No ice, ice, baby

Self-healing materials have been high on the agenda for some time. Now, research published in Applied Materials Today [Zhuo, Y. et al. Appl Mater Today (2020); 10.1016/j.apmt.2019.100542] could lead the way to such materials for specific applications. Researchers at the Norwegian University of Science and Technology (NTNU) in Trondheim have developed transparent coatings that can undergo self-healing very quickly. Additionally, the materials are durable and resistant to ice formation and so could have utility as protective coatings for a wide range of surfaces that are susceptible to environmental damage and in particular water ingress caused by ice.

The team explains that ice formation on various kinds of infrastructure can ultimately lead to damage and breakdown of functional surfaces, such as solar panels, sensors, and even windows. "Mechanically durable and transparent ice-phobic coatings are highly desired, the team writes." Current self-healing materials require special treatment and often high temperatures, which precludes their use in many practical settings, such as solar panel installations.

The team's material was prepared as a polydimethylsiloxane-urea (PDU) coating consisting of polydimethylsiloxane (PDMS) chains and urea groups. They started with commercially available isophorone diisocyanate (IDI) and bis(3-aminopropyl)-terminated PDMS (H2N-PDMS-NH2). They used three molecular weights of the PDMS segment, i.e. 1000, 3000, and 5000?Daltons, to give them PDU1000, PDU3000, and PDU5000.

The chains serve as flexible segments and the urea groups as bonding sites. They explain how chain diffusion and bonding-site density are two critical factors in the nanoscale mechanics of self-healing; a conclusion based on atomistic modeling and simulations.

The researchers add that their chains contain asymmetric alicyclic segments that promote the necessary chain diffusion. Additionally, they tuned the molecular weight of those segments to ensure optimal concentration of hydrogen-bonding sites, again to facilitate self-healing. The urea groups, they explain, serve as dynamic crosslinkers (hydrogen bonds) in the coating. This ensures colorless transparency as well as allowing the self-healing of mechanically damaged areas in the coating.

The team explains that when they make a small incision in their coating, to simulate ice damage, the material can self-heal and restore more than 80 percent of its initial tensile strength within 45 minutes at room temperature. Critically, for many applications, the coating on glass shows a light transmittance of almost 90 percent (89.1%) in the visible region, which is remarkably close to uncoated glass (91.9%). At the end of life, the material can be recycled.