Confined Shear Alignment of Ultrathin Films of Cellulose Nanocrystals

Cellulose nanocrystals (CNCs) are a naturally abundant nanomaterial derived from cellulose which exhibit many exciting mechanical, chemical, and rheological properties, making CNCs attractive for use in coatings. Furthermore, the alignment of CNCs is important to exploit their anisotropic mechanical and piezoelectric properties. Here, we demonstrate and study the fabrication of submonolayer to 25 nm thick films of CNCs via solution-based shear alignment. CNC solution is forced through a sub-millimeter tall channel at high volumetric flow rates generating shear. The half-width at half-maximum of the spread in CNC alignment significantly improves from 78 to 17° by increasing the shear rate from 19 to 19,000 s^-1. We demonstrate that the film thickness is increased by increasing the volume of CNC solution flowed over the substrate and/or increasing the CNC solution concentration, with a degradation in film uniformity at higher (≥7 wt %) concentrations, likely due to CNC aggregates in the solution. Deposition of ultrathin aligned CNC films occurs within seconds and the technique is inherently scalable, demonstrating the promise of solution-based shear for the fabrication of ultrathin aligned CNC films, thereby enabling the future study of their inherent material properties or use in high-performance coatings and applications.