Cellulose nanocrystals (CNCs) have potential to become a renewable and cost effective alternative for biocompatible polymer nanocomposites with a high strength to weight ratio. However, these particles are usually very polydispersed and non-compatible with hydrophobic polymers. Aqueous suspensions of rod shaped CNCs were prepared by sulfuric acid hydrolysis of kraft wood pulp from a local paper mill and ashless cotton cellulose powder. Atomic force microscopy was used to image the particles from both sources. The cotton crystals are on average 104 nm long, and 13.9 nm wide; while the wood pulp crystals have an average length of 107 nm and a width of 12.1 nm. Aqueous CNCs suspensions phase separate into a chiral nematic – isotropic equilibrium which is dependent on the particle size, cellulose concentration and solvent properties. Rheological analyses have been used to characterize the fluid behavior of the nanocrystal suspensions for different cellulose concentrations, average lengths, and crystal ordering; including the monophasic and biphasic regions. The relative viscosity, G', and G'' were all measured. Samples of different crystal lengths were obtained by fractionation using the size dependent phase partitioning. Nanocomposites filled with CNCs have usually shown little improvement on their properties, presumably because of the poor compatibility with the polymeric matrix. Approaches to increase the dispersibility of CNCs within polymer matrices for new sustainable nanocomposites are currently being explored.