Highly Tough, Stretchable, Self-Healing, and Recyclable Hydrogels

Apr 12, 2019 - It remains a challenge to fabricate healable and recyclable polymeric materials with simultaneously enhanced tensile strength, stretcha...
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Highly Tough, Stretchable, Self-Healing, and Recyclable Hydrogels Reinforced by in Situ-Formed Polyelectrolyte Complex Nanoparticles Tao Yuan,† Xinming Cui,‡ Xiaokong Liu,† Xinxin Qu,† 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

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S Supporting Information *

ABSTRACT: It remains a challenge to fabricate healable and recyclable polymeric materials with simultaneously enhanced tensile strength, stretchability, and toughness. Herein, we report a simple approach to fabricate high-performance polymer hydrogels that not only integrate high tensile strength, stretchability, and toughness but also possess selfhealing and recycling capabilities. The polymer hydrogels are fabricated by mixing a positively charged polyelectrolyte mixture of poly(diallyldimethylammonium chloride) (PDDA)/branched poly(ethylenimine) (PEI) with a negatively charged polyelectrolyte mixture of poly(sodium 4styrenesulfonate) (PSS)/poly(acrylic acid) (PAA) in an aqueous solution followed by molding, drying, and rehydration. The (PDDA/PEI)−(PSS/PAA) hydrogels with in situ-formed PDDA−PSS nanoparticles have a tensile strength, strain at break, and toughness of 1.26 ± 0.06 MPa, 2434.2 ± 150.3%, and 19.53 ± 0.48 MJ/m3, respectively. The toughness of the (PDDA/PEI)− (PSS/PAA) hydrogels is ∼5.2 and ∼108 times higher than that of the PEI−PAA and PDDA−PSS hydrogels, respectively. Benefiting from the high reversibility of the hydrogen-bonding and electrostatic interactions, the (PDDA/PEI)−(PSS/PAA) hydrogels can efficiently heal from physical damage to restore their original mechanical properties at room temperature in water. Moreover, the (PDDA/PEI)−(PSS/PAA) hydrogels after being dried and ground can be recycled under a pressure of ∼3 kPa at room temperature in the presence of water to reuse the damaged hydrogels.

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INTRODUCTION Due to the light weight and easy processing advantages of polymeric materials, there has been an increasing trend of replacing the traditional metallic and ceramic materials with high-performance polymeric materials in diverse areas including the automotive industry, aerospace, tissue engineering, and so forth.1−5 The fracture resistance capability, which is mainly determined by the tensile strength and toughness of the materials, is a key performance indicator of polymeric materials. While the toughness of a material is determined by both the tensile strength and stretchability, it is an eternal dilemma to simultaneously enhance the tensile strength and stretchability of polymeric materials because these properties generally tend to be mutually exclusive.6−15 Homogeneous incorporation of inorganic (e.g., silica, clay, and graphene oxide) or stiff polymer nanofillers (e.g., cellulose nanocrystals) into polymer matrices is an effective approach to enhance the tensile strength of polymeric composite materials.12,15−22 However, the stiff nanofiller-reinforced polymer composites usually exhibit decreased stretchability because of the suppressed polymer chain mobility exerted by the nanofillers. Recently, a few studies have reported the simultaneous © XXXX American Chemical Society

enhancement on tensile strength and stretchability/toughness of nanofiller-incorporated polymer composites by deliberately designing the noncovalent interactions between the nanofillers and polymer matrices.23−25 The dynamic noncovalent interactions between the nanofillers and polymer matrices can significantly improve their interfacial interactions, acting as sacrificial bonds to dissipate energy and toughen the polymer composites. However, it is still highly desirable to develop new yet simple strategies to simultaneously strengthen and toughen polymeric materials. Meanwhile, self-healing capability and recyclability have become important properties of artificial materials because these beneficial properties can contribute to the realization of a sustainable society by significantly extending the service life of the materials and reducing raw material consumption and environmental pollution.26−33 In recent years, various self-healing and recyclable polymeric materials have been developed based on polymer networks cross-linked via reversible dynamic covalent bonds or nonReceived: January 10, 2019 Revised: March 3, 2019

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DOI: 10.1021/acs.macromol.9b00053 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. (a) Schematic illustration of the preparation process of the hydrogels comprising polyelectrolyte complexes. (b) Chemical structures of PSS, PDDA, PAA, and PEI. (c) Digital photographs of the (16%PDDA1/PEI)−(20%PSS1/PAA) (i), PEI−PAA (ii), and PDDA−PSS (iii) hydrogels.

covalent interactions.33−39 Self-healing capability/recyclability and high mechanical performance are also contrary properties that are difficult to reconcile in one material, especially when the autonomous self-healing capability at mild conditions is required for high-strength polymeric materials.14,21,38−45 This is because high mechanical performance relies on a highly dense cross-linking that in turn suppresses the mobility of polymer chains and thus decreases the healability/recyclability of the polymeric materials. Therefore, it remains a huge challenge to fabricate polymeric materials that not only integrate high mechanical strength, stretchability, and toughness but also can effectively self-heal and recycle under mild conditions. Until now, our and other groups have reported various selfhealing/healable polymer films based on layer-by-layer (LbL) assembly of polymers with complementary noncovalent interactions.46−51 The healing behavior of the LbL-assembled polymer films in water, which resembles the healing of hydrogels, benefits from the reversible break and reformation of the noncovalent bonds and the high mobility of polymer chains. Essentially, the LbL-assembled polymer films are polymer complexes formed at solid substrates.52−55 It is accordingly believed that complexing polymers with complementary interactions in a bulk solution can also generate noncovalently cross-linked polymer composites with healability.56−58 Moreover, the compositions and microstructures of the polymer complexing composites can be conveniently tailored by varying the polymer species, complexing protocols, and bonding strengths of the polymer interactions.59−62 Previous studies have demonstrated that polymeric complexes derived from strong polyelectrolytes exhibit quite different morphologies and micro-/nanostructures from those derived from weak polyelectrolytes because of the difference in bonding strength and complexing dynamics.59,62−64 Therefore, we believe that simultaneous complexation of multiple types of strong and weak polyelectrolytes with different bonding strengths can lead to polymer composites with well-controlled nanostructures that can enhance mechanical properties of the resultant polymer composites. Based on the above consideration, herein we demonstrated the fabrication of tough, highly stretchable, self-healable, and recyclable hydrogels based on the simultaneous complexation of a positively charged polyelectrolyte mixture of poly(diallyldimethylammonium chloride) (PDDA)/branched poly(ethylenimine) (PEI) with a negatively charged polyelectrolyte mixture of poly(sodium 4styrenesulfonate) (PSS)/poly(acrylic acid) (PAA), followed by

molding the complexes into desired shapes and equilibrating into water. The as-prepared (PDDA/PEI)−(PSS/PAA) hydrogels, which exhibit excellent mechanical properties with a tensile strength of 1.26 ± 0.06 MPa, a strain at break of 2434.2 ± 150.3%, and a toughness of 19.53 ± 0.48 MJ/m3, are PEI− PAA hydrogels reinforced with in situ-formed PDDA−PSS nanoparticles. Because of the reversibility of electrostatic and hydrogen-bonding interactions among these polyelectrolytes, the hydrogels can autonomously heal from mechanical damage at room temperature to restore their original mechanical properties and be recycled and reshaped under mild conditions.

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EXPERIMENTAL SECTION

Materials. PAA (Mw ca. 450 000), PEI (Mw ca. 750 000), and PSS (Mw ca. 70 000) were purchased from Sigma-Aldrich. PDDA (Mw ca. 100 000) and Allura Red AC were purchased from Adamas-beta and TCI, respectively. All chemicals were used without further purification. Deionized water was used for the preparation of aqueous polyelectrolyte solutions and the corresponding hydrogels. Solution pH of aqueous polyelectrolyte solutions was adjusted with 1 M HCl or 1 M NaOH. Characterizations. 1H NMR spectra were measured using a 500 MHz Bruker instrument. The tensile tests were performed using a 410R250 Tension Instrument (TEST RESOURCES Inc.) with a stretching speed of 100 mm/min at room temperature. Transmission electron microscopy (TEM) images were obtained using a Philips Tecnai F20 transmission electron microscope at 200 kV. The samples for the TEM measurement were stained by the vapor of ruthenium tetroxide (RuO4) solution for 30 min before TEM observation. ζPotential measurements were conducted on a Malvern Nano-ZS Zetasizer at room temperature. Fourier transform infrared (FT-IR) spectra were conducted on a Bruker VERTEX 80 V FT-IR spectrometer. Rheological tests were performed on a TA HR-2 rheometer (8 mm parallel steel plate). A rheological frequency sweep from 0.1 to 100 rad/s was performed with a shear strain of 0.1% in the parallel-plate geometry in the temperature range from 0.1 to 90 °C. Digital photographs and movies were captured by a Canon SX40 HS camera. Preparation of PSS/PAA and PDDA/PEI Mixture Solutions and PDDA/PEI/PSS Complexes. The 20%PSS/PAA mixture solution was prepared by adding 25 mL of aqueous solution of PSS (10.3 mg/ mL) and 25 mL of deionized water into 200 mL of aqueous solution of PAA (5 mg/mL) under stirring. The pH of the mixture solution was adjusted to 3.0 by adding NaOH. The final concentrations of PAA and PSS in the mixture solution were 4 and 1.03 mg/mL, respectively. The mass fraction of PSS in the PSS/PAA mixture was 20%. The 16%PDDA/PEI mixture solution was prepared by adding 25 mL of aqueous solution of PDDA (8 mg/mL) and 25 mL of B

DOI: 10.1021/acs.macromol.9b00053 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 1. Summary of the Compositions of the (X%PDDAM/PEI)−(Y%PSSN/PAA) Hydrogels Measured by 1H NMR hydrogel abbreviations

measured mass percent of PDDA in PDDA/PEI (%)

measured mass percent of PSS in PSS/PAA (%)

measured monomer molar ratio of PDDA to PSS

measured mass ratio of PDDA, PEI and PSS to PAA

(9%PDDA1/PEI)−(11%PSS1/PAA) (16%PDDA1/PEI)−(20%PSS1/PAA) (23%PDDA1/PEI)−(28%PSS1/PAA) (16%PDDA2/PEI)−(11%PSS1/PAA) (16%PDDA1/PEI)−(34%PSS2/PAA) (9%PDDA1/PEI)−(20%PSS2/PAA) (28%PDDA2/PEI)−(20%PSS1/PAA)

23 33 35 40 32 27 58

20 31 36 17 49 42 35

1:1 1:1 1:1 2:1 1:2 1:2 2:1

0.2:0.7:0.2:1.0 0.3:0.7:0.4:1.0 0.4:0.8:0.6:1.0 0.3:0.5:0.2:1.0 0.4:0.8:0.9:1.0 0.3:0.8:0.7:1.0 0.8:0.6:0.5:1.0

Figure 2. (a) Stress−strain curves of the PDDA−PSS hydrogel, PEI−PAA hydrogel, and (X%PDDA1/PEI)−(Y%PSS1/PAA) hydrogels with a fixed PDDA to PSS monomer molar ratio of 1:1 but varied mass fractions of PDDA and PSS. (b) Stress−strain curves of the (X%PDDAM/PEI)− (Y%PSSN/PAA) hydrogels with different PDDA to PSS monomer molar ratios. (c) Digital photographs demonstrating the high strength and toughness of the (16%PDDA1/PEI)−(20%PSS1/PAA) hydrogels: holding up a weight of 500 g and being stretched even after being twisted and knotted. The hydrogel in (c) has a width of 2.5 mm and a thickness of 1.6 mm. deionized water into 200 mL of aqueous solution of PEI (5 mg/mL) under stirring. The pH of the mixture solution was adjusted to 10.0 by adding HCl. The final concentrations of PEI and PDDA in the mixture solution were 4 and 0.8 mg/mL, respectively. The mass fraction of PDDA in the PDDA/PEI mixture was 16%. In the control experiments, mixture solutions of 11%PSS/PAA, 28%PSS/PAA, 34%PSS/ PAA, 9%PDDA/PEI, 23%PDDA/PEI, and 28% PDDA/PEI were prepared in a similar way. The 16%PDDA1/PEI/20%PSS1 complexes were prepared by adding 25 mL of aqueous solution of PDDA (8 mg/ mL) into 200 mL of aqueous solution of PEI (5 mg/mL) under stirring, followed by adding 25 mL of aqueous solution of PSS (10.3 mg/mL) into the mixture solution under stirring. The aqueous solution of the complexes was further stirred for 1 h at room temperature, and then, its pH was adjusted to 10.0 by adding HCl. Fabrication of the Polyelectrolyte-Based Hydrogels. Taking the (X%PDDAM/PEI)−(Y%PSSN/PAA) hydrogels for instance, their preparation process is illustrated in Figure 1a. First, equal volumes of the PSS/PAA and PDDA/PEI mixture solutions were mixed through peristaltic pumps into a beaker under stirring. The resultant precipitate was then collected after decanting the turbid supernatant. The precipitate was compression-molded via two pieces of glass slides at a pressure of ca. 3 kPa at room temperature for 3 days until the sample became nearly transparent. Finally, the resultant composite was incubated in water for 2 h to reach an equilibrium adsorption of water. In this way, the (X%PDDAM/PEI)−(Y%PSSN/PAA) hydrogels were fabricated, in which X% and Y% represent the feed mass fractions of PDDA and PSS in the corresponding PDDA/PEI and PSS/PAA mixtures used for the preparation of the hydrogels, respectively, and M:N represents the feed monomer molar ratio of PDDA to PSS. In control experiments, PEI−PAA, PDDA−PSS, PEI− ( 2 0 % PSS/PAA), ( 1 6 % PDDA/PEI)−PAA, and ( 1 6 % PDDA 1 / PEI/20%PSS1)−PAA hydrogels were fabricated in a similar way to that of the (X%PDDAM/PEI)−(Y%PSSN/PAA) hydrogels (see the Supporting Information).

complexation of the PSS/PAA and PDDA/PEI mixtures. The chemical structures of PSS, PAA, PDDA, and PEI are shown in Figure 1b. Aqueous solutions of PSS/PAA and PDDA/PEI mixtures were first prepared by dissolving the same charged polyelectrolytes, that is, PSS and PAA, and PDDA and PEI in water (Figure S1a,b). The concentrations of PAA and PEI in their corresponding PSS/PAA and PDDA/ PEI solutions were always kept at 4 mg/mL, whereas the concentrations of PSS and PDDA were varied as described. Aqueous solutions of PSS/PAA and PDDA/PEI mixtures have ζ-potentials of −31.5 ± 0.5 and 36.3 ± 1.0 mV, respectively. When aqueous solutions of the PSS/PAA and PDDA/PEI mixtures with equal volumes were mixed (i.e., the feed mass ratio of PAA to PEI is kept constant as 1:1) through peristaltic pumps to a beaker under stirring, precipitation occurred due to the electrostatic and hydrogen-bonding interactions among these polyelectrolytes (Figure S1c). The electrostatic and hydrogen-bonding interactions in the precipitate and the resultant hydrogels were further confirmed by Fourier transform infrared (FT-IR) spectroscopy (Figure S2). After decanting the turbid supernatant, the white precipitate was collected (Figure S1d and its inset) and molded into sheetlike polymer composites via two pieces of glass slides. Pressure of ∼3 kPa was applied onto the glass slides for 3 days at room temperature until the polymer composites became nearly transparent (Figures 1a and S3). Subsequently, the as-prepared transparent polymer composites were incubated in water to transform the polymer composites into hydrogels. It is found that 2 h of incubation is sufficient to achieve an equilibrium water adsorption (Figure S4). In this way, sheetlike translucent (PDDA/PEI)−(PSS/PAA) hydrogels were finally obtained. By varying the feed mass fractions of PDDA and PSS in the corresponding PDDA/PEI and PSS/PAA mixtures while keeping the mass ratio of PEI to PAA at 1:1, seven types of polyelectrolyte-based hydrogels were fabricated. For simplicity,

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RESULTS AND DISCUSSION Preparation of Polymeric Hydrogels Comprising Polyelectrolyte Complexes. Figure 1a shows the preparation process of the polymeric hydrogels based on the C

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Table 2. Summary of the Mechanical Properties of PDDA−PSS, PEI−PAA, and Various Kinds of (X%PDDAM/PEI)−(Y%PSSN/ PAA) Hydrogels (Measured at a Stretching Speed of 100 mm/min) hydrogel abbreviations PDDA−PSS PEI−PAA (9%PDDA1/PEI)−(11%PSS1/PAA) (16%PDDA1/PEI)−(20%PSS1/PAA) (23%PDDA1/PEI)−(28%PSS1/PAA) (16%PDDA2/PEI)−(11%PSS1/PAA) (16%PDDA1/PEI)−(34%PSS2/PAA) (9%PDDA1/PEI)−(20%PSS2/PAA) (28%PDDA2/PEI)−(20%PSS1/PAA)

tensile strength (MPa) 2.01 0.52 0.88 1.26 1.13 0.78 0.91 0.70 0.69

± ± ± ± ± ± ± ± ±

Young’s modulus (MPa)

0.11 0.04 0.05 0.06 0.08 0.05 0.08 0.06 0.07

18.26 0.28 0.32 0.36 0.45 0.33 0.41 0.42 0.37

± ± ± ± ± ± ± ± ±

1.26 0.03 0.03 0.03 0.04 0.02 0.04 0.05 0.03

strain at break (%) 14.8 1370.0 1874.1 2434.2 2030.3 2511.2 1952.5 2421.5 2265.2

± ± ± ± ± ± ± ± ±

1.8 120.2 85.2 150.3 100.8 138.7 90.9 112.3 90.3

toughness (MJ/m3) 0.18 3.77 10.68 19.53 14.69 12.27 11.29 11.21 10.30

± ± ± ± ± ± ± ± ±

0.02 0.52 0.38 0.48 0.65 0.37 0.30 0.28 0.25

(16%PDDA1/PEI)−(20%PSS1/PAA) hydrogels is 1.26 ± 0.06 MPa, which is ∼2.4 times higher than that of the PEI−PAA hydrogels and is slightly lower than that of the PDDA−PSS hydrogels. Meanwhile, the strain at break of the (16%PDDA1/ PEI)−(20%PSS1/PAA) hydrogels reaches 2434.2 ± 150.3% (i.e., 25.3 times its initial length), which is ∼1.8 and ∼164 times higher than that of the PEI−PAA and PDDA−PSS hydrogels, respectively. Consequently, the (16%PDDA1/PEI)− (20%PSS1/PAA) hydrogels exhibit the highest toughness of 19.53 ± 0.48 MJ/m3 among all the as-prepared hydrogels, which is ∼5.2 and ∼108 times higher than that of the PEI− PAA and PDDA−PSS hydrogels, respectively. The high strength and toughness of the (16%PDDA1/PEI)−(20%PSS1/ PAA) hydrogels suggest that the hydrogels can have excellent ability to withstand load and absorb energy before fracture. As shown in Figure 2c, the (16%PDDA1/PEI)−(20%PSS1/PAA) hydrogel (2.5 mm wide and 1.6 mm thick) can hold a weight of 500 g and be stretched even after being twisted and knotted. The mechanical properties of the (X%PDDAM/PEI)−(Y%PSSN/ PAA) hydrogels are strongly dependent on the amounts and ratios of the incorporated PDDA and PSS. When the feed monomer molar ratio of PDDA to PSS is fixed at 1:1, relative to the (16%PDDA1/PEI)−(20%PSS1/PAA) hydrogels, the increase or decrease of mass fractions of PDDA and PSS in the PDDA/PEI and PSS/PAA mixtures leads to decreased tensile strength and toughness of the resultant hydrogels (Figure 2a and Table 2). When the mass fraction of PDDA is fixed at 16% or the mass fraction of PSS is fixed at 20%, varying the feed monomer molar ratio of PDDA to PSS from 1:1 to 1:2 and 2:1 leads to a decreased tensile strength and toughness of the resultant hydrogels (Figure 2b and Table 2). Therefore, among all the (X%PDDAM/PEI)−(Y%PSSN/PAA) hydrogels, the (16%PDDA1/PEI)−(20%PSS1/PAA) hydrogel exhibits the optimized mechanical properties in terms of tensile strength, stretchability, and toughness. Strengthening and Toughening Mechanism of the (X%PDDAM/PEI)−(Y%PSSN/PAA) Hydrogels. The structures of the (16%PDDA1/PEI)−(20%PSS1/PAA), PEI−PAA, and PDDA−PSS xerogels were examined by transmission electron microscopy (TEM) to understand the mechanism for the simultaneous enhancement on strength, stretchability, and toughness of the (16%PDDA1/PEI)−(20%PSS1/PAA) hydrogels. The samples for the TEM measurements were stained with ruthenium tetroxide (RuO4). As shown in Figure 3a,b, the PEI−PAA and PDDA−PSS xerogels exhibit homogeneous structures. In contrast, the (16%PDDA1/PEI)−(20%PSS1/PAA) xerogel exhibits a distinct two-phase structure with nanoparticles of 72.4 ± 18.9 nm in diameters homogeneously dispersed in the matrix (Figure 3c). The phase separation of

these hydrogels with different compositions are denoted (X%PDDAM/PEI)−(Y%PSSN/PAA), in which X% and Y% represent the feed mass fractions of PDDA and PSS in the corresponding PDDA/PEI and PSS/PAA mixtures used for the preparation of the hydrogels, respectively, and M:N represents the feed monomer molar ratio of PDDA to PSS. The compositions of these 7 types of ( X% PDDA M/PEI)− (Y%PSSN/PAA) hydrogels are the same as the corresponding precipitates if water contents are not considered. Therefore, the compositions of the hydrogels can be deduced from the supernatants of the mixing solution containing PDDA, PEI, PSS, and PAA polyelectrolytes, whereas the compositions of supernatants were calculated based on 1H NMR analysis (Figure S5, Supporting Information). The compositions of the (X%PDDAM/PEI)−(Y%PSSN/PAA) hydrogels are summarized in Table 1. The (16%PDDA1/PEI)−(20%PSS1/PAA) hydrogel is translucent with a water content of 41.9 ± 1.2 wt% (Figure 1c(i)). In control experiments, PEI−PAA and PDDA−PSS hydrogels were also prepared via the same procedure as that of the (X%PDDAM/PEI)−(Y%PSSN/PAA) hydrogels. As shown in Figure 1c(ii,iii), the as-prepared PEI−PAA hydrogel is transparent, whereas the PDDA−PSS hydrogel is opaque. Mechanical Properties of the ( X% PDDA M /PEI)− Y% ( PSSN/PAA) Hydrogels. The mechanical properties of the (X%PDDAM/PEI)−(Y%PSSN/PAA) hydrogels with different fractions of PDDA and PSS were measured by tensile tests using the hydrogel sheets (gauge length 4 mm, width 5 mm, thickness 1.8 mm) as specimens at the stretching speed of 100 mm/min. The stress−strain curves of the hydrogels are shown in Figure 2a,b. Their mechanical properties in terms of tensile strength, strain at break, Young’s modulus, and toughness are summarized in Table 2. The PDDA−PSS and PEI−PAA hydrogels are taken for comparison. As shown in Figure 2a and Table 2, the PDDA−PSS hydrogels are stiff and brittle. Although the PDDA−PSS hydrogels exhibit the highest tensile strength of ∼2.0 MPa and Young’s modulus of ∼18.3 MPa, the very low strain at break of ∼14.8% leads to the lowest toughness of ∼0.18 MJ/m3 among all the tested hydrogels. Because of the very low toughness, the PDDA−PSS hydrogels can easily break under stress. In contrast, the PEI−PAA hydrogels are soft but ductile. Compared with the PDDA−PSS hydrogels, the PEI−PAA hydrogels exhibit a lower tensile strength of ∼0.52 MPa and a Young’s modulus of ∼0.28 MPa. However, the PEI−PAA hydrogels have a much higher strain at break of ∼1370%, which leads to a high toughness of ∼3.8 MJ/m3. Intriguingly, all the as-prepared (X%PDDAM/PEI)− (Y%PSSN/PAA) hydrogels exhibit simultaneously enhanced tensile strength and stretchability compared to the PEI−PAA hydrogels (Figure 2a,b). Specifically, the tensile strength of the D

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(20%PSS1/PAA) hydrogels are mainly composed of the PDDA−PSS complexes, which play a significant role in simultaneously enhancing the tensile strength and toughness of the hydrogels. It should be mentioned that PSS also has electrostatic interactions with PEI. However, PEI has a lower charge density than PDDA because the degree of ionization of the weak polyelectrolyte of PEI is dependent on solution pH. As a result, the formation of PDDA−PSS complexes is faster than the formation of PEI-PSS complexes.65 This fact also supports that the phase-separated structures in (16%PDDA1/ PEI)−(20%PSS1/PAA) are dominantly composed of PDDA− PSS complexes. In a control experiment, (16%PDDA1/ PEI/20%PSS1)−PAA hydrogels were prepared based on the complexation between PAA and the preformed PDDA/PEI/ PSS complexes (see the Experimental Section). As indicated in the TEM image of Figure 3d, the (16%PDDA1/PEI/20%PSS1)− PAA hydrogel also exhibits a phase-separated structure, but with nonuniformly dispersed PDDA−PSS nanoparticles being observed. Compared with the PEI−PAA hydrogels, the (16%PDDA1/PEI/20%PSS1)−PAA hydrogels also exhibit simultaneously enhanced tensile strength and stretchability (Figure S7). However, their tensile strength and stretchability are still much lower than those of the (16%PDDA1/PEI)−(20%PSS1/ PAA) hydrogels because of the nonuniformly dispersed PDDA−PSS nanoparticles. Note that the (16%PDDA1/ PEI/20%PSS1)−PAA hydrogel has the same feed mass ratio of PDDA, PEI, PSS, and PAA as that of the (16%PDDA1/PEI)− (20%PSS1/PAA) hydrogel. These results can further confirm that the simultaneous complexation of PSS/PAA and PDDA/ PEI mixtures is important to fabricate PDDA−PSS nanoparticle-reinforced (16%PDDA1/PEI)−(20%PSS1/PAA) hydrogels that possess simultaneously enhanced tensile strength, stretchability, and toughness. The dynamic mechanical behaviors of the (16%PDDA1/ PEI)−(20%PSS1/PAA) hydrogels were systematically investigated with the rheological tests to understand the dynamic interactions among PSS, PAA, PDDA, and PEI polyelectrolytes in the hydrogels.11 The rheological behaviors of the PEI−PAA and (16%PDDA1/PEI)−(20%PSS1/PAA) hydrogels were performed from 0.1 to 100 rad/s at temperatures ranging from 0.1 to 90 °C. The as-obtained results were processed based on the time−temperature superposition principle by horizontally shifting the individual curves of different temperatures toward the curve corresponding to the reference temperature of 25 °C.11 In this way, the rheological master curves of the two hydrogels are constructed (Figure 4a,b). Meanwhile, the shifting factors (aT) corresponding to the individual curves of different temperatures are taken to generate the Arrhenius plots (Figure 4c,d) based on the Arrhenius equation, aT = AeEa/RT, where A is a constant and R is the ideal gas constant. As shown in Figure 4a, storage modulus (G′) of the PEI−PAA hydrogels is larger than the loss modulus (G″) until the shear frequency reaches 800 rad/s, and then, G′ becomes lower than G″. Therefore, the PEI−PAA hydrogels are in a gel state when the shear frequency is below 800 rad/s, and afterward the gel− sol transition occurs. The Arrhenius plot for the PEI−PAA hydrogels shows two apparent activation energy values (Ea) of 27.3 and 62.4 kJ/mol (Figure 4c). This result indicates that two kinds of physical interactions with different bonding strengths exist in the PEI−PAA hydrogels, which can be assigned to the hydrogen-bonding and electrostatic interactions between the amine/protonated amine groups of PEI and the carboxylic acid/carboxylate groups of PAA.66,67 For

Figure 3. TEM images of the PEI−PAA (a), PDDA−PSS (b), ( 1 6 % PDDA 1 /PEI)−( 2 0 % PSS 1 /PAA) (c), and ( 1 6 % PDDA 1 / PEI/20%PSS1)−PAA (d) hydrogels, which were selectively stained with RuO4.

the (16%PDDA1/PEI)−(20%PSS1/PAA) hydrogels can be explained as follows. (i) The electrostatic interactions between the strong polyelectrolytes of PDDA and PSS are stronger than the electrostatic and hydrogen-bonding interactions between the weak polyelectrolytes of PEI and PAA. Therefore, the formation of PDDA−PSS complexes will be faster than the formation of PEI−PAA and other types of complexes.65 (ii) Based on the molecular structures of the four kinds of polyelectrolytes (Figure 1b), it can be deduced that the PDDA−PSS complexes are much more hydrophobic than the PEI−PAA complexes. This conclusion is supported by the fact that PEI−PAA and PDDA−PSS complexes have water contents of 43.4 ± 4.3 and 27.2 ± 1.4 wt%, respectively. Meanwhile, the concentrations of PDDA and PSS are much lower than those of PEI and PAA in the resultant PDDA/PEI and PSS/PAA mixture solutions. Accordingly, simultaneous complexation between the strong (i.e., PDDA and PSS) and weak (i.e., PEI and PAA) polyelectrolytes leads to polymer composites with phase separation. Because RuO4 selectively stains the phenyl rings of PSS, it is confirmed that the observed nanoparticles in the TEM image of the (16%PDDA1/PEI)− (20%PSS1/PAA) hydrogels are mainly composed of the PDDA−PSS complexes, whereas the matrix is mainly composed of the PEI−PAA complexes. To verify the aboveproposed structure of the (16%PDDA1/PEI)−(20%PSS1/PAA) hydrogels, PEI−(20%PSS/PAA) and (16%PDDA/PEI)−PAA hydrogels were prepared, and their structures were also examined with TEM. Like the PEI−PAA hydrogels, the PEI−(20%PSS/PAA) and (16%PDDA/PEI)−PAA hydrogels that include only one kind of strong polyelectrolyte (i.e., either PSS or PDDA) exhibit homogeneous structures without phase separation being observed (Figure S6). Compared with the PEI−PAA hydrogels, the stress−strain curves of the PEI− (20%PSS/PAA) and (16%PDDA/PEI)−PAA hydrogels show an enhanced tensile strength but a slightly decreased stretchability. Meanwhile, both the tensile strength and stretchability of the PEI−(20%PSS/PAA) and (16%PDDA/PEI)−PAA hydrogels are much lower than those of the (16%PDDA1/PEI)− (20%PSS1/PAA) hydrogels. Taken together, it can be clarified that the in situ-formed nanoparticles in the (16%PDDA1/PEI)− E

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Macromolecules

Figure 5. Schematic illustration of the simultaneously strengthening and toughening mechanism of the (16%PDDA1/PEI)−(20%PSS1/PAA) hydrogels.

absorbing cross-linkers to effectively dissipate energy and toughen the hydrogels. Healability and Recyclability of the (16%PDDA1/PEI)− 20% ( PSS1/PAA) Hydrogels. Benefiting from the dynamic and reversible hydrogen-bonding and electrostatic interactions, the (16%PDDA1/PEI)−(20%PSS1/PAA) hydrogels exhibit excellent self-healing and recycling properties. As shown in Figure 6a(i), two (16%PDDA1/PEI)−(20%PSS1/PAA) hydrogel cuboids, with one of them stained via a red dye of Allura Red AC, were both cut into two pieces by a razor blade. The two pieces of differently colored hydrogels were brought into contact and then incubated in water at room temperature for different times to achieve complete healing from damage. The two pieces of fractured hydrogels can be rapidly healed together within 5 min at room temperature. Although the fracture can be still observed at the contact position, the 5 min-healed hydrogel can be stretched to a length of 3.8 times longer than its initial length (Movie S1). As indicated in Figure 6b, the (16%PDDA1/PEI)−(20%PSS1/PAA) hydrogels can be better healed on elongating the incubation time in water. The 14 hhealed hydrogel almost completely restores its initial mechanical properties (Figure 6b). The 14 h-healed hydrogel reaches ∼95 and ∼100% of the tensile strength and strain at break of the pristine hydrogels, respectively. As shown in Figure 6a(ii−iv), the 14 h-healed hydrogel can be stretched to a length of 6.6 times higher than its initial length without fracture and can hold a weight of 500 g as the pristine hydrogel does. Under stretching, the 14 h-healed hydrogel breaks, but the break does not occur at the healed position (Movie S2), further confirming the complete healing of the fractured hydrogels. In water, the hydrogen-bonding and electrostatic interactions among PAA, PSS, PEI, and PDDA in the fractured surfaces of the (16%PDDA1/PEI)−(20%PSS1/PAA) hydrogel can dynamically break and reform. In this way, the polyelectrolytes in the fracture surfaces can rebuild hydrogen-bonding and electrostatic interactions to finely heal the fractured hydrogels (Figure 6c). To the best of our knowledge, the (16%PDDA1/PEI)−(20%PSS1/PAA) hydrogel with a toughness of 19.53 ± 0.48 MJ/m3 is the toughest healable hydrogel compared with the previously reported hydrogels with healability (Table S1). Based on the same mechanism for healing, the (16%PDDA1/PEI)−(20%PSS1/PAA) hydrogels can also be recycled for repeated usage. As shown in Figure 6d, the hydrogels were first dried and then ground into powders. The powders were transferred into a heart-shaped Teflon mold followed by adding several drops of water into the mold. After a compression molding process under a pressure of ca. 3 kPa

Figure 4. (a, b) Frequency dependence of the storage modulus G′ and loss modulus G″ of the PEI−PAA (a) and (16%PDDA1/PEI)− (20%PSS1/PAA) (b) hydrogels. The as-obtained results were processed based on the time−temperature superposition principle by horizontally shifting the individual curves corresponding to different temperatures toward the curve corresponding to the reference temperature of 25 °C. (c, d) Arrhenius plots for the shift factors of the PEI−PAA (c) and (16%PDDA1/PEI)−(20%PSS1/PAA) (d) hydrogels. The apparent activation energy values (Ea) shown in (c) and (d) are calculated from the slopes of the curves.

the (16%PDDA1/PEI)−(20%PSS1/PAA) hydrogels, G′ is larger than G″ over the whole shear frequency range from 1.5 × 10−3 to 5.4 × 103 rad/s (Figure 4b), suggesting that the hydrogels are always in a gel state in the whole shear frequency range. Meanwhile, G′ of the (16%PDDA1/PEI)−(20%PSS1/PAA) hydrogels is also larger than that of the PEI−PAA hydrogels over the whole shear frequency range. This result is in good agreement with the much enhanced mechanical tensile strength and toughness of the (16%PDDA1/PEI)−(20%PSS1/ PAA) hydrogels compared with the PEI−PAA hydrogels. The Arrhenius plot for the (16%PDDA1/PEI)−(20%PSS1/PAA) hydrogels exhibits three apparent Ea values of 26.7, 63.6, and 136.2 kJ/mol (Figure 4d), which correspond to three kinds of physical interactions within the hydrogels. Compared with the PEI−PAA hydrogels, except two similar Ea values of 26.7 and 63.6 kJ/mol, a much higher Ea value of 136.2 kJ/mol of the (16%PDDA1/PEI)−(20%PSS1/PAA) hydrogels can be assigned to the much stronger electrostatic interactions with PDDA− PSS nanoparticles. In light of the phase-separated microstructure of the (16%PDDA1/PEI)−(20%PSS1/PAA) hydrogels and the involved three kinds of polyelectrolyte interactions, the strengthening and toughening mechanism of the (16% PDDA1 /PEI)− (20%PSS1/PAA) hydrogels is depicted in Figure 5. In the (16%PDDA1/PEI)−(20%PSS1/PAA) hydrogels, PEI mainly interacts with PAA through hydrogen-bonding and weak electrostatic interactions to form the matrix, whereas PDDA mainly interacts with PSS through strong electrostatic interactions to in situ form PDDA−PSS nanoparticles that are homogeneously dispersed in the PEI−PAA matrix. Moreover, electrostatic interactions and polymer chain entanglements still exist among PSS and PDDA in the surface of the PDDA−PSS nanoparticles and PEI and PAA in the matrix. In this scenario, the PDDA−PSS nanoparticles can not only act as nanofillers to strengthen the (16%PDDA1/PEI)− (20%PSS1/PAA) hydrogels but also act as deformable energyF

DOI: 10.1021/acs.macromol.9b00053 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

neously dispersed in the PEI−PAA matrix. The PDDA−PSS nanoparticles act as deformable polymer nanofillers that can simultaneously strengthen and toughen the (16%PDDA1/PEI)− (20%PSS1/PAA) hydrogels. The tensile strength, strain at break, and toughness of the (16%PDDA1/PEI)−(20%PSS1/PAA) hydrogels reach 1.26 ± 0.06 MPa, 2434.2 ± 150.3%, and 19.53 ± 0.48 MJ/m3, respectively, which are 2.4, 1.8, and 5.2 times higher than the corresponding values of the PEI−PAA hydrogels. The (16%PDDA1/PEI)−(20%PSS1/PAA) hydrogel exhibits the optimized mechanical performance among all the prepared hydrogels. Variations of the PDDA and PSS fractions or the PDDA/PSS ratios can lead to hydrogels with welltailored mechanical properties. Because of the high reversibility of the hydrogen-bonding and electrostatic interactions within the (16%PDDA1/PEI)−(20%PSS1/PAA) hydrogels, they can autonomously and efficiently heal damages at room temperature in water to restore their original mechanical strength and toughness. Meanwhile, the (16%PDDA1/PEI)−(20%PSS1/PAA) hydrogels can even be recycled and reshaped under a mild condition after being dried and ground. Self-healing and recyclable hydrogels with high stretchability and toughness are potentially useful in diverse areas such as tissue engineering, stretchable electronics, and wearable or implantable devices. We believe that the present work provides a new and effective strategy to fabricate self-healing and recyclable polymer composite materials with simultaneously enhanced tensile strength, stretchability, and toughness by simply mixing multiple kinds of polymers with complementary interactions. Considering the wide ranges of polymers that can be used, selfhealing and recyclable polymer composites with well-tailored mechanical properties can be fabricated by simultaneous complexation of multiple kinds of polymers with complementary interactions. The nanostructures generated during the complexation process are important to simultaneously enhance the tensile strength, stretchability, and toughness of the hydrogels or polymer composite materials. Moreover, together with the PEI−PAA hydrogels, the ( X% PDDA M/PEI)− (Y%PSSN/PAA) hydrogels with in situ-formed nanoparticles provide a model system to obtain in-depth understanding of the influence of well-dispersed polymer nanoparticles on the mechanical properties of the polymer composites.

Figure 6. (a) Digital photographs of the undyed and dyed (16%PDDA1/PEI)−(20%PSS1/PAA) hydrogel cuboids that were cut into two pieces (i), and the separated pieces with different colors were well healed at room temperature for 14 h (ii). The healed hydrogel with a width of 4 mm and thickness of 1.1 mm can be stretched to a length 6.6 times longer than its original length without fracture (iii) and hold a weight of 500 g (iv). (b) Stress−strain curves of the pristine and 1, 2, 6, 10, and 14 h-healed hydrogels, which were previously cut into two pieces. (c) Schematic illustration of the selfhealing process of the (16%PDDA1/PEI)−(20%PSS1/PAA) hydrogels. (d) Digital photographs of the (16%PDDA1/PEI)−(20%PSS1/PAA) hydrogel sheets that were cut into small pieces (i) and then dried in a vacuum oven at room temperature for 24 h (ii). The dried hydrogel pieces were ground into powders (iii), and the powders were reshaped into a heart-shaped hydrogel (iv). (e) Stress−strain curves of the pristine, recycled hydrogels, and the recycled hydrogel followed with a cut-healing process.

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for 24 h at room temperature, a piece of heart-shaped (16%PDDA1/PEI)−(20%PSS1/PAA) hydrogel was obtained. The tensile strength and strain at break of the recycled hydrogel are as high as 1.02 MPa and 2258.9%, respectively, which reach ∼81 and ∼93% of the corresponding original values of the pristine hydrogels (Figure 6e). The recycled (16%PDDA1/PEI)−(20%PSS1/PAA) hydrogels can also be healed to achieve the same mechanical properties as the recycled hydrogels.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00053. Experimental section, digital images, FT-IR spectra, water content data, 1H NMR spectra, TEM images, stress−strain curves, mechanical properties and selfhealing capability of our work and some typical selfhealing hydrogels (PDF) 5 min-healed hydrogel stretched to a length of 3.8 times longer than its initial length (AVI) Under stretching, 14 h-healed hydrogel breaks (AVI)

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CONCLUSIONS In summary, we have demonstrated a simple yet effective strategy for the fabrication of self-healing and recyclable hydrogels with integrated high mechanical strength, stretchability, and toughness. The hydrogels were fabricated based on simultaneous complexation of oppositely charged strong polyelectrolytes of PDDA and PSS with weak polyelectrolytes of PEI and PAA in an aqueous solution. Due to the mismatched interactions between the strong and weak polyelectrolytes and the different hydrophobicity of their complexes, the as-fabricated (16%PDDA1/PEI)−(20%PSS1/ PAA) hydrogels have a phase-separated structure in which the in situ-formed PDDA−PSS nanoparticles are homoge-

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xiaokong Liu: 0000-0002-0355-2658 Junqi Sun: 0000-0002-7284-9826 G

DOI: 10.1021/acs.macromol.9b00053 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Notes

(19) Sun, G.; Li, Z.; Liang, R.; Weng, L. T.; Zhang, L. Super Stretchable Hydrogel Achieved by Non-Aggregated Spherulites with Diameters