**2. Fundamentals of conductive self-healing ECs**

In the interest of providing comprehensive information, this section offers a concise overview of EC's self-healing strategies and materials design. However, the main emphasis of this chapter revolves around exploring the uses of conductive selfhealing ECs in flexible electronic devices.

#### **2.1 Self-healing chemistry strategies**

The healing strategies for epoxy materials can be broadly categorized into two groups based on their structures and chemical compositions: intrinsic and extrinsic approaches. These two categories are distinguished by the mechanisms they employ to achieve healing functionality.

Intrinsic self-healing approaches do not rely on external healing agents. Instead, they possess an inherent ability to self-heal, which is facilitated by the thermal diffusion of molecules, reorganization of the polymer matrix, and the reformation of reversible/dynamic bonds or supramolecular interactions at the damaged interfaces. The efficiency of self-healing in epoxy polymers is dependent on critical factors such as the chain mobility, dissociation rate, and association rates. The majority of intrinsic recovery strategies are based on: (1) reversible covalent bonds (RCBs), which include Diels-Alder (DA) reactions [19], disulfide bonds (S∙S) [20], boronated ester bonds [21], and imine bonds [22]. These RCBs offer a particularly favorable approach for designing self-healing epoxy polymers with enhanced mechanical properties, thanks to their robust bonding strength. According to Krishnakumar et al. [23], they demonstrated the potential of graphene oxide in facilitating healing of epoxy vitrimer nanocomposites, achieving approximately 90% restoration of the original strength through disulfide bonds. However, this healing process necessitates heating the cracked region to activate the bond exchange mechanism. At the earliest, Hao et al. [24] have showcased an innovative approach that involves a vanillin-based hyperbranched epoxy resin (VEHBP), integrating the synergistic effects of disulfide and imine dynamic covalent bonds (**Figure 2a**). This integration results in an epoxy resin that is both recyclable and malleable, while maintaining a notably high glass transition temperature (*T*g), along with remarkable improvements in creep resistance and mechanical properties. Remarkably, the inclusion of 5% VEHBP in the dynamic

#### **Figure 2.**

*(a) Synthetic routes of an epoxy resin incorporating a hyperbranched structure derived from vanillin, featuring disulfide and imine dynamic covalent bonds. Reprinted with permission from [24]. Copyright 2023 American Chemical Society. (b) Diagram illustrating the fabrication process of bilayered microcapsules encapsulating epoxy and (C2H5)2O·BF3 dual agents through the utilization of MOF-stabilized Pickering emulsion and solvent precipitation techniques. Reprinted with permission from [25]. Copyright 2023 American Chemical Society.*

covalent epoxy resin led to substantial enhancements in key characteristics. These enhancements encompassed a remarkable glass transition temperature of 175°C and a creep temperature of 130°C. Notably, the mechanical properties saw significant boosts, including a remarkable 34.1% increase in tensile strength, a 19.7% rise in storage modulus, and an impressive 173.3% surge in tensile toughness when compared to the pristine resin. Importantly, the distinctive hyperbranched architecture of VEHBP, coupled with the dual dynamic bonds, imbued these materials with exceptional self-healing capabilities, the potential for reprocessing, and a positive environmental footprint due to their degradability. These properties mark a significant stride forward in the realm of designing and crafting high-performance epoxy covalent adaptable networks. Liu and co-workers [26] conducted a study involving the synthesis of a range of ester-exchange catalysts, which were then analyzed for their potential in generating dynamic covalent bonds. In their investigation, they introduced metal-ion-free triethanolamine as an ester-exchange catalyst. This catalyst was utilized to develop epoxy resins possessing optimal characteristics suitable for

### *Electrically Conductive Self-Healing Epoxy Composites for Flexible Applications: A Review DOI: http://dx.doi.org/10.5772/intechopen.1003037*

the manufacturing of traditional electrical equipment. Furthermore, they combined 3,3′-dithiodipropionic acid (DTDPA), a compound containing carboxylic acid groups and S∙S bonds, with MHHPA to create an epoxy vitrimer featuring dual dynamic bonds. By manipulating the ratio of the curing agents, they fine-tuned the structure of the dynamic covalent bonds within the crosslinking network. The results highlighted that the repair efficiency for surface scratches exceeded 90%. After fracture repair, the mechanical strength was rejuvenated to over 70%, and even instances of electrical damage exhibited some degree of restoration. Zou et al. [27] also investigated self-healing epoxy coatings utilizing reversible crosslinking network based on the DA reactions, which incorporated MXene flakes responsive to near-infrared (NIR) light. Their observations revealed that the presence of MXene enabled the epoxy to rapidly convert NIR light energy into heat, resulting in a self-healing capability of approximately 15 mins or/and (2) supramolecular chemistries primarily focus on reversible noncovalent interactions (RNBs), which encompass hydrogen bonds [28], metal-ligand coordination [29], ionic interaction [30], and host-guest interactions [31]. Liu et al. [32] developed a room-temperature self-healing epoxy coating with high elasticity in which the damaged coating can be repaired within 5 mins even under aqueous immersion thanks to reassociation of UPy hydrogen bonding units. In a study by Hu et al. [33], they introduced self-healable photocured epoxy acrylate resins that were formulated using photocured blends containing bisphenol-A epoxy diacrylate, a host-guest complex formed by 6-monomethacryl-substituted β-cyclodextrin and acrylamide-azobenzene, as well as butyl acrylate. The researchers demonstrated a unique approach to mending these light-responsive host-guest interactions. They achieved this by subjecting the material to UV light exposure at 365 nm, followed by a subsequent heat treatment at 120°C. This sequential process effectively enabled the damaged films to undergo self-healing. While not as strong as RCBs, these interactions have the advantage of creating mechanically dynamic and resilient systems, making them highly desirable for self-healing polymer designs. Due to their reversible nature, intrinsic self-healing epoxy materials can undergo multiple healing cycles through these interactions or reactions.

In contrast to intrinsic self-healing, extrinsic self-healing epoxy materials involve the incorporation of dispersed healing agents within an epoxy polymer matrix. These agents consist of reactive fresh precursors and catalysts. When damage occurs, these agents are released and initiate a process of spontaneous polymerization and reconstruction of the cross-linking network through chemical reactions, thereby repairing the affected areas [34]. Epoxy composites containing healing agent microcapsules have been also reported in several studies [35–38]. For instance, Zhu et al. [39] utilized a solvent evaporation technique to develop self-healing wave-absorption microcapsules, wherein epoxy resin cores were enclosed by hybrid walls composed of carbonyl iron powder and ethyl cellulose. As a result of this novel design, the material exhibited remarkable self-healing properties when subjected to electromagnetic irradiation. This improvement can be attributed to the increased mobility of the epoxy resin, achieved through the conversion of electromagnetic energy into thermal energy. This innovative approach shows significant potential for advancing self-healing epoxy materials, with promising applications across various fields. In a recent investigation, a novel approach was employed to achieve remarkable selfhealing properties in epoxy resin. By incorporating 15 wt% microcapsules, the epoxy resin demonstrated an impressive 95% healing efficiency within a mere 10 mins at room temperature [25]. Simultaneously, the composite's interlaminar shear strength reached an outstanding 92% improvement within just 5 mins. This groundbreaking

study marks the first instance of achieving rapid self-healing across diverse damage modes in both epoxy resin and carbon fiber/epoxy resin composites. This achievement was realized through the utilization of a unique self-healing system incorporating single-component double-shell microcapsules (**Figure 2b**). The innovation primarily revolved around the design of double-shell microcapsules, loaded with both epoxy resin and a catalyst ((C2H5)2O·BF3). These capsules were ingeniously created by combining the principles of Pickering emulsion templating and solvent precipitation techniques. Noteworthy was the strategic use of metal-organic frameworks (MOFs) with precisely controlled sizes and hydrophilic properties. These MOFs acted as effective stabilizers for the Pickering emulsion, ensuring the stability and size control of the inner microcapsules. Furthermore, their distinctive properties provided an ideal platform for host-guest interactions, enabling the efficient capture, storage, and release of the cationic catalyst.

## **2.2 Self-healing performance assessment**

Self-healing efficiency refers to the capacity of epoxy materials to restore their original functionalities and quality after experiencing damage. Typically, the evaluation of self-healing performance involves subjecting the materials to induced damage followed by conducting a mechanical test to initiate and quantify the self-healing process. This evaluation can be broadly categorized into three aspects: self-healing efficiency, healing rate, and damage volume.
