The world of smart healable materials
Introduction
We live in an age of remarkable advancements in science and technology, where separate fields and disciplines are overlapping in new and exciting ways. This combined approach is exemplified in polymer science, which draws upon the knowledge of organic and physical chemistry, materials science and biochemistry, as well as electrical and mechanical engineering, allowing researchers in these fields to collaborate on the creation of uniquely inventive materials. One such type of material that has been undergoing tremendous growth and development in the past decade is that of Smart Materials.
A smart material is one in which a key material property is altered in a controlled fashion in response to the introduction of a pre-determined external stimulus. These stimuli-responsive materials might be utilized to undergo such changes as specimen shape, mechanical rigidity/flexibility, opacity, porosity, or may even be used for the controlled release of a specified molecular component for drug delivery purposes. This review, however, will focus on those materials that undergo mechanical healing in response to an applied stimulus. Such healable polymers and composite materials offer the ability to prolong the functional lifetime of their resulting structures and coatings.
In order to achieve the mechanical strength required for many structural applications, highly cross-linked polymeric materials are necessary. The trade off for this gain in mechanical strength is that the resulting materials tend to be brittle and are therefore more prone to developing cracks through normal usage, ultimately failing. This road to system failure begins with microscopic cracks that are imperceptible with current damage detection techniques; as these cracks propagate and coalesce, they result in the formation of macroscopic cracks in the material. The damage continues to propagate throughout the sample, eventually ending in catastrophic and irreversible failure of the system. This has led to an in-depth exploration of mechanisms to retard and repair the microscopic cracks before irreversible damage is done to the material. Smart materials are able to achieve healing, either intrinsically via reversible bonds present in the material itself, or extrinsically via a pre-added healing agent, in response to some external stimulus.
This field of stimuli-responsive healable materials is a relatively new one, beginning in the early 1990s, with the majority of the research occurring in the past decade. The Second International Conference on Self-Healing Materials took place in Chicago, Illinois from June 28th to July 1st, 2009, building on the successes of the prior meeting. In addition to the multitude of papers and patents that have been a published in this area, a number of reviews also exist that focus on various aspects of the research, from both the synthetic chemistry and materials engineering points of view [1], [2], [3], [4], and a comprehensive book on the subject has been released [5].
This review will present a comprehensive view of the field of stimuli-responsive healable materials. It will begin with an examination of the healing of polymeric materials, briefly discussing the conventional techniques for repair and maintenance of composite materials. It will continue with a detailed analysis of the various systems that have been proposed and investigated over the past two decades, as outlined in Table 1, with particular emphasis on work published in the past two years. These systems will be introduced according to the particular stimulus responsible for initiation of healing to occur, moving from mechanical damage to heat, electricity, electromagnetic field, ballistic impact and light. This discussion will cover the work done in the early days of healable materials and will include current trends and potential future directions.
Section snippets
Conventional healing methods
When polymer composites used as structural materials become damaged, there are only a few methods available to attempt to extend the functional lifetime of that material. Ideal repair methods are ones that can be executed quickly and effectively on site, eliminating the need to remove a component for repair. The mode of damage must also be taken into consideration, as repair strategies that work well for one mode might be completely useless for another. For example, matrix cracking can be
Hollow glass fibers
It is well known that the mechanical properties of polymeric materials can be greatly improved with the addition of a reinforcing fiber or other filler, giving the composite desired high strength and stiffness-to-weight ratios [8]. The main disadvantage to these composites, however, is their poor performance under impact loading; this is an indication of their susceptibility to damage, which manifests mainly in the form of delamination. Utilizing reinforcing fillers that contain a healing agent
Diels-Alder polymers
While the non-covalent interactions of supramolecular chemistry have long been utilized for their inherently reversible nature, they are far weaker than their covalent counterparts. It is therefore desirable when designing a structural healable polymeric material to utilize the stronger covalent bond in a reversible fashion. The most widely used and intensely studied reversible covalent bonds are found in the Diels-Alder (DA) cycloaddition reaction.
First documented by Diels and Alder in 1928
Electrical stimulus
A particularly promising alternative route for the initiation of healing that has been reported upon in recent years is the incorporation of a conductive component into the polymer matrix which can undergo resistive heating upon application of an electrical stimulus. When a crack forms in the material, the number of pathways available for the electrons to travel becomes limited; this results in a corresponding increase in resistance. With a constant applied electric field, heat is generated at
Magnetic particles
As an alternative to using conductive materials that require electrical contacts to accomplish inductive heating within a polymer composite material, magnetic compounds can be incorporated into the material and heated remotely (without contacts) via electromagnetic induction. This method of inductive heating has been studied mainly in the field of biomedical materials, where magnetic nanoparticles are injected directly into solid tumors and subsequently heated by exposure to a high-frequency
Ionomers
Another form of reversible interaction that has been utilized in polymer systems as reversible cross-linking is the ionic interaction. Polymers that contain up to 15% ionic content are called ionomers and these materials have been examined for their ability to restore cross-linking after damage to a material, thereby restoring mechanical strength. It has been shown that increased ion content in a polymer leads to an increase in the ultimate tensile strength and fracture resistance of the
Reversible photocyclization
Reversible covalent bonding can be brought about through cycloaddition reactions, as previously discussed with the Diels-Alder reaction, which are thermally initiated. The alternative type of cycloaddition reactions are photoinitiated, undergoing cyclization upon irradiation of a certain wavelength of light and cleavage upon irradiation of a shorter wavelength of light. Photochemical reactions are commonly used tools of organic synthesis, as they are a low cost, simple to initiate and
Future outlook
The field of healable materials has reached an exciting crossroads. While systems involving encapsulated liquid healing agents have been thoroughly studied and continue to be improved upon, creative alternate approaches are becoming more prevalent in the literature. There is now an international effort towards a common end goal of autonomic reversibly healable materials, and the research being done covers many new avenues to afford such intelligent materials.
Current research trends, as
Conclusions
The field of stimuli-responsive healable materials is a fascinating one, with an increasing number of research papers being published every year. From the initial studies on healing in concrete structures via embedded glass fibers to the most recent work on healing using shape memory alloy wires in a polymer composite, the different avenues being explored to achieve the common end goal of prolonged functional lifetimes for composite structural materials are astounding. As researchers deepen
Acknowledgments
The authors wish to acknowledge support from NextGen Aeronautics as well as Mitsubishi Chemical—Center for Advanced Materials, University of California, Santa Barbara.
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