Mechanics and manufacturing of crosslinked cellulose nanocomposites

Abstract

Cellulose nanocrystals (CNCs) are naturally derived and renewable nanostructures with exceptional mechanical and chemical properties. Consequently, CNCs provide a compelling platform to study the mechanical properties and manufacturing processes of nanocomposites, toward sustainable, high-performance structural materials. This thesis presents the formulation, mechanics and additive manufacturing of CNC composites with high hardness and toughness. A gel precursor is formulated, combining CNCs, oligomers and solvent; and net-shape forming and additive manufacturing of macroscopic parts are achieved by a UV and thermal curing sequence. Characterization of CNC composites by nanoindentation and bi-modal atomic force microscopy (AFM) reveals a nanoscale grain structure and fracture toughening mechanism.

By quantitative analysis of AFM images and robust statistical treatment of nanoindentation data, the measured mechanical properties are correlated with the microstructure of the composite. The composites are observed to have modulus, hardness and fracture toughness of around 9 GPa, 0.6 GPa and 5 MPa-m¹/², exceeding most conventional polymers in performance. Rheological characterization reveals the effect of shear history applied during processing, on the microstructure of the composites. Rheology coupled with in-situ infrared spectroscopy shows that CNCpolymer composite gels display the distinctive features of colloidal glasses and that intrinsic chemical additives can be used to tune their behavior during extrusion. A complementary study is performed on the photopolymerization kinetics and process control of interpenetrating polymer networks (IPNs) using a custom-built linear shear rheometer.

Photopolymers are formulated with dual monomer systems, that respond to separate wavelengths of light. The mechanical and chemical properties enabled by IPNs, and their potential for nanocomposite manufacturing are explored. Using cellulose as a model system, this thesis presents a route towards formulation, processing and bulk fabrication of nanocomposites, and a fundamental understanding of the structure-property relationships from the nano to the macro scale, arising at high loading fractions of nanomaterial fillers.