Direct-Ink-Write Crosslinkable Bottlebrush Block Polymers for On-The-Fly Control of Structural color

The biological world offers exquisite examples of how simultaneous control over molecular composition and long-range macroscopic order gives rise to material properties unique to living organisms. One such phenomenon is structural color, found in various animals, arising from the periodic ordering of domains with different refractive indices on the nanometer scale. Structural color presents an eco-friendly alternative to synthetic color, as it avoids the environmental pollution associated with synthetic dyes. Additionally, it offers high brilliance and dynamic properties that are challenging to achieve through synthetic methods. Among various materials, bottlebrush block copolymers (BBCPs) show significant promise in mimicking biological structural color due to their ability to access a wide range of nanoscale morphologies, including lamellar, cylindrical, and spherical structures, resulting in visible-range structural coloration. In this study, we demonstrate "on-the-fly" modulation of structural color during printing by combining a UV-assisted DIW 3D printer with photo-crosslinkable BBCP chemistry. A key innovation is to realize dynamic control of assembly kinetics through programming the rate of photo-crosslinking, which serves to kinetically arrest assembly and lock in desired structural color “on-the-fly”, or as we print. We then validate two key aspects of this hypothesis by combined coarse-grained simulations, rheological characterizations, and experimental structural analysis. First, using the implicit side-chain model developed by our team, we elucidate an evaporation-driven assembly pathway whereby the structural color evolves from blue to red due to backbone extension. This inference is affirmed by scanning electron microscopy and ultra-violet visible spectroscopy. Second, by rheology and in situ imaging, we show that the crosslinking timescale matches with the evaporation-driven assembly timescale, supporting the idea that assembly is arrested by crosslinking during evaporation-driven structural evolution. Enabled by this mechanistic understanding, we program the temporal profile of UV irradiance to demonstrate modulation of structural color “on the fly” as to access much of the visible spectrum and to create color gradients using a single ink material.