Letter Cite This: ACS Macro Lett. 2019, 8, 500−505
pubs.acs.org/macroletters
One-Step Synthesis of Healable Weak-Polyelectrolyte-Based Hydrogels with High Mechanical Strength, Toughness, and Excellent Self-Recovery
ACS Macro Lett. Downloaded from pubs.acs.org by CALIFORNIA STATE UNIV BAKERSFIELD on 04/15/19. For personal use only.
Xu Fang and Junqi Sun* State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, P. R. China S Supporting Information *
ABSTRACT: Excellent self-recovery is critically important for soft materials such as hydrogels and shape memory polymers. In this work, weak-polyelectrolyte-based hydrogels with high mechanical strength, toughness, healability, and excellent self-recovery are fabricated by one-step polymerization of acrylic acid and poly(ethylene glycol) methacrylate in the presence of oppositely charged branched polyethylenimine. The synergy of electrostatic and hydrogen-bonding interactions and the in situ formed polyelectrolyte complex nanoparticles endow the hydrogels with a tensile strength of ∼4.7 MPa, strain at break of ∼1200%, and toughness of ∼32.6 MJ m−3. The hydrogels can recover from an ∼300% strain to their initial state within 10 min at room temperature without any external assistance. Moreover, the hydrogels can heal from physical cut at room temperature and exhibit a prominent shape-memory performance with rapid shape recovery speed and high shape-fixing and shape-recovery ratios.
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toughness, and fast self-recovery by a convenient one-step fabrication process. Recently, Gong and co-workers developed a series of tough hydrogels cross-linked via electrostatic interactions by copolymerization or sequential homopolymerization of two kinds of oppositely charged strong polyelectrolyte monomers followed by dialysis against water to remove mobile counterions.25,31 For example, the polyion complex (PIC) hydrogels synthesized by sequential homopolymerization of 3-(methacryloylamino)propyl-trimethylammonium chloride (MPTC) and sodium pstyrenesulfonate (NaSS) are soft. After dialysis against water, mobile counterions are removed to strengthen the electrostatic interactions, and the resultant PMPTC/PNaSS hydrogels exhibit a high tensile strength of ∼3.7 MPa and toughness of ∼14.8 MJ m−3. The self-recovery of the hydrogels against ∼300% strain takes 2 h at room temperature.31 In our previous work, the layer-by-layer (LbL)-assembled poly(acrylic acid) (PAA)/poly(allylamine hydrochloride) (PAH) weak polyelectrolyte film with high Young’s modulus and excellent recovery was fabricated for the construction of an energetic bilayer actuator.32 The actuator with the PAA/PAH film as the driving layer can drive a walking device carrying a load 120 times heavier than the actuator to walk steadily and rapidly under periodic alternation of the surrounding humidity. Based on these results,31,32 we believe that the complexation of oppositely
ydrogels are three-dimensionally cross-linked polymer networks containing large amounts of water.1−3 The fabrication of hydrogels with high mechanical strength, toughness, and excellent self-recovery has attracted the attention of scientists for decades because of their applications in sensing, electronic devices, soft machines, and tissue engineering.4−12 The excellent self-recovery is crucial to improve the durability and reliability of hydrogels with high mechanical strength and toughness because self-recovery can effectively retard degradation of mechanical properties after a long-term and repeated usage of the hydrogels.13−16 The mechanical strength, toughness, and self-recovery of hydrogels can be effectively improved by incorporating reversible noncovalent interactions as sacrificial bonds. However, the full recovery of tough hydrogels with high mechanical strength in megapascal order still takes a long time, up to several hours even with the assistance of external stimuli.17−21 The unsatisfied recovery of hydrogels with high mechanical strength is mainly attributed to the heterogeneity of the hydrogel network and the inefficient dissociation/association process of the sacrificial bonds in the hydrogels.22−24 Moreover, the currently employed methods for the fabrication of recoverable hydrogels with high mechanical strength and toughness usually require multistep processes involving posttreatments such as dialysis, solvent exchange, coordination with metal ions, and so forth to further tailor the network structure and interactions of the hydrogels.13,15,20,25−30 Therefore, it is challenging to fabricate hydrogels that can simultaneously possess high mechanical strength (in megapascal order), high © XXXX American Chemical Society
Received: March 14, 2019 Accepted: April 11, 2019
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DOI: 10.1021/acsmacrolett.9b00189 ACS Macro Lett. 2019, 8, 500−505
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Figure 1. (a) Schematic illustration for the synthesis and structure of weak-polyelectrolyte-based hydrogels through a one-step polymerization method. Digital photos to demonstrate the high strength, toughness, and self-recovery of the PEI0.15/AA1/MPEG0.04 hydrogels: (b−e) the hydrogel strip before (b) and after stretching (c), knotting (d), and twisting (e). (f) The fully recovered hydrogel from consecutive stretching, knotting, and twisting (from c to e) can lift a 2 kg weight. (g, h) The hydrogel after the aforementioned treatments (from c to f) can recover to its initial state by relaxing at room temperature for 10 min (g) or in 50 °C water for 10 s (h).
poly(AA-co-MPEG) and protonated amine groups of PEI and the hydrogen-bonding interactions between carboxylic acid groups of poly(AA-co-MPEG) are the main driving forces for the formation of hydrogels (Figure S1, Supporting Information).35−38 As shown in Figure 1b, a piece of a PEI0.15/AA1/ MPEG0.04 hydrogel strip with a width of 3 mm and thickness of 2 mm can sequentially withstand various deformations such as stretching, knotting, and twisting (Figure 1c−e). The hydrogel that fully recovers from the above deformations can lift a 2 kg weight without any damage (Figure 1f and Movie S1). After undergoing the above-mentioned deformations and loadbearing tests, the hydrogel can completely recover to its initial shape at room temperature within 10 min (Figure 1g). Alternatively, the full recovery of the hydrogel can also be achieved within 10 s in 50 °C water (Figure 1h). All these results demonstrate that the PEI0.15/AA1/MPEG0.04 hydrogels possess remarkable load-bearing capability, high mechanical strength, toughness, and excellent self-recovery. The mechanical properties of the PEI0.15/AA1/MPEGy hydrogels can be tailored by varying the molar ratios of MPEG from 0 to 0.06. Figure 2a shows the typical stress−strain curves of the PEI0.15/AA1/MPEGy hydrogels. To compare the mechanical properties of our hydrogels with the previously reported tough hydrogels,20,25,31 stretching speed was set at 50 mm/min for tensile tests. All the hydrogels have a high tensile strength in the megapascal level. In particular, the PEI0.15/AA1/ MPEG0 hydrogels exhibit an ultrahigh tensile strength of ∼12.0 MPa with a strain at break of ∼306%. The X-ray diffraction (XRD) spectrum of the PEI0.15/AA1/MPEG0.04 hydrogels shows no diffraction peak, indicating that there is no crystalline phase in the hydrogels (Figure S2, Supporting Information). The increase of the MPEG ratio in the PEI0.15/AA1/MPEGy hydrogels leads to an obvious decrease of tensile strength
charged weak polyelectrolytes can provide a convenient way to fabricate hydrogels with high mechanical strength, toughness, and excellent self-recovery because of the following facts: (i) the coiled weak polyelectrolyte chains are helpful for enhancing elasticity of the hydrogels and (ii) the number of counterions tightly associated with weak polyelectrolytes is very limited, and no dialysis step is necessary for further enhancing the mechanical strength of the hydrogels.32−34 Herein, we demonstrate the fabrication of weak-polyelectrolyte-based hydrogels with high tensile strength, toughness, and excellent self-recovery through one-step polymerization of acrylic acid (AA) and poly(ethylene glycol) methacrylate (MPEG) in the presence of oppositely charged weak-polyelectrolyte-branched polyethylenimine (PEI). The hydrogels at ∼300% strain require only 10 min to fully recover to their initial state at room temperature without any external assistance. The hydrogels show self-healing ability after damage because of the dynamic nature of the electrostatic and hydrogen-bonding interactions. Moreover, the thermosensitivity of hydrogen bonds endows the hydrogels with prominent shape-memory function with rapid shape recovery rate and high shape-fixing and recovery ratios. As shown in Figure 1a, the aqueous solution comprising AA and MPEG monomers, PEI weak polyelectrolytes, and initiators of Irgacure 2959 was irradiated by UV light at 365 nm for 2 h to fabricate the weakpolyelectrolyte-based hydrogels. The monomer molar ratio between PEI and AA in solution for polymerization is 0.15:1, while the monomer molar ratio between MPEG and AA is 0:1, 0.02:1, 0.04:1, and 0.06:1, respectively. For simplicity, the hydrogels are denoted as PEIx/AA1/MPEGy, where x and y represent the monomer molar ratio of PEI and MPEG to AA, respectively. The copolymerization of AA and MPEG produces poly(AA-co-MPEG) random copolymers. The electrostatic interactions between carboxylate groups of PAA segments in 501
DOI: 10.1021/acsmacrolett.9b00189 ACS Macro Lett. 2019, 8, 500−505
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ACS Macro Letters
hydrogels. The resultant PEI0.15/AA1/PEG0.04 hydrogels have a lower tensile strength, strain at break, and toughness than those of the PEI0.15/AA1/MPEG0 hydrogels. Meanwhile, the strain at break and toughness of the PEI0.15/AA1/PEG0.04 hydrogels are much lower than those of the PEI0.15/AA1/MPEG0.04 hydrogels (Figure S5 and Table S1, Supporting Information). These results indicate that the MPEG segments in the poly(AA-coMPEG) copolymers are crucial in improving the mechanical properties of the PEI0.15/AA1/MPEGy hydrogels. Moreover, the incorporation of MPEG segments can improve the homogeneity of the PEI0.15/AA1/MPEGy hydrogel network (Figure S6, Supporting Information). With the increase of MPEG ratio, the PEI0.15/AA1/MPEGy hydrogels turn from opaque for PEI0.15/AA1/MPEG0 hydrogels to highly transparent for PEI0.15/AA1/MPEG0.06 hydrogels (Figure S7, Supporting Information). The PEI ratio can also have an influence on the mechanical properties of the PEIx/AA1/MPEG0.04 hydrogels. The PEI0/AA1/MPEG0.04 hydrogels are too weak for tensile tests. The strength and strain of the PEIx/AA1/MPEG0.04 hydrogels significantly increase with increasing PEI ratio from 0.05 to 0.15. However, further increase of PEI ratio over 0.15 leads to a dramatic decrease of tensile strength and strain at break of the hydrogels (Figure S8 and Table S1, Supporting Information). Therefore, the PEI0.15/AA1/MPEG0.04 hydrogels have the well-optimized mechanical properties of high mechanical strength, extensibility, and toughness. As far as we know, the PEI0.15/AA1/MPEG0.04 hydrogels have the highest toughness and tensile strength compared with previously reported hydrogels fabricated by one-step polymerization without post-treatment (Figure S9, Supporting Information).41−45 The PEI0.15/AA1/MPEG0.04 hydrogels with high mechanical strength and toughness were subjected to cyclic tensile tests to evaluate their self-recovery. As shown in Figure 3a, the first loading−unloading curve of the hydrogel shows a large hysteresis, with ∼76% of total energy being dissipated when the hydrogel is stretched to ∼300% strain. When the second loading−unloading test was conducted immediately, the hydrogel exhibits an obvious decrease in hysteresis loop and elastic modulus, which originates from the partially fractured physical network of the hydrogel during the first loading cycle. With increasing the resting time, the stress−strain curves of the hydrogel gradually recover to the first loading−unloading curve. The hydrogel can achieve ∼100% of the original dissipated energy and elastic modulus after 10 min rest (Figure 3a and 3b). Moreover, the hydrogels were further subjected to 10 successive loading−unloading cycles to undergo sufficient fatigue. Figure 3c shows obvious drops in both hysteresis loops and tensile strength in the second cycle, and the downward tendency becomes slow in the following cycles. This result is due to the fact that the broken sacrificial networks in the first loading− unloading cycle cannot recover immediately, and only a small number of sacrificial bonds break in the following tests at the same strain in the subsequent loading−unloading cycles. However, after resting at room temperature for ∼10 min, the hydrogel was again subjected to 10 successive loading− unloading cycles at the strain of ∼300% (Figure 3d). The hysteresis curves can almost overlap with those in the original hydrogels in Figure 3c, indicating that the hydrogels have excellent resilience and fatigue resistance. Note that the hydrogels with a tensile strength of megapascal order usually take a rest of 1 to 4 h at room temperature to gain a full recovery from an ∼300% elongation. Table S2 in the Supporting
Figure 2. (a,b) Stress−strain curves (a) and toughness (b) of the PEI0.15/AA1/MPEGy hydrogels with y being 0, 0.02, 0.04, and 0.06. (c,d) TEM images of the PEI0.15/AA1/MPEG0 (c) and PEI0.15/AA1/ MPEG0.04 (d) hydrogels.
from ∼12.0 MPa to ∼3.7 MPa but a remarkable increase in strain at break from ∼306% to ∼1451%. As shown in Figure 2b, the introduction of MPEG can also enhance the toughness of the hydrogels. Among these hydrogels, the PEI0.15/AA1/MPEG0.04 hydrogels have a tensile strength of ∼4.7 MPa, strain at break of ∼1200%, and the highest toughness of ∼32.6 MJ m−3. The structures of the PEI0.15/AA1/MPEGy hydrogels were investigated by transmission electron microscopy (TEM) to clarify the mechanism of MPEG in tailoring mechanical properties of the hydrogels. As indicated in Figure 2c, the PEI0.15/AA1/ MPEG0 hydrogels negatively stained with uranyl acetate show an obvious two-phase structure with bright nanoparticles of ∼20.9 nm in diameter homogeneously dispersed in the hydrogels. As uranyl acetate preferentially stains the carboxylic acid groups of PAA in hydrogels,39 the stained areas are a hydrogen-bonded PAA network, while the unstained nanoparticles are in situ formed insoluble PAA/PEI complexes. The in situ formed PAA/PEI complex nanoparticles in hydrogels are further confirmed by precipitates that form when aqueous solutions of PEI and PAA are mixed together (Figure S3a−c, Supporting Information). These PAA/PEI complex nanoparticles serve as stiff nanofillers to strengthen the PEI0.15/ AA1/MPEG0 hydrogels. In contrast, the nanoparticles in PEI0.15/AA1/MPEG0.04 hydrogels have a diameter of ∼5.2 nm (Figure 2d). The TEM images in Figure 2c,d and Figure S4 indicate that a higher MPEG content leads to smaller complex nanoparticles in PEI0.15/AA1/MPEGy hydrogels. The poly(AAco-MPEG) is bottle-brush-like with PAA as backbones and MPEG as branches because the MPEG chain is much longer than that of AA.40 The incorporation of MPEG can effectively isolate the long PAA chain into short PAA segments and disrupt their electrostatic interactions with PEI (Figure S3d−f, Supporting Information). Therefore, the PAA/PEI complex nanoparticles in the PEI0.15/AA1/MPEGy hydrogels become smaller and softer with the increase of MPEG content. As a result, the tensile strength of the PEI0.15/AA1/MPEGy hydrogels decreases, and their strain at break increases with increasing MPEG contents. In a control experiment, copolymerized MPEG was replaced with poly(ethylene glycol) (PEG) for the fabrication of PEI0.15/AA1/PEG0.04 hydrogels which have the same compositions with that of the PEI0.15/AA1/MPEG0.04 502
DOI: 10.1021/acsmacrolett.9b00189 ACS Macro Lett. 2019, 8, 500−505
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their initial mechanical properties. The stress relaxation of the hydrogels was measured by stretching them to ∼25% strain at 50 °C and then maintaining the strain for 9 min. The stress relaxation curves indicate that the residual stress of the hydrogel gradually decreases with time but still keeps ∼75% of its initial stress at the stress relaxation time of 9 min (Figure S10, Supporting Information), demonstrating that the electrostatic interactions and part of the retained hydrogen bonds can ensure the integrity and elasticity of the hydrogels at 50 °C. As shown in Figure 3e, the self-healing capacity of the PEI0.15/ AA1/MPEG0.04 hydrogels is investigated by cutting the hydrogel strip into two parts. The cut surfaces were immersed in a 3 M NaCl solution for 2 min and then were brought into contact for 12 h at room temperature. Finally, the healed hydrogels were immersed into water to remove NaCl. Figure 3f and 3g show that the healed hydrogels can be stretched again and hold a 500 g weight. As indicated in Figure S11 (Supporting Information), the healed hydrogels have a tensile strength of ∼2 MPa and cannot completely restore their initial mechanical performance. However, compared with the previously reported healable hydrogels with healing process being conducted at room temperature, the healed PEI0.15/AA1/PEG0.04 hydrogels with a tensile strength of ∼2 MPa still possess satisfactory mechanical strength.20,46−54 The saline solution can break partial electrostatic and hydrogen-bonding interactions on the cut surfaces and facilitate the chain mobility. When bringing into contact, the electrostatic and hydrogen-bonding interactions can be partially rebuilt via chain interdiffusion at the damaged surfaces to heal the fractured hydrogels. Because of the thermosensitivity of hydrogen bonds and the excellent self-recovery, the PEI0.15/AA1/MPEG0.04 hydrogels were investigated as shape-memory polymer materials. As shown in Figure 4a, a strip of the PEI0.15/AA1/MPEG0.04
Figure 3. (a−d) Self-recovery and fatigue resistance of the PEI0.15/ AA1/MPEG0.04 hydrogels. The fast self-recovery capability (a) and the recovery of elastic modulus and hysteresis loop (b) at different relaxing time. Ten successive loading−unloading cycles of original (c) and recovered (d) samples after resting for 10 min at room temperature. (e−g) Self-healing of the PEI0.15/AA1/MPEG0.04 hydrogels. (e) The hydrogel is cut into two parts. (f, g) The healed sample can withstand stretching (f) and lift a weight of 500 g (g).
Information summarizes the recently reported tough hydrogels with a tensile strength of megapascal order that exhibit good selfrecovery properties. It can be recognized that the one-step synthesized PEI0.15/AA1/MPEG0.04 hydrogels have the highest toughness and the rapidest self-recovery at room temperature without any external assistance. Generally, the toughness of hydrogels is attributed to the introduction of an energydissipating mechanism in which weak sacrificial bonds can break to dissipate energy,17,25 while the self-recovery of hydrogels requires elastic retraction arising from strong cross-linking and the reassociation of the previously cleaved sacrificial bonds.14,17,25,31 In the PEI0.15/AA1/MPEG0.04 hydrogel, the in situ formed complex nanoparticles serve as strong cross-linkers to strengthen the hydrogels and make them highly recoverable. Upon stretching, the coiled polymer chains in the hydrogels are extended, and the weak hydrogen bonds dissociate to effectively dissipate energy. After stress release, the deformed hydrogel recovers to its original state due to the elastic contraction of the chain skeleton enhanced by PAA/PEI complex nanoparticles. Meanwhile, the fractured networks can quickly repair through reforming hydrogen bonds between carboxylic acid groups of PAA. The recovery process of the deformed PEI0.15/AA1/ MPEG0.04 hydrogels can be significantly accelerated by recovering in 50 °C water (Figure 1h). When temperature is raised to 50 °C, a higher amount of hydrogen bonds in the hydrogels are in the broken state, while PAA/PEI complex nanoparticles based on electrostatic interactions remain stable, leading to high mobility of polymer chains and good elastic contraction in hydrogels. After cooling to room temperature, the broken hydrogen bonds in the hydrogels reform to fully recover
Figure 4. Shape-memory performance of the PEI0.15/AA1/MPEG0.04 hydrogels. (a) Digital images of a piece of hydrogel before (i) and after (ii) being fixed into a temporary shape. (iii) Digital image of a spiral hydrogel recovering to its permanent shape in 50 °C water for 15 s. The scale bar is 1 cm. (b) The shape-recovery ratio (Rr) of the hydrogels as a function of time in water of 37 and 50 °C. (c) The shape-fixing ratio (Rf) and shape-recovery ratio (Rr) of the hydrogels in 50 successive shape-memory cycles.
hydrogel was deformed into a spiral by wrapping it on a glass rod at 50 °C. Then, the spiral was cooled to 10 °C for 5 min to fix the temporary shape. The spiral hydrogel was transferred into 50 °C water, and the spiral hydrogel quickly recovered to the initial strip shape within 15 s, with a shape-recovery ratio of ∼99% (Movie S2, Supporting Information). The rapid and highly efficient shape recovery is attributed to the aforementioned 503
DOI: 10.1021/acsmacrolett.9b00189 ACS Macro Lett. 2019, 8, 500−505
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (NSFC grant 21774049) and the Program for JLU Science and Technology Innovative Research Team (2017TD-10).
excellent self-recovery of the hydrogels. Specifically, the hydrogen bonds in the spiral hydrogel can quickly break at 50 °C to allow for the efficient recovery of the spiral hydrogel to its initial state. Figure 4b shows that the spiral hydrogel can completely recover to its initial state at the human body temperature of ∼37 °C within ∼3 min. The shape-memory PEI0.15/AA1/MPEG0.04 hydrogels exhibit excellent durability against repeated shape-memory cycles. As indicated in Figure 4c, the hydrogels still maintain a shape-fixing ratio of ∼94% and a shape-recovery ratio of ∼99% after 50 shape-memory cycles. In summary, we have reported a new kind of weakpolyelectrolyte-based hydrogel with high strength, toughness, and excellent self-recovery by one-step polymerization of AA and MPEG in the presence of PEI. The MPEG segments in the poly(AA-co-MPEG) copolymers can control the interactions between PAA and PEI and are important to tailor the mechanical properties of the resultant hydrogels. Because of the synergy of electrostatic and hydrogen-bonding interactions and the reinforcement effect of the PAA−PEI complex nanoparticles, the PEI0.15/AA1/MPEG0.04 hydrogels exhibit a high tensile strength of ∼4.7 MPa, toughness of ∼32.6 MJ m−3, and excellent self-recovery such that the deformed or stretched hydrogels can fully recover to their initial states within 10 min at room temperature. Meanwhile, the hydrogels have prominent shape-memory function with high shape-fixing and shaperecovery ratios and rapid shape recovery. The dynamic nature of electrostatic and hydrogen-bonding interactions endows the hydrogels with the ability to heal from physical damage. The excellent self-recovery and healability can significantly enhance the durability and reliability of the hydrogels for long-term usage. Moreover, the one-step polymerization method provides a practically simple and convenient route for the fabrication of the PEI0.15/AA1/MPEG0.04 hydrogels. Given the wide choice of weak polyelectrolytes or monomers, we believe that this study will open a new avenue for one-step fabrication of various tough hydrogels with well-tailored mechanical properties and desired functions.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.9b00189.
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Materials, Synthetic procedures, instrumentation, and supporting figures and tables (PDF) Movie showing a hydrogel that fully recovers from the above deformations can lift a 2 kg weight without any damage (AVI) Movie showing a spiral hydrogel being transferred into 50 °C water and the spiral hydrogel quickly recovering to the initial strip shape within 15 s, with a shape-recovery ratio of ∼99%(AVI)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Junqi Sun: 0000-0002-7284-9826 Notes
The authors declare no competing financial interest. 504
DOI: 10.1021/acsmacrolett.9b00189 ACS Macro Lett. 2019, 8, 500−505
Letter
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DOI: 10.1021/acsmacrolett.9b00189 ACS Macro Lett. 2019, 8, 500−505
