Self-healing materials are commonly found in nature, yet man-made materials do not typically have the ability to repair themselves after damage and regenerate their function. We spoke to Professor Wolfgang H. Binder about his research into self-healing and stress reporting, which could help to improve the reliability of a wide variety of materials.
Opening up new horizons in self-healing materials The vast majority
of engineering materials have traditionally been designed with an emphasis on damage prevention rather than damage management, but now the emergence of self-healing materials promises to open up new horizons. Based at the University of Halle in Germany, Professor Wolfgang H. Binder and his research group are investigating self-healing principles, which could be used to improve existing materials. “We are developing concepts to introduce self-healing into these materials,” he outlines. A second aim is to introduce stress reporting capabilities into these materials. “When a material is mechanically deformed or damaged, we want to restore its initial properties. But we also want to see where the damage occurred.” This is an important issue for the aerospace industry in particular, where safety depends to a large degree on rapidly identifying which specific areas on a structure have been subject to high levels of stress. Professor Binder and his colleagues use mechanochemistry to induce a chemical reaction from mechanical stress. “We transform the energy of the stress into a chemical reaction,” he explains. On the one hand this shows where stress has occurred, while it can also help to start the process of
healing any damage. “Stress reporting is very important in certain application areas, such as automotive and aerospace engineering. We want to know where stress has occurred, so that we can essentially visualise it with relative ease.”
The question is how you can introduce a material with which I can achieve self-healing in a living organism. A second point surrounds how self-healing materials can be more effectively implemented in a structure..... Self-healing Many biological materials have self-healing properties, so Professor Binder and his colleagues are drawing a lot of inspiration from the natural world in research. For example, one healing principle that researchers are using is hydrogen bonds, versatile structures which are very common in nature, especially in bio-materials. “When we look at biomaterials – for instance spider silk – we see that the versatility of hydrogen bonds is very important.” Researchers are taking principles from nature and transforming them for technical purposes, although Professor Binder says that they may work differently in synthetic materials. “We aim to understand
Fig. 1: Repeated self-healing with H-bonds : reversible, and multiple self-healing1-7.Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.
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how a hydrogen bond works in a synthetic polymer material.” A hydrogen bond is non-covalent, so it’s relatively weak in comparison to a covalent bond. The bonding energy of a hydrogen bond is also much lower than for a covalent
chemical bond, which is an important consideration in terms of the project’s wider goals. “This means that at normal temperature, this bond can open and close all the time, without us even realising it has happened.” This opening and closing of the bond could be used to induce self-healing in certain classes of materials. “When you cut a material and put the two edges together, the hydrogen bonds find each other on these two different parts of the material, and they reorganise and restructure.” The second self-healing mechanism of interest in the project is based on the use of capsules. In this case, capsules containing an epoxy resin are introduced into a material, which are then used to heal damage when it occurs. “If a material breaks then the capsules are ruptured, and the contents are delivered to the crack site.” Once the resin has been delivered to this location, Professor Binder says that a chemical reaction is then initiated to effectively glue a crack together. “We are using the click reaction, a very efficient and easy reaction, which works at room temperature,” he explains. “We use graphene, together with copper nanosized nanoparticles, as a catalyst. We have developed this in the programme, and we can run the chemistry at room temperature and even below.” This is an important consideration, as while some groups use heat to induce materials to self-heal, the emphasis in Professor Binder’s group is on achieving it at room temperature. The idea is to develop a new concept and incorporate it into existing
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materials. “The materials we are adding are new, but they are only present in relatively small quantities in the main structure,” he stresses. This will help minimise disruption to industry, which could encourage more companies to consider these new concepts. “For industry, entirely changing a portfolio of materials is often simply too expensive and too risky,” points out Professor Binder. “We are taking existing materials, and adding self-healing and stress reporting properties.” A number of tests have also been developed to assess the effectiveness of these self-healing concepts in terms of regenerating the function of materials. One very simple test involves first cutting a material, then putting the two parts together and allowing it to heal, before making some stress-strain measurements. “So you stress the material and check how strong it is, then you can measure the recovery in percentage terms,” he explains. With a second set of tests the material is stressed, then its colour is monitored by fluorescence spectroscopy. “You check how much chemistry has been produced by this mechanical stress which you have put on the material, then you measure it. So you can quantify the stress.”
The wider context here is the issue of the reliability and performance of engineering materials, which has important financial and environmental implications. While a microcrack on a material may be only nanometres in length, a scale not even be visible to the naked eye, it can develop further and lead to significant problems if left unchecked. “With these micro-cracks, you want to take action before they develop into macrocracks.” Stress reporting therefore needs to be continuous, rather than just in response to specific events. “The system can report the repeated stress on materials. With it, we can understand the history of stress in a material and assess the level of stress it has been subjected to.”
Industrial applications This research has attracted the attention of industry, including automotive and aerospace companies, who are keen to both improve the reliability of materials and reduce maintenance costs. The idea is often not to use self-healing materials throughout a whole structure, but rather in specific parts which may be more vulnerable to damage or scratches, such as the steering wheel of a car. “A German automotive manufacturer is using a coating on the steering
wheel, so that when it is scratched, the scratch is removed by itself after a certain period of time,” outlines Professor Binder. There has also been contact with representatives of the aircraft industry, particularly with respect to optical fibres. “These optical fibres are used to transmit information in an aircraft.” A large volume of information is transmitted between different parts of an aircraft during flight, so it’s essential that these optical fibres function effectively. The optical fibres are bound together with certain parts of the aircraft during the manufacturing process, which Professor Binder says requires careful monitoring. “We want to see whether any of these fibres have been broken during the manufacturing process. For this we need a reporting system which is embedded into the fibres, in order to visualise where any damage has happened,” he explains. This can act as a kind of initial quality check, ensuring that optical fibres are still functioning effectively. “Stress reporting is very important here.” The group’s research could also be relevant to other areas of industry, such as the chemical sector, while there are still many other avenues of research to explore. One major area of interest to Professor Binder is whether self-healing can be incorporated
Fig. 2: Mechanochemical healing : force induced chain breakage and catalyst activation, allowing stress detection and stress-induced repair8-12. The concept can be extended to self-healing epoxy-resin systems13,14, using encapsulated reagents15. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.
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Mechanochemical and supramolecular self-healing polymers
Fig. 3: Graphene based self-healing allowing stress-location by fluorogenic “click”-reactions9,12,16. Reproduced with permission from the Royal Society of Chemistry, (RSC).
Project Objectives
One of the most outstanding properties of biological materials is their ability to self-heal and regenerate function upon the infliction of damage by external mechanical loads. Man-made materials generally do not have this healing ability, as engineering materials were and are developed on the basis of the ‘damage prevention’ paradigm rather than a ‘damage management’ concept. However, self-healing materials certainly offer enormous possibilities, in particular for applications where long-term reliability in poorly accessible areas, such as tunnels, underground infrastructures, high-rise buildings or space applications, is important. The objective of the Priority Programme is the conceptual design of synthetic self-healing materials and the elucidation of generic, fundamental material-independent principles (e.g. following a sequence of crack generation and propagation, mobility and transport of material, interface bonding and immobilisation of the transported material).
Project Funding
Funding by the European Union’s Seventh Framework Programme for research, technological development and demonstration under Grant Agreement No. 313978 (IASS, “Improving aircraft safety by self healing structures and protecting nanofillers”) as well as Grant Nos. DFG-Bi 1337/8-1 and DFG-Bi 1337/8-2 within the SPP 1568 (“Design and Generic Principles of Self-Healing Materials”) by the “Deutsche Forschungsgemeinschaft (DFG).
into a living system. “The question is how you can introduce a material with which I can achieve self-healing in a living organism,” he outlines. A second point surrounds how self-healing materials can be more effectively implemented in a structure. “This is quite a large step, and I think that 3-D printing is a key technology here,” says Professor Binder. “With 3-D printing technology you can basically introduce a material in any position within your structure, technologically or optically.”
This opens up the possibility that a wider range of materials could have self-healing properties in future. While economic realities mean that the concept is unlikely to be introduced into all materials, he believes that self-healing could bring significant benefits in certain areas. “It makes sense if you have a self-healing tyre for example. A tyre can be used for a longer period, and in this way you can really enhance sustainability,” he points out.
Fig. 4 : Click-chemistry based self-healing nanocomposites15. © Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.
Contact Details
Professor Wolfgang H. Binder Martin Luther University Halle-Wittenburg Full Professor of Macromolecular Chemistry Faculty of Natural Sciences II von Danckelmannplatz 4, D-06120 Halle T: +49 (0)345 55 25930 E: wolfgang.binder@chemie.uni-halle.de W: http://www.natfak2.uni-halle.de/forschung/ polymers/institute_of_chemistry/binder/?lang=en Professor Wolfgang Binder
(1) Chen, S.; Yan, T.; Fischer, M.; Mordvinkin, A.; Saalwächter, K.; Thurn-Albrecht, T.; Binder, W. H. Opposing Phase-Segregation and Hydrogen-Bonding Forces in Supramolecular Polymers. Angewandte Chemie International Edition 2017, 56, 13016-13020. (2) Chen, S.; Binder, W. H. Dynamic Ordering and Phase Segregation in Hydrogen-Bonded Polymers. Acc. Chem. Res. 2016, 49, 1409-1420. (3) Chen, S.; Mahmood, N.; Beiner, M.; Binder, W. H. Self-Healing Materials from V- and H-Shaped Supramolecular Architectures. Angew. Chem., Int. Ed. 2015, 54, 10188-10192.
Wolfgang Binder is Professor of Macromolecular Chemistry at Martin Luther Universitat, Halle-Wittenberg. His research centres around the preparation of functional polymers and the transfer of the generated molecules into areas of biomimetic polymers, self-healing polymers and nanostructured materials.
(4) Yan, T.; Schröter, K.; Herbst, F.; Binder, W. H.; Thurn-Albrecht, T. What Controls the Structure and the Linear and Nonlinear Rheological Properties of Dense, Dynamic Supramolecular Polymer Networks? Macromolecules 2017, 50, 2973-2985. (5) Yan, T.; Schröter, K.; Herbst, F.; Binder, W. H.; Thurn-Albrecht, T. Unveiling the molecular mechanism of self-healing in a telechelic, supramolecular polymer network. Scientific Reports 2016, 6, 32356. (6) Herbst, F.; Binder, W. H.: Self-healing polymers via supramolecular, hydrogen bonded networks. In Self Healing Polymers: from Principles to Application; Binder, W. H., Ed.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2013; pp 275-300. (7) Herbst, F.; Seiffert, S.; Binder, W. H. Dynamic supramolecular poly(isobutylene)s for self-healing materials. Polym. Chem. 2012, 3, 3084-3092. (8) Michael, P.; Biewend, M.; Binder, W. H. Mechanochemical Activation of Fluorogenic CuAAC “Click” Reactions for Stress-Sensing Applications. Macromolecular Rapid Communications 2018, 0, 1800376. (9) Döhler, D.; Michael, P.; Binder, W. H. CuAAC-Based Click Chemistry in Self-Healing Polymers. Accounts of Chemical Research 2017, 50, 2610-2620. (10) Michael, P.; Binder, W. H. A Mechanochemically Triggered “Click” Catalyst. Angew. Chem., Int. Ed. 2015, 54, 13918-13922. (11) Hu, M.; Peil, S.; Xing, Y.; Döhler, D.; Caire da Silva, L.; Binder, W. H.; Kappl, M.; Bannwarth, M. B. Monitoring crack appearance and healing in coatings with damage self-reporting nanocapsules. Materials Horizons 2018, 5, 51-58. (12) Döhler, D.; Rana, S.; Rupp, H.; Bergmann, H.; Behzadi, S.; Crespy, D.; Binder, W. H. Qualitative sensing of mechanical damage by a fluorogenic “click” reaction. Chem. Commun. 2016, 52, 11076-11079. (13) Guadagno, L.; Vertuccio, L.; Naddeo, C.; Calabrese, E.; Barra, G.; Raimondo, M.; Sorrentino, A.; Binder, W. H.; Michael, P.; Rana, S. Self-healing epoxy nanocomposites via reversible hydrogen bonding. Composites Part B: Engineering 2019, 157, 1-13. (14) Raimondo, M.; Nicola, F. D.; Volponi, R.; Binder, W.; Michael, P.; Russo, S.; Guadagno, L. Self-repairing CFRPs targeted towards structural aerospace applications. International Journal of Structural Integrity 2016, 7, 656-670. (15) Rana, S.; Döhler, D.; Nia, A. S.; Nasir, M.; Beiner, M.; Binder, W. H. “Click”-Triggered Self-Healing Graphene Nanocomposites. Macromol. Rapid Commun. 2016, 37, 1715-1722. (16) Shaygan Nia, A.; Rana, S.; Döhler, D.; Osim, W.; Binder, W. H. Polymer 2015, 79, 21-28.
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