A molecular dynamics simulation on self-healing behavior based on disulfide bond exchange reactions

Highlights

• The disulfide exchange reaction induced self-healing are studied at the atomic scale.

• The effect of crosslink density on the self-healing efficiency is investigated.

• High crosslink density has a negative effect on the mobility of molecular chains.

• High crosslink density has a positive effect on the mechanical properties.

Abstract

The self-healing polymers based on dynamic covalent bonds or non-covalent interactions have been widely reported in experiments. However, the self-healing theory and mechanism still need to be explored further. In this paper, molecular dynamics simulations are performed to investigate the self-healing mechanism based on disulfide bond exchange reactions at the atomic scale. The microstructures of sample during self-healing process are tracked, and the mechanical properties varying with healing time are examined by uniaxial tension tests. In addition, the effect of crosslink density on the self-healing efficiency and mechanical properties is investigated. The results reveal that the high crosslink density has a positive effect on the mechanical properties and a negative effect on the mobility of molecular chains. The effect of healing time on healing efficiency is also studied, which exhibits the same tendency as the experimental results. Finally, the stress relaxation test is simulated to study the dynamic feature of exchangeable disulfide bonds. The results indicate that the system with shorter stress relaxation time has higher healing efficiency.

Introduction

Conventional thermosets are widely used in structural applications, biomedical materials, coatings and adhesives due to their mechanical properties, thermal stability and solvent resistance. Their irreversible crosslinked structure renders the polymers to be permanent, intractable and lack of recyclability at the same time [[1], [2], [3]]. Once the cracks or scratches form and expand, the lifetime of thermosets will be reduced irreversibly. Hence, the concept of self-healing polymeric materials, which can partially or fully repair the internal damage, is put forward to improve the lifetime of materials [4].

In recent years, the dynamic chemical reactions (e.g. radical-based chain transfer [5,6], Diels-Alder (DA) reaction [[7], [8], [9]], disulfide exchange [[10], [11], [12], [13]] and transesterification [14]) or non-covalent interactions (e.g. hydrogen bonds [15] and hydrophobic interaction [16]) are incorporated into polymers to prepare intrinsic self-healing polymers. For example, Montarnal et al. [14] reported recyclable and reparable epoxy networks using temperature-sensitive transesterification reactions. It revealed that the broken samples could be welded perfectly due to the exchange reactions. Ling et al. [17] synthesized a self-healing polyurethane taking advantage of reversible photo-dimerization and photo-cleavage habit of coumarin moieties. The mechanical strength of broken specimens can be repeatedly restored under continuous ultraviolet irradiation of 350 and 254 nm.

Among the aforementioned reversible covalent bonds, exchange reactions based on disulfide bonds have attracted much attention due to their moderate reaction conditions and responsiveness to diverse stimuli [[18], [19], [20], [21]]. For instance, Rekondo et al. [10] designed a new poly(urea-urethane) thermoset elastomer with aromatic disulfide, which showed a remarkable self-healing ability at room temperature without any catalyst or external stimulus. Xiang et al. [18] reported a feasible approach to prepare photo-crosslinked, self-healable and reprocessable rubbers by means of ultraviolet irradiation induced disulfide metathesis. The incorporation of dynamic disulfide bonds enhances the chain mobility, but weakens the mechanical properties of materials. It is a great challenge to balance these two contrasting effects in the design of self-healing polymers [[22], [23], [24], [25], [26], [27]]. Wang et al. [4] studied the effect of swelling index on the basic mechanical and self-healing performances, and designed a disulfide-containing poly(urea-urethane) network with high healing efficiency and strong mechanical properties. Liu et al. [25] prepared a series of polyurethane with different crosslinking density to tune the strength and self-healing efficiency. The self-healing polyurethane they prepared displayed a maximum Young's moduli of 112 MPa and an excellent self-repairing efficiency of 94%.

Until now, many strategies for designing self-healing polymers based on dynamic disulfide bonds have been proposed. However, the fundamental knowledge of the self-healing mechanism is still at the very beginning. A clear understanding of the underlying mechanism at the molecular level is essential for synthesis and optimization of self-healing polymers [[28], [29], [30], [31], [32], [33]]. Wool et al. [28] proposed a microscopic theory for crack healing in polymers in terms of the stages of crack healing. They found that the diffusion stage was the most important, which controlled the development of mechanical properties during healing. Yu et al. [33] developed a multiscale model to study thermo-induced surface welding, and described the evolution of chain density across the interface. Although these studies promoted our understanding about this kind of materials, it would be very helpful if we can delineate the macromolecular level details of dynamic disulfide bonds during healing process.

Molecular dynamics simulation is used to investigate the dynamic bonds in self-healing polymers, which can provide insightful information at the atomistic scale [[34], [35], [36], [37], [38], [39], [40], [41], [42]]. Ge et al. [38] employed bead-spring model to study the diffusion dynamics, and discussed the evolutions of interface structure and interface strength during healing. Yang et al. [39] investigated the macromolecular details of bond exchange reactions, and tracked the trace of active units across the interface during welding. The molecular dynamics method was also used to study the self-healing behavior based on DA reaction and photo depolymerization-polymerization reaction [35,41,42].

In this paper, all-atom models are established to study dynamic diffusion and the healing process at the interface based on disulfide bond exchange reactions. The variation of crosslink density during self-healing is examined. In addition, the effect of crosslink density on the mechanical properties and self-healing efficiency is discussed by uniaxial tension tests. The variation of healing efficiency with different healing time is also studied. Finally, the stress relaxation behavior due to disulfide exchange reactions is investigated at the atomistic scale.