Before the creation of self-healing buildings or cars, strong materials that can entirely self-repair in water-less ambience are required. Although self-healing materials function efficiently when they are wet and soft, scientists have analyzed that the self-repairing potential declines if the materials are dried out.
Researchers from the Osaka University are working to overcome this drawback by employing rigid materials that can repair about 99% of a cut on the surface in semi-dry conditions. The prototypes created by the researchers are the first to integrate chemical and physical approaches for self-healing, and were published in Chem on November 10.
The combination of physical and chemical self-healing enables materials to exhibit rapid and efficient self-healing even in a dried, hard state.
Akira Harada, Supramolecular Polymer Chemist, Osaka University
“Only a small amount of water vapor is needed to facilitate self-healing in the dried film state. In other words, water serves as a non-toxic glue in the self-healing process,” adds co-author Yoshinori Takashima, an associate professor at Osaka University.
Numerous approaches are used by material engineers to create self-healing materials. One approach is to physically implant microcapsules or pathways containing healing agents in the material, another is to use molecules like polyrotaxane (with the ability to change shape when damaged, i.e. stress relaxation) to build the material.
Chemical self-healing materials use reversible bonds ranging from reversible chemical reactions to intermolecular interactions like hydrogen bonding.
Chemical as well as physical and self-healing mechanisms were combined in the materials by Harada and his colleagues. They performed this by employing polyrotaxane as a backbone structure that was cross-linked by reversible interactions, between diols and boronic acid in this case.
Stress relaxation for healing from a shallow dent is enabled by the polyrotaxane structure, and the chemical self-healing of a deep cut is enabled by the reversible nature of the bonds. The materials were able to self-heal up to 80% of their strength within a time period of 10 minutes by employing the combined approach.
In contrast, the materials could self-heal only up to 30% of their strength after one hour without the combined approach.
Recent research on supramolecular polymeric materials has demonstrated that smart design leads to smart function on a macroscopic scale. Polymeric materials, both tough and self-healable, can open up a new frontier in materials science.
Masaki Nakahata, Assistant Professor in engineering science at Osaka University.
The researchers proposed that the materials created by them can be employed in a broad array of applications such as external coatings of buildings and cars and in medical applications such as self-healing resins and adhesives. They intend to proceed with the development of a harder material that can self-repair on its own even under ambient conditions without the need for adding any cues from outside.