Creating structural color with polymers

NCCR Bio-Inspired Materials researchers at the Adolphe Merkle Institute (AMI) have drawn inspiration from nature to develop nanoscale structures that display bright colors with potential industrial applications. These so-called photonic particles could be employed in a broad range of products, from coatings to cosmetics.

The dazzling hues of the wings of some insects and birds originate from tiny structures that diffract light differently. Mimicking those structures will allow scientists to create new materials that produce color in a more efficient, economical, and sustainable way, says Dr. Ilja Gunkel, a group leader in the AMI Soft Matter Physics group. Part of Gunkel’s research is inspired by how animals such as butterflies and beetles get their vibrant hues. These colors are called ‘structural’ because they’re produced by light scattering from a regular array of structures, for example scales on a butterfly wing. The scales are typically made of natural polymers that arrange into microscopic layers, which reflect light in ways that produce bright colors.

To imitate these structures, Gunkel and his colleagues created a comb-shaped polymer, a type of polymer consisting of a main polymer chain and a series of side chains made of a different molecule. Under the right conditions, these polymers self-assemble into onion-like microstructures made of layers that are about 160 nanometers thick.

Using a specific protocol, which they detailed in the journal Macromolecular Rapid Communications, the researchers succeeded in producing spheres that measure about 10 micrometers in diameter and show intense coloration. The first particles produced were only green, but now the researchers are able to create particles of all different colors. “The trick is simple,” Gunkel explains. When making the comb-like polymer, the researchers add different amounts of the additional molecule to the main polymer chain. This results in onion-like structures with layers of either increased or decreased thickness, which reflect different portions of the light spectrum.

The particles are non-iridescent, meaning that their hue does not depend on the angle they’re viewed from. “Typically, one wants the color of a device or a wall to look exactly the same, no matter under which angle one looks at it,” Gunkel adds. What’s more, the particles can be suspended in water or used as a powder. Due to these characteristics, the particles hold promise as non-iridescent photonic pigments, he says. “We show that we can create structural color with a simple and scalable process.” However, the particles are not yet ready for practical applications, Gunkel notes. One reason is that the color is not visible to the naked eye, but is only detectable using a microscope. Also, light bounces multiple times between different particles — a phenomenon called scattering, which makes the particles appear white.

To get around the problem of scattering, the team is again drawing inspiration from nature: animals often combine structural colors with pigments, which absorb scattering light. The researchers now plan to introduce such pigments in their onion-like particles. “We want to have pigments in the center of a particle to make sure that only a certain portion of light is reflected — and that only reflection, rather than scattering, takes place,” Gunkel says. In the future, he adds, the particles may find use as paints or colorants that are more stable than traditional ones, which tend to fade over time. “We have all the ingredients, we just need to get the pigments incorporated into our onion-like particles.”

Reference: Moriceau, G.; Kilchoer, C.; Djeghdi, K.; Weder, C.; Steiner, U.; Wilts, B. D.; Gunkel, I. Photonic Particles Made by the Confined Self-Assembly of a Supramolecular Comb-Like Block Copolymer. Macromolecular Rapid Communications 2021, 42 (24), 2100522. https://doi.org/10.1002/marc.202100522.

Author: Giorgia Guglielmi