2023-11-26 15:36:00
Professor Ahn Seok-gyun’s research team in the Department of Applied Chemical Engineering at Pusan National University printed structural colors that imitate nature through 3D printing and created anisotropic mechanical discoloration.[1]It was announced on the 7th that it had succeeded in implementing .
‘Structural color’ is the color of an object that is not caused by pigments or dyes, but by the nanostructure of the object. In other words, the nanostructure of an object reflects or scatters light of a specific wavelength, causing color to appear. Structural color is a bright, shiny color that is observed in nature in the feathers of male peacocks, the wings of butterflies, and the skin of chameleons.
Methods for artificially creating structural colors include creating repeated structures hundreds of nanometers in size using photonic crystals, photonic glass, and cholesteric liquid crystals. Among them, ‘cholesteric liquid crystal’ is a principle in which rod-shaped liquid crystal molecules form a uniformly oriented helical structure to reflect light of a specific wavelength.
Conventional cholesteric liquid crystal elastomer[2]orientation[3] Methods include surface orientation and anisotropic drying, but there are limitations in designing them with the desired structure and pattern. In comparison, the advantage of producing cholesteric liquid crystal elastomers using 3D printing is that the output shape and orientation direction can be freely designed.
Research on combining cholesteric liquid crystal elastomers with 3D printing technology has recently received a lot of attention from academia, but no research has been reported on implementing anisotropic mechanochromism using them.
Accordingly, the research team first used direct ink writing.[4] A cholesteric liquid crystal elastomer was produced by combining 3D printing and photocuring. Uniquely, the cholesteric liquid crystal elastomer printed through 3D printing showed different reflective colors depending on the viewing direction, unlike previously known cholesteric liquid crystal elastomers. The researchers explained that this is because, unlike existing cholesteric liquid crystal elastomers in which the spiral axis is formed perpendicular to the substrate, the spiral axis was formed at an angle through the printing process.
Conceptual diagram of direct ink writing printing process of cholesteric liquid crystal elastomer and example of optical behavior of printed structure: (a) Conceptual diagram and orientation schematic of 3D printing process of cholesteric liquid crystal elastomer, (b) Image of reflected color according to observation direction , (c) an image observed from the front of a sample stretched in the vertical (left) and horizontal (right) directions with respect to the printing direction, (d) an image taken at each elongation of a cholesteric liquid crystal elastomer printed in a concentric square array, ( e) Images taken by elongation of cholesteric liquid crystal elastomers printed in the shape of Pusan National University’s English initials (PNU). (The black arrow in each picture indicates the printing direction.) Source: Pusan National University
What’s even more interesting is that the 3D printed cholesteric liquid crystal elastomer reflects different colors depending on the direction in which it is stretched. If stretched perpendicular to the printed direction, there is no significant change in the angle of the spiral axis, and only the grid spacing narrows, causing the film, which reflected red before stretching, to reflect blue. On the other hand, when stretched in the printed direction, the angle at which the spiral axis is tilted decreases and at the same time the grid spacing narrows, resulting in a much smaller color change before and following stretching due to the trade-off effect.
By taking advantage of the fact that the reflected color can vary depending on the combination of printing direction and stretching direction, various mechanochromic patterns can be created with just one ink. The research team used this to create an hourglass that reflects different colors and a ‘PNU’ pattern, which is the English initials of Pusan National University.
Professor Ahn Seok-gyun of Pusan National University, who was in charge of the research, said, “This is the first study that combines cholesteric liquid crystal elastomers with 3D printing technology to freely design desired shapes and patterns and develop different structural colors depending on the viewing direction and stretching direction. “It is significant in terms of achievement,” he said. In particular, he expected that “it might be used not only for security technology and building structure stability diagnosis technology, but also for the development of optical elements for displays and holograms, such as virtual reality (VR) and augmented reality (AR) technology.”
The research results were published in the October 31st online edition of ‘Advanced Functional Materials’, an academic journal in the materials field.
Paper name: Direct-Ink-Written Cholesteric Liquid Crystal Elastomer with Programmable Mechanochromic Response
#Terminology
[1] Mechanical discoloration: A technology that changes color by applying physical force (tensile force, compression force, etc.)
[2] Cholesteric liquid crystal elastomer: A form in which liquid crystal molecules bound to a polymer chain are aligned in a three-dimensional helical structure and then weakly cross-linked. It exhibits rubber-like properties and can develop structural color.
[3] Orientation: Multiple molecules or nanoparticles arranged in a specific direction
[4] Direct-ink-writing: A 3D printing process that extrudes high-viscosity ink to create a two-dimensional or three-dimensional structure as if writing by hand.
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