TU DELFT. Self-healing polymer materials. Imaging automatic repair with the Olympus LEXT

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1 TU DELFT Imaging automatic repair with the Olympus LEXT Self-healing polymer materials Things around us have two features in common: Firstly, each item is shaped to a specific geometry for functional and/or aesthetic reasons. Secondly, they are made out of one or more materials, determined by a whole host of requirements from physical needs through to desired tactile or visual effects. A number of key questions impinge on both of these features: What does the product need to do? What is the desired lifespan? What conditions/stresses will it be subjected to? As a result, selecting the right materials and their grade is always a balance between many requirements and another consideration will also soon be entering the fray self-healing capabilities! Steven Mookhoek and colleagues at the Delft University of Technology and The Dutch Polymer Institute are working on introducing such properties to a range of materials and have been using the Olympus LEXT confocal laser scanning microscope to get a better grasp of the underlying processes. Introduction As a result of exposure to external elements such as physical stresses, UV light, temperature, wind and water erosion, oxygen, etc, a material s properties degrade overtime and although substances have different degradation profiles, it is essentially an unavoidable process. Stresses cause micro-damage to the material and eventually the damage accumulates and leads to failure or to a surface state which is deemed unacceptable in the case of coatings. To make the product last longer two different strategies are traditionally used: 1) Over-dimension of materials 1

2 (making them stronger or better than necessary) - one of the most common strategies to combat mechanical degradation effects. 2) Providing a coating to the material to combat the effects of chemical degradation. There is now, though, a growing number of research groups around the world looking a third option self-healing properties. Repair, replace or leave alone Potentially, the ability of a material or even an entire structure to heal its own minor cracks and breaks would make the world a safer and more efficient place. For example, if micro-cracks started to develop on an aircraft wing they could very quickly lead to a disastrous failure and loss of life. But if the very process of cracking induced a self-healing substance embedded in the metal to fill the cracks and stop them creeping further, disaster could be averted. Self-heal (SH) processes would ideally return a material to its initial strength, but this is not always possible, hence in a critical component like a plane wing it would also be essential that the selfheal event signalled that material degradation had happened. Therefore a proper repair or replacement can be made and avert any chance of critical failure. In less critical or even cosmetic components, SH processes could actually prolong the lifespan of a material considerably and remove the need to replace sections. Basic examples of such systems are already on the market certain car paints are available that can remove scratches from their own surface. One such system requires that the scratched paint is exposed to direct sunlight for a number of hours enabling the paint to soften enough to flow back into the scratched areas. Other concepts for SH coating systems were designed at the Delft University of Technology (TU Delft), making use of moisture in air to close cracks in topcoats. For this purpose a layer of very hydroscopic clay particles placed just below a layer of a common top-coat, swell on contact with the moisture in air and fill the scratches re-establishing a continuous top layer. Research into SH 2

3 properties requires instrumentation to study surfaces very closely such as the scanning electron microscope and scanning probe microscope. The Olympus LEXT confocal laser scanning microscope though, offers many of the desirable properties of these traditional metrology instruments, but with the added benefit of being very quick and easy to use. Methods The Olympus OLS3100 LEXT is a confocal laser scanning microscope (clsm) designed for ultra-precise measurement and observation with the highest levels of reliability. No sample preparation is required, and samples can be placed directly on the microscope stage as they are. Both 3D observation and high-precision 3D measurements are possible in real time. What is more, the LEXT features much higher resolution than conventional optical devices, but just as many different observation methods, ensuring quicker, more accurate specimen analysis. LEXT utilises low wavelength optical technology with a 408nm laser in combination with confocal scanning to exceed known resolution limits of optical imaging systems. By developing a special optical system which minimises the aberrations associated with short wavelength and maximises the transmission at 408nm Olympus achieved unequalled image quality and signal response. The advanced XY scanner used in the LEXT makes the scanning process faster and the results more reproducible compared to conventional scanner technologies, since the moving mass is much lower. The result is a worldleading plane resolution which clearly recognizes individual lines spaced only 0.12µm apart and 0.01µm in height, for ultra-precise measurements of the micro fabrication surfaces. All this with a remarkable reproducibility of 3σ n-1 = 0.02µm (XY direction) and 3σ n-1 = Lµm (height; L= measurement height). 3

4 Results Steven Mookhoek and colleagues in the Faculty of Aerospace Engineering at TU Delft, are using the Olympus LEXT in their work on liquid-based self healing mechanisms in polymer materials. In these systems, microcapsules act as a storage medium for liquid monomers inside a polymer matrix and release their reactive contents upon fracture of the surrounding material. They are investigating the fundamental properties of different microcapsule contents, shapes, sizes and densities, in a range of polymer items. Example combinations include epoxy resins stored in poly (urea formaldehyde) (PUF) microcapsules embedded in a piece of polystyrene. There are a large number of different variables to be tested and therefore investigations involve many samples, as a result fast and accurate analysis is essential. Ensuring distribution To ensure that the liquid capsules in each sample are distributed correctly, Steven Mookhoek and colleagues use x-ray (micro)tomography to give a recreation of the 3D nature of the sample - as shown in figure 1. Furthermore, this technique allows them to quantify the liquid release upon fracture inside the crack since it shows how many microcapsules have rupture with the crack. Invoking healing The healing potential of a material is tested by controlled fracture experiments. In such experiments stress is typically applied to each sample using a dual cantilever beam for example, following the creation of small weak point using a razor blade. The samples are either statically or dynamically (fatigue) loaded to failure, controlling the crack propagation. After failure the crack-surfaces of the tested samples are clamped back together and are allowed to heal over varying time intervals. Subsequently, 4

5 the healed samples are loaded again, recording their toughness following self healing. Comparison of the healed fracture toughness to the initial test value results in a healing efficiency for the selected material system and healing time. Visualising repair processes Once the experiments are complete, the surfaces of the fracture-planes are visualised and analysed using the Olympus LEXT confocal laser scanning microscope. Steven commented, The Olympus LEXT has proven to be an excellent instrument for our research as it enables us to visualise fractureplanes on the samples very quickly and easily in high resolution. The images are comparable to SEM, but there is absolutely no preparation required, meaning that we can view and analyse all our samples in the time it may take to do one sample on an SEM. He continued, We can do all the measurements we need to do with a great degree of accuracy and repeatability including width, depth, height, area and volume. Moreover, we can do these in a non-vacuum environment, which is important as some of the chemicals used in our experiments are toxic and when a vacuum is applied they are pulled out of the sample. This means that the SEM needs to be decontaminated after use, increasing its downtime. Steven also noted that the LEXT had some additional features that have made it even more useful. The built-in analysis tools within the operating software, provide an excellent measurement and data manipulation resource, and the laser can be operated in non-confocal mode, to provide a unique imaging resource (see figure x). Results to be proud of With such powerful imaging and analysis capabilities, the researchers at TU Delft have forged ahead on their projects and have discovered, amongst other things, that microcapsules not only rupture at the fracture plane, but also inside the material close to the crack due to the distributed 5

6 stresses. The affected zone is close to a factor of 2.5 times the capsule diameter. It was observed that the propagated crack only showed small deviation due to the embedment of the liquid-containing structures. It also demonstrated that microcapsules do not always fracture mid-plane, instead they can fracture anywhere throughout their profile. The images and analysis data have enabled the team to accurately interpret the factors affecting the healing mechanism and as a result they have been able to take their research forward more quickly. Discussion Although some self-healing systems have started being commercialised, there is a lot of research left to do to discover suitable SH systems and apply them to everyday situations. A number of groups around the world are making serious inroads into the science behind SH properties and there is a requirement for fast, efficient and accurate measurement systems. The Olympus LEXT fulfils many of the requirements for the team at TU Delft and The Dutch Polymer Institute, and this article shows the kind of results and images that have enabled Steven Mookhoek and colleagues to stride forward in their investigation of SH in polymers. There is also a dividend for other research teams at the university, as the flexibility of the LEXT makes it applicable to a broad range of research projects. Images: Image 1 Image 2 3D x-ray tomographic reconstruction of a microcapsule/polymer composite showing filled microcapsules in purple and empty microcapsules in green Olympus LEXT 3D visualisation of a fracture-plane of a microcapsule/polymer composite, showing the ruptured microcapsules 6

7 Image 3 Image 4 SEM image of a similar fracture Olympus LEXT 2D non-confocal image surface of urea-formaldehyde microcapsules on substrate Image 5 Image 6 Olympus LEXT 3D visualisation of Olympus LEXT urea-formaldehyde microcapsules on substrate Glossary Self healing (SH) is a property enabling a material to partially or totally mend damage that has occurred to its structure or surface Tomography is a method of creating more content rich images using slices or sections, which can even be recombined to create 3D reconstructions. Contents Abstract: Selecting the right material(s) for an object is a very important and multifaceted process. One feature though is becoming increasingly popular self healing. A number of different self healing mechanisms have been developed, and the structural level actions of these are currently being researched by Stephen Mookhoek and colleagues. Integral to their investigation is the Olympus LEXT OLS3100 confocal laser scanning microscope. 7

8 Please contact: OLYMPUS LIFE SCIENCE EUROPA GMBH Microscopy Esther Ahrent Department Manager Marketing Communication Tel: Fax: microscopy@olympus-europa.com 8