Chiral self-assembly of cellulose nanocrystals for photonic films

Abstract

The self-assembly of nanoscale building blocks offers a scalable route to functional materials, but understanding how the properties of the large-scale structure emerge from the behaviour of individual sub-units is a persistent challenge. A notable example is the self-assembly of cellulose nanocrystals (CNCs) into nanostructured films. CNCs are elongated colloidal particles that spontaneously form a cholesteric liquid crystal phase in aqueous suspension. This helicoidal configuration can be preserved as the suspension dries, resulting in films with vibrant structural colour. Although CNCs have attracted growing interest from different research communities over the past few decades, many fundamental questions about CNC self-assembly remain unanswered. This thesis focuses on two of these questions: First, how does the chirality of the CNC mesophase arise? The chiral crystalline structure of native cellulose is widely believed to play a crucial role, but most CNCs are not strongly twisted and exhibit considerable polydispersity in particle size and shape. A detailed analysis of the morphology of individual CNCs was performed, and related to the ensemble behaviour of CNC suspensions. These results indicated that composite CNC particles are essential for the transfer of chirality across length-scales. Second, how does kinetic arrest influence the visual appearance of CNC films? It is well-known that drying of CNC suspensions traps the systems in a non-equilibrium configuration, but the mechanism of kinetic arrest (whether colloidal gelation or glass transition) is unclear. By tuning the both the suspension formulation and particle morphology, the kinetic arrest transition and its effect on the visual appearance of photonic CNC films was both explained and controlled. These findings deepen our understanding of CNC self-assembly and open up new avenues of enquiry for research on bio-sourced nanomaterials.