The crystals that comprise CNC interact with one another based on their size, charge and shape. Since nanoparticles are at a scale where they are in constant motion, the resulting energy allows the particles to self-organize.
Due to the CNC crystals’ spindle shape, they can form liquid crystals: a state where there are domains of order within a fluid. As the fluid is concentrated, the spindles self-orient and form layers of crystals where each layer is oriented in the same direction. At a particular concentration, there is a phase separation into a phase where the domains exist and a phase where they do not. As the fluid is further concentrated, more of the fluid becomes organized until a film with only an ordered structure remains on a given surface
This ordered structure creates a hard, smooth and tightly packed film. The packing is particularly tight for CNC because the crystals contain a twist, allowing the spindles to interlock. Due to the chirality of CNC the twists in each crystal are in the same direction, resulting in even more effective packing.
CNC is an inherently strong material because of its high crystallinity. Each crystal has a stiffness that is of the order of 150 GPa and a tensile strength that is of the order of 10 GPa. These numbers are comparable to those of Kevlar™ as well as hard metals and their alloys. Many crystals are also inherently hard and incompressible and CNC is not an exception.
The crystals of CNC, however, are nanometric and are not strongly interconnected except in the structures laid down by nature from which they are isolated or when they are aggregated or incorporated into matrices and films.
The challenge is to use CNC to impart greater stiffness, tensile strength, hardness and incompressibility to materials and therefore harness the power of what nature has provided. This challenge, which is now being met in various ways, is to ensure that the nanometric particles can be controlled in the way that they are distributed and that they are compatible with matrices to which they are part or to which they adhere.
Fluids containing CNC are shear thinning, meaning they decrease in viscosity with the application of shear. This property provides the basis for several types of application. The degree of shear thinning is dependent on the rigidity, charge and shape of the crystal. These parameters amplify the effect of a small amount of CNC by changing the effective particle diameter and can be affected and tuned by the interplay of other components of the system.
Methods of preparation that affect the rigidity, charge and shape of the crystal will also affect the rheology. For example, using methods that result in a carboxylated or uncharged crystal product will not yield the same properties as CNC. The ongoing uniformity and purity of the product is also critical to obtaining consistent rheological properties. By the nature of the process used to make it, CNC is both uniform and contaminant-free.
Like all cellulose, CNC is comprised of a linear, long-chain glucose polymer that is rich in oxygen, particularly hydroxyl groups. These hydroxyl groups develop the hydrogen bonds that give the CNC its inherent strength while providing a reactive surface of hydroxyl groups on two of the crystal’s facets. Though not all of the hydroxyl groups are equally reactive and accessible, they adequately allow a multitude of reactions. The hydroxyl groups are also the reason why CNC, unlike carbon nanotubes and other materials, is inherently hydrophilic.
CNC surface is also comprised of acidic groups attached to its surface which allows for reaction with a variety of bases. Though many traditional products (such as cellulose acetate, carboxymethylcellulose and cellulose ethers) take advantage of the reactivity of cellulose, CNC allows for the bonding of a variety of hydrophobic structures. This makes the material compatible with a wide range of solvents and polymer matrices.
CNC forms solids with structural colour. As the ordered CNC fluid becomes a solid, its colour is created by the interaction of light with the layered structures that are developed. Many animals and plants use structural colour rather than pigments or dyes to create vibrant, iridescent and durable displays.
As the layers develop, the charge that is on the crystal surfaces keeps the layers separated. The chiral twist in the crystals that make up a given layer results in a helical structure of layers wherein the orientation of the crystals in each layer is offset. This helix has periodic layers of crystals with the same orientation.
The periodicity of CNC is of the order of the wavelength of light, which in turn is reflected and amplified through constructive interference. Because only one helical twist exists in the layers, the reflected light is polarised. The layer periodicity can be tuned by the addition of salts, by ultrasound and by the strength of the magnetic field. The observed colour is also affected by the viewing angle. Under polarised light, even the dynamic, layered structures within the fluids can be observed as birefringent patterns.