Tough and Fluorescent Self-Healing Materials Could Enhance Creation of Functional Polymers

Self-healing is the ability of a material to recover from physical damage. When damaged by an operational use, self-healing materials can fully or partially recover its original set of properties. This ability has found a wide range of applications in materials which are prone to damage from cracking or microcracking.

Self-Healing Polymers

Experts have used both physical and chemical approaches to construct self-healing polymers. These include shape-memory effects, covalent-bond reformation, diffusion and flow, and heterogeneous self-healing systems.

The most common types of self-healing materials are polymers or elastomers, although it can cover all classes of materials like ceramics and metals. Fluorescent elastomers that possess self-healing performance constitute an emerging type of functional polymeric material. However, constructing such materials efficiently still needs to be explored.

Fluorescent materials are very useful since they can be used for solar cells, organic field-effect transistors (OFETs), and organic light emitting diodes (OLEDs). However, one of the challenges in using fluorescent materials is their short lifetime during usage.

Synthesis of Tough and Fluorescent Elastomers

At the RIKEN Center for Sustainable Resource Science (CSRS), a group of experts has succeeded in creating a type of self-healing material which can emit a high amount of fluorescence when absorbing light. The study, entitled "Synthesis of Tough and Fluorescent Self-Healing Elastomers by Scandium-Catalyzed Terpolymerization of Pyrenylethenylstyrene, Ethylene, and Anisylpropylene," can pave the way for developing new materials like organic solar cells which are more durable than conventional ones.

Led by Zhaomin Hou, experts at RIKEN CSRS copolymerized ethylene and anisylpropylene in 2019. This success was made possible by using a rare-earth metal catalyst.

The binary copolymer they produced displayed remarkable self-healing properties against material damage. It also has soft components made from alternating units of ethylene and anisylpropylene. When combined with hard crystalline units of ethylene-ethylene chains, these components can act as physical cross-linking points. These can form a nano-phase-separated structure that plays a vital role in the self-healing process.

From this success, the research team incorporated a luminescent unit called styrylpryrene into a monomer. They used this to form polymers that also included anisylpropylene and ethylene. Such a process has led to the synthesis of a type of material which is both tough and bright.

Compared to conventional fluorescent materials, the self-healing materials developed by the team can be expected to last for a longer period of time with increased reliability. They did not only prove to be tough, but they also exhibited self-healing properties without the need for external stimuli or energy.

The resulting copolymer has tensile strength that can be fully recovered within 24 hours. Additionally, its self-healing ability can be observed in water, acidic, and alkaline solutions, making it useful in various environments.

The researchers were also able to transfer a 2D image onto the fluorescent self-healing film, made possible through photolithography. Although the image is invisible under natural light, it can be recognized under UV light. This makes the material applicable for film used as an information storage device. Even with the images, the film maintained its remarkable self-healing and elastomeric properties.