From Nanoscale Self-Assembly towards Practical Applications: Sustainable Coloration with Liquid Crystalline Cellulosics

Abstract

The liquid-crystalline self-assembly of cellulose at the nanoscale is a phenomenon that can result in striking structural colouration. This naturally inspired method of generating colour with a twist has the potential to bring sustainability to the commonly toxic dye and pigment industry, enabling its much sought-after green transition. In this thesis, the cholesteric phase of cellulose nanocrystals and hydroxypropyl cellulose is leveraged to produce such structurally coloured bio-based materials. However, several key questions remain to be addressed before these cellulose-based reflectors can be adopted for practical use. The top-down method of cellulose nanocrystal production results in particles of significant polydispersity in size and shape. This leads to an inhomogeneous appearance of solid samples, further complicated by the impractically long drying times required to anneal defects. By altering the self-assembly behaviour of the cellulose nanocrystals using xanthan gum, films of greater homogeneity are cast in a fraction of the time. This is despite the composite samples exhibiting a highly arrested microstructure that leads to greater apparent flexural strength and interesting optical properties. Similar difficulties with homogeneity in the solid-state arise when processing the lyotropic hydroxypropyl cellulose. It is shown here that homogeneity in the cross-linked and uncrosslinked solid state can be achieved by annealing the cholesteric phase using the shear stresses applied during 3D printing and precisely controlling the drying parameters. Such processing also allowed for the control and further understanding of the spontaneous wrinkling that occurs when the spacing of cholesteric layers is distorted. These wrinkles could be leveraged to reduce the angular dependence of the hydroxypropyl cellulose. Furthermore, by carefully controlling the dimensions and drying conditions of solid samples of pure hydroxypropyl cellulose, a full-colour palette could be produced, shedding light on the mechanisms behind colour retention. Finally, through the detailed chemical and physical analysis of glutaraldehyde cross-linked hydroxypropyl cellulose catalysed with different acids, the stability of the reactants during the reaction was significantly improved, resulting in samples of superior optical quality. This grasp of the reaction mechanism enabled the controlled modulation of the cholesteric pitch in solid samples and provided a significant step in the innovation of greener routes for cross-linking. The investigations in this thesis provide additional routes to produce solid structurally coloured bio-based materials and open new avenues to tune their properties. This research work is, therefore, a significant step forward in the development of cellulosic materials for photonic applications and contributes to the shift toward sustainable colouration.

Date of Award	28 Jan 2024
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Coskun Kocabas (Supervisor) & Ahu Gumrah Parry (Supervisor)