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Unconventional Tough Double-Network Hydrogels with Rapid Mechanical Recovery, Self-healing and Self-gluing Properties Haiyan Jia, Zhangjun Huang, Zhaofu Fei, Paul J. Dyson, Zhen Zheng, and Xinling Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11241 • Publication Date (Web): 26 Oct 2016 Downloaded from http://pubs.acs.org on October 27, 2016

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ACS Applied Materials & Interfaces

Unconventional Tough Double-Network Hydrogels with Rapid Mechanical Recovery, Self-healing and Self-gluing Properties Haiyan Jia, Zhangjun Huang, Zhaofu Fei, Paul J. Dyson, Zhen Zheng and Xinling Wang* H. Jia, Dr. Z. Huang, Prof. Z. Zheng, Prof. X. Wang School of Chemistry and Chemical Engineering, State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Dongchuan Road No. 800 Shanghai 200240, China E-mail: [email protected] Prof. P. J. Dyson, Dr. Z. Fei Institut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015, Lausanne, Switzerland.

KEYWORDS: Polymers, Materials, Hydrogels, Double-networks, Mechanical recovery, Self-healing, Self-gluing, Shape memory.

ABSTRACT: Hydrogels are polymeric materials which have a relatively high capacity for holding water. Recently, a double network (DN) technique was developed to fabricate hydrogels with a toughness comparable to rubber. The mechanical properties of DN hydrogels may be attributed to the brittle sacrificial bonding network of one hydrogel, facilitating stress dispersion, combined with ductile polymer chains of a second hydrogel. Herein, we report a novel class of tuneable DN hydrogels composed of a polyurethane (PU) hydrogel and a stronger, dipole-dipole and H-bonding interaction reinforced (DHIR) hydrogel. Compared to 1

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conventional DN hydrogels, these materials show remarkable improvements in mechanical recovery, modulus and yielding, with excellent self-healing and self-gluing properties. In addition, the new DN hydrogels exhibit excellent tensile and compression strength and possess shape memory properties, which make them promising for applications in engineering, biomedicine and other domains where load-bearing is required.

1. Introduction

There is considerable interest in tough hydrogels with good mechanical properties, i.e. tensile and compression strengths in the order of MPa,1-10 and fracture energies in the kJ·m-2 range.11,12 These materials are essential in various load-bearing applications, such as soft robotics,11,13,14 structural biomaterials,15,16 sensors17,18 and smart actuators,19,20 where properties including strength, fatigue resistance, stiffness, toughness as well as self-healing are necessary.1,21-23 To date, a variety of tough hydrogels have been developed, such as tetra-polyethylene glycol (PEG) hydrogels,24-25 slide-ring hydrogels,26 macro-molecular microsphere composite hydrogels,27,2 sacrificial lamellar bilayer hydrogels,28 hydrophobic modified hydrogels,30 and double network (DN) hydrogels.6,8,30

DN hydrogels are composed of a rigid and brittle heterogeneous polyelectrolyte network integrated with a soft and ductile relatively high molecular mass polymer network, and have greatly improved the performance of hydrogels.1,9 For instance, DN hydrogels prepared from poly2-acrylamido-2-methyl-1-propanesulfonic acid/polyacrylamide (pAMPS/pAAm) have extremely high tensile strengths (1 – 10 MPa at 1000 – 2000 %) with compression strengths of 2.2 – 17.2 MPa.1,9 A further advance was made with the introduction of DN hydrogels 2


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formed from oppositely charged polyelectrolytes. These materials not only show tough mechanical performance (Young’s modulusmax = 7.9 MPa, fracture stressmax = 5.1 MPa), but also have the ability to rebuild at room temperature.2 DN hydrogels based on a Ca2+-alginate-pAAm hybrid structure may be elongated by 2000%, have a fracture energy of ca. 9000 J·m-2 and a mechanical recovery of 74% at 80 °C over a 24 hour period.11 Another interesting system comprises the thermo-recoverable DN hydrogels prepared from Agar/pAAm, which displays 90% recovery in elastic modulus at 100 °C within 10 minutes.8

DN hydrogels are ideally suited to biomedical applications as the compressive fracture stress of some systems is close to that of cartilage. Nevertheless, such applications require the material to withstand consecutive, high level loading-unloading processes in a short time together with self-healing properties following injury. Currently, most DN hydrogels have a low fatigue resistance as they contain a stress-dissipating sacrificial bond network.9,31-32 Nevertheless, it has been proposed that DN hydrogels could be combined with self-healing materials,33-36 e.g. by replacing the sacrificial bond network with reversible bond networks. However, maintaining strength with reversible bonds is challenging and the absence of sacrificial bonds and energy dissipating mechanisms in the second network could also lead to poor self-healing properties and weaken mechanical recovery. Indeed, tough hydrogels with the ability to undergo rapid mechanical recovery at room temperature, with simultaneous yielding free, high modulus (>1 MPa) and self-healing properties, are rare.26

To have all the criteria required for ideal biomaterials in a single hydrogel material, we prepared DN hydrogels composed of a strong first network and an even stronger second

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network. Specifically, the first network comprises a polyurethane (PU) hydrogel with excellent strength and elongation properties together with a 5-armed crosslinker and soft segments with high molecular weight (PEG 20,000) to further enhance the mechanical properties. The second network is composed of a dipole-dipole and hydrogen-bonding reinforced (DHIR) hydrogel with excellent mechanical energy dissipation properties. Herein, we report on the preparation of these new DN hydrogels, their rapid mechanical self-recovery at room temperature, modulus and yielding properties, which appear to be superior to other related materials and, importantly, their temperature or solvent induced self-healing and self-gluing properties, and excellent tensile, compression and shape memory properties. We believe these DN PU/DHIR hydrogels have considerable potential as load-bearing materials and might lead to new applications for hydrogel materials.

2. Results and Discussion

The route used to prepare the DN PU/DHIR hydrogels is illustrated in Scheme 1. In the first step dried PU gels (first network) are immersed in a DMSO solution containing the second network monomers, i.e. acrylonitrile (AN), AAm, PEGDA575 (polyethylene glycol diacrylate, Mn=575), 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) and Iragure2959 at 50 °C for 3 hours, during which time the PU gels become swollen (see Experimental for full details and Figure.S1 for FTIR characterization). Next, the monomers absorbed within the PU gels are polymerized with UV irradiation to form the second network (see Scheme S1 for the molecular structures of the two networks). Following dialysis in water, hydrophobic AN-AN dipole pairing and hydrogen bonds in the resulting pAAm are presumably formed which

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reinforce the PU/DHIR hydrogels. The DN hydrogels are termed PU/DHIR-α-β, where PU denotes the first network, DHIR the second network, α denotes the PEGDA575 content in mol%, with respect to the monomers of second network, and β denotes the concentration of the second network monomer solution in wt%. Based on the work of Zhang et al.,7 the formulation of the second network was optimised with: AN 65%, AAm 30%, AMPS 5% with various concentrations of PEGDA575. A series of PU/DHIR-α-β hydrogels were prepared with α ranging from 0 – 0.300 mol% and β ranging from 0 – 20 wt% (see Tables 1 and S2), which exhibit different structural and mechanical properties. Here, because the common swelling and diffusion process is difficult to control the exact molar ratio of the two networks and also causes a large amount of waste for unreacted second-network monomers8, dried PU hydrogels were applied to soak in the second network monomers to avoid these problems. The drying process has little influence to the performance of the hydrogel, due to polyurethane is a property stable elastomer, as well as PEG is a flexible polymer.

Scheme 1. Preparation and reversible non-covalent bonds of the PU/DHIR hydrogels. 5



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The extent of the dipole-dipole and hydrogen bonding interactions within the hydrogels can be controlled by applying a force, varying the temperature or changing the solvent. The presence of the 2-acrylamido-2-methyl-1-propanesulfonic acid (APMS) and acrylamide (AAm) units increases the electrostatic interactions between the second networks. Both components initially react to form ammonium salts (see Figure.S2).37 Once the salts are co-polymerized, the resulting polymer is zwitterionic, allowing the electrostatic interactions to come into play,38 although due to the relatively low amount of AMPS employed, these interactions are negligible.

Based on the tensile and compression strength and storage modulus of the PU/DHIR-0.300-β hydrogels, where β = 0, 5, 10, 20%, (Figure.S3 and Table S1), the optimal monomer concentration for the second network in the DN system was found to be 20 wt%. Consequently, a series of PU/DHIR-α-20 hydrogels were prepared with α varying from 0 to 0.300 mol% and their mechanical properties characterised, including compression strength at 85%, tensile strength, elongation at break, Young’s modulus, fracture energy and equilibrium water content (EWC), see Table 1.

The PU/DHIR-α-20 hydrogels exhibit extraordinary mechanical properties (Table 1 and Figure. 1). The fracture compression strength of the PU/DHIR-α-20 hydrogels exceeded the limit of the electromechanical tester (10000 N ≈ 270 MPa* 36 mm2). For example, the compression strength of the PU/DHIR-0.244-20 hydrogel at 85% strain is 160 MPa, approximately 5 times the value of the single network DHIR-0.244-20 hydrogel (33 MPa) and 130 times that of the single network PU hydrogel (1.2 MPa), see Figure.1a. Moreover, during

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the compression process no water appears to be extruded from the PU/DHIR-α-20 and PU hydrogels, whereas some water is extruded from DHIR hydrogels, presumably due to the low density of crosslinks of the latter. The PU/DHIR-α-20 hydrogels also exhibit excellent fracture tensile strength, the highest value of 4.79 MPa at 815% strain corresponding to PU/DHIR-0.244-20, considerably stronger than that of the single network DHIR-0.244-20 (0.664 MPa at 239%) and PU (0.080 MPa at break) hydrogels (Figure.1b). Thus, the PU/DHIR-α-20 hydrogels have a mechanical strength in the range of human ligament (~13 MPa),39 muscle (~1 MPa),40 cartilage (~20 MPa),41,42 and skin (~5 MPa).43 Table 1. Mechanical properties of the PU/DHIR-α-20% hydrogels (α = 0 – 0.300 mol%). Tensile Elongation Young’s Fracture EWCa/% PEGDA575 Compression content/mol%

strength at 85%

strength/MPa

at break/%

modulus/MPa

energy/J·m-2

strain/MPa

0

25.5 ± 5.7

1.52 ± 0.57

516 ± 56

0.30 ± 0.11

649 ± 39

89.16 ± 0.38

0.075

51.1 ± 9.4

2.68 ± 0.40

567 ± 49

0.72 ± 0.39

1465 ± 121

87.97 ± 0.29

0.150

68.6 ± 28.6

3.42 ± 0.41

677 ± 35

0.78 ± 0.53

2040 ± 136

85.42 ± 0.87

0.188

133.4 ± 30.5

4.22 ± 0.67

762 ± 67

2.02 ± 0.61

2154 ± 147

82.77 ± 0.33

0.244

159.8 ± 29.8

4.79 ± 0.55

815 ± 59

2.14 ± 0.43

2493 ± 138

82.35 ± 0.26

0.300

167.4 ± 46.1

3.53 ± 0.63

470 ± 58

2.54 ± 0.71

995 ± 109

78.29 ± 0.15

a

EWC = equilibrium water content.

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Figure 1. Mechanical properties of the DN PU/DHIR-α-20 hydrogels in comparison to DHIR-0.244-20 and PU single network hydrogels. (a) compression and (b) tensile stress–strain curves of the PU, DHIR-0.244-20 and PU/DHIR-0.244-20 hydrogels. Loading-unloading curves of the PU/DHIR-0.150-20 (c) and PU (d) hydrogels without resting times.

The PU/DHIR hydrogels present excellent fatigue resistance and rapid and complete self-recovery properties, which are critical parameters for load-bearing applications. As an example, the successive loading-unloading curves of the PU/DHIR-0.15-20 hydrogel without resting times are shown in Figure.1c. A distinct hysteresis loop is observed for the first loading-unloading cycle of PU/DHIR-0.15-20, indicating that the PU/DHIR hydrogels are able to dissipate energy effectively by changing their network structure. In addition, no yielding phenomenon can be observed, suggesting there is no breakage of the covalent bonds in the first network, which is distinct from traditional DN hydrogels.8,27 Furthermore, the hysteresis loops of the subsequent loading-unloading cycles are significantly smaller than that of the first. The hysteresis loop and tensile strength at 300% for the following sequential cycles are approximately constant. After the first cycle there appears to be less physical interaction fracture required to cause energy dissipation. Nevertheless, the hydrogel 8


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self-recovers rapidly at room temperature, presumably due to the short recovery time of the non-covalent interactions. In contrast, the PU hydrogel displays over-lapped and statistically insignificant hysteresis loops in continuous loading-unloading cycles, suggesting that an energy dissipation mechanism does not take place (Figure.1d). The elongation of the DHIR-0.244-20 hydrogel is insufficient (245%) for the anti-fatigue test at ~300% strain (Figure.1b).

Physical (non-covalent) crosslinks are known to contribute to plasticity whereas covalent crosslinks result in elasticity,1,7,44 and consequently, an optimum balance between the two is needed to fabricate high-performance DN hydrogels. This balance can be achieved by varying the crosslinking density of the second hydrogel network and the ratio of the second network with respect to the first network. The mechanical properties of the PU/DHIR-α-20 hydrogels varies with the concentration of the PEGDA575 crosslinker (see Table 1 and Figure.S4). As the concentration of the crosslinker is increases, the Young’s modulus and EWC increase, whereas the tensile strength, elongation at break, facture energy initially increase and then decrease. The PU/DHIR-0.244-20 hydrogel was found to have the optimal comprehensive mechanical properties in the range tested, with the highest tensile strength, elongation at break and fracture energy of all the materials tested.

Compression strengths at 85% of the PU/DHIR-α-20 hydrogels are much higher than their corresponding tensile strengths, suggesting their compactness leads to the outstanding stiffness of the materials, with the crosslinker content contributing significantly to the enhanced stiffness. Indeed, the crosslinking density of the second network is known to impact

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on the mechanical properties of DN hydrogels.9 The fracture energy of DN hydrogels has been shown to decrease as the second network crosslinking density increases.9,

40

This

observation has been attributed, at least in part, to inhomogeneity of the network caused by the increased crosslinking density. Our results are in agreement with this suggestion (Figure.S5) and the crosslinking density is optimum when the PEGDA575 crosslinker content, α, is 0.244 mol%, i.e. the PU/DHIR-0.244-20 hydrogel exhibits the most comprehensive mechanical strength.

Cyclic tensile studies were undertaken to verify the energy dissipating properties of the PU/DHIR-α-20 hydrogels resulting from the reversible sacrificial dipole-dipole and H-bonding interactions. The successive loading-unloading curves of the PU/DHIR-α-20 hydrogels, where α equals 0, 0.150 and 0.244 mol% at 300%, strain after successive time intervals, are shown in Figure. 2a-c. The noticeable hysteresis loops are indicative of energy dissipation via the sacrificial bonds. Figure.2d-f shows the relationship between resting time and the hysteresis ratio and stress loss. Here, the hysteresis ratio corresponds to the area ratio of the hysteresis loops relative to the first cycle (note that the stress loss is the lost tensile stress of the following cycles at 300% strain relative to the initial data set).

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Figure 2. Cyclic tensile studies of the PU/DHIR-α-20 hydrogels. Top: loading-unloading curves of PU/DHIR-α-20 hydrogels at different recovery times, α = 0 (a), 0.150 (b) and 0.244 (c) mol%, respectively. Bottom: effect of resting time dependence of hydrogels on hysteresis (blue) and stress (red) loss, α = 0 (d), 0.150 (e) and 0.244 (f) mol%, respectively.

The hysteresis loops of the PU/DHIR-0-20 hydrogel (Figure.2a) show complete recovery to the original state after 10 minutes at room temperature. As the crosslinker content in second network increases, the time required for complete recovery becomes longer (Figure.2b-c). Specifically, the PU/DHIR-0.150-20 hydrogel requires 40 minutes to completely recover and the PU/DHIR-0.244-20 hydrogel requires almost 6 hours for full recovery. PU/DHIR-α-20 hydrogels show fast-recovery, compared to the self-recoverable hydrogels reported up to now. For example, the oppositely charged polyelectrolytes hydrogels need about 120 min for complete self-recovery, besides, there is significant residual strain after each loading;5 Agar/PAM DN hydrogels show 65% recovery of energy loss and 90% recovery of elastic modulus at 100 ºC after 10 min, but almost no recovery at 50 ºC;8 The work on reloading of 11



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alginate-polyacrylamide hydrogels were recovered to 74% after stored at 80 ºC for 1 day.11 The extended recovery time may be explained by the increase in AN-AN and H-bonding interactions in the DHIR hydrogels, due to the increase in the chemical crosslinking density.7 In contrast, the stress loss of the PU/DHIR-0-20 hydrogel at each cycle was found to be negligible and recoverable, suggesting that the covalent bonds are maintained during hysteresis. Besides, due to the water evaporation during the 6 hours of rest, density of the hydrogels increased, resulting in the stress level higher than the first loading, which is a common phenomenon during the long time recovery test.45 Comparing the first cycles of these three hydrogels (Figure.S7), the data further suggest that increasing the chemical crosslinking density can promote the formation of dipole-dipole and H-bonding interactions.

Figure 3. First, immediate second, third and fourth loading curves of the PU/DHIR-0-20 hydrogel at large deformation. Stretching conventional DN hydrogels, such as pAMPS-pAAm,9,46 to a large deformation (>300 % strain), often leads to the rupture of the first network to dissipate energy. When stretched again, the loss in mechanical strength of a hydrogel is then observed. Figure. 3 shows the first to fourth loading curves for the PU/DHIR-0-20 hydrogel at large deformation. 12


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On the first loading, the hydrogel fractures abruptly without any yielding or necking phenomenon, and the relevant fracture elongation is smaller than that of PU hydrogels (Figure.1b). During the second stretch, no loss of strength was observed, in spite of a small energy loss (strain < 300%) resulting from the breakage of dipole-dipole and H-bond interactions. A plateau region is observed in the second and third loading curves at large deformations (strain > 400%). On the fourth loading, a plateau region was not observed and an apparent strength loss takes place. Based on these observations it would appear that the fracture point of the PU/DHIR hydrogel mainly depends on the fracture of the DHIR network as the fracture elongation during the first loading is smaller than that of PU hydrogels. Moreover, the reinforcing mechanism of the DN PU/DHIR hydrogels seems different to conventional DN hydrogels. Soft and ductile PU networks, unlike rigid and brittle polymers, help to increase the fracture elongation of the DHIR materials to overcome the yielding phenomenon of the DN hydrogels, enhancing their load-bearing properties.

To further establish the sensitivity of the dipole-dipole and H-bonding interactions to temperature, solvent and loading, the variation in storage modulus (E’) and loss modulus (E’’) of the PU/DHIR-α-20 hydrogels were determined. A temperature range of 15 °C to 70 °C was used to perform the dynamic mechanical analysis (DMA), see Figure. 4. The values of the storage modulus (E’) and loss modulus (E’’) of the PU/DHIR hydrogels were found to decrease as the temperature is raised from 10 to 70 °C. Within the temperature range of 15 – 60 °C, the E’ values of the PU/DHIR-α-20 (α = 0, 0.112, 0.150, 0.244 and 0.300) hydrogels decrease to 1/4, 1/11, 1/25 and 1/40 of the initial values, suggesting that the physical crosslinking is weakened due to the temperature-induced rupture of the dipole-dipole and 13



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H-bonding interactions. As the second network crosslinker content is reduced, the E” values of the respective hydrogels at 15 °C decrease from 0.668 to 0.006 MPa. Modulus loss of hydrogels is usually attributed to weak and reversible physical interactions,21,1,38 in agreement with the observation that the second network crosslinking density facilitates the dipole-dipole and H-bonding interactions in the range of 0 – 0.300 mol%.

Figure 4. (a) Storage modulus (E’) and (b) loss modulus (E’’) of the PU/DHIR-α-20 hydrogels (α = 0, 0.112, 0.150, 0.244 and 0.300) from 15 to 70 °C.

The interesting mechanical properties and the reversible dipole-dipole and H-bonding interactions of the DN hydrogels further encouraged us to investigate their self-healing and self-gluing properties using dialysis or temperature-change methods. Self-healing polymers are considered a key smart technology47 and a large number of self-healing polymers have been reported.47,48 The PU/DHIR-0.150-20 hydrogel was cut into two pieces and dyed with red and blue (Figure. 5a-b). Subsequently, the cut surfaces were dipped into dimethylsulfoxide (DMSO), to completely exchange the water by DMSO, and the cut surfaces of the two pieces were pressed together and immersed in water. After 12 hours the hydrogel showed almost the same modulus as the original sample, including the high tensile 14


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stress of 1.5 MPa and large deformation of 220% (Figure.5c-e). The mechanism of self-healing may be attributed to the large number of AN-AN dipole pairs and H-bonds in the gels able to repair the cut surface. The healing process is initiated when the DN hydrogels are immersed in DMSO, which weakens these bonds as water is replaced by DMSO solvent. As the adjacent surfaces are immersed in water, the DMSO is displaced, regenerating the strong non-covalent interactions. Since a process employing DMSO is not suitable for some applications, self-healing was also induced through heating-cooling processes. To demonstrate the affects, the cut surfaces of PU/DHIR-0.150-20 hydrogel were pressed together in water at 50 °C for 3 hours, before cooling to 10 °C in water (Figure S9), to afford a healed hydrogel. Since the contact limitation between the cut surfaces through this heating method, the self-healed tensile strength is about 1.2 MPa (Figure. S9d), a little smaller than that obtained from immersion in DMSO and water successively. Here, the self-healing may be attributed to the sensitivity of the dipole-dipole and H-bonding interactions to temperature.

Figure 5. Various tests demonstrating the strength of the self-healing and self-gluing 15



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PU-DHIR-0.150-20 gels. Top: photographs of (a) two pieces of representative PU-DHIR-0.150-20 DMSO gels dyed with pink and blue colours, (b) following self-healing before testing with load and tensile stress. (c) A weight of 0.5 kg suspended from the self-healed strip sample and (d) tensile tests. (e) Stress-strain curves of the original, self-healed and self-glued material. Bottom: photographs showing that fragments (f) of the PU-DHIR-0.150-20% hydrogel can be self-glued into different shapes, such as star (g) and bear (h) shapes. (i) Load testing involving a weight of 0.5 kg suspended from the self-glued bear-like hydrogel demonstrating its strength.

For the self-gluing experiments, fragments of the PU/DHIR-0.150-20 hydrogel were placed in a mould and then heated to 60 °C for 1 hour, and then cooled to 15 °C for 1 hour. A bulk star-shaped hydrogel was obtained, and by repeating the heating-cooling process other shapes can be formed (Figure.5f-h). The mechanical strength of the self-glued hydrogel was tested (Figure.5e) with 0.82 MPa of fracture tensile stress and almost the same modulus with the original one. In addition, Figure.5i shows the self-glued bear-shape hydrogel (1.5 mm in thickness and 20 mm in diameter) with a 0.5 kg weight suspended from it. Note that solvent dialysis can be also used to induce self-gluing.

The shape memory behaviour of the PU/DHIR-α-20 hydrogels in response to temperature is shown in Figure.S8. The fixed shapes can be easily erased by immersing the hydrogel in water at 60 °C for 5 minutes. Afterwards, new shapes can be written into the hydrogels in a two-step procedure, i.e. mechanically reshape and then fix the shape by cooling. As an example, Figure. 6 shows spring, spiral and zig-zag shapes formed by the erasing-writing

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process. The mechanism of temperature sensitive shape memory has previously been explained7 and presumably applies to the PU/DHIR-α-20 hydrogels. Briefly, non-covalent crosslinking from dipole-dipole and H-bonding interactions are able to lock a hydrogel in a temporary shape below the transition temperature. As the temperature is raised the non-covalent crosslinks successively break and the shapes are unlocked, allowing the hydrogels to return to their original shape.

Figure 6. Varied shapes of a single PU/DHIR-0.150-20 hydrogel fixed by sequential heating-cooling cycles. (a) Initially the sample is written into spring shape. (b) The spiral shape was erased by heating to 60 °C and written into spiral shape. (c) The spiral shape was erased and a zig-zag shape was obtained using the same procedure.

Conclusions

The DN PU/DHIR-α-20 hydrogels fabricated in this study comprise a ductile and robust PU hydrogel network combined with a stronger second DHIR hydrogel. The resulting DN hydrogels not only show a range of excellent strength properties (compression strength of 25.5 – 167.4 MPa at 85% strain, fracture tensile strength of 1.52 – 4.79 MPa, elongation of 470-815%, Young’s modulus of 0.30 – 2.54 MPa and fracture energy of 649 – 2493 J·m-2), but also exhibit rapid mechanical recovery at room temperature (from between 5 minutes and 17



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6 hours). The mechanical properties and mechanical recovery speeds of the PU/DHIR-α-20 hydrogels can be tuned by simply adjusting the crosslinking density of the second network.

The PU/DHIR-α-20 hydrogels also have self-healing properties and can be reconstructed by heating-cooling or solvent-dialysis procedures. These properties may be attributed to temperature/solvent modulated non-covalent dipole-dipole and H-bonding interactions. The hydrogels inherit their shape memory properties from DHIR network.

These PU/DHIR-α-20 hydrogels combine several favourable mechanical and structural properties within a single hydrogel material, which could be useful in load-bearing applications. In principle other soft and ductile hydrogels, such as polyacrylic acid hydrogels, polyacrylamide hydrogels, could be applied as the first network, with stronger hydrogels, such as phase-separation-induced hydrogels and PU-urea supramolecular hydrogels employed as the second network (since their mechanical strength is similar to that of DHIR hydrogels). Consequently, the DN hydrogel system reported here might open new avenues in hydrogel research and application development.

3. Experimental Materials: Hexamethylene diisocyanate (HDI), stannous octanoate, ethanol absolute, N,N,N’,N’’,N’’-pentakis(2-hydroxypropyl)diethylenetriamine (PHPDTA) and AMPS (98%) were obtained from Adamas Reagents, Ltd. AAm 99% was purchased from TCI, PEGDA575 99%

from

Aladdin

and

2-hydroxy-4’-(2-hydroxyethoxy)-2-methylpropiophenone

(Irgacure2959, 98%) from Dow. AN (98%), dichloromethane, DMSO and Linear PEG20000 (hydroxyl value = 5.6 mg KOH per g) were purchased from Shanghai Chemical Reagent 18


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Corporation. Dichloromethane and PEG20000 were dried before use otherwise all other reagents were used as received. Preparation of PU hydrogel: The PU hydrogel was fabricated using a one-pot method.24 PEG20000 (0.5 g, 0.025 mmol), HDI (0.0084 g, 0.05 mmol), catalyst (stannous octoate, 0.5 mg) and CH2Cl2 (5 mL) were placed in a glass mould (100*100*2 mm). The reaction mixture was stirred in and ultrasonic bath (50 W, 40 kHz) at room temperature for 30 min and then maintained at 50 °C for 4 h. next, a PHPDTA DMSO solution (0.1 M, 100 µL) was added. The subsequent polymerization was performed at 50 °C for 6 h. The gels were immersed in a large volume of absolute ethanol and deionized water successively for 3 days to remove the organic solvent and residual monomers and then dried.

Preparation of PU/DHIR-α α-β hydrogels: The second network monomer solution was prepared by adding the appropriate amount of AN, AAm, AMPS, PEGDA575 and Iragure2959 to DMSO (see Table S2 for concentrations of each reagent). The dried PU gels were then immersed in the second network monomer solution. After degassing for 30 min the mixture was sealed under N2 and heated to 50 °C for 3 h until the PU gels were fully swollen with the second network monomer solution. The impact of swelling time of PU gel in the second network monomer solution is illustrated in Figure.S6. Next, the gel was photo-polymerized to form the PU/DHIR-α-β gel using UV light (365 nm, 36 W) for 1 h. The resulting hydrogels were dialysed in deionized water for 3 days to remove DMSO and residual monomers.

Characterization of the hydrogels: Attenuated total reflection Fourier transform infrared 19



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(ATR-FTIR) spectroscopy (Perkin Elmer, Paragon 1000, USA) was used to characterize the hydrogels (see Figure.S1 for further details).

Equilibrium water content (EWC) of the hydrogels: The EWC was calculated by the following equation: EWC =

mwet − mdry mwet

× 100 %

where mwet and mdry denote the weight of the fully swollen hydrogel and weight of the completely dried hydrogel. Three independent data sets were used to calculate the EWC for each specimen.

Determination of the mechanical properties of the hydrogels: The mechanical properties were tested on a universal test machine (Instron 4465, USA) at room temperature on samples completely swollen in deionized water. For each sample, five parallel specimens were tested and the data is presented as the mean with standard deviation. For the compression tests the hydrogels were cut into cylinders of ca. 6 mm in diameter and ca. 6 mm in length. The applied compression speed was 2 mm/min. For the tensile tests the hydrogels were cut into dumbbell shapes (GBT 528-20094) with a 2 mm width, 10 mm length and 2 mm thickness. For the standard test, the applied strain rate was 100 mm/min, whereas for the tensile anti-fatigue tests, the strain rate was 150 mm/min. The method used for the tearing tests was taken from Gong et al.21,49 Briefly, the samples were cut into a trouser shape (GBT 529-2008 A, 1/2 size) with 7.5 mm width, 50 mm in length, 20 mm notch length and about 2 mm in thickness (see Figure.S5). The applied strain was 180 mm/min and the fracture energy (G) was calculated from the following equation: 20


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G=

2F w

where F denotes the constant stretching force during the tear and w refers to the thickness of samples.

Dynamic mechanical analysis (DMA) of the hydrogels: Samples were prepared as described above for the compression test. Elastic and loss modulus of the hydrogels were measured using a DMA 242 C/1/G (Netzsch, Germany) in compression mode (10 Hz) over a temperature range of 15 to 70 °C with a heating rate of 3 °C/min, a load of 2 N, and an amplitude of 120 um.

Shape memory properties: The samples were sealed as described in the preparation and the hydrogel strips were wrapped around a cylinder and then heated to 70 °C for 1 h. The strips were then immersed in water at 10 °C to fix the spring shape. The shape memory behaviour of the hydrogels was determined from photographs.

Self-healing and self-gluing properties: Solvent induced self-healing: the hydrogels were cut into two pieces and then the cut surfaces were immersed in DMSO to replace the water in hydrogels. Next, the cut surfaces were pressed together and dialyzed in deionized water for 24 h.

Temperature induced self-healing: the hydrogels were cut into two pieces and then the cut surfaces were pressed together. Subsequently, the hydrogels were heated to 70 °C for 1 h then cooled to 10 °C for 1 h.

Self-gluing properties: the PU/DHIR-0.150-20% hydrogel was crushed into fragments. 21



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Different shaped metal moulds were filled with the fragments and the fragments self-glued either by heating to 70 °C and then cooling to 10 °C for 1 h or via dialysis in DMSO for 24 h and then deionized water for 24 h.

ASSOCIATED CONTENT

Supporting Information Mechanical properties of PU/DHIR-0.3-β (β = 0, 5, 10, 20%) hydrogels; formulation of the second network monomer solution of PU/DHIR-α-β hydrogels; FTIR spectra and molecular structure of PU gels, DHIR gels and PU/DHIR-0.244-20 gels; tearing test, the first hysteresis loop and shape memory behaviour of the PU/DHIR-α-20 hydrogels; fracture tensile strength of the PU/DHIR-0.150-20 hydrogels in response to swelling time; solvent induced self-healing properties.

AUTHOR INFORMATION

Corresponding Author E-mail: [email protected].

Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENT We thank National Science Fund of China (20974061) and the Shanghai Leading Academic Discipline Project (no. B202).

22


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Table of Content

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Unconventional Tough Double-Network Hydrogels ... - ACS Publications

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