Origin of vacuum-assisted chiral self-assembly of cellulose nanocrystals

To produce functional cellulose nanocrystal (CNC) films with desirable optical and mechanical properties, it is critical to understand and control the chiral self-assembly of CNCs during solvent removal. Here, the formation mechanism of CNC chiral nematic liquid crystal phase during vacuum-assisted self-assembly (VASA) was investigated. To elucidate the structural evolution of CNC aggregations on filter paper, CNC suspensions were "frozen" at various filtration stages in a polyacrylamide matrix. In addition, the flow rate of CNC suspension was monitored in situ. We found that disordered-to-ordered CNC self-assembly occurs at the interface between the filter paper and suspension over four stages. Tactoids develop in the concentrated CNC suspension close to the filter paper, which is also observed in evaporation-induced self-assembly (EISA) of CNC films. However, compared to EISA, VASA promotes helical axes of CNC tactoids along the flow-field direction, leading to faster liquid crystal formation with long-range order via the nucleation growth.