Capillary and Localized Magnetic Effects in Cellulose Nanocrystal Thin Films
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Bioderived materials such as cellulose nanocrystals (CNCs) have inherent structural organization that can be exploited for the control of dimensionality, periodicity, and functionality of biocompatible and photonic materials. However, precise control and tunability of organized cellulose nanocrystal-based materials is challenging due to the random organization of the chiral nematic structure, called tactoids, during evaporation induced self-assembly (EISA) of solid films. Lack of fundamental assessment of CNC-centric materials has limited applications for the sustainable, bioderived material since control over its fundamental behavior orients the entire understanding for the chiroptic potential of those materials. This work focuses on the control of lyotropic LC alignment of CNCs by (i) utilizing geometric confinement for asymmetric evaporation that induces directional flow of LC suspension, (ii) direct chemical modification of the surface groups for control of inter-particle interactions, and (iii) applying magnetic fields for localized CNC alignments and global patterned formation. Firstly, we utilized tunicate-inspired hydrogen-bonding-rich 3,4,5-trihydroxyphenethylamine hydrochloride (TOPA) for physical crosslinking of nanocrystals and polyethylene glycol (PEG) as a relaxer of internal stresses in the vicinity of the capillary surface. The CNC/TOPA/PEG film is organized as a left-handed chiral structure parallel to flat walls, and the inner volume of the films displayed transitional herringbone organization across the interfacial region. Secondly, we utilized polyethyleneimine (PEI) coated Fe3O4 magnetic nanoparticles to produce localized magnetic patterns in CNC thin films under weak magnetic fields. The resulting CNC films showed tailored optical patterns controlled by localized magnetic patterns while retaining a iridescent optical appearance. The significance of this work being that the tuning and control over the materials’ magnetic properties allows for a bottom-up approach towards a controlled assembly of larger scale materials with localized pattens. Overall, this work advances the dynamic control over CNC films to widen the range of applications available for multi-stimuli response, biocompatible films and gels of interest in biosensing and tunable optical reflectors.