S.E. Trujillo, M. Fedai, A.L. Kwansa, Y.G. Yingling
North Carolina State University,
United States
Keywords: cellulose nanocrystals, liquid crystals, coarse grained, molecular dynamics, self-assembly, mesogen, structural coloration, phase diagram
Summary:
Self-assembly into liquid crystalline (LC) mesophases enables a broad range of functional materials, particularly in optics, where long-range orientational order can produce phenomena such as selective reflection and structural coloration. A prominent bio-derived LC system is cellulose nanocrystals (CNCs). Cellulose, a linear polymer of beta-D-glucose, is the most abundant biopolymer in the world and serves as the main structural component of plant cell walls. When extracted via acid hydrolysis into rod-like CNCs, it exhibits unique optical properties arising from a self-assembled hierarchical structure. The self-assembly behavior of CNCs is governed by a multitude of factors, and understanding its mechanisms across larger scales continues to be a challenge, particularly at the mesoscale where collective ordering and phase transitions emerge. High-resolution structures obtained via experimental techniques have been instrumental in elucidating the static atomic structure of cellulose, providing a critical foundation for the development of computational CNC models. While experimental methods enable the precise characterization of CNC architecture, they inherently capture only static snapshots and cannot directly explore the dynamic structural changes or the complex intra- and intermolecular interactions that govern self-assembly processes. To address these limitations, we modeled large-scale CNC-like systems and performed coarse-grained molecular dynamics (CGMD) simulations which enable access to collective dynamics and phase behavior at length and time scales inaccessible to atomistic simulations. To systematically explore the parameter space and uncover the key determinants of self-assembly behavior we vary temperature and pressure conditions. Phase diagrams, constructed based on final equilibrated structures, reveal how these environmental conditions influence supramolecular organization and optical properties. Our results demonstrate that temperature acts as a primary control parameter, systematically shifting the pressure thresholds and stability ranges of ordered phases by regulating the balance between thermal disorder and anisotropic interactions. At low to moderate temperatures, CNCs form highly ordered LC phases, such as cholesteric or partially smectic structures, or exist in transitional states characterized by locally high orientational order. In contrast, higher temperature simulations consistently yield isotropic, disordered phases across all pressure conditions studied. Preliminary analysis of molecular chirality within the observed ordered phases revealed stability of cholesteric and smectic phases at moderate molecular chirality ranges, highlighting the robustness of ordered assemblies. This approach enables us to directly address how temperature shifts the pressure at which ordered phases emerge, and further probe into how molecular properties such as chirality and interaction strength affect the stability of ordered phases. This predictive framework enables optimized design and tuning of CNC-based materials through cholesteric pitch control, LC phase stability windows, and processing conditions to tailor optical responses, facilitating the sustainable development of optical materials.