Mechanical Properties of Microspheres Assembled from Carboxylated Cellulose Nanocrystal Rods

The elastic modulus of carboxylated cellulose nanocrystal (cCNC) microbeads made by spray-drying were measured by Atomic Force Microscopy (AFM) combined with a 1 μm Si microsphere probe method. Use of a Si microsphere on the cantilever probe tip to measure deformations of cCNC microbeads is shown to eliminate spurious contributions from localized mechanical responses that plague conventional cantilever sharp probe tips. The findings are consistent with the Hertz model. The the Young’s modulus of cCNC microbeads depends on spray drying parameters. Spray drying from dilute cCNC suspensions yields particles with a Young’s modulus of 18.02 MPa. Higher cCNC feed concentrations yield denser particles characterized by an elastic modulus of 24.55 MPa. Doping dilute cCNC suspensions with citric acid results in aerosol-phase esterification and crosslinking of the cCNC microbeads. Crosslinking in this manner yields a stiffer microbead with a Young’s modulus of 27.97 MPa. cCNC-derived microbeads are stiffer than microbeads derived from collagen, hyaluronic acid, alginate, dextran or pectin, but they are more elastic than urethane-acrylate crosslinked beads, and cellulose beads reconstituted from dissolved cellulose by emulsion-precipitation.