Buckling Mechanics Modulus Measurement of Anisotropic Cellulose Nanocrystal Thin Films

Bioderived materials have become an increasingly desirable alternative to materials sourced from nonrenewable resources. Cellulose nanocrystals (CNCs), often derived from wood pulp, can be used to manufacture thin films with applications ranging from optical, protective, and aesthetic coatings to sensors and batteries. Quantifying the mechanical properties of CNC films is a necessary step toward improving the quality of these bioderived films. Because CNCs are highly anisotropic and can subsequently form highly ordered, aligned structures, an experimental method that can succinctly determine how film properties change with particle orientation is of interest. Here, spin coating an aqueous solution of CNCs onto a silicone elastomer results in a radially aligned particle assembly. As the local orientation of these aligned, high aspect ratio particles changes with respect to a uniaxially applied compression, the mechanical response of the particle assembly was observed to vary significantly. Applying a lateral compression to a radially aligned CNC film/elastomer bilayer caused surface buckles to align orthogonal to the compression direction. The wavelength of these wrinkles, coupled with the thickness of the film and the modulus of the substrate, is dictated by the modulus of the film. The modulus as a function of local CNC alignment and position for each film was thus determined in a single experiment. These experiments measured a higher modulus for the film where the orientation of the CNC particles is aligned parallel to the uniaxial compression direction. Coarse-grained modeling of closely packed, high aspect ratio particle assemblies supporting the experimental results agrees with the observed trend. Characterizing the mechanical properties of CNC films can allow for these green materials to be further developed for industrial-scale implementation; additionally, an experimental method is proposed for concisely capturing a range of modulus values in an anisotropic particle film.