Letter pubs.acs.org/macroletters
Ultrastretchable, Self-Healable Hydrogels Based on Dynamic Covalent Bonding and Triblock Copolymer Micellization Peng Wang,† Guohua Deng,*,† Lanying Zhou,† Zhiyong Li,‡ and Yongming Chen*,‡ †
Key Laboratory of Functional Molecular Engineering of Guangdong Province, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, P. R. China ‡ Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, School of Materials Science and Engineering, Sun Yat-Sen University, Guangzhou 510275, P. R. China S Supporting Information *
ABSTRACT: Excellent mechanical properties and remarkable self-healing ability are difficult to unify in one hydrogel. We integrated acylhydrazone bonds and Pluronic F127 (PF127) micelle cross-linking, as two kinds of dynamic cross-links, in one system and developed hydrogels with superior stretchability, high toughness, and good self-healing ability. The hydrogel could stretch up to 117 times its initial length and self-heal approximately 85% of its initial strength within 24 h. The toughness of the hydrogel, indexed by the work of extension, W, reached 14.1 MJ m−3. Energy dissipation occurred from the simultaneous decomposition of the PF127 micelles and chain sliding facilitated by the reconfiguration of the acylhydrazone bonds. This unique combination and dynamics led to pronounced hysteresis in the loading−unloading cycles, as well as good recovery and self-healing of the hydrogel. Dynamic cross-linking of the covalent acylhydrazone bonds was comparable to those of physical interactions, such as coordination and ionic bonding. ydrogels are “soft and wet” materials that consist of crosslinked polymer networks swollen in water. Although hydrogels have been extensively explored and used in a variety of applications, such as sensing,1 drug delivery,2,3 actuation,4 and tissue engineering,5,6 their relatively poor mechanical properties have impeded their use in real-world applications that require excellent mechanical performance. Many efforts have been made to improve the mechanical properties of hydrogels by introducing effective energy dissipation mechanisms or implementing homogeneous cross-linking in hydrogel polymer networks.7 Over the past few decades, tough hydrogels, such as double-network (DN) hydrogels,8−13 nanocomposite hydrogels,14 slide-ring hydrogels,15 tetra-arm poly(ethylene glycol) hydrogels,16 nanostructured hydrogels,17 and macromolecular microsphere composite (MMC) hydrogels,18 have been developed. To fabricate these tough hydrogels, physical bonds, such as hydrogen bonding,19,20 ionic interactions,10,21 hydrophobic interactions and micelle cross-linking,22,23 have been used as dynamic cross-links. These physical bonds are more attractive than static covalent bonds for energy dissipation because physical bonds can readily break and reform, resulting in a hydrogel that is quite stretchable, selfhealable, and recoverable. Recent progresses have shown that integrating two or more kinds of physical interactions in one system is a promising strategy to obtain tough hydrogels with a diversity of unique properties. For example, by combining hydrophobic association, multiple hydrogen bonds, and SDS
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© 2017 American Chemical Society
(sodium dodecyl sulfate) micelle encapsulation, Vlassak and coworkers prepared an extremely stretchable and self-healable hydrogel.24 This hydrogel could stretch up to 100 times its initial length and completely self-heal within 30 s. Employing an amphiphilic triblock copolymer containing strong hydrophobic domains and sacrificial hydrogen bonds, Gong’s group fabricated tough physical DN hydrogels with high stiffness, fatigue resistance, and high mechanical performance in concentrated saline solution.11 However, unification of excellent mechanical properties, such as ultrastretchability and high toughness, with good self-healing ability in one hydrogel is still a challenge. The combination of dynamic covalent bonds with other physical dynamic cross-links has rarely been reported and might supply more diversity of such properties. In this work, we report that by integrating acylhydrazone bonds and micelle cross-linking two kinds of dynamic crosslinks, in one hydrogel, can lead to superior stretchability and good self-healing ability. This system allowed the hydrogel to stretch up to 117 times its initial length and to self-heal approximately 85% of its initial strength within 24 h without external energy input. The toughness of the hydrogel, indexed by the work of extension, W, reached 14.1 MJ m−3, which is comparable to those of many DN hydrogels. As shown in Received: July 15, 2017 Accepted: July 31, 2017 Published: August 3, 2017 881
DOI: 10.1021/acsmacrolett.7b00519 ACS Macro Lett. 2017, 6, 881−886
Letter
ACS Macro Letters Figure 1, we designed two polymer gelators: (1) a three-armed poly(ethylene oxide) (PEO) (G3) with acylhydrazine termini
Figure 1. Preparation of the hydrogel by condensation of PF127 aldehydes (Mn = 12 600) and three-armed PEO acylhydrazines (Mn = 3000) in phosphate buffer (pH 6).
and (2) a three-block copolymer PEO99-b-PPO65-b-PEO99 (PF127) (G2) with aldehyde end functional groups (see the Supporting Information for synthetic procedures). By stoichiometrically mixing G2 and G3 (1:1, acylhydrazine:aldehyde) in pH 6 phosphate buffer, a hydrogel formed in several minutes. In the polymer networks, the acylhydrazone bonds and PF127 micelles were connected. The acylhydrazone bonds were formed by condensation of the acylhydrazine and aldehyde end groups, while the micelles were generated by selfassembling PF127 in water. The acylhydrazone bonds are strong due to their covalent nature, but their formation and breakage are reversible under mild acidic or neutral conditions. As reported by us and later by others, acylhydrazone bonds can lead to self-healable and adaptive materials or dynamers.25−37 Meanwhile, PF127 is a typical amphiphilic triblock copolymer and self-assembles into micelles in water. Upon covalent polymerization into the hydrogel networks,23 the micelles acted as dynamic macro-cross-linkers. Due to the deformation of the micelles and the corresponding internal rearrangements of physical association, the PF127 micelle cross-linking incorporated an energy dissipation mechanism into the hydrogels making them tough.23,38,39 Prior work on self-healing materials also showed that the physical microphase of polymers in the solid state contributed to excellent mechanical properties.40−42 Combining the two dynamic cross-linkings in one system in the present strategy is a novel way to prepare hydrogels with high mechanical performance, resulting in a hydrogel that is not only self-healable but also extremely stretchable and tough. Tensile tests were performed using dumbbell-shaped hydrogel samples. All the samples were prepared in Teflon molds in pH buffers with solid contents of 10, 15, or 20 wt % and aged at room temperature for 48 h before testing. The typical nominal stress−strain curves are shown in Figure 2. The hydrogel with 15 wt % solid content in pH 6 buffer showed extremely high stretchability and good mechanical strength. The hydrogel showed yielding behavior at low strain (Figure 2a,b), which is common for physically cross-linked hydrogels9,24,43,44 and elastomers.45 The elastic modulus of the hydrogel (15 wt %) was calculated as 27.71 ± 0.17 kPa from the stress−strain curve over the low-strain region (
