Similar to colorful flowers, butterflies, chameleons, and other animals often display vibrant colors, although there are differences between these two phenomena. The colors of flowers mainly come from pigments, where pigments absorb or reflect specific wavelengths of light to produce colors. On the other hand, the colors of butterflies or chameleons originate from subtle structures on their bodies, such as nano "photonic crystals" (PC), which involve light scattering, diffraction, and interference. This phenomenon is known as "structural color." Structural color offers various advantages, such as resistance to fading, water washing, and adjustable colors. However, achieving structural color conveniently and efficiently through currently popular 3D printing technology is still not feasible.

Chameleon.  The picture comes from the Internet

    Recently, the research group led by Ying Diao at the University of Illinois Urbana-Champaign (UIUC) in the United States published a paper in the journal Science Advances, demonstrating the 3D printing of vibrant structural colors using only one type of "ink." The crucial aspect of this work lies in the use of a polymer material known as "bottlebrush copolymer" in their "ink."

3D printing of a colorful chameleon image.   Image source: Sci. Adv.

    "Bottlebrush copolymer" is a type of branched or grafted polymer with high grafting density side chains attached to the main chain backbone, resembling the shape of a bottlebrush. Due to its unique structure, it can form complex but highly ordered materials. By controlling the selection of monomers and the synthesis process, it allows for the design and fabrication of "DIY nanomaterials" with specific size, shape, and composition. As a result, it has broad research and application potential in various fields, such as surfactants, photonic crystals, coatings, and nanomedicine.

Schematic diagrams of three "bottlebrush copolymers."  Image source: Chem. Soc. Rev. [1]

Preparation of PDMS-b-PLA bottlebrush copolymer.   Image source: Sci. Adv.

    The copolymer white solid powder (Figure E) was dissolved in tetrahydrofuran (THF) to prepare a solution with a concentration of 100 mg/mL, which served as the required "ink" for printing. In the solution, the branched chains of the bottlebrush polymer entangled with each other, forming micelles. When the solution was drop-cast to form a film, the solvent evaporated, causing microphase separation between polymer molecules, resulting in an ordered layered structure and leading to significant random colors.

“Colorful thin films formed after drying the 'bottlebrush copolymer' solution”.   Image source: Sci. Adv.

    The bottlebrush copolymer can exhibit a wide range of vibrant colors, which can be controlled by adjusting the printing speed, applied pressure, and substrate temperature. In other words, these factors influence the polymer's microphase separation process. The researchers loaded the 'ink' into a 3D printer and, under a pressure of 30 kPa, obtained a two-dimensional color matrix by varying the substrate temperature (25 °C, 50 °C, and 70 °C) and printing speed (15 to 480 mm/min). The printed lines displayed consistent colors. However, slight color variations were observed at the beginning of printing and around corners (Figure C)."

3D printing complex structural colors.   Image source: Sci. Adv.

3D printing complex structural color patterns.   Image source: Sci. Adv.

    With the increase in printing speed, a significant blue shift in the reflected wavelength is observed, while higher temperatures cause a noticeable red shift. Through SEM and SAXS characterization, the color variations result from the different domain spacings in the printed layered thin films. The changes in domain spacing are attributed to the "head-to-head" arrangement of the bottlebrush copolymer molecules, meaning that each structural unit consists of a double layer, and the layer backbone exhibits significant elasticity. The variation in domain spacing is caused by the expansion and compression of the molecules.

Microscopic structural characterization.   Image source: Sci. Adv.

"Utilizing polymers to achieve these vivid colors and applying them in eco-friendly coatings and highly selective optical filters pose a challenge," says Ying Diao. "Accurate control of polymer synthesis and processing is required to form ultra-thin ordered structures, creating structural colors observed in the natural world." Currently, the colors obtained through this method are still limited and not well-suited for large-scale printing. Therefore, "we are collaborating with Damien Guironnet, Charles Sing, and Simon Rogers from the American Institute of Chemical Engineers to develop more easily controllable polymers and printing processes, bringing us closer to producing vivid colors found in nature." [2]

Tunable structural color of bottlebrush block copolymers through direct-write 3D printing from solution

Bijal B. Patel, Dylan J. Walsh, Do Hoon Kim, Justin Kwok, Byeongdu Lee, Damien Guironnet, Ying Diao

Sci. Adv., 2020, DOI: 10.1126/sciadv.aaz7202

References:

1. Verduzco R, Li X, Pesek S L, et al. Structure, function, self-assembly, and applications of bottlebrush copolymers. Chem. Soc. Rev., 2015, 44, 2405-2420.

https://pubs.rsc.org/en/content/articlelanding/2015/cs/c4cs00329b#!divAbstract

2. Researchers mimic nature for fast, colorful 3D printing

https://news.illinois.edu/view/6367/809468

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