The French Academy of Science reported the existence of an inherent self-healing capacity in certain cement-based materials over 180 years ago. But research interest in these materials, often polymers which can automatically repair damage to themselves, escalated in the past decade as scientists began to realize their vast potential. Self-healing materials can provide protection against natural wear and tear, reduce inefficiencies, and limit costs and waste.

Most self-healing materials however require external triggers to work such as heat or pressure. Healing typically occurs through irreversible or reversible covalent-bond formation, hydrogen bonding, metal-ligand interactions, or ionic interactions. Self-healing materials generally cannot function in water, acid, or alkaline solutions because this disrupts the conventional self-healing mechanisms, and many also lack sufficient toughness to make them optimal for real-world situations.

Now, an ethylene-based self-healing material offers a solution for many of these problems.

As researchers report in the Journal of the American Chemical Society, the polymer can repair itself in a variety of environments. It also exhibits a high degree of elasticity, toughness, and shape memory that make it ideal for a wide range of practical applications in fields as varied as architecture, electronics, and medicine.

“My group has been working on the design and synthesis of new catalysts to achieve chemical syntheses that were difficult to do before,” says Zhaomin Hou, a chemist at the RIKEN Center for Sustainable Resource Science in Wako, Japan, and senior author of the article. “The most exciting point of this work is that the new copolymers we made can autonomously self-heal, not only in a dry environment but also in water and acid and alkaline solutions, without the need for any external energy or stimulus.”

Researchers have long predicted that copolymerization of ethylene with polar functional olefins would provide an efficient, practical route for synthesizing functionalized polyolefins with beneficial properties. Typically, however, because of the differences in reactivity between ethylene and polar olefin monomers, the copolymerization process forces a trade-off between the molecular weight and polar-monomer incorporation. This tradeoff can produce materials that have either high molecular weight or high polar-monomer content, but not both, limiting the material’s ability for practical applications.

Hou and his colleagues envisaged that a scandium catalyst they previously developed could lead to unique copolymers of ethylene and anisylpropylenes with unprecedented compositions and structures. They had previously found that scandium ions in catalysts interact with heteroatoms like oxygen, and as such could enhance the olefin polymerization activity of heteroatom-functionalized olefins by bringing the olefin unit to the active catalyst site. By increasing the polymerization activity of functional olefins, they could undertake copolymerization of ethylene with anisylpropylenes.

Their predictions were correct. The end result produced by their scandium catalyst are polymers composed of unique multi-blocks of relatively long alternating ethylene-anisylpropylene sequences interspersed with shorter ethylene-ethylene segments. The alternating blocks are highly flexible and form a three-dimensional network from the ethylene-ethylene crystalline nanodomains as cross-linking points. The network is resistant to water, acid, and alkaline solutions, enabling elasticity and self-healability under various conditions. As Hou says, “We were astounded by the special properties that these copolymers exhibited.”

In experiments, the researchers cracked the ethylene film with a razor blade and showed that, within five minutes in either air or water at 25°C, the material had self-healed at the micrometer level. In a similar experiment, they shaped the material into letters spelling “RIKEN,” and then heated the letters from 20°C to 50°C to deform them into an unreadable scramble, which they fixed at 20°C. When they placed the deformed letters back into a 50°C water bath, within five seconds the letters popped back into place, once again neatly spelling “RIKEN.”

“The ability of this system to control the copolymerization process as well as the copolymer microstructure represents a significant advancement in the field of transition-metal-catalyzed copolymerization of ethylene with polar monomers,” says Changle Chen, a chemist at the University of Science and Technology of China in Hefei, who was not involved in the work. “The unique properties of the copolymer products, especially the remarkable self-healability, are quite surprising considering the simplicity of both monomers.”

“This work really opens up new possibilities in the designing of novel polyolefin materials with desirable functionalities, and will certainly stimulate further research activities in related fields,” Chen adds.

The new material can easily be synthesized from ethylene and various heteroatom-containing olefins, Hou says, and he and his colleagues expect that it will eventually find application in coatings, paint, sealing, and medical implants, for example. “From the results we observed, it is not difficult to imagine the potential for this new class of polymers,” Hou says. “We look forward to working with experts and companies in various areas to find practical applications for them.” 

Read the abstract in the Journal of the American Chemical Society.