Solid-Phase Structural Coloration from Cholesteric Phases of Hydroxypropyl Cellulose-Parameter Space for a Full Color Palette

Hydroxypropyl cellulose (HPC) is a sustainable, cost-efficient and biocompatible cellulose derivative which can self-assemble into cholesteric mesophases at highly concentrated water solutions. Such cholesteric solutions reflect iridescent and metallic colors in the visible range (1,2,4). Retaining the vibrant coloration in the solid phase for the HPC has been widely studied by offering cross-linkers that are mostly toxic such as glutaraldehyde (3, 4), with the assistance of acid catalysts and the exact mechanism of cross-linking is still a subject of debate and ongoing research.<br/>In this presentation, we will discuss the mechanisms behind retaining the solid-phase structural coloration in the HPC without cross-linking and by controlling the parameter space during the evaporation process to achieve a full color palette.(5) Our investigation revealed that during the evaporation process, at the early stages of the evaporation the HPC readily forms a disordered dense skin that controls the balance between the kinetic arrest of the cholesteric order and its thermal expansion. Increasing the sample thickness, applying higher curing temperatures, and exposing the samples to higher humidity during the evaporation all result in red-shift in the pitch values and the final coloration. Furthermore, we will also showcase our new findings which stems from the idea of implementing the steric repulsion that leads to fabricating solid phases of HPC with predictable coloration by adding linear dicarboxylic acids to the solution. The dicarboxylic acid additives causes further steric interactions between the cholesteric layers. Therefore changing the aliphatic chain length of the acid from 3 to 5 by using malonic acid (MA), succinic acid (SA) and glutaric acid (GA) allows an additional fine tuning ability at the molecular level for the color manipulation.<br/>References<br/>Werbowyj, R. S., & Gray, D. G. (1976). Liquid Crystalline Structure In Aqueous Hydroxypropyl Cellulose Solutions. <i>Molecular Crystals and Liquid Crystals</i>,<i> 34</i>(4), 97-103. https://doi.org/10.1080/15421407608083894<br/>Godinho, M. H., Gray, D. G., & Pieranski, P. (2017). Revisiting (hydroxypropyl) cellulose (HPC)/water liquid crystalline system. <i>Liquid Crystals</i>, 1-13. https://doi.org/10.1080/02678292.2017.1325018<br/>Chan, C. L. C., Bay, M. M., Jacucci, G., Vadrucci, R., Williams, C. A., De Kerkhof, G. T., Parker, R. M., Vynck, K., Frka-Petesic, B., & Vignolini, S. (2019). Visual Appearance of Chiral Nematic Cellulose-Based Photonic Films: Angular and Polarization Independent Color Response with a Twist. <i>Advanced Materials</i>,<i> 31</i>(52), 1905151. https://doi.org/10.1002/adma.201905151<br/>Balcerowski, T., Ozbek, B., Akbulut, O., & Dumanli, A. G. (2023). Hierarchical Organization of Structurally Colored Cholesteric Phases of Cellulose via 3D Printing. <i>Small</i>,<i> 19</i>(8), 2205506. https://doi.org/10.1002/smll.202205506<br/>Ren, H., Balcerowski, T., & Dumanli, A. G. (2023). Achieving a full color palette with thickness, temperature, and humidity in cholesteric hydroxypropyl cellulose. <i>Frontiers in Photonics</i>,<i> 4</i>. https://doi.org/10.3389/fphot.2023.1134807