Chemical and physical aspects of self-healing materials

Chapter 8 - Supramolecular network-based self-healing polymer materials

Self-healing materials have attracted attention due to their ability to regain their structure and function after damage. In recent years, significant progress has been made in achieving various functions through supramolecular chemistry. This review describes an overview of the strategies used to prepare self-healing and self-restoring materials utilizing reversible and movable crosslinks. Reversible crosslinks, consisting of noncovalent bonds, can reversibly undergo repeated cleavage and reformation. Therefore, self-healing can be achieved by effectively regenerating reversible crosslinks between polymeric chains. Reversible crosslinks exploit many kinds of dynamic covalent bonds and noncovalent bonds, such as hydrogen bonds, metal coordination bonds, ionic interactions, π–π stacking, van der Waals forces, and hydrophobic interactions. Movable crosslinks exhibit self-restoring properties. Self-restoring materials can regain their original shape and mechanical properties after a cycle of loading and unloading external stress. Movable crosslinks consist of polymer chains that penetrate macrocyclic units and have self-restoring properties due to their sliding motion along the polymeric chains. In addition, multiple reversible cross-links produce synergistic effects to simultaneously achieve high toughness and effective self-healing. We believe that self-healing and self-restoring materials will play a substantial role in realizing a sustainable society.

Concept of self-healing in polymeric materials

Technologies for room-temperature self-healing polymer materials and their applications in energetic materials

The development and characterization of self-healing polymeric materials have been inspired by biological systems in which damage triggers an autonomic healing response. This is an emerging and fascinating area of research that could significantly extend the working life and safety of the polymeric components for a broad range of domestic as well as defence. Polymers and structural composites are used in a variety of applications, which include transport vehicles (cars, aircrafts, ships, and spacecrafts), civil and defence engineering, and electronics. However, these materials during their use are susceptible to damage induced by mechanical, thermal, chemical, UV radiation, or a combination of these factors. This could lead to the formation of micro-cracks deep within the structure where detection and external intervention are difficult or impossible. The presence of the micro-cracks in the polymer matrix can affect both the fiber- and matrix dominated properties of a composite and it is leads to catastrophic failure of any structure. However, many conventional repair methods are not effective for healing invisible micro-cracks within the structure during its service life. The concept has been demonstrated through US Air force and European Space Agency investments in self-healing polymers. Self-healing materials have the capability of repairing or recovering themselves when suffering mechanically or thermally induced damage, which can occur autonomously or be activated by external stimuli (e.g. heat) once or multiple times. Hollow fiber and microcapsules /nano-capsule approaches are the most studied technological approaches to the development of self-healing polymers. Embedded microcapsules filled with a liquid healing agent ruptured and release the healing agent into the damaged area and react with already incorporated catalyst causes polymerization of the healing agent, thus repairing the damage autonomically. The reversibility of supramolecular interactions makes them well suited for incorporation into a healable material, allowing for a system capable of undergoing multiple cycles of mending. A reverse Diels–Alder approach ensures the reformation of broken bonds on heating. Using this concept the recycling of thermoset-based plastics and composites is possible.

Noncovalent interactions can maintain the three-dimensional structures of biomacromolecules (e.g., polysaccharides and proteins) and control specific recognition in biological systems. Supramolecular chemistry was gradually developed as a result, and this led to design and application of self-healing materials. Self-healing materials have attracted attention in many fields, such as coatings, bionic materials, elastomers, and flexible electronic devices. Nevertheless, self-healing materials for biomedical applications have not been comprehensively summarized, even though many reports have been focused on specific areas. In this Review, we first introduce the different categories of supramolecular forces used in preparing self-healing materials and then describe biological applications developed in the last 5 years, including antibiofouling, smart drug/protein delivery, wound healing, electronic skin, cartilage lubrication protection, and tissue engineering scaffolds. Finally, the limitations of current biomedical applications are indicated, key design points are offered for new biological self-healing materials, and potential directions for biological applications are highlighted.

Self-Healing Hydrophilic Porous Photothermal Membranes for Durable and Highly Efficient Solar-Driven Interfacial Water Evaporation

The ability to finely tune the solution viscosity of an aqueous system is critical in many applications ranging from large-scale fluid-based industrial processes to free-standing hydrogels important in regenerative medicine, controlled drug delivery, and 'green' self-healing materials. Herein we demonstrate the use of the macrocyclic host molecule cucurbit[8]uril (CB[8]) to facilitate reversible cross-linking of multivalent copolymers with high binding constants (K(a) > 10(11)-10(12) M(-2)) leading to a supramolecular hydrogel. Multivalent copolymers were prepared by free radical polymerization techniques and contained either pendant methyl viologen (a good first guest for CB[8]) or naphthoxy derivatives (good second guests for CB[8]). A colorless solution of the two multivalent copolymers bearing first and second guests, respectively, can be transformed into a highly viscous, colored supramolecular hydrogel with the cross-link density being easily controlled through CB[8] addition. Moreover, the cross-links (1:1:1 supramolecular ternary complexes of CB[8]/viologen/naphthoxy) are dynamic and stimuli-responsive, and the material properties can be modulated by temperature or other external stimuli. Rheological characterization of the bulk material properties of these dynamically cross-linked networks provided insight into the kinetics of CB[8] ternary complexation responsible for elastically active cross-linking with a second guest dissociation rate constant (k(d)) of 1200 s(-1) for the ternary complex. These materials exhibited intermediate mechanical properties at 5 wt % in water (plateau modulus = 350-600 Pa and zero-shear viscosity = 5-55 Pa·s), which is complementary to existing supramolecular hydrogels. Additionally, these supramolecular hydrogels exhibited thermal reversibility and subsequent facile modulation of microstructure upon further addition of CB[8] and thermal treatment. The fundamental knowledge gained from the study of these dynamic materials will facilitate progress in the field of smart, self-healing materials, self-assembled hydrogels, and controlled solution viscosity.

Royal Australian Chemical Institute Awards.

The aim of this thesis is to execute a bottom-up design of the intrinsically self-healing nanocomposites. We briefly introduced the self-healing materials in chapter 1, covering classification and basic self-healing mechanism. In chapter 2, we have synthesized polyborosiloxane (PBS) according to the last century recipe as the self-healing supramolecular matrix. Additionally, we provided the long existing recipe with exclusive supplementary details, such as reaction kinetics, structural refinement and characterization, mechanical performance, glass transition temperature, etc. We have denoted the refined PBS, in which the end groups of PBS primarily exhibit in the well-defined borono structure (B(OH)2), rPBS. In chapter 3, the molecular interaction of PBS molecules that leads to the self-healing behavior was investigated with modern techniques. Besides the well-known physically cross-linked H-bonds, PBS molecules also cross-link via forming reversible covalent boroxane structures (B-O-B). Chapter 4 and 5 focus on the reinforcing effects of rigid nanoclay particles (Cloisite 20A) and flexible graphene oxide (GO), respectively. As PBS molecules bear the hydrophilic borono end groups, the incorporation of natural clay and graphene oxide leads to softened nanocomposites due to their hydrophilic surfaces. GO platelets efficiently drive the GO/rPBS nanocomposite into thixotropic material readily at a mass fraction of 0.001. However, natural clay surface is not as hydrophilic as GO. The softening effect is not only less pronounced, but also quickly compensated due to the formation of the particle network. Nevertheless, it results in a paste-like thixotrope when the mass fraction of natural clay reaches 0.6. Cloisite 20A and OMGO are surface modified natural clay and GO, with almost the same alkyl modifier. They behave as ordinary reinforcement additives and may efficiently enhance the rheological properties of the nanocomposites.

Natural Self-Healing Keeping the Green World Alive: The Repair Cycle of Photosystem II Josef Komenda, Franck Michoux, and Peter J. Nixon Regeneration of 11-cis-Retinal in the Retinoid Visual Cycle Gennadiy Moiseyev and Jian-Xing Ma Biological Repair Mechanisms: A Short Overview Alessandro Sinigaglia Artificial Self-Healing Self-Healing of Gold Nanoparticles during Laser Irradiation: An Embryonic Case of Systems Nanotechnology Vincenzo Amendola and Moreno Meneghetti Self-Healing in Two-Dimensional Supramolecular Structures: Utilizing Thermodynamic Driving Forces Markus Lackinger Pursuit of Long-Lasting Oxygen-Evolving Catalysts for Artificial Photosynthesis: Self-Healing Materials and Molecular-Reinforced Structures Serena Berardi, Andrea Sartorel, Mauro Carraro,and Marcella Bonchio Dynamic Self-Assembly of Nanoscale Components for Solar Energy Conversion Ardemis A. Boghossian, Moon-Ho Ham, Jong Hyun Choi, and Michael S. Strano Self-Healing Nanocomposites: Role and Activation of Inorganic Moieties and Hybrid Nanophases Ivano Alessandri Thermoreversibility in Polymeric Systems: Chemical and Physical Aspects C. Toncelli, D. De Reus, A.A. Broekhuis, and F. Picchioni Self-Repairing by Damage-Triggered Smart Containers Dmitry G. Shchukin and Dmitry O. Grigoriev Self-Healing Process in Glassy Materials Francois O. Mear, Daniel Coillot, Renaud Podor, and Lionel Montagne Basic Principles of Self-Healing in Polymers, Ceramics, Concrete, and Metals Martin D. Hager and Ulrich S. Schubert Frontiers of Self-Healing at the Nanoscale Design of a Repair-and-Go System for Site-Specific Healing at the Nanoscale German V. Kolmakov, Todd Emrick, Thomas P. Russell, Alfred J. Crosby, and Anna C. Balazs Autopoietic Self-Reproduction as a Distinctive Feature of Structural and Dynamic Organization in Microcompartment Systems: From Self-Assembled Micelles to Synthetic Cells Pasquale Stano Molecular Motors and Self-Healing at the Neuromuscular Synapse Alf Mansson, Marlene Norrby, and Sven Tagerud Fault-Tolerant Approach to the Configuration of Programmable Logic at the Nanoscale Gianluca Tempesti, Andre Stauffer, and Joel Rossier Index

Self-healing materials received significant attention in recent years among researchers. One of the greatest challenges in modern material sciences is the development and manufacture of engineering materials with self-healing (self-repair) properties. In such materials, stresses of a certain magnitude induced by chemical, physical, or thermal sources cause mechanical deformation of the material, which in turn triggers a response in the material, leading to the potential repair (self-healing) of the physical damage. Nature is an excellent source to show humankind how to achieve these properties for manmade materials. By a short look at nature, it is obvious that a dynamic system is required to achieve a self-healing material. Consequently, the important characteristic aspect of a self-healing material is the availability of a structure, which can dynamically respond to an external stimulus in order to restore the initial properties of the material. Considering the highly complex chain structure of polymers, polymers are ideally suitable to serve as molecules for dynamic and therefore self-healing properties. Among different methods to achieve this ability in engineering material micro/nano-containers containing healing materials have been widely investigated. In this chapter, the focus is on the methods, which are available for the production of these containers.

Over the past ten years, a broad range of self-healing materials, systems that can detect when they have been damaged and heal themselves either spontaneously or with the aid of a stimulus, has emerged. Although many unique compositions and components are used to create these materials, they all employ basic chemical reactions to facilitate repair processes. Kinetically controlled ring-opening reactions and reversible metal–ligand interactions have proven useful in autonomic self-healing materials, which require no stimulus (other than the formation of damage) for operation. In contrast, nonautonomic self-healing materials, which require some type of externally applied stimulus (such as heat or light) to enable healing functions, have capitalized on chemistries that utilize either reversible covalent bonds or various types of noncovalent interactions. This review describes the underlying chemistries used in state-of-the-art self-healing materials, as well as those currently in development.

This chapter outlines fundamental and more advanced concepts in self-healing of materials. Although the primary focus is on molecular processes and chemical reactions leading to self-healing materials, physical processes associated with shape memory metals and metal oxides are also discussed. Specifically, the role of the following chemical reactions and physical processes is discussed: covalent bonds, reversible cycloaddition reactions, exchange reactions, stable free radical-mediated reshuffle reactions, heterocyclic compounds and carbohydrates in polyurethanes, supramolecular chemistry, hydrogen bonding, metal–ligand coordination, π–π stacking, ionic and host–guest interactions, chemo-mechanical self-healing, encapsulation and remote self-healing.

The behaviour of autonomic self-healing cementitious materials depends on a set of interacting mechanical, chemical and transport processes. A summary is provided of a set of component models developed to simulate these processes along with a description of a linked experimental programme of work. The component models are brought together in a coupled finite element formulation that solves Navier-Stokes and mass-balance equations for healing agent transport and uses elements with embedded strong discontinuities to represent cracks. A compact description is also provided of a new cohesive-zone damage-healing model for discrete concrete cracks. This model simulates evolving curing-fronts within the body of healing-agent using a two-level recursive time-stepping scheme. The formulation naturally accounts for the dependency of the healing response on the crack opening displacement (COD) its rate. The crack-front model is embedded in a damage-healing solution algorithm that addresses simultaneous cracking and healing, as well as re-cracking and re-healing. Validations undertaken of full coupled finite element model are discussed and an illustrative example presented. A new extension is described of the curing-front model that allows healing in wide cracks (i.e. COD>0.5mm) to be simulated. A new parametric study, for a vascular system with cyanoacrylate as the healing agent, is also presented from which a set of graphs are produced that show the expected healing in a crack for a given relative COD and time. The graphs should be useful to researchers working on cementitious self-healing materials.