Continuous phase separation of cellulose nanocrystals to facilitate the preparation of chiral photonic films

Abstract

Cellulose nanocrystals (CNCs) are promising nanomaterials with unique optical properties for advanced anti-counterfeiting applications. However, the wide particle size distributions of CNCs and the strict self-assembly conditions of the chiral photonic films (CPFs) severely hampered their practical use. Hence, a continuous phase separation strategy was developed to achieve stepwise fractionation of CNCs and further facilitate the preparation of the CPFs. Spectroscopic and microscopic analyses revealed that the CNC particles were graded into various CNC fractionations with different properties after the continuous phase separation. The CNCs in the anisotropic phase had relatively high length, diameter, aspect ratio, surface charge, and crystallinity than these in the isotropic phase. CPFs with uniform chiral nematic liquid crystal structure and vibrant structural colors could be easily prepared from the anisotropic CNCs without the time-consuming evaporation-induced self-assembly (EISA) process. Subsequently, these CPFs were crushed into chiral photonic micro-flakes and embedded into polymer matrices to fabricate films and coatings. The resulting composites demonstrated anti-counterfeiting capabilities and easily distinguishable features under circularly polarized light. This environmentally friendly and scalable phase separation strategy significantly improves the preparation efficiency and reproducibility of CNC-based CPFs, providing a promising pathway for their application in anti-counterfeiting materials.