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Self-Healing and Recyclable Hydrogels Reinforced with in Situ-Formed Organic Nanofibrils Exhibit Simultaneously Enhanced Mechanical Strength and Stretchability Tao Yuan, Xinxin Qu, Xinming Cui, and Junqi Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08208 • Publication Date (Web): 13 Aug 2019 Downloaded from pubs.acs.org on August 13, 2019

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ACS Applied Materials & Interfaces

Self-Healing and Recyclable Hydrogels Reinforced with in Situ-Formed Organic Nanofibrils Exhibit Simultaneously Enhanced Mechanical Strength and Stretchability Tao Yuan, † Xinxin Qu, † Xinming Cui, ‡ and Junqi Sun*,†

†State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, P. R. China ‡Department of Pathology, College of Basic Medical Science, Jilin University, Changchun 130021, P. R. China

KEYWORDS: hydrogels, materials science, polymer complexes, self-healing materials, recyclable polymers ABSTRACT: In this work, self-healing and recyclable polymer hydrogels with simultaneously enhanced mechanical strength and stretchability are fabricated through the complexation of poly(acrylic acid) (PAA) with complexes of branched poly(ethylenimine) and 1-pyrenybutyric acid (PEI-PYA) to generate PAA/PEI-PYA complexes, which are further molded, dried and rehydrated. The in situ-formed PYA nanofibrils with aggregated structures during the

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complexation process enable the simultaneous enhancement of the tensile strength and stretchability of the PAA/PEI-PYA hydrogels. The PAA/PEI-PYA hydrogels have a tensile strength of 1.13 ± 0.04 MPa and stretchability of 2970 ± 154%, which are 2.2- and 2.1-times higher than those of the PAA/PEI hydrogels. Meanwhile, the damaged PAA/PEI-PYA hydrogels can be efficiently healed or recycled at room temperature to regain their original mechanical strength and integrity because the dynamic nature of hydrogen-bonding and electrostatic interactions among PAA, PEI and PYA endows the hydrogels with excellent healing and recycling capacity. This strategy of using aggregated nanofibrils to simultaneously enhance the tensile strength and stretchability of hydrogels can be extended to PAA/PEI hydrogels reinforced with aggregated nanofibrils of 9-anthracenecarboxylic acid and N,N′di(propanoic acid)-perylene-3,4,9,10-tetracarboxylic diimide, demonstrating its generality for fabricating hydrogels with enhanced mechanical properties. INTRODUCTION Polymer materials with high mechanical strength and stretchability have received extensive attention because of their potential applications in diverse areas including stretchable electronic devices, soft robotics, load-bearing materials, and so forth.1-6 However, the high mechanical strength and stretchability of polymer materials are always contradictory because materials with high mechanical strength generally have strong interactions among polymer chains, which will suppress the mobility of polymer chains and decrease their stretchability.7-15 Polymer materials with high stretchability are usually achieved at the expense of low mechanical strength.8,10,16-20 For example, Bao and co-workers have reported a highly stretchable elastomer by cross-linking


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poly(dimethylsiloxane) chains via coordination complexes.10 Although the reported elastomer can be stretched to ca. 27 times of its original length, it possesses a tensile stress of only ~0.25 MPa. Therefore, it is challenging to fabricate polymer materials combined with high mechanical strength and stretchability. As a type of high-performance natural polymer composite, spider silk displays a unique combination of high mechanical strength and stretchability.21 These unique mechanical properties arise from its hierarchical architectures where highly organized β-sheet nanocrystals are arranged within semi-amorphous protein matrices that consist of less ordered β-structures.21-23 Under stretching, the hierarchical architectures gradually rupture and hydrogen-bonding interactions among them reversibly break and reform to dissipate energy, leading to the high strength and stretchability of spider silk. Wood, which is widely used in the construction industry, also shows a hierarchical architecture of cellulose nanofibrils embedded in lignin-hemicellulose matrix.24 These cellulose nanofibrils are semicrystalline assemblies of cellulose molecules and they can interact with hemicelluloses through hydrogen-bonding interactions. When wood is loaded, the nanofibrils slide on each other and the hydrogen-bonding interactions undergo reversible break and reformation to dissipate energy and endow them with high toughness.25,26 Inspired by the unique mechanical properties of spider silk and wood, we believe that polymer materials combined with high mechanical strength and stretchability can be fabricated by uniformly incorporating nanofillers with hierarchical structures into polymer matrices. Polymer materials can suffer from physical damages in practical usage, resulting in deterioration of their original mechanical strength.20,27,28 One of the most effective approaches to resolve this problem is endowing polymer materials with self-healing and recycling capability.8,10,29-32 When the self-healing polymer materials are seriously damaged and become difficult to restore their initial mechanical strength, they can be recycled to avoid wasting of materials.33-36 Recently, by


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exploiting the reversibility of supramolecular interactions, a large variety of self-healing and/or recyclable polymer materials have been developed, significantly prolonging service life of the polymer materials and reducing environmental pollution.6,8,10,15,37-43 Our group and others have demonstrated that complexation of polymers with complementary noncovalent interactions in solution can produce polymer complexes that can be processed into polymer composites with selfhealing and/or recycling capability.28,33,42-46 Molecular self-assembly provides a convenient method for the fabrication of nanostructures, such as nanofibrils, nanoribbons and nanotubes.47-51 We believe that the co-assembly of complementary polymers and molecules with self-assembly ability in solution can generate self-healing/recyclable polymer composites dispersed with molecular assemblies possessing hierarchical structures because the growth of the molecular assemblies are restricted in network of polymeric complexes. The molecular assemblies with hierarchical structures are expected to simultaneously enhance the tensile strength and stretchability of the polymer composites. In this work, we demonstrate the fabrication of mechanically robust and highly stretchable polymer hydrogels with excellent self-healing and recycling capacity by complexation of poly(acrylic acid) (PAA) and branched poly(ethylenimine) in the presence of 1-pyrenybutyric acid (PYA), following by shape molding and water adsorption. The as-prepared PAA/PEI-PYA hydrogels reinforced with hierarchical PYA nanofibrils exhibit a high tensile strength of 1.13 ± 0.04 MPa, strain at break of 2970 ± 154%, and toughness of 18.17 ± 0.86 MJ/m3, which are 2.2-, 2.1-, and 4.8-times higher than the corresponding values of the PAA/PEI hydrogels. Meanwhile, the hydrogels exhibit excellent self-healing and recycling capability due to the dynamic nature of hydrogen-bonding and electrostatic interactions. We further prove that self-healing and recycling PAA/PEI hydrogels with simultaneously enhanced tensile strength and stretchability can also be fabricated by dispersing the hydrogels with


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hierarchical nanofibrils of 9-anthracenecarboxylic acid (Anth) and N,N′-di(propanoic acid)perylene-3,4,9,10-tetracarboxylic diimide (PBI), respectively. RESULTS AND DISCUSSION Fabrication of the PAA/PEI-PYA Hydrogels. The chemical structures of PAA, PEI, PYA, and the preparative process of the PAA/PEI-PYA hydrogels are shown in Figure 1a. Aqueous solution of PEI-PYA complexes was prepared by directly mixing aqueous solutions of PEI and PYA. There exist electrostatic and hydrogen-bonding interactions between amine/protonated amine groups of PEI and carboxylate/carboxylic acid groups of PYA. When aqueous solutions of PAA and PEI-PYA complexes were homogeneously mixed in a beaker under stirring, precipitation immediately generated because of the hydrogen-bonding and electrostatic interactions among PAA, PEI, and PYA (Figure S1). After decanting the turbid supernatant, the precipitate was collected and molded into sheet-like polymer composites at room temperature for 3 days. Finally, PAA/PEI-PYA hydrogels were obtained after the molded composites were saturated in water for 2 h (Figure S2). The hydrogels with different compositions are denoted as PAA/PEI-PYAx%, where x% represents the measured mass fraction of PYA in the corresponding xerogels. The exact mass fraction of PYA within the corresponding PAA/PEI-PYAx% xerogels was determined using calibration curve of PYA in aqueous solution of NaOH (0.1 M) and KBr (2.5 M) (Figure S3). The PAA/PEI-PYA2.1% hydrogel with water content of 43.1 ± 0.8 wt% looks yellow and shows blue luminescence under UV irradiation, indicating that PYA molecules are successfully loaded within the hydrogel (Figure 1b(i) and (ii)). In a control experiment, PAA/PEI hydrogels were prepared by using the method for the fabrication of PAA/PEI-PYAx% hydrogels. As shown in Figure 1b(iii), the PAA/PEI hydrogel is colorless and looks transparent.


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Figure 1. (a) Schematic diagram for the preparation process of PAA/PEI-PYA hydrogels reinforced with aggregates of PYA nanofibrils. Inset in (a) shows the chemical structures of PAA, PEI and PYA. (b) Digital photographs of the PAA/PEI-PYA2.1% hydrogel under visible (i) and UV (ii) light, and PAA/PEI hydrogel under visible light (iii). Mechanical Properties of the PAA/PEI-PYAx% Hydrogels. The mechanical properties of the PAA/PEI-PYAx% hydrogels with various mass fractions of PYA were characterized by uniaxial tensile tests. The corresponding stress-strain curves, which were measured at a stretching speed of 100 mm/min, are presented in Figure 2a. Table 1 summarizes the mechanical properties of the PAA/PEI-PYAx% hydrogels. Compared with PAA/PEI hydrogels, the tensile strength and strains at break of the PAA/PEI-PYAx% hydrogels simultaneously increase with increasing mass fraction of PYA in the hydrogels from 0.8 wt% to 2.1 wt%. The PAA/PEI-PYA2.1% hydrogels exhibit a maximum tensile strength of 1.13 ± 0.04 MPa and strain at break of 2970 ± 154%, which are 2.2 and 2.1 times higher than that of the PAA/PEI hydrogels. Consequently, the PAA/PEI-PYA2.1% hydrogels also exhibit a maximum toughness of 18.17 ± 0.86 MJ/m3, which is 4.8 times higher than that of the PAA/PEI hydrogels. As shown in Figure 2b, the PAA/PEI-PYA2.1% hydrogel can be stretched even after being knotted and twisted, and hold up a weight of 500 g without fracture,


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Figure 2. (a) Stress-strain curves of the PAA/PEI and PAA/PEI-PYAx% hydrogels with different mass fraction of PYA. (b) Digital photographs of the PAA/PEI-PYA2.1% hydrogels that are knotting, twisting, and lifting up a 500 g weight. The hydrogel has a width of 2.5 mm and a thickness of 1.8 mm.

Table 1. Mechanical Properties of PAA/PEI and PAA/PEI-PYAx% hydrogels with different mass fraction of PYA. Hydrogel abbreviations

PAA/PEI PAA/PEI-PYA0.8% PAA/PEI-PYA1.6% PAA/PEI-PYA2.1% PAA/PEI-PYA2.5% PAA/PEI-PYA3.1%

Tensile strength (MPa)

Strain at break (%)

Young’s modulus (MPa)

Toughness (MJ/m3)

0.52 ± 0.04 0.74 ± 0.03 0.89 ± 0.06 1.13 ± 0.04 0.97 ± 0.03 0.67 ± 0.05

1370 ± 120 1963 ± 106 2574 ± 112 2970 ± 154 2706 ± 133 1593 ± 164

0.28 ± 0.03 0.31 ± 0.04 0.32 ± 0.02 0.34 ± 0.04 0.35 ± 0.03 0.38 ± 0.05

3.77 ± 0.52 7.93 ± 0.64 12.57 ± 0.75 18.17 ± 0.86 14.72 ± 0.82 6.65 ± 0.88

demonstrating that the hydrogel has a high strength and toughness. When the mass fraction of the hydrogels exceeds 2.1 wt%, the tensile strength and strain at break of the PAA/PEI-PYAx% (x = 2.5 and 3.1) hydrogels decrease with further increasing mass fractions of PYA. Nevertheless, all the PAA/PEI-PYAx% hydrogels with x being no larger than 3.1 show a simultaneous enhancement of tensile strength and strain at break compared to the PAA/PEI hydrogels.


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Strengthening and Toughening Mechanism of the PAA/PEI-PYAx% Hydrogels. The selfassembly behavior of the PYA molecules in aqueous solution was investigated. As shown in Figure 3a, precipitate of PYA nanofibrils with diameters of 91.2 ± 21.0 nm can generate after adding aqueous HCl solution into the aqueous PYA solution (pH ≤ 7.0), indicating that the PYA molecules can self-assemble into nanofibrils in acidic aqueous solution. X-ray diffraction (XRD) measurements were performed on PYA nanofibrils obtained from aqueous PYA solution, PAA/PEI and PAA/PEI-PYAx% hydrogels with different mass fraction of PYA molecules. The XRD spectrum of PYA nanofirils in Figure 3b shows a characteristic diffraction peak at 2 of 23.3°, indicating that the PYA nanofibrils are composed of PYA nanocrystals. The diffraction peak at 23.3° corresponds to a d spacing of 3.8 Å, which suggests that the crystallized PYA nanofibrils are constructed via - stacking interactions of PYA molecules. As shown in Figures 3b and S4, besides PAA/PEI hydrogel, all the PAA/PEI-PYAx% hydrogels show the same diffraction peaks of PYA crystalline, although the diffraction intensity decreases with decreasing mass fractions of PYA in the corresponding hydrogels. This result indicates that all the PAA/PEI-PYAx% hydrogels incorporated with PYA molecules contains crystallized PYA. The structures of the crystallized PYA molecules within the PAA/PEI-PYAx% xerogels were further characterized by transmission electron microscopy (TEM). The low-magnification TEM image of the PAA/PEI xerogel shows that the structure of the xerogel is homogeneous without nanofibrils being observed (Figure S5). In contrast, the low-magnification TEM image of the PAA/PEI-PYA2.1% xerogel in Figure 3c shows dispersed aggregates of nanofibrils. The enlarged TEM images in Figure 3d and its inset clearly show that the aggregates of nanofibrils are composed of parallel aligned nanofibrils with an average diameter of 7.9 ± 1.8 nm and length of 51.5 ± 15.8 nm. The high-resolution TEM (HR-TEM) image in Figure 3e indicates that each single


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Figure 3. (a) SEM image of PYA nanofibrils prepared from aqueous PYA solution. (b) XRD patterns of the PYA nanofibrils prepared from aqueous PYA solution, PAA/PEI-PYA2.1% and PAA/PEI hydrogels. (c-e) TEM (c, d) and HR-TEM (e) images of the PAA/PEI-PYA2.1% xerogels.

nanofibril is PYA nanocrystals that are self-assembled PYA molecules via - stacking interactions. These nanofibrils can further assemble with each other through hydrogen-bonding interactions of carboxylic acid groups on the surface of the nanofibrils to generate aggregates of PYA nanofibrils. The aggregates of PYA nanofibrils have strong electrostatic and hydrogenbonding interactions with the PAA/PEI polyelectrolyte network. Due to the restriction of the PAA/PEI polyelectrolyte network, the PYA nanofibrils become aggregated and have a smaller diameter than the PYA nanofibrils generated in aqueous PYA solution. As shown in Figure S6a, the TEM image of PAA/PEI-PYA3.1% xerogel also shows dispersed hierarchical nanofibrils of 10.9 ± 2.5 nm in diameter and 61.2 ± 16.2 nm in length, which are larger than nanofibrils in the PAA/PEI-PYA2.1% xerogel. Meanwhile, the HR-TEM image in Figure S6b also confirms that the


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nanofibrils in PAA/PEI-PYA3.1% xerogel are crystalized. We observed that the hierarchical PYA nanofibrils exist in all the PAA/PEI-PYAx% hydrogels, and their diameter and length increase with increasing mass fraction of PYA. Based on the above-mentioned results, we propose that the aggregates of PYA nanofibrils play a significant role in simultaneously enhancing the tensile strength and stretchability of the PAA/PEI-PYA2.1% hydrogels.21-24,52,53 As depicted in Figure 4, PAA/PEI hydrogels are constructed by PAA and PEI through hydrogen-bonding and electrostatic interactions between them. The in situ-formed aggregates of PYA nanofibrils serve as nanofillers to strengthen the PAA/PEI-PYA2.1% hydrogels because PYA nanofibrils are rigid and have electrostatic and hydrogen-bonding interactions with PAA/PEI hydrogels. Meanwhile, we propose that upon stretching, the hydrogen-bonding interactions among neighboring PYA nanofibrils can break, and the individual PYA nanofibril can slide with respect on each other to effectively dissipate energy.24-26,53-55 In this way, the stretchability of the PAA/PEI-PYA2.1% hydrogels can be enhanced with the simultaneous improvement of tensile strength.

Figure 4. Schematic illustration for strengthening and toughening of the PAA/PEI-PYAx% hydrogels by aggregates of PYA nanofibrils.


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Figure 5. (a) Chemical structure of CNC. (b) TEM image of the PAA/PEI-CNC1.0% xerogel. (c) Stress-strain curves of the PAA/PEI and PAA/PEI-CNCx% hydrogels with different feed mass fraction of CNC. (d) Stress and strain of the PAA/PEI-CNCx% hydrogels as a function of CNC content. In a control experiment, cellulose nanocrystals (CNC) without aggregates were uniformly incorporated into PAA/PEI hydrogels. The resultant hydrogels are denoted as PAA/PEI-CNCx%, where x% represents the feed mass fraction of CNC in the corresponding mixing solutions. The chemical structure of CNC is shown in Figure 5a. The TEM image of the PAA/PEI-CNC1.0% xerogel in Figure 5b indicates that the individual CNC fibrils with a diameter of 7.1  1.0 nm are homogeneously dispersed in the xerogel. Compared with the PAA/PEI hydrogels, the tensile strength of all the PAA/PEI-CNCx% hydrogels is enhanced because of the strong electrostatic and hydrogen-bonding interactions of CNC and PAA/PEI hydrogels (Figure 5c and 5d). However, the strain at break of PAA/PEI-CNCx% hydrogels decreases with the incorporation of CNC in the


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hydrogels. Compared with aggregates of PYA nanofibrils, we believe that the individual CNC fibrils cannot slide to each other to effectively dissipate energy and enhance the stretchability of the hydrogels. The decreased stretchability of the PAA/PEI-CNCx% hydrogels supports that sliding of the rigid PYA nanofibrils enables the simultaneous enhancement on stretchability and tensile strength of the PAA/PEI-PYAx% hydrogels. Compared with PAA/PEI-PYA2.1% hydrogels, the larger aggregates of PYA nanofibrils in PAA/PEI-PYA3.1% hydrogels can cause concentration of stresses, which decreases the tensile strength and stretchability of the PAA/PEI-PYA3.1% hydrogels. Healability and Recyclability of the PAA/PEI-PYA2.1% Hydrogels. The healability and recyclability of the mechanically strongest PAA/PEI-PYA2.1% hydrogels were examined at room temperature. As shown in Figure 6a(i), two pieces of the PAA/PEI-PYA2.1% hydrogels with one of them stained with Allura Red were cut into halves. The two pieces of cut hydrogels with different colors were brought into contact and then incubated in water at room temperature for 9 h to heal the damaged hydrogels. It is observed that the re-connected hydrogels are well healed after 9 h of incubation (Figure 6a(ii)). The healed hydrogels can be stretched and lift up a weight of 500 g without fracture at the healed region. Figure 6b depicts the stress-strain curves of the separated PAA/PEI-PYA2.1% hydrogels that were healed for different time. The tensile strength and stretchability of the healed PAA/PEI-PYA2.1% hydrogels gradually restore to the initial state with elongating the healing time. The 9 h-healed hydrogel can completely restore to its original mechanical properties. The excellent self-healing ability of the PAA/PEI-PYA2.1% hydrogels originates from the reversibility of electrostatic and hydrogen-bonding interactions and high mobility of polymer chains. When the damaged surfaces of the PAA/PEI-PYA2.1% hydrogel are brought into contact in water, the reversible noncovalent interactions can partially break to


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facilitate the migration of polymer chains near the damaged areas. After the polymer chains reentangle and noncovalent interactions reform, the damaged PAA/PEI-PYA2.1% hydrogel is healed with its original mechanical properties being restored. As a comparison, the fractured PAA/PEI hydrogel can be completely healed after 6 h immersion in water at room temperature. This result indicates that the same hydrogen reinforced with PYA nanofibrils is difficult to be healed because of the suppressed mobility of polymer chains. The PAA/PEI-PYA2.1% hydrogels can also be recycled because of the dynamic nature of the noncovalent interactions among PAA, PEI and PYA nanofibrils. As shown in Figure 6c, the small pieces of the hydrogels were dried and ground into powders, which were then transferred into a heart shape Teflon mold and added with several drops of water. The powders of the hydrogels were compressed under a pressure of ca. 3 KPa for 24 h at room temperature to obtain a recycled PAA/PEI-PYA2.1% hydrogel. The stressstrain curves in Figure 6d indicate that the 24 h-recycled hydrogel can reach 90% and 97% of the tensile strength and stretchability for the pristine hydrogel, respectively. The stress-strain curves in Figure S7 reveal that the PAA/PEI-PYA2.1% hydrogel after three cycles of recycling shows no obvious decrease of tensile strength and stretchability compared with the hydrogel after the first recycling process. The recycled PAA/PEI-PYA2.1% hydrogels retain the excellent self-healing capability of the pristine hydrogels.


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Figure 6. (a) Digital photographs of the un-dyed and dyed PAA/PEI-PYA2.1% hydrogel sheets (i) and the separated hydrogel sheets with different colors that are well healed at room temperature for 9 h (ii). The healed hydrogel can be stretched (iii) and hold a weight of 500 g with a width of 4 mm and thickness of 1.2 mm (iv). The red hydrogel was stained with Allura Red AC. (b) Stressstrain curves of the original and 1, 2, 3, 5, 7, and 9 h-healed samples, which were previously cut into two pieces. (c) Digital photographs of the PAA/PEI-PYA2.1% hydrogel sheet that was cut into small pieces (i) and ground into powders (ii). The powders were reshaped into hydrogel (iii) and the recycled hydrogel can still be healed after being cut into two pieces (iv). (d) Stress-strain curves of the original, recycled hydrogels, and the recycled hydrogel after being cut and healed. Generality of in Situ Self-Assembled Hierarchical Nanofibrils to Strengthen and Toughen Hydrogels. To demonstrate the generality of in situ self-assembled hierarchical nanofibrils in simultaneously enhancing the tensile strength and stretchability of the PAA/PEI hydrogels, aqueous PAA solution was mixed with either aqueous solution of PEI and 9-anthracenecarboxylic


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acid (Anth) complexes or aqueous solution of PEI and N,N′-di(propanoic acid)-perylene-3,4,9,10tetracarboxylic diimide (PBI) complexes. The precipitates were collected to prepare PAA/PEIAnthx% and PAA/PEI-PBIx% hydrogels, where x% represents the feed mass fraction of Anth and PBI in the corresponding mixing solutions. The chemical structures of Anth and PBI are shown in Figure 7a. Stress-strain curves in Figure 7b and 7c reveal that the tensile strength and strain at break of the PAA/PEI-Anthx% and PAA/PEI-PBIx% hydrogels are simultaneously enhanced compared with those of the PAA/PEI hydrogels. The PAA/PEI-Anthx% and PAA/PEI-PBIx% hydrogels exhibit the highest tensile strength and strain at break when the feed mass fraction of Anth and PBI are 4.7% and 2.4%, respectively. The PAA/PEI-An4.7% hydrogel shows a tensile strength of 1.02 ± 0.05 MPa and strain at break of 2470 ± 146%, and the PAA/PEI-PBI2.4% hydrogel exhibits a tensile strength of 1.19 ± 0.06 MPa and strain at break of 2160 ± 1578%. TEM image of the PAA/PEI-Anth4.7% hydrogel shows aggregates of nanofibrils comprising of Anth nanofibrils with an average diameter of 7.7 ± 1.5 nm and length of 45.6 ± 12.3 nm (Figure 7d). Meanwhile, the TEM image of the PAA/PEI-PBI2.4% hydrogel also shows aggregated nanofibrils comprising of PBI nanofibrils with an average diameter of 8.2 ± 2.4 nm and lengths of 76.3 ± 23.1 nm (Figure 7e). These results demonstrate that the complexation of PAA with PEI-Anth or PEIPBI complexes can also generate hydrogels containing in situ self-assembled aggregates of nanofibrils. These aggregates of nanofibrils can simultaneously enhance the tensile strength and stretchability of the hydrogels in the same way as PYA nanofibrils in PAA/PEI-PYAx% hydrogels. Moreover, the PAA/PEI-Anth4.7% and PAA/PEI-PBI2.4% hydrogels can efficiently repair damages and be recycled at room temperature to restore their original mechanical properties (Figure S8).


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Figure 7. (a) Chemical structures of Anth and PBI molecules. (b, c) Stress-strain curves of the PAA/PEI hydrogel and PAA/PEI-Anthx% hydrogels with different feed mass fraction of An (b) and PAA/PEI-PBIx% hydrogels with different feed mass fraction of PBI (c). (d, e) TEM images of PAA/PEI-Anth4.7% (d) and PAA/PEI-PBI2.4% (e) xerogels. CONCLUSIONS In summary, we have developed a novel strategy for the fabrication of self-healing and recyclable PAA/PEI-PYA2.1% hydrogels with simultaneously enhanced mechanical strength and stretchability by employing in situ self-assembled nanofibril aggregates as nanofillers. The hydrogels were prepared by mixing of PAA with PEI-PYA complexes during which PAA interacts with PEI to form the hydrogel matrices and the liberated PYA molecules in situ self-assemble into nanofibril aggregates. The comparison with cellulose nanocrystal-reinforced PAA/ PEI hydrogels implies that PYA nanofibrils can slide with respect on each other during stretching of the


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PAA/PEI-PYA2.1% hydrogels. On one hand, the rigid aggregates of PYA nanofibrils can strengthen the hydrogel; on the other hand, the sliding of the PYA nnaofibrils and the reversible breakage of hydrogen-bonding and electrostatic interactions among PAA, PEI and PYA nanofibrils can efficiently dissipate energy. In this way, the simultaneous enhancement of tensile strength and stretchability of PAA/PEI-PYA2.1% hydrogels is achieved. The PAA/PEI-PYA2.1% hydrogels have a tensile strength of 1.13 ± 0.04 MPa, strain at break of 2970 ± 154%, and toughness of 18.17 ± 0.86 MJ/m3, which are 2.2-, 2.1- and 4.8-times higher than that of the PAA/PEI hydrogels. Benefiting from the reversibility of hydrogen-bonding and electrostatic interactions, the fractured PAA/PEI-PYA2.1% hydrogels can finely heal themselves at room temperature in water to restore their original mechanical properties. Meanwhile, the hydrogels can be recycled at room temperature for repeated usage. By replacing PYA nanofibrils with aggregates of An and PBI nanofibrils, PAA/PEI-Anth and PAA/PEI-PBI hydrogels with simultaneously enhanced tensile strength and stretchability can be fabricated, demonstrating the generality of aggregated organic nanofibrils in simultaneously enhancing tensile strength and stretchability of hydrogels. The selfhealing and recyclable PAA/PEI-PYA2.1% hydrogels with high stretchability and mechanical strength are potentially useful in fabricating stretchable electronics and wearable devices with elongated service life. Moreover, the complexation approach provides a convenient way for the fabrication of polymeric hydrogels with enhanced tensile strength, stretchability and toughness. We believe that the in situ self-assembled aggregates of organic nanofibrils can also serve as nanofillers for the fabrication of other types of polymer materials with well-tailored mechanical properties.

EXPERIMENTAL SECTION


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Materials. PAA (Mw ca. 450 000) and PEI (Mw ca. 750 000) were purchased from SigmaAldrich. PYA (97%), 3,4,9,10-perylenetetracarboxylic dianhydride (98%), 3-aminopropanoic acid (98%), and imidazole (99%) were purchased from Alfa Aesar. Anth was purchased from J&K Scientific Ltd.. All chemicals were used without further purification. Solution pH was adjusted with 1 M HCl or 1 M NaOH. Synthesis of PBI. The PBI was synthesized following the protocol reported previously.56 Typically, 3,4,9,10-perylenetetracarboxylic dianhydride (3.92 g, 10.0 mmol), 3-aminopropanoic acid (5.34 g, 60.0 mmol) and imidazole (28 g, 411.7 mmol) were mixed in a 250 mL roundbottomed flask and heated to 120 °C for 12 h with stirring under a controlled nitrogen atmosphere. The reaction mixture was cooled to room temperature and dispersed in 200 mL hot water, followed by filtering the aqueous solution. Then, 100 mL ethanol was added into the filtrate followed by addition of 200 mL HCl (2 M). The mixture was stirred for 2 h, and then the precipitate was collected by filtration and washed with water and ethanol three times, followed by drying under vacuum at 60 °C for 12 h to give the product of PBI (68.3% yield). 1H NMR (500 MHz, D2O, δ): 7.48-6.07 (8H, perylene), 4.25-3.15 (4H, N-CH2-), 2.69-1.68 (4H, -CH2-COOH) (Figure S9). Preparation of PEI-PYA, PEI-An and PEI-PBI Complexes. An aqueous PYA solution (28 mg/mL) was prepared by dissolving PYA (140 mg) in a 0.1 M aqueous NaOH solution (5 mL). Subsequently, the aqueous PYA solution was added into 250 mL aqueous PEI solution (4 mg/mL) with intense stirring to produce the aqueous PEI-PYA complex solution. The pH of the complex solution was adjusted to 10.0 by adding HCl. The final concentration of PEI and PYA in the complex solution was 4 mg/mL and 0.56 mg/mL, respectively. The mass fraction of PYA in PEIPYA was 12.3%, and these complexes were denoted as PEI-PYA12.3%. Complex solutions of PEIPYA7.4%, PEI-PYA10.7%, PEI-PYA13.8%, PEI-PYA16.7%, PEI-An7.4%, PEI-An9.1%, PEI-An10.7%, PEI-


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PBI2.0%, PEI-PBI4.8%, PEI-PBI7.4%, PEI-CNC1.0%, PEI-CNC2.0%, and PEI-CNC3.8% were prepared in a similar way with that of PEI-PYA12.3%. Preparation of PYA Nanofibrils. An aqueous solution of HCl (1 M) was gradually added into 100 mL aqueous solution of PYA (0.56 mg/mL) under stirring. Precipitate of PYA nanofibrils was obtained when the solution pH was below 7.0. The precipitate was collected via centrifugation, followed by washing with deionized water three times and drying under vacuum at 40 °C for 12 h. Hydrogel Fabrication. Taking the PAA/PEI-PYA hydrogel for instance, its preparation process is illustrated in Figure 1a. First, equal volumes of aqueous PAA solution (4 mg/mL, pH 3.0) and aqueous PEI-PYA solution were mixed through peristaltic pumps into a beaker under stirring. The precipitate of PAA/PEI-PYA complexes was then collected and compression-molded via two pieces of glass slides at a pressure of ca. 3 KPa in air for 3 days. Finally, the as-prepared polymer composites were immersed in water for 2 h to adsorb of water. In this way, a sheet of PAA/PEI-PYA hydrogel was fabricated. PAA/PEI, PAA/PEI-CNC, PAA/PEI-Anth, and PAA/PEI-PBI hydrogels were fabricated in a similar way to that of the PAA/PEI-PYA hydrogel. Content of PYA in the PAA/PEI-PYA Hydrogels. First, the PAA/PEI-PYA hydrogels were dried and ground into powders. Subsequently, the powders (20 mg) were added into 5 mL of aqueous solution of NaOH (0.1 M) and KBr (2.5 M), followed by sonicating for 12 h. After filtration, the absorbance at 343 nm of the solution was measured, and the mass fraction of PYA in PAA/PEI-PYA xerogels was determined using calibration curve of PYA in aqueous solution of NaOH (0.1 M) and KBr (2.5 M).

ASSOCIATED CONTENT


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Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Supplementary Figures. FT-IR spectrum of PAA/PEI xerogel (Figure S1); water content of the PAA/PEI-PYA2.1% hydrogel as a function of immersion time in water (Figure S2); calibration curve of PYA in mixture solution of NaOH (0.1 M) and KBr (2.5 M) (Figure S3); XRD patterns of PAA/PEI-PYAx% hydrogels (Figure S4); TEM images of PAA/PEI and PAA/PEI-PYA3.1% xerogels (Figure S5 and S6); stress-strain curves of the original, healed and recycled PAA/PEIPYA2.1%, PAA/PEI-Anth4.7% and PAA/PEI-PBI2.4% hydrogels (Figure S7 and S8); 1H NMR of PBI (Figure S9). AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (NSFC grant 21774049).

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Table of Contents Graphic


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Self-Healing and Recyclable Hydrogels ... - ACS Publications

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