Self-healable and reprocessable acrylate-based elastomers with exchangeable disulfide crosslinks by thiol-ene click chemistry

Highlights

• •Self-healable acrylate-based elastomers with dynamic covalent bonds.

• Acrylate-based elastomers prepared by thiol-ene click chemistry.

• Thiol-ene click chemistry performed by radical and base catalytic pathways.

• Good self-healing and reprocessing properties.

• No need for additional self-healing catalyst in alkali system.

Abstract

Non-reprocessable thermosets and elastomers are causing more and more environmental issues. Acrylate-based elastomers with self-healing and reprocessing capability are highly desired. As dynamic covalent chemistry provides a new ground to address these issues, the present article synthesized a novel kind of acrylate-based elastomers with exchangeable disulfide crosslinks, particularly, by virtue of the thiol-ene click reactions between acrylate monomers and polysulfide oligomers. We also investigated and compared thiol-ene click reactions via different approaches, including by radical pathways, such as redox-initiator system and photo-initiator system, or via catalytic processes mediated by bases. These novel acrylate-based elastomers are capable of self-healing and reprocessing in a rapid and efficient manner, and the self-healing and reprocessing efficencies are related to the elastomer structures formed by different synthetic strategies. Notably, the base system without the extra addition of self-healing catalyst are found especially efficiently and the mechanical properties of acrylate-based elastomers can be restored more than 90% with a habitation of 5 h. Such rapid and efficient restoration suggests a highly promising application as recoverable adhesives in practice. Those results open up a novel avenue for the designation and synthesis of acrylate-based elastomers and thermosets with self-healing and reprocessing properties, among other desirable functions.

Introduction

Owing to their excellent thermomechanical and solvent resistance abilities, acrylate-based thermosets have wide applications in many fields including biomedical devices, coatings and adhesives [1,2]. However, traditional acrylate-based thermosets cause serious environmental issues because they cannot be reprocessed or reshaped by using heat or solvents due to the crosslinked network structure [[3], [4], [5], [6]], which isn't reprocessable in nature. In addition to incineration and landfill, methods for treating those thermosetting polymers include mechanical recycling [7], pyrolysis [8], chemical recycling [9] and supercritical fluid processes [10]. However, these methods have many drawbacks such as requiring harmful solvents for pyrolysis, or strong acid (sulfuric acid/nitric acid) for destroying the chemical bonds. Obviously, these thermosets that allow self-healing, environmentally friendly reprocessing and recycling while still keeping their original properties, are attractive [11]. This has recently been addressed by the introduction of dynamic covalent chemistries.

Dynamic covalent chemistry has been applied for obtaining unconventional polymer networks with intriguing properties [3,12]. For instance, under external stimuli, dynamic covalent bonds can enable network rearrangement, which endows the polymers with self-healing ability. Thus, reprocessing and recycling those materials become possible [13,14]. Recently, typical dynamic covalent bonds are fabricated by the formation of β-hydroxyl esters [15,16], imine bonds [17,18] or disulfide bonds [19,20]. Usually, disulfide bond exchange reactions have attracted much attention because of their mild reaction conditions and ability to respond to external stimuli [20,21]. By those reactions, various self-healing materials such as polyurethane [[22], [23], [24]], epoxy resin [25,26], vulcanized polybutadiene rubber [27] and epoxidized natural rubbers [28] can be facilely prepared. However, to be best knowledge of the authors, there is no systematic study of the dynamic covalent bond system in crosslinked acrylate-based elastomers.

As a few typical thiol-ene click reactions have been used to synthesize polymers in recent years, and the thiol-ene click reactions attract more and more attentions worldwide [[29], [30], [31]]. In addition to these attributes intrinsic in click reactions, thiol-ene click reaction can be performed via a few approaches such as by a radical pathway (termed thiol-ene radical reaction) [32], and a catalytic approach under nucleophiles or strong bases (termed thiol-ene Michael addition) [33]. In general, the thiols and initiators may significantly influence thiol-ene click reactions [29]. By employing appropriate thiols and initiators, so far many polymers have been synthesized via an efficient step-growth of chains. Also, the thiol-ene networks themselves can act as benchmark polymer materials to react with other substances for forming functional polymers such as elastomers, coating and adhesives [34]. Zhang had synthesized photo-crosslinkable, self-healable and reprocessable rubbers via thiol-ene click polymerization between double bonds in polybutadiene and thiols in polysulfide [19]. Bowman had also reported remoldable thiol−ene photocurable acrylate networks using bisphenol A glycerolate di (norbornenyl ester) and pentaerythritol tetrakis- (3-mercaptopropionate) for incorporating transesterification [35]. Oh had synthesized dual sulfide–disulfide crosslinked networks by photoinduced thiol-ene click-type radical addition [36].

In the present work, we demonstrate a versatile method to prepare self-healable and reprocessable elastomers from universal acrylate monomers using thiol-terminated polysulfide cross-linkers containing abundant disulfide bonds by click chemistries, particularly, via thiol–ene radical reaction using redox-initiators or photo-initiators, or via thiol-ene Michael addition mediated by alkali. These acrylate-based elastomers with disulfide bonds can undergo disulfide exchange reactions under heating or catalysis, forming new covalent bonds, which leads to the reconstruction of elastomer networks. We also demonstrated that the self-healing properties of acrylate-based elastomers prepared by thiol-ene Michael addition are better than those prepared by thiol–ene radical reaction. It is probably because alkali does not only catalyze Michael addition, but also promote the exchange of disulfide bonds.