Recent Advances in Smart Self-healing Polymers and Composites

Guoqiang Li, Harper Meng

• Publisher: Woodhead Publishing
• Year: 2015
• Print ISBN: 9781782422808
• eBook ISBN: 9781782422921
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

Recent Advances in Smart Self-Healing Polymers and Composites examines the advances made in smart materials over the last few decades and their significant applications in aerospace, automotive, civil, mechanical, medical, and communication engineering fields.  Based on a thorough review of the literature, the book identifies "smart self-healing polymers and composites as one of the most popular, challenging, and promising areas of research. Readers will find valuable information compiled by a large pool of researchers who not only studied the latest datasets, but also reached out to leading contributors for insights and forward-thinking analogies. - Examines the advances made in smart materials over the last few decades - Presents significant applications in aerospace, automotive, civil, mechanical, medical, and communication engineering fields - Compiled by a large pool of researchers who not only studied the latest datasets, but also reached out to leading contributors for insights and forward-thinking analogies

1

Overview of crack self-healing

G. Li^1,2; H. Meng^{2 1} Louisiana State University, Baton Rouge, LA, USA
² Southern University, Baton Rouge, LA, USA

Abstract

The last several decades have witnessed remarkable advances in smart materials, which will play a significant role in the areas of aerospace structures, transportation vehicles, maritime and offshore structures, wind turbine blades, pipelines, pressure vessels, civil engineering structures, medical devices and implants, household products, sports equipment, and other areas. Basically, smart materials could find applications in all existing man-made structures or devices simply because there is a need for these structures to be intelligent and this need in turn drives research and development in smart materials. It is highly desired that man-made structures be reliable for their design lifetimes. However, poor quality products and uncontrolled factors such as accidental loading (for instance impact loading), design faults, construction defects, and lack of maintenance lead to structural failure at loads well below design level or at service life well before design lifetime. Therefore, self-healing is an anticipated feature for any man-made structure. Smart materials are and will continue to be the focal material for self-healing applications. In this chapter, we will first review the existing self-healing systems and then provide an outlook for future development in this emerging area of study.

Keywords

Self-healing

Crack

Shape memory polymer

Biomimetic

Composite

Intrinsic healing

Extrinsic healing

Acknowledgments

This study was financially supported by the Army Research Office under grant number W911NF-13-1-0145, National Science Foundation under grant numbers CMMI 1333997 and CMMI 0900064, and the Cooperative Agreement NNX11AM17A between NASA and the Louisiana Board of Regents under contract NASA/LEQSF(2011-14)-Phase3-05.

1.1 Review of existing self-healing systems

While all structure designs take proactive measures to prevent premature failure, such as using safety factors greater than 1, premature failures do occur due to accidental over loading, environmental attacks, design faults, construction defects, and lack of maintenance. Therefore, proactive maintenance by active intervention (repair) is needed to restore some or all of the lost structural capacity, which may eventually extend the service life of the structures. Hence, repair or self-healing is not just a way of passively restoring structural service; it can actively manage the structural life by repair or healing at the right time and right place [1]. With the rapid advancement in materials science and engineering, as well as understanding of self-healing mechanisms and modeling capability, self-healing is evolving toward engineering practice instead of engineering dream. Self-healing is gradually changing the paradigm of structural design, which can be seen in a number of recent books and review articles [1–33].

Almost all types of polymers, including thermoplastic, thermoset, and elastomers, have either demonstrated self-healing capability or have shown potential to be healed. Among them, thermoplastics are capable of autonomously and repeatedly self-healing damage at a molecular level by simply heating the materials above their melting (or bonding) temperature and then cooling them down. The mechanisms involved include molecular interdiffusion, randomization, recombination of chain ends, and so on. Therefore, self-healing of thermoplastics is relatively simple. Because thermoplastics are not used as widely as thermosets in structural applications due to their low stiffness and thermal instability, a self-healing study of thermoplastics is not as highly desired as one of thermosetting polymers. Therefore, most recent self-healing studies are focused on thermoset polymers. It is well known that thermoset polymers are chemically or physically cross-linked polymers (chemical bonds between polymer chains, intermolecular van der Waals bonds, dipole–dipole interactions, and molecular entanglement). These cross-links serve as molecular anchors that prevent molecular motion of the polymer chains. This is how thermoset obtains its strength, stiffness, and thermal stability and why it behaves in a brittle manner under mechanical loading. Once one chain fractures, the force is transferred to its neighbors through the cross-linked network, leading to crazing, cracking, and ultimately macroscopic fracture at a relatively small strain. Therefore, the special molecular structure suggests that thermosets are brittle and the failure of thermosets involves molecular-length scale fractures such as cross-links or chemical bonds in the main polymer chains. Compared to thermoplastics, thermosets do not possess the chain mobility that is so heavily used in the self-healing of thermoplastics. As a result, the development of self-healing thermosets has followed distinctly different routes and represented a real challenge.

While a number of self-healing approaches and strategies have been explored for thermosetting polymers, and the literature is exploding, generally, healing can be divided into two categories. One is extrinsic healing based on incorporation of an external healing agent; the other is intrinsic healing by the polymer itself. As indicated by Li [1], both extrinsic healing and intrinsic healing can be further divided into physical healing (molecule entanglements) and chemical healing (reestablishment of chemical bonds). Chemical healing can be further divided into subcategories based on the type of chemical bonds (covalent bonds, hydrogen bonds, and so on).

1.1.1 Intrinsic self-healing

Intrinsic healing systems include polymers with a thermally reversible covalent bond (TRCB) [34], ionomer [35], supramolecule chemistry (hydrogen bonds, metal–ligand coordinations, π–π stacking interactions) [36], thermosetting epoxy with unreacted epoxide [37], and polymers with dynamic covalent bond exchange (DCBE) [38]. However, not all self-healing polymers are suitable for applications in load-carrying structures. The reason is that in load-bearing structures, the polymer matrix must have sufficient strength and stiffness, even using fibers as reinforcement. One polymer that stands out is ionomer. As compared to other intrinsic self-healing polymers, which are usually gels or elastomers and do not have sufficient mechanical strength and/or stiffness, ionomers have sufficient mechanical strength and stiffness, suggesting an applicability to load-carrying structures (tensile strength in tens of MPa and modulus in several GPa) [39]. It is interesting to note that ionomer has been proven to work at low temperatures under ballistic impact and pressure (DuPont Surlyn 8920, at −30°C with 3 MPa of pressure by 197 m/s projectile) [40]. Ionomers are thermoplastic ionic polymers; that is, hydrocarbon polymers bearing pendant carboxylic acid groups that are either partially or completely neutralized with metal or quaternary ammonium ions. The presence of ionic clusters and the order-to-disorder transition phenomenon of the clusters has been related to healing effects in ionomeric materials. The healing ability in the ballistic case arises from local heating above the ionomer melting temperature, producing an elastic response that closes the hole, followed by a slower process involving H–H and ionic bond breaking and reformation. Actually, as long as the fractured pieces can be brought together and the ionomer is locally melted, ballistic impact is not a must; sawing also caused healing [40]. However, all the studies on ionomer self-healing are for panels under free boundary conditions. If the panel is under tensile stress, such as the skin of an aircraft or fuel tank at high altitude, the fractured ionomer pieces due to ballistic impact will be pulled away from each other. Therefore, healing may not be possible.

1.1.2 Extrinsic self-healing

1.1.2.1 Extrinsic self-healing in conventional structural thermosetting polymers

Conventional thermosetting polymers used in lightweight engineering structures such as air...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• List of contributors
• Woodhead Publishing Series in Composites Science and Engineering
• Preface
• 1: Overview of crack self-healing
• 2: Modeling of self-healing smart composite materials
• 3: Solid-state healing of resins and composites
• 4: Microcapsule-based self-healing materials
• 5: Microvascular-based self-healing materials
• 6: Reversible cross-linking polymer-based self-healing materials
• 7: Supramolecular network-based self-healing polymer materials
• 8: Self-healing coatings
• 9: Self-sensing and self-healing in composites
• 10: Rubber-like polymeric shape memory hybrids with repeatable heat-assisted, self-healing, and joule heating functions
• 11: Shape memory polymer-based self-healing composites
• 12: Self-healing materials with embedded shape memory polymer fibers and wires
• Index