Photonic properties of liquid crystalline hydroxypropyl cellulose in the solid-state

As we transition towards a more sustainable society, high-performance bio-based materials are increasingly required. In this context, cellulosic materials hold exceptional promise as they are derived from abundant and bio-sourced feedstock, and they are able to self-assemble into nanoscale architectures. More specifically, several cellulose derivatives exhibit a cholesteric liquid crystalline phase in solution, displaying vibrant structural colours. Amongst them, water-soluble hydroxypropyl cellulose (HPC) is ideal for developing sustainable photonic materials to replace toxic dyes and pigments. The development of photonic HPC materials has been, so far, hindered by the difficulty to retain colouration in solid-state films. In this study, three methods to preserve the colour of HPC-water mesophase in the solid-state were developed, and each of these methods yielded different optical or mechanical behaviour, resulting in materials with a variety of functionalities. Firstly, by combining chemical crosslinking and heat treatment, photonic films across the entire visible spectrum were obtained and instead of the typical metallic iridescence associated with HPC mesophases, these films appeared matte and angular independent. Secondly, a functionalised HPC polymer was exploited as an ink to achieve direct 3D printing of photonic structures, which colour could be tuned by exploiting the lyotropic and thermotropic properties of HPC. Lastly, a strategy to develop HPC gels using supramolecular chemistry was explored, leading to photonic gels crosslinked via host-guest and π-π interactions. Through these studies, a toolbox has been developed to tailor the optical properties of HPC and other cellulosic liquid crystals, paving a path towards sustainable cellulose-based photonic systems.