Evaporative Self-Assembly of Cellulose Nanocrystals with Binary Polymers for Asynchronous Phase Transition and Dual Structural Colors

Drying colloidal suspensions provides a simple yet powerful process to create complex nanostructures with tailored properties. Its scalability, reliability, and versatility have attracted tremendous interest in materials science. Here we describe the heterogeneous self-assembly of rodlike cellulose nanocrystals (CNCs) with binary polymers of polyethylene glycol (PEG) and dextran, displaying an evaporation driven multiphase transition with asynchronous kinetics and dual structural colors. This is achieved by mixing an aqueous CNC suspension with PEG and dextran, where the dispersed CNCs maintain cholesteric organization with the existence of binary polymer mixtures. During evaporation, the dispersed CNCs undergo phase transition from isotropic to cholesteric and further be preserved into solid films with the helical pitch compressed into the visible range. Meanwhile, the two polymers, PEG and dextran, are mutually miscible at low concentrations but become immiscible as evaporation proceeds, leading to liquid-liquid phase separation at the critical concentration. This evaporation driven heterogeneous self-assembly of CNC and binary polymers results in an asynchronous multiphase transition, characterized by fast and slow kinetics. The obtained CNC-PEG-dextran composite films exhibit dual structural colors with unequal affinity of CNC within the percolating binary polymer network, suggesting the partition difference between the PEG-rich and dextran-rich domains. Our findings introduce an alternative way to construct hierarchical photonic structure through evaporative self-assembly of polymers and nanoparticles, offering a flexible and scalable route that can be applied to a wide variety of colloidal systems.