Fabrication of Poly(acrylic acid)/Boron Nitride Composite Hydrogels with Excellent Mechanical Properties and Rapid Self-Healing Through Hierarchically Physical Interactions
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Many living tissues possess excellent mechanical properties and self-healing ability. To mimic these living tissues, a series of novel composite hydrogels, poly(acrylic acid)/surface-modified boron nitride nanosheets (PAA/BNNS-NH2) were fabricated simply through hierarchically physical interactions: molecular-scale metal coordination interaction between –COOH of PAA and Fe3+ and nanoscale H-bond between –COOH of PAA and –NH2 of BNNS-NH2. The composite hydrogels exhibit both excellent mechanical properties (including enhanced fracture stress, elongation, toughness, Young’s modulus, and dissipated energy) and rapid healing ability without any external stimulus. Especially, the B0.5P70 (the hydrogel with BNNS concentration of 0.5 mg mL− 1, the water content of 70 wt%) exhibits a fracture stress of ~ 1311 kPa and toughness of ~ 4.7 MJ m− 3, almost ~ 3 times and ~ 8 times to B0P70, respectively. The excellent properties, combined with the simple preparation method, endow these composite hydrogels with potential applications.
KeywordsSelf-healing hydrogel Boron nitride nanosheets Hierarchically physical interactions
Composite hydrogel with the BNNS-NH2 concentration of x mg mL− 1 and water content of y wt%
Poly(acrylic acid)/surface-modified boron nitride nanosheets
Graphene oxide/poly(acrylic acid)
Scanning electronic micrographs
Hydrogels with three-dimensional networks formed by covalent bonds and/or physical interactions crosslinking containing a large amount of water possess high hydrophilicity, water-holding capacity and unexceptional biocompatibility [1, 2, 3, 4], enabling to be one of the most popular biomaterials. However, most hydrogels have poor mechanical property, which largely limited the applications. It is well known that many living tissues, such as muscle, ligament, and skin, possess excellent mechanical property and significant ability to heal wounds autonomously [5, 6, 7]. Inspired by these living tissues, materials with high mechanical properties and self-healing ability have been explored for various applications [8, 9], including tissue engineering, drug release, wound dressing, contact lenses, sensors, and actuators [2, 10, 11, 12]. Ihsan et al. reported a polyampholyte hydrogel self-healed through re-forming the ironic bonds at fracture surface . Zhang et al. designed a PVA self-healable hydrogel with fast self-healing process through hydrogen bonds . Tao et al. prepared a cold resistance self-healing hydrogel crosslinked by dynamic catechol-borate ester bonding which enable to self-heal at both room temperature and low temperature . However, all these self-healable materials have a common weakness: poor mechanical property [15, 16, 17, 18, 19] largely limited the applications.
In order to improve the mechanical property of the hydrogels, some inorganic nanomaterials have been introduced to the crosslinked systems. Han et al. reported a supermolecular hydrogel by using graphene oxide nanosheets to reduce the temperature for self-healing . Si et al. exploited a new ultrahigh-water-content, super-elastic, and shape-memory nanofiber-assembled hydrogels . The flexible SiO2 nanofibers were introduced to enhance mechanical property and to accelerate shape memory and pressure response. Especially, Duan et al. developed poly(vinyl alcohol)/boron nitride nanosheet (PVA/BNNS) composite hydrogels with enhanced mechanical properties . Gao et al. fabricated a nanocomposite hydrogel filled with exfoliated montmorillonite which dramatically improved the fracture elongation . Zhong et al. designed graphene oxide (GO)/poly(acrylic acid) (PAA/GO) nanocomposite hydrogels which significantly enhanced the mechanical properties . Novel composite self-healing hydrogels with enhanced mechanical property are still highly pursued although exploited hydrogels have advanced significantly in recent years. Boron nitride nanosheets, “white graphene”, exhibit many excellent properties including superb mechanical properties, extraordinary chemical inertness, and remarkable non-toxicity [24, 25, 26]. Notably, surface-modified BN nanosheets served as nanofillers in the nanocomposite hydrogels enhance mechanical and thermal properties and have been reported in recent works [27, 28]. Therefore, the development of a novel composite hydrogel with surface-modified BN nanosheets is still highly pursued.
Here, the novel composite hydrogels are fabricated from poly(acrylic acid) (PAA) and amino groups surface-modified boron nitride nanosheet (BNNS-NH2) through hierarchically physical interactions: molecular-scale metal coordination interaction between –COOH of PAA and ferric ion (Fe3+) and nanoscale H-bond between –COOH and BNNS-NH2 were reported. The introduction of BNNS-NH2 enhanced the mechanical property and accelerated self-healing process of the hydrogels. This work provides a new route to prepare hydrogels with excellent mechanical properties and rapid self-healing ability.
Potassium persulfate (KPS; 99.0%) and FeCl3·6H2O (99.0%) were purchased from J&K Chemical Technology, and acrylic acid (AA; 98.0%) was purchased from Sigma-Aldrich. All these chemicals were used as received without any purification. Rhodamine B (95.0%) was purchased from Sigma-Aldrich. BNNS-NH2 was obtained by our previous work . Deionized water was used throughout the experiments.
Preparation of BNNS-NH2 Dispersion
BNNS-NH2 was prepared according to our previous work . In order to make BNNS-NH2 steadily dispersed in the polymer network, it is indispensable to prepare the BNNS-NH2 water dispersions. To obtain the stable BNNS-NH2 dispersions, magnetic stirring and ultrasound bath were utilized at room temperature. The BNNS-NH2 dispersions with concentration of 1.0, 0.8, 0.5, and 0.1 mg mL− 1 were obtained by the following procedure. The 100 mg, 80 mg, 50 mg, and 10 mg of BNNS-NH2 were added in 100 mL of deionized water, respectively, under magnetic stirring (1000 rpm) for 24 h at room temperature in air ambient to obtain mixtures, and then the mixtures were sonicated (20 kHz) at room temperature for 2 h in air ambient to get stable dispersions. For prohibiting loss of the water solution, the obtained dispersions were preserved in sealed vessels with different marks for following preparation of self-healing hydrogels.
Preparation of Self-Healing Hydrogel
PAA as the common polymer with abundant –COOH groups enables to establish the amount of intrachain and interchain hydrogen bonds which endow the polymer to possess superior elasticity and favorable strength . In addition, metal coordination interactions are set up between –COOH of PAA and ferric ion (Fe3+). The two kinds of reversible non-covalent bonds equipped the hydrogel with self-healing property. The hydrogels crosslinked by non-covalent bonds always possess inferior mechanical properties. In order to enhance the strength of the hydrogel, BNNS-NH2 was introduced to the polymer three-dimensional network, which established hydrogen bonds between –NH2 of BNNS-NH2 and –COOH of PAA. Here, the composite PAA/BNNS-NH2 hydrogels were abbreviated as BxPy, in which B represents BNNS-NH2, x is the content of the BNNS-NH2 (mg mL− 1), P means PAA/BNNS-NH2 composite hydrogel, and y refers to the water content of the PAA/BNNS-NH2 composite hydrogel (mass fraction, wt%). The hydrogels were prepared according to a procedure described below. Typically, 10 mL of AA, 0.25 g of FeCl3·6H2O (1.05 mol% of AA), and 0.1 g of KPS (0.25 mol% of AA) were dissolved in BNNS-NH2 dispersions with different concentrations or deionized water under magnetic stirring (1000 rpm) at room temperature for 10 min under air ambient to form a homogeneous mixture. After that, N2 was bubbled into the mixture to remove oxygen (10 min), and then polymerization was carried out at 25 °C in water bath for 6 h to form hydrogels. Hydrogels prepared as aforementioned procedure and parameters from BNNS-NH2 dispersions with the concentration of 1.0, 0.8, 0.5, and 0.1 mg mL− 1 were denoted as B1P90, B0.8P90, B0.5P90, and B0.1P90, respectively, while hydrogels prepared from deionized water was named as B0P90.
It is well known that the hydrogels with different water contents possess entirely different mechanical properties. In order to characterize the influence of water content to the mechanical properties of the hydrogels, the hydrogels with different water contents were prepared as follows. Firstly, the BxP90 hydrogels were prepared as the aforementioned procedure and parameters. Then, the as-prepared BxP90 hydrogels were exposed in air at room temperature for different times depending on the final water content of the hydrogels. Thereinto, the obtained drying hydrogels with different water contents were labeled as BxP70, BxP50, and BxP25, respectively. The water content was calculated by the formula: water content = Ww/Wt, where the Ww is the weight of the water and Wt is the whole weight of the hydrogel. On the other hand, the crosslinking densities of BxP90 hydrogels were calculated from the results of rheological measurements, and it is well known that the higher crosslinking density leads to the more robust mechanical property. To verify the theory, it is indispensable to carry out the tensile tests. However, the BxP90 hydrogels were so soft that the electrical universal material testing machine cannot recognize the sample exhibiting no load, so the composite hydrogels with lower water content were highly required to fabricate. The hydrogels with different water contents were cut into different shapes or sizes for the following various tests.
In order to characterize the mechanical properties of the hydrogels, the as-prepared hydrogels were cut into a flaky shape (50 mm × 2 mm × 2 mm) and tested by the electrical universal material testing machine with a 200 N load cell under a speed of 50 mm min− 1 at 25 °C and a humidity of approximately 45%. The tensile stress (σ) representing strength was calculated by the equation: σ = F/(a × b), where F, a, and b were force of loading and width and thickness of hydrogels, respectively. The strain (ε) representing stretchability was defined as the change of the length, illustrated by the formula: ε = (l − l0)/l0 × 100%, where l and l0 represent the lengths after and before testing, respectively. Stiffness was characterized by Young’s modulus which was obtained from the slope of the stress-strain curve at the low strains. The toughness of the samples was illustrated as the area under stress-strain curves. The cyclic tensile tests were performed at the same experimental condition which aimed to obtain the dissipated energy. The dissipated energy was characterized by the area between the loading-unloading curves and X-axis.
The Fourier-transform infrared (FTIR) spectra was carried out to record the samples’ FTIR characters, which were recorded on a Thermo Scientific Nicolet 6700 spectrometer in attenuated total reflection (ATR) mode, with a resolution of 4 cm− 1 within the range 400–4000 cm− 1. The morphology of the hydrogels after the frozen drying process was observed on scanning electronic micrographs (SEM, Carl Zeiss AG, ZEISS EV0 MA15). In order to analyze the viscoelasticity of the hydrogels and calculate the crosslinking density, the rheological measurements were carried out by using a rheometer (HAAKE MARS III Thermo Fisher Scientific Limited, China) to measure the storage moduli (G’) and loss moduli (G”). The tensile tests were carried out to analyze the mechanical properties of the samples, which were conducted using an electrical universal material testing machine with a 200 N load cell (Instron 2360).
Results and Discussion
Here, Ge, R, and T are the terrace value of G’, gas constant, and absolute temperature, respectively. The crosslinking density is shown in Fig. 4b. With increase in the concentration of BNNS-NH2, the crosslinking density increased, which demonstrates that BNNS-NH2 also served as a crosslinker in the composite hydrogels through hydrogen bond interactions between –COOH of PAA and –NH2 of BNNS-NH2. However, the crosslinking density decreased when the BNNS-NH2 concentration is over 0.5 mg mL− 1 which corresponded with the results of the mechanical properties . It is illustrated that the excess BNNS-NH2 leads to reunion of the nanosheets which impairs the enhancement to the composite hydrogels such as B0.8Py and B1.0Py [41, 42].
The toughness is observed in Fig. 5b. It is clear that toughness increases with decreasing water content, similar to the trend of Young’s modulus. Without BNNS-NH2, the toughness of B0P70 was about ~ 0.5 MJ m− 3, and with BNNS-NH2, the toughness of B0.5P70 increased to ~ 4.7 MJ m− 3, almost eight times to that of B0P70. The B0.5P25 exhibited the highest Young’s modulus of ~ 17.9 MPa, highest tensile strength of ~ 8491 kPa, and highest toughness of ~ 10.5 MJ m− 3, which is far higher than that of B0P25.
The stiffness of most polymer hydrogels decreases with increase in the corresponding toughness. According to the Lake-Thomas model [42, 43], toughness increases but stiffness decreases with decreasing crosslinking density. In this work, a novel type hydrogel with both high stiffness and high toughness (B0.5Py) (Fig. 5) has been fabricated, which is different from the conventional hydrogels (high stiffness/low toughness or low stiffness/high toughness). The exceptional properties can be ascribed to the existence of hierarchical interactions: metal coordination interactions in molecular scale and hydrogen bonds in nanoscale.
In summary, the novel composite hydrogels have been fabricated through hierarchically physical interactions: the metal coordination interaction in molecular scale and hydrogen bond in nanoscale. The hydrogels exhibit enhanced stiffness (about 17.9 MPa), toughness (about 10.5 MJ m− 3), extension, and self-healing ability. The reversibility of metal coordination interaction and hydrogen bond interaction is responsible for the enhanced mechanical properties and self-healing efficiency. Combined with facile preparation, enhanced mechanical properties and self-healing ability make these composite hydrogels suitable for many potential applications.
This work was financially supported by Foundation of Sichuan Youth Science and Technology (2016JQ0036), Fok Ying Tung Education Foundation (161103), Open Funds of State Key Laboratory of Petroleum Pollution Control (PPC2017008) and State Key Laboratory Oil and Gas Reservoir Geology and Exploitation (PLN1201, SWPU), Natural Science Foundation of Nanchong City (NC17SY4015), and Innovative Research Team of Southwest Petroleum University (2017CXTD01).
Foundation of Sichuan Youth Science and Technology (2016JQ0036).
Fok Ying Tung Education Foundation (161103).
Open Funds of State Key Laboratory of Petroleum Pollution Control (PPC2017008) and State Key Laboratory Oil and Gas Reservoir Geology and Exploitation (PLN1201, SWPU).
Natural Science Foundation of Nanchong City (NC17SY4015).
Training Program of Innovation and Entrepreneurship for Undergraduates (201710615006).
Innovative Research Team of Southwest Petroleum University (2017CXTD01).
Availability of Data and Materials
The datasets supporting the conclusions of this article are available in the [repository name] repository [unique persistent identifier and hyperlink to datasets in http:// format].
In this work, YW designed the experimental strategy. YW, SX, and MG performed the experiment. SX and MG did the tests. TZ, WL, and DL guide the theoretical analysis for the results of the tests. SX, YW, and TZ accomplished the whole manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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