PDMAAm DN gels also show UV-activated luminescence due to the unique G4-quartets structure in G4·K+ supramolecular first network. Taken together, G4·K+/PDMAAm DN gels outperform LMWG-based hybrid DN gels in terms of mechanical, self-recovery, and fatigue resistance properties. We believe that our design strategy for G4·K+/PDMAAm DN gels will provide a new avenue for the development of mechanically tough and multifunctional LMWG-based hydrogels.
Figure 11. XRD curves of G4·K+ SN gel and G4·K+/PDMAAm DN gels at different stains.
that upon freeze-drying G4·K+/PDMAAm DN gels at various strains, all gel specimens displayed the two common characteristic peaks of 4.5° and 26.8° corresponding to G4-quartets, and the peaks did not shift as the strain increased. This indicates that the π−π stacking distance between two planar G-quartet motifs remains very stable and unchanged, i.e., G4-fibers do not experience lengthening during deformation. However, the intensity of the peak of 26.8° became weaker as the stain increased from 0 to 5 mm/mm, suggesting that the stacking structure of G4-quartets in G4-fibers is partially destroyed to some extent, explaining that the toughness of G4·K+ supramolecule network can not be recovered completely even when repairing the gel at 100 °C for 60 min (max. Rtoughness = 85%). XRD results also suggest that the fracture of G4·K + supramolecule network involves two processes: (a) physical debonding of the hydrogen bond interactions by “pulling-out” G4 fibers from G4·K+ supramolecule network. Such “chain pulling-out” mechanism without chain scission also interprets the continuous fracture of the first G4·K+ network. The “pulling-out” G4 fibers are accompanied with a large amount of energy dissipation, mainly contributing to the high toughness of
4. EXPERIMENTAL SECTION 4.1. Materials. Guanosine (G, 98%), KOH (GR, 95%), boric acid (AR), and N,N′-methylenebisacrylamide (MBA) were purchased from Aladdin (Shanghai), Inc. 2-hydroxy-4′-(2-hydoxyethoxy)-2-methylpropiophenone (Irgacure 2959) and N,N′-dimethylacrylamide (DMAAm) were purchased from TCI Shanghai, Inc. All chemicals and solvents purchased were of the highest available purity, and unless otherwise stated, they were used as received. 4.2. Preparation of G4·K+/PDMAAm DN Gels. G4·K+/ PDMAAm hybrid DN gels were synthesized by a one-pot method as reported in our previous work with modification.32 The composition of DN gels was referred to as Gx-Dy-KBz-MBAh: where x is guanosine concentration (mol/L); y is a mass ratio of DMAAm to 1752
DOI: 10.1021/acs.chemmater.8b00063 Chem. Mater. 2018, 30, 1743−1754
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Chemistry of Materials H2O (wt%); z is a molar ratio of guanosine to B(OH)3 (mol:mol); h is a molar ratio of MBA to DMAAm (mol %). For example, G0.6-D50KB1:0.5-MBA0.03 meant G was 0.6 mol/L, DMAAm was 50 wt % to DI water, B(OH)3 was 0.5 molar ratio of G, and MBA was 0.03 mol % to DMAAm. Briefly, for synthesis of G0.6-D50-KB1:0.5-MBA0.03, 0.8497 g of G, 0.0927 g of B(OH)3, 5 g of DMAAm, 0.1121 g of Irgacure 2959 (1 mol % of DMAAm), 115 μL of MBA (20 mg/mL aqueous solution, 0.03 mol % of DMAAm), and 5 mL of DI water were added into a reactor, and the reactor was gradually heated up to 90 °C in an oil bath for about 2 min to dissolve all the reactants and form GB borate diesters, and then 210 μL of KOH (400 mg/mL aqueous solution) was added to the solution and heated 4 min. The resulting solution was sealed under N2 protection after three degassing cycles and could be easily injected into a plastic mold (D = 8.5 mm), followed by cooling the solution at room temperature for 30 min to form the G4· K+ first supramolecular network. Subsequently, the plastic mold was placed under a UV lamp (wavelength of 365 nm and intensity of 8 W/ cm2) for 2 h. After polymerization, G4·K+/PDMAAm hybrid DN gels were successfully prepared by this simple and facile process. The PDMAAm SN gels were also synthesized by the same process except for without G, B(OH)3, and KOH added. All the gels were stored at 4 °C. Before mechanical testing, the gel specimens were allowed to stay at room temperature for about 30 min. 4.3. Mechanical Tests. The mechanical properties of the G4·K+/ PDMAAm DN gels were tested by a commercial test machine with a 100 N load cell. The tensile tests of the G4·K+/PDMAAm DN gels (diameter = 8.5 mm and length = 60 mm) were pulled up by the load cell at a constant velocity of 100 mm/min at room temperature. For the tearing testing, the G4·K+/PDMAAm DN gels were cut into a trouser shape (40 mm in length, 10 mm in width, and 2 mm in thickness) with an initial notch of 20 mm. The two arms of the specimens were clamped, in which one of arms was fixed, while the other one was pulled at 50 mm min −1. The tearing energy (T) is defined as the work required to tear a unit area, as estimated by27 T=
Jie Zheng: 0000-0003-1547-3612 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Q.C. is grateful for financial support, in part, from National Nature Science Foundation of China (21504022), the Joint Fund for Fostering Talents of NSFC-Henan Province (U1304516), Henan Province (NSFRF1605, 2016GGJS-039 and 17HASTIT006), and Henan Polytechnic University (72105/001 and 672517/005). J.Z. thanks financial support from NSF (DMR-1607475) and in part from NSF (CBET1510099).
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2Fav w
where Fav is the average force of peak values during steady-state tear and w is the width of the specimen. For cycle test, the G4·K+/ PDMAAm DN gels were first loaded to λ = 6 with a crosshead speed of 100 mm min−1 and unloaded with the same speed. The dissipated energies (Uhys) were estimated by the area between loading− unloading curves. 4.4. Characterizations. G4·K+/PDMAAm DN gels were frozendried for XRD measurements. X-ray diffraction (XRD) patterns were recorded on a Smart-Lab XRD with Cu−Kα radiation over a 2θ range of 3−90° with a scanning rate of 5°/min. For luminescence spectroscopic analysis, spectra were recorded on a FLS920 luminescence spectrophotometer (Edinburgh Instruments, EI) to excite G4·K+/PDMAAm DN gels, and luminescence emission spectra were collected from 335 to 600 nm.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b00063. Effects of different network components, molar ratios of cross-linker/network monomers on mechanical properties, hysteresis tests, and self-recovery tests (PDF)
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DOI: 10.1021/acs.chemmater.8b00063 Chem. Mater. 2018, 30, 1743−1754
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