Thermotropic liquid crystal polymers (PCL)

In liquids, the individual atoms or molecules are distributed randomly, and knowledge of the position of any one of them gives no indication of the position of the others.

In crystalline solids, atoms (or molecules) occupy positions defined by the crystal lattice, and knowledge of the position of one atom (or molecule) defines the position of the others.

Liquid crystals (LCs) have the characteristics of both liquids and crystalline solids.

At short distances (around 10 nm), LCs are highly organized. In small regions of space, called domains, atoms and molecules are arranged relative to each other, as in crystalline solids. This organization is so high that CL refract electromagnetic radiation such as X-rays or visible light, giving them a diffuse, translucent appearance indicative of the presence of a multiphase liquid.

However, even within domains, interatomic or molecular forces are relatively moderate, and domains can be easily deformed by shear stresses.

At greater distances, there are few interatomic or molecular interactions, and the material appears like a conventional liquid. LCs therefore represent a new state of matter, distinct from both liquids and solids.

In each of these domains, the physical properties depend on the direction of measurement. They are therefore highly anisotropic. When the CL is in a relaxed state, this anisotropy is hardly noticeable, as the domains are randomly oriented. Conversely, under shear stress or flow-induced stress, CL becomes highly anisotropic. This effect is at the root of existing and potential applications.

Depending on temperature, materials capable of forming liquid crystals can also take solid or liquid form (see box).

At sufficiently low temperatures, they will form crystalline solids; conversely, at sufficiently high temperatures, above the so-called "clearing temperature", where the energy of atoms and molecules is greater than the interatomic or molecular forces, they will form isotropic liquids.

For liquid crystal polymers, the clearing temperature is generally higher than the decomposition temperature. Its interest is therefore purely academic.