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Innovative Self-Healing Coatings Developed by Waseda University Researchers

Self-healing coatings represent a significant advancement in material technology, capable of autonomously repairing damages such as scratches and cracks. A team of researchers from Waseda University has introduced a novel method for creating self-healing films formed from alternating layers of highly cross-linked organosiloxane and linear polydimethylsiloxane (PDMS). These new films demonstrate enhanced durability compared to traditional self-healing PDMS materials, boasting superior hardness and better thermal stability, while also self-repairing at mild temperature conditions. This breakthrough has the potential to lead to stronger, more dependable, and easier-to-maintain self-healing materials.

Polysiloxane materials, particularly those based on PDMS elastomers, have shown self-healing capabilities primarily due to the presence of silanolate (Si-O-) groups. This unique feature arises from the dynamic nature of the siloxane (Si-O-Si) bonds, enabling them to break and reform in order to repair damaged areas. The ability to self-heal makes these materials particularly attractive for various applications, including protective coatings in sectors such as optics, electronics, and aerospace engineering.

To further enhance the properties of PDMS materials, researchers have explored the combination of these polymers with inorganic fillers such as nanoparticles or nanosheets. The integration of nanosheets often leads to a layered structure that improves thermal, mechanical, and gas barrier properties. Additionally, films with an oriented structure have shown improved crack-healing abilities, which can be attributed to the directed diffusion of the polymer in the in-plane direction.

The advancements made by the Waseda University team mark a critical step forward in the development of self-healing siloxane materials. Their study, published on January 6, 2025, in Volume 61, Issue 16 of the journal Chemical Communications, details the work of Professor Atsushi Shimojima, Research Associate Yoshiaki Miyamoto, and Assistant Professor Takamichi Matsuno. They successfully fabricated a composite film that integrates highly cross-linked organosiloxane (silsesquioxane) with grafted PDMS layers using a self-assembly technique.

“The demand for maintenance-free and durable applications is high, and our self-healing material, which is less prone to degradation and features high hardness, could replace traditional materials,” states Miyamoto, who is a principal author of the research.

The fabrication process began with the application of a solution containing 1,2-bis(triethoxysilyl)ethane, Pluronic P123 (a PEO-PPO-PEO triblock copolymer comprised of poly(ethylene oxide) and poly(propylene oxide)), along with a PEO-PDMS-PEO block copolymer onto a silicon or glass substrate. This was accomplished using either spin-coating or drop-casting methods, resulting in the creation of a thin film with a lamellar structure.

Subsequently, the film underwent calcination in air at 170 °C for four hours. This heating process eliminated the PEO and PPO components, leaving a multilayer structure made up of silsesquioxane and PDMS layers.

To incorporate self-healing capabilities into the film, Si-O- groups were introduced. This addition facilitates the rearrangement and reconnection of the siloxane (Si-O-Si) networks. The self-healing process was initiated by immersing the film in a solution of tetrahydrofuran, water, and potassium hydroxide (KOH), where hydroxide ions (OH-) extracted protons (H+) from silanol (Si-OH) groups, converting them into Si-O- ions. The resulting film can effectively mend micrometer-scale cracks when subjected to heated conditions of 80 °C at 40% relative humidity for a duration of 24 hours.

This innovative film shows remarkable enhancements over conventional PDMS-based materials. The addition of cross-linked organosiloxane layers not only increases rigidity but also creates a barrier that mitigates the evaporation of cyclic siloxanes, addressing some inherent shortcomings of traditional PDMS materials. While standard self-healing PDMS elastomers possess a hardness of approximately 49 MPa, the new self-healing film achieves an impressive hardness of 1.50 GPa.

“Our multilayered design significantly enhances both hardness and heat resistance relative to existing self-healing siloxane-based materials, signaling the game’s potential for more enduring and reliable applications,” adds Miyamoto.

With its combination of high hardness and effective self-healing capabilities, this innovative material is poised for use in protective coatings, flexible electronics, and other applications requiring prolonged performance and durability.

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