Ming Qiu Zhang - Extrinsic and Intrinsic Approaches to Self-Healing Polymers and Polymer Composites

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Ming Qiu Zhang and Min Zhi Rong

Materials Science Institute, Zhongshan University Guangzhou, China

This edition first published 2022

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[Hardback]: 9781119629955

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Cover Images: courtesy of Ming Qiu Zhang. Min Zhi Rong, Dong Yu Zhu and Nan Nan Xia

The global study of self‐healing polymers and polymer composites started roughly at the beginning of this century based on the consensus that it represents a next‐generation technology, which would greatly improve the performance of products, including, but not limited to, their reliability and durability. Two decades have past since then. When summarizing the present status, we find there has not yet been a landmark example of a commercial application. It turns out that our prediction of the development of the newly emerging innovation made in our book Self‐Healing Polymers and Polymer Composites , which was published by Wiley in 2011, was too optimistic. Nowadays, scientists are still working hard in both theoretical and applied research. In this context, an updated book is necessary to reflect the latest achievements, which fits with the vigor and vitality of the study activities.

Being involved in the early stage of the research, we witnessed the evolution track of the movement. In general, the strategies of self‐healing polymers and polymer composites are classified into two categories: extrinsic and intrinsic. The former makes use of the healing vessels (e.g. microcapsules, micropipelines, and vascular networks) embedded in the target materials to be repaired, which would be broken upon damaging the materials, releasing fluidic healing agent to the cracked sites and rebinding the cracks via chemical/physical interactions across the interface. In contrast, the latter operates by means of reversible intra‐ and/or intermacromolecular interactions, and no additional healing agent is required. A few groups investigated intrinsic self‐healing of polymers as early as the 1970s from the angle of polymer physics. Entanglement of molecular chains acted as the main mechanism of wound healing. In 2001, Professor Scott White and his coworkers at the University of Illinois at Urbana‐Champaign published a paper about self‐healing in terms of healing capsules. Afterwards, extrinsic self‐healing attracted plenty of research interest worldwide. In the meantime, intrinsic self‐healing via chemical reversible covalent and non‐covalent interactions grew so fast that it was ahead of extrinsic self‐healing once again with respect to the materials species and properties to be restored. Nevertheless, chemistry has made a greater contribution this time, as more and more chemists have become involved, focusing on molecular design and synthesis as well as functionalities recovery. Intrinsic self‐healing ability has been a standard feature of materials in some cases. In addition, the development of intrinsic self‐healing has created other new accompanying techniques, such as reprocessing, reshaping and recycling, topology rearrangement, and processing difficult‐to‐process materials of traditionally non‐reworkable thermosetting polymers. The new possibilities go beyond the scope of classic polymer engineering and enrich the measures of material diversification.

The style of writing and organization of our abovementioned book is followed herein, so that this book will provide the reader with an overall view of the ongoing research, and more importantly, inspiration for the development of novel materials and functionalities.

Chapter 1serves as an introduction to the field of self‐healing polymers and polymer composites. In addition to the general scope, the achievements made by different laboratories are carefully reviewed. Personal viewpoints of the authors are addressed showing the challenges, trends, and future directions. In Chapters 2– 8, the self‐healing strategies proposed by the authors' group are introduced, which are structured in accordance to the chemical reactions responsible for crack healing. Chapter 2is dedicated to the healing reaction governed by addition polymerization and the self‐healing systems based microcapsules and plastic tubes as well, which are able to work without manual intervention. High healing efficiency, determined by static fracture, impact and fatigue tests, is acquired for epoxy composites. To improve healing speed and widen windows of processing and operation of self‐healing thermosetting materials, a few fast hardeners are introduced to work with epoxy monomer forming new groups of healing agent, as discussed in Chapter 3. Taking advantage of cationic polymerization, not only repair of cracks is possible at and below room temperature like the epoxy‐mercaptan pair, but also crack healing is greatly accelerated. Besides, redox cationic polymerization is used for healing of thermoplastics. Chapter 4describes the self‐healing polymeric materials for advanced engineering applications, driven by anionic polymerization. The healing system consists of epoxy‐loaded microcapsules and imidazole latent hardener. The latter can be well pre‐dissolved in an uncured composites' matrix, leading to homogenous distribution of the reagent on a molecular scale. The major concern of Chapter 5lies in usage of small molecule monomers as healing agent instead of epoxy monomer. Accordingly, nucleophilic addition, ring‐opening reaction, atom transfer radical polymerization, and free radical polymerization prove effective in rebinding the cracked planes. Unlike the extrinsic self‐healing approaches surveyed in Chapters 2– 5, Chapter 6deals with design, synthesis and characterization of intrinsic self‐healing epoxy, in which thermally reversible Diels–Alder bonds account for crack healing via chain reconnection. In addition to the remendability, the cured version of the novel epoxy has similar mechanical performance to conventional epoxy. In Chapter 7, the intrinsic self‐healing via reversible C─ON bonds is analyzed. Because of the synchronous fission/radical recombination of C─ON bonds, the polymers containing this type of reversible covalent bond can be self‐healed in a one‐step fashion, which prevents material distortion during healing as for those containing Diels–Alder bonds. Lastly, Chapter 8demonstrates the application of reversible S─S bonds in self‐healing polymers. The disulfide bonds in the tailor‐made polymers as well as the commercial silicone elastomer and vulcanized rubber can be triggered under the stimuli of heating and sunlight, offering satisfactory healing efficiency.