A self-healing PDMS elastomer based on acylhydrazone groups and the role of hydrogen bonds
Graphical abstract
Introduction
Self-healing refers to the ability to autonomously heal damage, which also means without any external intervention [1]. So far, the major findings in this field can be divided into two categories: extrinsic self-healing and intrinsic self-healing [2]. The former requires a pre-embedded healing agent, represented by the epoxy system [3], which has the problem that the healing agent may be exhausted. In contrast, the latter heals crack using the structural characteristics of polymers, providing a material with the advantage of healing repeatedly and reversibly. For the intrinsic self-healing polymers, the reported healing modes include supramolecular interactions (such as hydrogen bonding [4], [5], π-π stacking [6], [7], and metal coordination [8], [9]) and dynamic covalent bonds (such as acylhydrazone bonds [10], [11], [12], [13], disulfide bonds [14], [15], [16], Diels–Alder reactions [17], [18], [19], and imine bonds [20]). The acylhydrazone group, arising from the reaction between acylhydrazine and aldehyde, has dynamic characteristics, provided by both the reversible acylhydrazone bond and the hydrogen bonding sites [21]. The acylhydrazone bond can be activated under mild conditions, showing both temperature and pH responsiveness. Lehn's group [22], [23], [24], [25] performed a comprehensive and in-depth study of linear polyacylhydrazones. They investigated the change of mechanical and optical properties caused by the exchange reaction of acylhydrazone bonds [22], [23], [24]. Later, double dynamic self-healing polymers were also prepared [25].
In our previous work, the acylhydrazone bond was used to construct dynamic polymer gels based on poly(ethylene glycol) (PEG), having self-healing properties [10], [11], [12], exhibiting sol–gel transitions in response to pH changes, and demonstrating rapid adhesion between hydrogel and organogel [13]. White and coworkers achieved restoration of large-scale damage by developing a vascular-like repair system involving the two-stage chemistry of rapid acylhydrazone formation and slow polymerization [26]. Self-healing bulk materials or elastomers [14], [15], [17], [18] are more difficult to fabricate due to the nature of the very slow diffusion of segments in polymer melts or solids. Recently, Schubert and coworkers [27] prepared an acylhydrazone bond-crosslinked self-healing bulk material for the first time by using 2-hydroxyethyl methacrylate as the backbone. The material can heal a deep scratch as long as 1 cm in length under 125 °C. The healing mechanism was attributed to the exchange reaction between acylhydrazone bonds, but the role of hydrogen bonds existing in plenty in this system was omitted. It is worth noting that hydrogen bonds can easily form between C=O and N-H in acylhydrazone groups, which is similar to other multiple hydrogen bonds systems formed between C=O and N-H [28], [29], [30], [31], [32].
In this study, a self-healing bulk material crosslinked by acylhydrazone groups was prepared. Here, polydimethylsiloxane (PDMS), one of the common commercial elastomers, was employed as the backbone to replace PEG because the latter is in the semi-crystalline state (hindering the self-healing behavior) at room temperature. The self-healing properties of the PDMS elastomers were investigated in detail. The healing mechanism and the role of H-bonds were explored with the help of rheology, variable-temperature FTIR, and other characterization methods. The present study will provide some guidance for designing self-healing materials based on acylhydrazone groups.
Section snippets
Materials
Bis(3-aminopropyl)-terminated Polydimethylsiloxane (H2N-PDMS-NH2, Mn = 2500 g/mol) was purchased from Sigma-Aldrich. Methyl acrylate, terephthalaldehyde, and dichloroacetic acid were obtained from J&K Scientific Ltd. Hydrazine hydrate (80% in water, w/w), methanol, toluene, chloroform, acetic acid (HAc), triethylamine (Beijing Chemical Reagent Co.). All the chemicals were used as received.
Sample preparation
Synthesis of tetra-acylhydrazine-terminated PDMS (A4). Tetra-acylhydrazine-terminated PDMS (A4) was
Properties of the organogel
The gelation of A4 and terephthalaldehyde (B2) in toluene was investigated. When the gelator mass concentration was fixed at 20 wt%, a gel with equimolar functional groups was formed in 6 h. A room temperature dynamic frequency sweep also indicated the formation of a gel (Fig. 2a). The gel exhibited reversible gel–sol phase transition in dichloroacetic acid and triethylamine (Fig. 2b) and could be repaired with acetic acid (HAc) (Fig. 2c). In summary, a self-healing PDMS organogel based on
Summary
In summary, a self-healing PDMS gel based on acylhydrazone groups was successfully prepared from tetra-acylhydrazine-terminated PDMS and terephthalaldehyde. The PDMS elastomer was further fabricated through solution casting. The elastomer had excellent acid- and heat-assisted self-healing properties. The rheological study showed that the system presented a reversible transition near 80 °C. Through comparative study of the gelation process of the non-equimolar system and the temperature response
Acknowledgements
This work was supported by the NSFC (Grant Nos. 21374127 and 21674122), the National Basic Research Program of China (973 Program, No. 2014CB643601).
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