December's Paper of the Month

The dynamic nature of self-healing molecules can be attributed to the reversible cross-linking of functional end-groups. 

The dynamic nature of self-healing molecules can be attributed to the reversible cross-linking of functional end-groups. 

Self-healing in materials is the process by which materials reassemble after applications of stress. In everyday life, we often suffer from small bumps and grazes which we very quickly recover from. This process is an essential feature of living systems which helps to avoid sustaining permanent damage. 

The idea of self-healing has been of great interest in the materials field in recent years. If these properties could be emulated in synthetic materials, the applications would be vast. This year alone has seen many devastating earthquakes and tsunamis, the development of self-healing materials in buildings could help save millions of lives. 

December’s Paper of the Month comes from Yan et al., from Martin Luther University Halle-Wittenberg, who have been working on recreating self-healing in synthetic materials by incorporating it as an intrinsic dynamic network. 

Self-healing occurs when dynamic bond interactions break when stress is applied, but - with time - reform and restore the strength of the material. These types of bonds can be incorporated into the load carrying molecular backbone of a polymer which encourages the self-healing properties. 

For such a system to work the bond interactions of the material must be dynamic, thus weak bond interactions, which are more readily formed and broken, can be exploited. The molecular scaffold should also incorporate a matrix in which the reformation of bonds can occur. 

Previous work suggested that the time for self-healing is related to the time of relaxation of molecular processes. The relationship between the molecular relaxation process and self-healing has been analysed but the processes behind the self-healing were not elucidated. 
To uncover these processes, Yan et al., used a reversible network of telechelic polymers and conducted small angle x-ray scattering and rheological experiments. 

For clear analysis, the network formed by the molecules needs to be well controlled and structured but this is often difficult to achieve. In their experiments, Small Angle X-ray Scattering (SAXS) was used to prove the molecules form a network of small aggregates, which form through self-assembly and become weaker at elevated temperatures. They used a custom made HFS91 Capillary Stage adapted for vacuum in their heated Small-Angle X-ray Scattering experiments. This enabled them to investigate the relaxation process of the material. 

They used telechelic polyisobutylene (PIB) as the relaxation processes of the material were well separated. This in turn allowed them to analyse how the healing process was related to the relaxation of the molecule.
Their experiments underlined the molecular processes of healing, allowing them to envision common design rules. With more work, their findings can be used to create better self-healing molecules in the future. 

By Tabassum Mujtaba

Yan, T. et al. Unveiling the molecular mechanism of self-healing in a telechelic, supramolecular polymer network. Sci. Rep. 6, 32356; doi: 10.1038/srep32356 (2016).