Stretching the imagination: scientists create “liquid crystal elastomers” with amazing properties

We’ve all sat there in a dull moment at work stretching an elastic band between our fingers and watching it return to its original shape and size as we let it go. But how many of us would have thought of combining the elasticity of rubber with the optical properties of the liquid crystals commonly used in watches, laptops and calculators? On Monday 5th April at the Institute of Physics Condensed Matter and Materials Physics Conference in Warwick, Professor Mark Warner from the University of Cambridge will describe how he did just that when he mathematically predicted a new range of physical phenomena in materials known as ’liquid crystal elastomers’.

The molecular structure of a liquid crystal elastomer is similar to that of conventional rubber as it consists of long chains of molecules that can slide past each other easily and so allow the material to be stretched with little effort. Attached to these chains like the branches of a tree are smaller rod-like molecules that are usually found in liquid crystals. They allow the material to interact with light and can align the long chains and give unexpected mechanical properties, such as the ability to change colour when they are stretched and the ability to drastically change their shape either when they are heated or – for certain versions of the materials – when light falls on them. They have a variety of potential uses, for example they could provide the basis for a laser which only needs a small amount of power to operate and can change its wavelength (colour) just by being stretched. Alternatively the natural twisting of their internal structure means liquid crystal elastomers could act as a new system for detecting the difference between right-handed and left-handed forms of drugs. Many drugs have these two so-called ’chiral’ forms which are the mirror image of each other, and the liquid crystal elastomer will only alter its internal twisting when it comes into contact with one form. The other form will have no effect on it and as a consequence will not be absorbed. Separating out the different forms during the manufacturing process is extremely important for the pharmaceutical industry as the right-handed version of a particular drug can produce a different medical effect to the left-handed version.

“Strangest of all these properties was the prediction and experimental discovery that certain shape changes could be imposed with little or no energy cost. This has been christened ‘soft elasticity’ and places these materials between liquid and solid in an elastic classification of matter – as I will explain in my lecture. The secret is to think very carefully about the idea of changing shape, which is so central to defining the solid state. Whenever you sit down and think for a moment there seem to be no shortage of new phenomena that these new types of materials would have, but which are not found in existing solids or liquids. My aim is to try and find more and more of these phenomena and look at the uses they might be put to” says Professor Warner.

Since his first predictions in liquid crystal elastomers in the late 1980’s, Professor Warner – who won the 2003 Agilent Europhysics Prize for this work (in conjunction with Prof Heino Finkelmann from the Institut für Makromolekulare Chemie in Germany who simultaneously and independently created them experimentally) – has watched interest in these materials increase.

Some of the liquid crystal elastomers which are now being produced can significantly change their length within 10 milliseconds (10 thousandths of a second) of light being shone on them. “So there’s an enormous possibility for a light-activated sensor, or a light-activated actuator to make something move, in a system where you don’t want to feed in heat or electricity” explains Professor Warner.

Despite the variety of experimental investigations taking place world-wide, Professor Warner thinks liquid crystal elastomers – which are relatively straightforward to make – may still have other as yet undiscovered properties which could allow even more potential applications. “There always seem to be new twists to this story and I don’t think I’ve got remotely near the end of it” he says.