The self healer

Materials World magazine
,
1 Feb 2018

A chance discovery could lead to more sustainable glass-like components for the automotive and technology sectors. Kathryn Allen reports.

A new type of glass that can heal itself at room temperature has been discovered by accident. Graduate student Yu Yanagisawa, from the University of Tokyo, Japan, was preparing polyether-thioureas glass as an adhesive, when he discovered its self-healing properties. Yanagisawa hopes it could be used as an environmentally friendly material, reducing waste in devices or applications requiring glass or glass-like components. 

Polyether-thioureas is a low-weight, transparent and mechanically rigid polymer, which, when cut, can heal under compression, without the need for high temperatures. Yanagisawa observed that the edges of a cut or crack in the material could adhere to each other after manual compression for 30 seconds. 

According to the researchers, led by Professor Takuzo Aida from the University of Tokyo, this material is the first hard substance of this type that can self-heal at room temperature – Yanagisawa found the material could self-heal at around 21°C – while most healable materials requiring temperatures of 120°C or more.

Compressed healing

Aida told Materials World, ‘Our glassy polymers are composed of short-chain polymers that are reversibly connected with each other by high-density hydrogen bonds and are therefore very rigid just like long-chain polymer materials.’ Yanagisawa continued, ‘Upon applying a strong external force, our polymeric materials break into two pieces, where non-covalent hydrogen bonds are preferentially cleaved off, so that many unpaired groups are generated at the fractured surfaces. Because such unpaired groups are eager to find their partners, healing occurs when two fractured surfaces are compressed together.’

As this process is reversible, the material’s properties do not alter and it can heal repeatedly.  

Market potential 

Discussing the availability of polyether-thioureas, the team explained that the material comprises inexpensive compounds, which are synthesised by a one-step reaction. However, the researchers clarified that, ‘apart from the self-healing nature, our products do not surpass existing glassy polymers that are already in the market. As they have not been used for the exterior components of smartphones, our current products, without another big jump, may not be usable for smartphones.’ 

According to Aida, this is because glassy polymers are less robust than inorganic glass. In addition, high mechanical robustness and a self-healing ability tend to be mutually exclusive, due to a large number of hydrogen bonds – which allow self-healing – also resulting in structural crystallisation that causes the polymeric material to become brittle. The challenge is to create robust, glassy polymers that are self-healing.

He refers to the work of Professor Ludwik Leibler, Research Director at the National Centre for Scientific Research, France, in overcoming this issue. In 2011 Leibler designed vitrimers – a novel organic polymer material. ‘[Vitrimers] are mechanically robust due to 3D crosslinking, but they can self-heal,’ said Aida. ‘Nevertheless, the healing requires heating with an embedded catalyst that allows dynamic covalent bonds to exchange just like non-covalent bonds.’ Aida and Yanagisawa’s material does not require this heat, but, so far, does not match the robustness of inorganic glass. 

Industry application

However, self-healing materials are already being used in the smartphone industry. In 2015, LG, South Korea, launched their G Flex 2 with a self-healing polymer coating on the back to fix scratches. But, LG included a disclaimer, stating that the coating’s recovery rate will vary depending on scratch depth, ambient temperature and wear and tear. (For more information on smart coatings see here). 

Despite not being ready for smartphone applications, Aida’s research could, for example, aid the development of durable, transparent materials for car windows.  

To read Mechanically robust, readily repairable polymers via tailored noncovalent cross-linking, visit bit.ly/2Cqm3fw