The interplay between order and disorder in cholesteric hydroxypropyl cellulose films

Abstract

Structural colouration originates from the physical interaction between light and nanostructured materials, unlike pigments and dyes, where colour is achieved by selective absorption. The formation of a cholesteric liquid crystalline phase is one route to achieve such structuring on the nanoscale. An especially promising candidate is hydroxypropyl cellulose (HPC), a bio-sourced, bio-compatible and water-soluble polymer, which spontaneously assembles into a cholesteric mesophase reflecting bright and intense colours. Previous work has often assumed that the structure is a perfect monodomain, which would reflect metallic and iridescent colour. However, in macroscopic HPC films, a multitude of domains of various orientations coexist and scatter light multiple times, which results in a more matte appearance.

The aim of this thesis is to develop modelling methods for characterising the interplay between order and disorder in polydomain cholesteric films. First, an open-source Python toolkit (PyLlama) was developed to model the optical response of arbitrary multilayer stacks, and applied to the specific case of a monodomain cholesteric liquid crystal. These results were combined with a statistical method (Monte Carlo) that simulates the penetration of light inside polydomain cholesteric films and the resulting angular-dependent scattering. The developed numerical model allowed for an understanding of the structural arrangement of HPC films from their interactions with light. The numerical results were found to be in good agreement with experimental measurements on HPC films, validating this approach. Lastly, the mechanochromic behaviour of large-scale HPC films is investigated as a practical example of the importance of nanoscale structure to the macroscopic optical response, with a potential application as a pressure sensor.