Nanocellulose fibers, derived from the load bearing cellulose fibers in plant cell walls, have diameters ranging from 4 to 40 nm. In bundles, nanocellulose fibers possess Young’s moduli of 20 GPa and tensile strengths of 500 MPa, which means that a rope with a 6 mm diameter could lift a sedan weighing 1250 kg. The high stiffness and strength, which is due to an extended chain conformation of crystalline and amorphous regions and the hydrogen bonds between the fibers, make nanocellulose an excellent choice as a renewable, biodegradable reinforcement material.
To date, primarily isotropic nanocellulose foams, which have a structure similar to a bath sponge, have been non-directionally frozen and freeze-dried. In contrast, anisotropic foams with a honeycomb-like structure can be prepared by freeze casting, which is a directional freezing technique. The advantage is that in one direction, the direction parallel to the long pore axis, honeycombs are significantly stiffer and stronger than isotropic foams and possess aligned, open porosity.
Freeze-cast nanocellulose-reinforced composites were prepared at different compositions including chitosan, nanoclay, and hydroxyapatite. Compression testing revealed that Young’s modulus and yield strength increased with the inclusion of nanocellulose. Additionally, at a porosity of 98.6%, freeze-cast structures of nanocellulose-reinforced clay achieved a Young’s modulus of 2.1 MPa and a yield strength of 0.032 MPa, which is over 80 times the modulus and 30 times the strength of the isotropic foams with the same composition.
Fundamental structure-property-processing correlations that result from these studies will aid the design of mechanically efficient scaffolds for multiple applications ranging from packaging to tissue engineering.