Polymer Liquid Crystals
Materials that are crystalline (i.e. made of crystals) are typically strong. For example, steel is made up of crystals of iron atoms. On the other hand, liquids do not have a structure made of crystals and, therefore, are weak. Consequently, the term liquid crystals seem to be a contradiction. This term is used because polymer liquid crystals (PLCs) have properties, which are between those of solids and liquids. Like solids, in which the atoms arrange themselves into ordered structures to from crystals, the polymer molecules that form PLCs naturally align themselves to give some degree of order. However, this order is easily manipulated, making them also similar to liquids in that their form can be readily changed.
Electrodes are used to apply an electric field that changes the structure of PLCs. The molecules of the PLCs have a positive and a negative end. Therefore, each end of the molecule will be attracted to the electrode with the opposite charge, causing the molecule to align parallel to the electric field. This alignment of molecules occurs to a much greater extent than the arrangement that naturally forms in the PLC. The high amount of alignment then allows light to pass through the PLC. By turning the electric field on and off, the PLC can be transformed from a light to dark color or from transparent to translucent. By varying the color PLCs can be used to create displays like digital watches and computer screens. The ability to change from transparent to translucent allows PLC to be used in applications like the one seen in the figure below.
This figure shows two PLC panels. The panel on the left has no electric field applied. As a result, very little light can be transmitted through the panel. Instead, most of the light is reflected, causing the panel to appear white. The panel on the right has an electric field applied to it so that the molecules of the PLC become aligned and allow light to pass through; therefore, the couch on the inside of the building can be seen. These types of panels can be used as windows, which will provide privacy at the flip of a switch.
Collins, Peter J., Liquid Crystals: NatureÕs Delicate Phase of Matter, Princeton University Press: Princeton, New Jersey, c1990