Inspired by nature, self-healing materials represent the forefront of recent developments in materials chemistry and engineering. This review outlines the recent advances in the field of self-healing polymers. The first part discusses thermodynamic requirements for self-healing networks in the context of conformation changes that contribute to the Gibbs free energy. The chain flexibility significantly contributes to the entropy changes, whereas the heat of reaction and the external energy input are the main contributors to enthalpy changes. The second part focuses on chemical reactions that lead to self-healing, and the primary classes are the covalent bonding, supramolecular assemblies, ionic interactions, chemo-mechanical self-healing, and shape memory polymers. The third part outlines recent advances using encapsulation, remote self-healing and the role of shape memory polymers. Recent developments in the field of self-healing polymers undeniably indicate that the main challenge will be the designing of high glass transition (Tg) functional materials, which also exhibit stimuli-responsive attributes. Build-in controllable hierarchical heterogeneousness at various length scales capable of remote self-healing by physical and chemical responses will be essential in designing future materials of the 21st century.

The concept of self-healing synthetic materials emerged a couple of decades ago and continues to attract scientific community. Driven primarily by an opportunity to develop lifelike materials on one hand, and sustainable technologies on the other, several successful approaches to repair mechanically damaged materials have been explored. This review examines chemical and physical processes occurring during self-healing of polymers as well as examines the role of interfaces in rigid nano-objects in multicomponent composites. The complex nature of processes involved in self-healing demands understanding of multi-level molecular and macroscopic events. Two aspects of self-healing are particularly intriguing: physical flow (macro) of matter at or near a wound and chemical re-bonding (molecular)of cleaved bonds. These events usually occur concurrently, and depending upon interplay between kinetics and thermodynamics of the processes involved, these transient relations as well as efficiency are critical in designing self-healing materials. This review examines covalent bonding and supramolecular chemistry in the context of molecular heterogeneities in repair processes. Interfacial regions in nanocomposites also facilitate an opportunity for supramolecular assemblies or covalent bonding which, if designed properly, are capable of self-repairs.

Self-healing materials are notable for their ability to recover from physical or chemical damage. We report that commodity copolymers, such as poly(methyl methacrylate)/n-butyl acrylate [p(MMA/nBA)] and their derivatives, can self-heal upon mechanical damage. This behavior occurs in a narrow compositional range for copolymer topologies that are preferentially alternating with a random component (alternating/random) and is attributed to favorable interchain van der Waals forces forming key-and-lock interchain junctions. The use of van der Waals forces instead of supramolecular or covalent rebonding or encapsulated reactants eliminates chemical and physical alterations and enables multiple recovery upon mechanical damage without external intervention. Unlike other self-healing approaches, perturbation of ubiquitous van der Waals forces upon mechanical damage is energetically unfavorable for interdigitated alternating/random copolymer motifs that facilitate self-healing under ambient conditions.

rearrangements. While one challenge is to incorporate chemical modifications enabling chemical bond reformations, equally important is the coordination of chemical events with polymer network physical changes within interfacial regions resulting from mechanical damage.Recent studies suggested that two types of dynamic bonds are highly attractive to achieve self-healing in polymers: reversible covalent bonds and supramolecular chemistry. Reversible covalent bonds offer higher bonding energies, allowing the material to regain physical properties similar to conventional polymers prior to damage. Advantages and disadvantages of reversible covalent bonding were summarized in several review articles. [5] Self-healing involving dynamic dissociation/association using supramolecular chemistry offers fast reversibility under ambient conditions reaching exceptionally quickly equilibrium state, bonding directionality, and remarkable sensitivity. Since broad-range of network structures can be obtained via supramolecular interactions, applications in soft electronics, sensing, biomedical technologies, 3-4D printing, and reprocessable materials are highly attractive and may open many new technological opportunities (Figure 1). Mechanical integrity using supramolecular chemistry is achieved by the formation of multiple noncovalent interactions between multiple associative groups covalently attached to polymer side chain or chain ends of macromolecular building blocks. These interactions include H-bonding, metal-ligand complexation, hostguest, ionic interactions, π-π stacking, and hydrophobic interactions. They are depicted in Figure 1. In contrast to covalent rebonding, mechanical properties of supramolecular polymer networks are achieved by the presence of these secondary interactions forming noncovalent crosslinks, which bind liquid-like building blocks into plastic or rubbery polymers in hydrated and nonhydrated states. The dynamic nature of these noncovalent bonds provides an opportunity for self-healing attributed to reassociation of supramolecular components, similar to secondary interactions found in living systems that regulate many living functions. The rule of thumb in designing these networks is that the degree of association is determined by the equilibrium constant (K a ) values, whereas dynamics is driven by rate constants of the association-dissociation reactions, k a and k d , respectively. [6] Molecular structural and electronic features along with network heterogeneities are of particular importance in these networks. Mechanical strength and elasticity of supramolecular polymers are determined by not only the K a values, but also stacking and clustering of the associative groups which may result in heterogeneities leading to the Recent advances of supramolecular chemistry utilized in the development of self-healing polymers have revealed that the rate and equilibrium constants of bond dissociation/re-association, bonding directionality, chain relaxation time, decay rate of chain relaxation after damage, and clu...

Hierarchical multiphase fibrous morphologies provide strength and elasticity for biological species, facilitating responses to environmental changes. Wound closure of leaves is one example. If polymers can be formed in a similar manner by introducing multiphase-separated morphologies, self-healing in a variety of commodity materials can be achieved. In these studies, we demonstrate the role of phase morphologies, interphases, and viscoelasticity-driven shape memory effects on self-healing. We synthesized phase-separated polycaprolactonepolyurethane fibrous thermoplastic polymers in which microphase separation facilitates the formation of stable interfacial regions between hard and soft segments. Self-healing can be repeated many times. This behavior is attributed to the shape memory effect, given that micron-scale interphase reduces chain slippage, enabling entropic energy storage during damage. Chemically identical but nanophase-separated copolymers do not exhibit this behavior. These studies show that self-healing can be achieved by morphology control and facilitated by thermal or other volume-induced transitions.