# Fabrication of Self-Healable Magnetic Nanocomposites via Diels - Alder Click Chemistry
## 1 Introduction  
The concept of smart materials, such as substances which self-heal without foreign mediation, is a promising substitute to material design that is insensitive to defects to sustain safety and/or performance. Such systems have the capability to heal themselves without outside mediation and restore their performance by using the properties intrinsically installed within them. So far, several approaches for the fabrication of autonomous self-healing materials, such as microscale vascular [1, 2], capsule [3, 4], and reservoir [5, 6] systems, have been established. For these materials, the effectiveself-healing process often resorts to the release of an active ingredient which could be catalyst, monomer or crosslinking agent at the crack tip to efficiently block the crack advancement [7]. However, the irreversible autonomous systems often provide the self-repair function only once because of the consumption of the limited healing agents, leading to a potential disadvantage for their long-term use [8; 9]. Therefore, the development of an efficient and versatile strategy for self-repairing materials capable of healing repeated damage events is greatly required to overcome this drawback from the irreversible systems. Recently, there is a growing attention on reversible self-healing systems. These self-healing polymers with inherently reversible interactions are capable of undergoing infinite break and healing cycles upon outside stimulations, and thus can sufficiently provide high self-healing proficiency to rebuild the damaged area at the same location. A number of strategies have been built up to develop reversible self-healing systems by utilizing chemical reactions including cycloaddition [10], metathesis reaction [11; 12], metal-ligand complexation [13], ring-opening [14], as well as complexation/decomplexation reactions [15; 16]. Among them, the Diels\(-\)Alder (DA) reaction was often recognized as one of the most favorable reaction types applied for reversible self-repair systems since this click reaction can be easily triggered by a thermal treatment. For example, furan and maleimide serving respectively as electron-rich and electron-deficient moiety undergo a thermally reversible cycloaddition reaction in the temperatures range from 100 to 150 \({}^{\circ}\)C, which is suitable for daily applications [17; 18; 19; 20; 21; 22; 23; 24; 25]. Despite the continuous progress of the DA systems, it should be noted that most studies focused on their synthetic routes and self-healing abilities. However, specific efforts with the DA based organic/inorganic hybrid systems are less common. In principle, heat activated self-repair polymers are superior matrix materials used in hybrid systems. The incorporation of inorganic nanomaterials for the fabrication of hybrid materials would further offer multifunctional properties that create expanded opportunities for developing advanced applications. For example, self-healing nanohybrids based on DA reactions with the incorporation of MWCNTs (multiwalled carbon nanotubes), GOs (graphene oxide), or inorganic nanoparticles showed potential for practical use [26; 27; 28; 29; 30; 31; 32]. In particular, carbon nanotubes have received a great interest due to their superior electrical, thermal, and mechanical properties compared to those of traditional fiber materials [33; 34]. The decoration of carbon nanotubes with magnetic nanoparticles, such as iron oxides, can further enhance or diversify the optical, electromagnetic, and electrochemical properties of the carbon nanotubes [35; 36]. The magnetic nanotubes-based hybrid systems can be used in various applications, e.g., electrochemical sensing, rechargeable batteries, and electromagnetic interference shielding [37; 38; 39; 40]. Developing novel carbon nanotubes/iron oxides materials with self-repairability is highly desirable to improve the long-term durability of these application fields.