Self-healing polymers are a relatively new class of “smart” materials that have the ability to repair damage autonomously, much the way biological systems can heal cuts and bruises. The applications for these self-healing polymers are numerous, and include automobile coatings, luggage, cell phones, and factory machinery. Not only can they repair micro-damage without any human intervention or even diagnosis, they can prevent small damage from becoming major damage and extend the life span of the coating, device, or machine.
Traditionally, however, these polymer gel-based materials needed to be soft and wet (water serves as a non-toxic clue in the self-healing process) which is good for rainy outdoor environment but limits them in dry conditions such as interiors or machinery that can’t get wet. Researchers from Osaka University in Japan recently developed a new self-healing polymer solution that works in mostly dry conditions and with harder materials. Only a small amount of water vapor is needed to facilitate self-healing in the dried film state. It does this by combining physical and chemical self-healing mechanisms rather than using one or the other.
Using a combination of physical and chemical self-healing processes, researchers found that both the gel state and the dried gel state of the material exhibited rapid and efficient self-healing properties. (Image source: Osaka University)
The research was published in the November 10 issue of Chem.
Senior author Akira Harada, a supramolecular polymer chemist at Osaka University, and co-author Yoshinori Takashima, an associate professor at Osaka University, told Design News that prior to their research, self-healing polymers have been categorized into two types: physical approach and chemical approach. The physical mechanism often relies on polyrotaxane, a macromolecule composed of ring molecules threaded by an axle polymer, with bulky stoppers at both ends. The ring molecules are able to move along the axle.
The physical approach includes microcapsules or vascular networks filled with healing agents embedded in the materials and a stress relaxation mechanism in the materials based on polyrotaxane. The material is embedded with microcapsules or pathways filled with healing agents, or they use a material like polyrotaxane, which changes shape in response to damage.
Chemical self-healing materials use reversible bonds ranging from reversible chemical reactions to intermolecular interactions such as hydrogen bonding. Rather than rely on one method or another, the work of Osaka University researchers advocates a new design principle of self-healing materials through a combination of physical and chemical self-healing mechanisms.
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“We utilized polyrotaxane as a backbone structure, imparting materials with physical self-healing ability,” Professor Harada told Design News . “Our material design is polyrotaxane networks cross-linked by reversible interactions: in this case, boronate-diol interactions. The polyrotaxane structure enables stress-relaxation in recovering shallow dents. In addition, the reversible nature of the bonds enables chemical self-healing from a deep cut. The combination of physical and chemical self-healing enables multifunctional self-healing