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Prog. Polym. Sci. 33 (2008) 479–522 CSIRO M The development and characterization of self-healing synthetic polymeric materials have been inspired by biological 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 480 4.4. Self-healing via reversible bond formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 489 ARTICLE IN PRESS www.elsevier.com/locate/ppolysci 0079-6700/$ - see front matter Crown Copyright r 2008 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.progpolymsci.2008.02.001 �Corresponding author. Tel.: +613 9545 2893; fax: +61 3 9545 2829. E-mail address: Dong.Yang.Wu@csiro.au (D.Y. Wu). 2. Fracture mechanics of polymeric materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 483 3. Traditional repair methods for polymeric materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 485 3.1. Repair of advanced composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 485 3.1.1. Welding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 485 3.1.2. Patching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 485 3.1.3. In-situ curing of new resin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 485 3.2. Repair of thermoplastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 485 4. Self-healing of thermoplastic materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 486 4.1. Molecular interdiffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 486 4.2. Photo-induced healing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 487 4.3. Recombination of chain ends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 488 could significantly extend the working life and safety of the polymeric components for a broad range of applications. An overview of various self-healing concepts for polymeric materials published over the last 15 years is presented in this paper. Fracture mechanics of polymeric materials and traditional methods of repairing damages in these materials are described to provide context for the topic. This paper also examines the different approaches proposed to prepare and characterize the self-healing systems, the different methods for evaluating self-healing efficiencies, and the applicability of these concepts to composites and structural components. Finally, the challenges and future research opportunities are highlighted. Crown Copyright r 2008 Published by Elsevier Ltd. All rights reserved. Keywords: Polymeric materials; Self-healing; Composite repair; Biomimetic repair Contents systems in which damage triggers an autonomic healing response. This is an emerging and fascinating area of research that Abstract Received 17 June 2007; received in revised form 30 January 2008; accepted 18 February 2008 Available online 4 March 2008 Dong Yang Wu�, Sam Meure, David Solomon anufacturing and Materials Technology, Gate 5, Normanby Road, Clayton South, Victoria 3168, Melbourne, Australia Self-healing polymeric materials: A review of recent developments . . . . . . . . . . . . . . . . . . . . . . . . . . . self-h onsi . . . . . . . . . . . . . . . . . . rials . . . . . . . . . . . . . . . . . . Polymers and structural composites are used in a radiation, This could de ex Th ma do et al. [2] dominated fat by more recen causes dela In the case the polymeric components, shorten the life of the vehicle, and potentially compromise passenger ARTICLE IN PRESS D.Y. Wu et al. / Prog. Polym. Sci. 33 (2008) 479–522480 mposites and found that matrix cracking mination and subsequent fiber fracture. Conference on Self-healing Materials organized by the Delft University of Technology of the Nether- matrix-dominated properties such as compressive strength are also influenced by the amount of matrix damage. Jang et al. [5] and Morton and Godwin [6] extensively studied impact response in toughened polymer co such interests were demonstrated through US Air force [11] and European Space Agency [12] invest- ments in self-healing polymers, and the strong presence of polymers at the First International damage. Chamis and Sullivan [3] and tly, Wilson et al. [4] have shown that then White et al. [9] in 2001 further inspired world wide interests in these materials [10]. Examples of igue life due to the redistribution of loads caused matrix tions in the topic by Dry and Sottos [8] in 1993 and have predicted reductions in fiber- properties such as tensile strength and for extending the working life and safety of the polymeric components. The more recent publica- ep within the structure where detection and ternal intervention are difficult or impossible. e presence of the microcracks in the polymer trix can affect both the fiber- and matrix- minated properties of a composite. Riefsnider conventional repair methods are not effective for healing invisible microcracks within the structure during its service life. In response, the concept of self-healing polymeric materials was proposed in the 1980s [7] as a means of healing invisible microcracks y mechanical, chemical, thermal, UV or a combination of these factors [1]. lead to the formation of microcracks techniques have been developed and adopted by industries for repairing visible or detectable da- mages on the polymeric structures. However, these variety of applications, which include transport vehicles (cars, aircrafts, ships, and spacecrafts), sporting goods, civil engineering, and electronics. However, these materials are susceptible to damage induced b safety. With polymers and composites being increasingly used in structural applications in aircraft, cars, ships, defence and construction industries, several 4.4.1. Organo-siloxane . . . . . . . . . . . . . . . 4.4.2. Ionomers . . . . . . . . . . . . . . . . . . . . 4.5. Living polymer approach. . . . . . . . . . . . . . . 4.6. Self-healing by nanoparticles . . . . . . . . . . . . 5. Self-healing of thermoset materials . . . . . . . . . . . . 5.1. Hollow fiber approach. . . . . . . . . . . . . . . . . 5.1.1. Manufacture and characterization . . 5.1.2. Assessment of self-healing efficiency . 5.2. Microencapsulation approach . . . . . . . . . . . 5.2.1. Manufacture and characterization of 5.2.2. Mechanical property and processing c 5.2.3. Assessment of self-healing efficiency . 5.3. Thermally reversible crosslinked polymers . . . 5.4. Inclusion of thermoplastic additives . . . . . . . 5.5. Chain rearrangement. . . . . . . . . . . . . . . . . . 5.6. Metal-ion-mediated healing . . . . . . . . . . . . . 5.7. Other approaches . . . . . . . . . . . . . . . . . . . . 5.7.1. Self-healing with shape memory mate 5.7.2. Self-healing via swollen materials . . . 5.7.3. Self-healing via passivation . . . . . . . 6. Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8. Insights for future work . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. Introduction of a transport vehicle, the propagation of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 489 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 491 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 493 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 493 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 494 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 494 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 494 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 497 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 498 ealing microcapsules . . . . . . . . . . . . . . . . . . . . . . . 499 derations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 503 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 504 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 508 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 509 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 511 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 512 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 513 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 513 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 513 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 514 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 514 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 515 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 515 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 516 microcracks may affect the structural integrity of lands in February 2007. ARTICLE IN PRESS D.Y. Wu et al. / Prog. Polym. Sci. 33 (2008) 479–522 481 Nomenclature e elongation to break Z fatigue-healing efficiency s fracture stress DK change in KI during fatigue cycling l wavelength A6ACA acryloyl-6-amino caproic acid BDMA benzyl dimethylamine CQ camphorquinone DA Diels–Alder DBTL di-n-butyltin dilaurate DCB double-cantilever beam DCPD dicyclopentadiene DETA diethylenetriamine Conceptually, self-healing polymeric materials have the built-in capability to substantially recover their load transferring ability after damage. Such recovery can occur autonomously or be activated after an application of a specific stimulus (e.g. heat, radiation). As such, these materials are expected to contribute greatly to the safety and durability of polymeric components without the high costs of active monitoring or external repair. Throughout the development of this new range of smart materials, the mimicking of biological systems has been used as a source of inspiration [13]. One example of biomimetic healing is seen in the vascular-style bleeding of healing agents following the original self-healing composites proposed by Dry and Sottos [8]. These materials may also be able to heal damage caused by insertion of other sensors/ DGEBA diglycidyl ether of bisphenol-A DMA dimethylaniline DSC differential scanning calorimetry E fracture energy EMAA poly(ethylene-co-methacrylic acid) ENB 5-ethylidene-2-norbornene ESR electron spin resonance GQ strain energy-release factor HOPMDS hydroxyl end-functionalized polydi- methyl-siloxane I molecular parameters KI stress intensity factor KIMax maximum stress intensity factor KIQ critical stress intensity factor LDPE low-density polyethylene MA methacrylic acid Mw molecular weight N number of cycles in a fatigue test NBE norbornene NMA nadic methyl anhydride NMR nuclear magnetic resonance OH hydroxyl group PBE polybisphenol-A-co-epichlorohydrin PC polycarbonate PDES polydiethoxysiloxane PEEK polyether–ether–ketone PET poly(ethylene terephthalate) PMMA poly(methyl methacrylate) PMEA poly(methoxy ethylacrylate) PROMP photo-induced ring-opening metathesis polymerization actuators, cracking due to manufacturing-induced residual stresses, and fiber de-bonding. An ideal self-healing material is capable of continuously sensing and responding to damage over the lifetime of the polymeric components, and restoring the material’s performance without nega- tively affecting the initial materials properties. This is expected to make the materials safer, more reliable and durable while reducing costs and maintenance. Successful development of self-healing polymeric materials offers great opportunities for broadening the applications of these lightweight materials into the manufacture of structural and critical components. Healing of a polymeric material can refer to the recovery of properties such as fracture toughness, tensile strength, surface smoothness, barrier properties PS polystyrene ROMP ring-opening metathesis polymeriza- tion SEM scanning electron microscopy TBC paratertbutylcatechol TCE 1,1,1-tris-(cinnamoyloxymethyl) ethane TDCB tapered double-cantilever beam TEGDMA triethyleneglycol dimethylacrylate TEM transmission electron microscopy TEMPO 2,2,6,6-tetramethyl-piperidine-1-oxy Tg glass transition temperature TGA thermo-gravimetric analysis UDME urethane dimethacrylate UF urea-formaldehyde UV ultraviolet light and even molecular weight. Due to the range of properties that are healed in these materials, it can be difficult to compare the extent of healing. Wool and O’Connor [14] proposed a basic method for describing the extent of healing in polymeric systems for a range of properties (Eqs. (1)–(4)). This approach has been commonly adopted as discussed in later sections, and has been used as the basis for a non-property-specific method of compar- ing ‘‘healing efficiency’’ (Eq. (5)) of different self- healing polymeric systems RðsÞ ¼ shealed sinitial (1) Rð�Þ ¼ �healed �initial (2) RðEÞ ¼ Ehealed Einitial (3) RðIÞ ¼ Ihealed I initial (4) Healing efficiency ¼ 100� Property valuehealed Property valueinitial (5) where R, s, e, E and I represent the recovery ratios relating to fracture stress, elongation at break, fracture energy and molecular parameters, respectively. This review briefly describes the fracture me- chanics of polymeric materials and the traditional methods of repairing damage in these materials to provide the context for our focus of highlighting major advancements in design and development of self-healing polymeric materials during the last 15 years. Tables 1 and 2 provide summaries of these developments. It can be seen that both molecular and structural approaches were investigated for self- healing of thermoplastic and thermoset materials although the research interests have been shifted to ARTICLE IN PRESS Table 1 Developments in self-healing polymers Matrix Healing type Healing method First report of method Best efficiency achieved Test method Healing conditions Thermoplastic Molecular Molecular interdiffusion (thermal) 1979 [67] 120% [67] Fracture toughness 7–8min at 115 1C Molecular interdiffusion (solvent) 1990 [44] 100% [44] Fracture toughness 4–5min at 60 1C Reversible bond formation 2001 [91] 100% [94] Puncture closure o1min at �30 1C ] ] ] ] 9] 0] ] ,162] 4] 2] 5] 1] D.Y. Wu et al. / Prog. Polym. Sci. 33 (2008) 479–522482 Recombination of chain ends 2001 [82 Photo-induced healing 2004 [77 Living polymer 2005 [95 Structural Nanoparticle healing 2004 [99 Thermoset Molecular Chain re- arrangement 1969 [18 Thermally reversible crosslinks 2002 [17 Ion-mediated healing 2006 [13 Structural Microencapsulation approach 1997 [120,121 Thermoplastic additives 2005 [18 Healing via passivation 1998 [20 Memory shape alloy 2002 [19 Healing via swelling 2005 [20 98% [88] Tensile strength 600 h at Ambient Molecular weight 600 h at Ambient 26% [77] Flexure strength 10min at 100 1C – – – Impeded Crack Growth [102] Visual inspection Ambient 100% [187] Visual inspection 10min at ambient 100% [189] Fracture toughness 150 1C 80% [169] Fracture toughness 30min at 115 1C then 6 h at 40 1C 75% [13] Tensile strength 12 h at ambient 213% [135] Fatigue resistance Ambient 93% [127] Fracture toughness 24 h at Ambient 14% [136] Tensile strength 24 h at ambient then 24 h at 80 1C 65% [186] Impact strength 1 h at 160 1C – – – – – – – – – ARTICLE IN PRESS ort o 3] 3] 9] ng fai D.Y. Wu et al. / Prog. Polym. Sci. 33 (2008) 479–522 483 Table 2 Developments in self-healing polymer composites Host matrix Healing type Healing method First rep method Thermoset composites Structural Microencapsulation approach 2001 [16 Thermoplastic additives 1999 [18 Hollow-fiber Approach 1996 [10 Fig. 1. Mode 1 openi thermoset-based systems in recent years. We will also describe and discuss the different approaches proposed to prepare and characterize the self- healing systems, the methods for evaluating self- healing efficiencies, the applicability of the concepts to composites and structural components, and the challenges and future research opportunities. 2. Fracture mechanics of polymeric materials Although thermal, chemical and other environ- mental factors can cause damage in polymers, impact and cyclic fatigue associated failures are receiving the most attention for structural applica- tions of polymeric materials [15]. Both of these failure mechanisms proceed via crack propagation, with a monotonic load experienced during impact- type incidents and cyclic loads experienced during fatigue. Crack propagation [16–18] and the me- chanics [19,20] associated with these failures in polymeric materials have been modeled and re- searched extensively. For a crack to propagate, the energy released during cracking must be equal to, or larger than the energy required to generate new surfaces on the material [1,21]. Although new models for crack f Best efficiency achieved Test method Healing conditions 80% [122,163] Fracture toughness 48 h at 80 1C 19% [136] Tensile strength 24 h at Ambient then 24 h at 80 1C 100% [183] Flexure strength 10min at 120 1C Tensile strength 10min at 120 1C 30% [186] Visual 2 h at 130 1C 93% [114] Flexure strength 24 h at Ambient lure in a material [1]. propagation are still being developed [22,23], most crack propagation modeling is based on a para- meter called the (KI) [24,25]. During crack opening- type failure growth (mode I in Fig. 1), KI is related to crack depth, material/crack geometry and the applied stresses. As the applied stress and crack geometry change during monotonic or cyclic load- ing, a critical stress intensity factor (KIQ) is reached and then crack growth occurs. During an impact damage incident (consisting of a monotonic load) the extent of crack propagation is related to the maximum stress intensity factor (KIMax) experi- enced. During fatigue-type damage crack propaga- tion is related to both KIMax and the change in KI during cycling (DK) [26]. In order to heal- cracked polymers, the fractured surfaces need to be resealed or alternatively crack growth must be impaired. Fig. 2 demonstrates a number of methods to retard crack growth [24,27]. Basically, crack growth retardation occurs when energy is dissipated within the loaded material without extending an existing crack. Intrinsic crack growth retardation can be achieved through selection of appropriate monomer and curing agent system [28,29], varying the ratio of curing components [30–32], or use of additives or ARTICLE IN PRESS D.Y. Wu et al. / Prog. Polym. Sci. 33 (2008) 479–522484 modifiers [33–35]. These intrinsic approaches to crack growth retardation provide alternative ave- nues for stress relief within the original structure, and they are generally used to improve the intrinsic properties of the virgin materials rather than to heal-damaged components. Fig. 2. Extrinsic mechanisms of cr Extrinsic crack growth retardation mechanisms are used as the primary method of repairing damage in both the traditional and the s