A self-healing PDMS elastomer based on acylhydrazone groups and the role of hydrogen bonds

Highlights

• Self-healing PDMS elastomers based on acylhydrazone groups were prepared.

• A reversible transition was observed around 80 °C.

• Role of H-bonds on self-healing properties was elucidated.

Abstract

A PDMS elastomer based on acylhydrazone groups with both acid- and heat-assisted self-healing properties was successfully prepared from tetra-acylhydrazine-terminated PDMS and terephthalaldehyde through solution casting. The good healing performance was obtained with catalytic acetic acid for 24 h at 25 °C or by annealing at 120 °C for 2 h. The elastomer exhibited a reversible transition near 80 °C observed by rheological measurements and variable-temperature FTIR, which corresponded to the dissociation and reconstruction of hydrogen bonds between acylhydrazone groups. Since the non-equimolar sample presented similar behaviors with the equimolar sample, it verifies that the reversible dissociation/reformation of hydrogen bonds dominates the heat-assisted self-healing process. This finding will enable better understanding of the contribution of hydrogen bonding interactions in acylhydrazone self-healing systems, thus promoting the development of self-healing bulk materials based on acylhydrazone groups.

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.