What are liquid crystals made of?

Asked by: Bill Devenport

Answer

In 1888, Friedrich Reinitzer was studying the substance cholesteryl benzoate when he discovered that it had two distinct melting points ' the solid changed first into an opaque liquid, then on raising the temperature further the material became a clear liquid. The intermediate, opaque liquid was termed the liquid crystal phase.

Most liquid crystals have common characteristics; firstly the molecules tend to be rod-shaped, have a rigid backbone and strong dipoles (or easily polarisable substituents). These rod-like molecules can align in different ways to give different degrees of order in the liquid crystal, (or 'mesogenic'), state.

Examples are the 'nematic' phase, in which the rods all point in the same direction but have little lateral order, and the 'smectic' phase, in which there is alignment and a degree of lateral order.

Many liquid crystals are simple polymeric organic compounds. We tend to think of polymers as flexible (polythenes etc.) so for liquid crystals the polymers have to incorporate a rigid section, either in the long polymer section (main chain liquid crystals) or attached to the polymer as a 'side chain'. But the basic elements involved are simply C, H, N, O (for example) - so that's what they're made of!

For those that don't know, in LCD displays, use is made of linear polarisers, familiar to most as the glass in polarising sunglasses. If you 'cross' two polarisers (i.e. arrange them at 90 degrees to each other) then light does not pass through. This is the basis of the LCD, but between the cross polars the liquid crystals are arranged with a 'twist'. This twist allows light to pass through. However, when an electric field is passed through the liquid crystals, the twist is removed, and so light cannot pass through ' the area appears black.
Answered by: Jan Skakle, Ph.D., Lecturer, University of Aberdeen, Scotland UK