  Due to their exceptional mechanical and chemical properties and their natural
abundance, cellulose nanocrystals (CNCs) are promising building blocks of
sustainable polymer composites. However, the rapid gelation of CNC dispersions
has generally limited CNC-based composites to low CNC fractions, in which
polymer remains the dominant phase. Here we report on the formulation and
processing of crosslinked CNC-epoxy composites with a CNC fraction exceeding 50
wt.%. The microstructure comprises sub-micrometer aggregates of CNCs
crosslinked to polymer, which are analogous to the lamellar structure of nacre
and promotes toughening mechanisms associated with bulk ductile behavior,
despite the brittle behavior of the aggregates at the nanoscale. At 63 wt.%
CNCs, the composites exhibit a hardness of 0.66 GPa and a fracture toughness of
5.2 MPa.m$^{1/2}$. The hardness of this all-organic material is comparable to
aluminum alloys, and the fracture toughness at the centimeter scale is
comparable to that of wood cell wall. We show that CNC-epoxy composite objects
can be shaped from the gel precursors by direct-write printing and by casting,
while the cured composites can be machined into complex 3D shapes. The
formulation, processing route, and the insights on toughening mechanisms gained
from our multiscale approach can be applied broadly to highly loaded
nanocomposites.
