Self-Healing Nanotextured Vascular Engineering Materials

Several main healing agents currently used in self-healing nanotextured materials are discussed in this section. These include dicyclopentadiene (DCPD) and Grubbs’ catalyst (Sect. 2.1) and dimethyl siloxane (DMS, a resin monomer) and dimethyl-methyl hydrogen-siloxane (curing agent) polymerized as poly(dimethyl siloxane) (PDMS, Sect. 2.2). Several other elastomers used for self-healing are discussed in Sect. 2.3. Self-healing agents can also comprise epoxy-hardener systems (Sect. 2.4), and gels (Sect. 2.5).

The spreading of droplets of liquid healing agents on both horizontal and tilted intact surfaces is considered and compared with that on porous nanofiber (NF) mats in Sects. 3.1 and 3.2, respectively. The intact surfaces and NF mats serve as macroscopic models of self-healing engineering materials with vascular networks, where the healing agents have been released from the NFs in a damaged domain. The spreading of droplets on NF mats show significant deviations from that on the intact surfaces because of the imbibition of liquid into the inter-fiber pores. The model macroscopic experiments with a single crack tip in Sect. 3.3 elucidate the self-healing mechanism, namely, that the epoxy resin and hardener released into the tip react with each other, yielding a cured and hardened epoxy that heals the crack tip. Then, in Sect. 3.4, a microfluidic chip-like setup comprising a vascular system of microchannels alternatingly filled with either a resin monomer or a curing agent is used to study the additional intrinsic aspects of the physical healing mechanism in self-healing engineering materials. The model demonstrates that, as a pre-notched crack propagates across the chip, the resin and curing agent are released from the damaged channels, wet the surrounding matrix, spread over the banks of the crack, mix, and finally polymerize. Moreover, the polymerized domains form a system of pillars, which stitch the crack banks on opposite sides, thus preventing further propagation of the crack.

Several fabrication methods are used to prepare the components of self-healing nanotextured vascular materials. These include the general method of electrospinning discussed in Sect. 4.1 and its variant of co-electrospinning discussed in Sect. 4.2; the latter is used to form core-shell nanofibers (NFs) that contain healing agents in their cores. Another variant of electrospinning used to form such NFs is the emulsion spinning, which is described in Sect. 4.3. Instead of electrospinning, another general method of solution blowing (see Sect. 4.4) can be used to form NFs. This method is industrially scalable, and its variant of coaxial solution blowing can be used to rapidly manufacture core-shell NFs with healing agents in the cores (Sect. 4.5). Another variant of solution blowing—emulsion blowing—has also been used to form core-shell NFs with healing agents in the core (Sect. 4.6). All the abovementioned chapters discuss the fabrication of essentially two-dimensional self-healing materials. The subsequent Sect. 4.7 describes the methods of fabrication of two- and three-dimensional self-healing composites with embedded nanotextured vascular systems based on core-shell NFs.

This chapter describes and discusses some of the characterization methods employed to verify the physical and chemical aspects of self-healing on the micro- and nanoscale levels. These methods are used to confirm that the healing agents are encased and released from nanofibers (NFs), spread, reacted, and solidified.

This Section is devoted to the description of several key ideas related to material failure, including several failure criteria (Sect. 6.1) and some necessary elements of the fracture mechanics, such as cracks, the Griffith theory, surface energy, and stress intensity factors for fracture in Modes I, II and III (Sect. 6.2).

In this Section the evaluation of the mechanical consequences of self-healing is discussed. In particular, the recovery of such mechanical properties as stiffness is addressed. The tensile testing of self-healing composites with different types of fibers are analyzed in Sects. 7.1 and 7.2.

In this Section the adhesion (blister) test is discussed in Sects. 8.1 and 8.2 regarding the performances of two types of nanotextured vascular self-healing materials and their effects on the adhesion and cohesion energies. In Sect. 8.3, double-cantilever beam and bending tests, applied for the mechanical characterization of self-healing materials, are discussed. Section 8.4 outlines the interfacial toughening by means of nanofibers (NFs) intended to prevent delamination and crack propagation. In addition, damage to interfacial layers toughened by NFs is characterized by impact testing. Sect. 8.5 provides comprehensive data on mechanical recovery in self-healing vascular materials. In addition to the essentially two-dimensional self-healing materials discussed in Sects. 7.1, 7.2, 7.3 and 7.4 and 8.1, 8.2, 8.3, 8.4 and 8.5, the mechanical behaviors of their three-dimensional counterparts are explored in Sect. 8.6.

This chapter begins with a brief discussion of the fundamental electrochemical aspects of corrosion cracking (Sect. 9.1). Section 9.2 discusses the capsule-based extrinsic self-healing approach.

In this chapter in Sects. 10.1 and 10.2, corrosion protection provided by co-electrospun and emulsion-spun nanofibers (NFs), respectively, is discussed. These NFs are embedded in protective coatings, forming composite materials that are deposited on steel substrates.

Mimicking natural vascular systems in engineering materials is achievable by using core-shell nanofibers (NFs) whose cores are filled with self-healing agents (e.g., resin monomer and curing agent, or epoxy and hardener). This is beneficial for the following reasons: (i) uniformly distributed versus localized healing elements (as in the case of microcapsules containing healing agents), and (ii) nanoscale instead of microscale healing elements (as in the case of microcapsules). Nanoscale healing elements can fit ply areas in layered composites and avoid weakening the surrounding matrix. Another benefit of NF-based self-healing systems is that the dispersion of additional components required for self-healing reactions in the surrounding matrix is unnecessary. Specifically, a self-healing system should use two types of interwoven NFs with two complementary healing agents present within their cores, namely, a resin and its curing agent or an epoxy and its hardener. When released from the damaged core-shell nanofibers, these components react with each other to form polymerized, solidified stitches that connect the crack banks. The stiffness and self-cohesion of the damaged material can thus be restored. However, the prevention of delamination and the recovery of adhesion to surfaces of different compositions remain issues to be resolved that require additional future research efforts.