Over the past 50 years, plastics have transformed our lives. As well as replacing traditional materials such as metals and wood, they have made possible the development of new technologies in medicine, electronics and optics. Now plastics based on liquid crystals may play an increasingly important role in advanced technology. Some of these novel materials have remarkable mechanical or electronic properties, which researchers are just starting to exploit.
A plastic consists of chains of repeating molecular units-usually, but not always, based on a back-bone of carbon atoms. These chains are called polymers. For example, polythene, or polyethylene, is created when many molecules of ethene (C2H4) fuse together in a head-to-tail fashion-a process called polymerisation. Polymers have a huge variety of different properties depending on their chemical structure. For example, some polyimides (which contain nitrogen atoms as well as carbon and hydrogen atoms) form rigid plastics that remain stable at temperatures above 300 °C, while certain poly-siloxanes (containing silicon, oxygen and hydrogen) are stable up to-200 °C and remain flexible at temperatures below -20 °C.
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Like other materials, polymers usually form a liquid phase above a certain temperature. However, the transition from solid to liquid is often quite complex because polymers can have different types of solid phases. In some plastics, the molecules form regular arrays as in a crystal, while in others the molecules form a tangle of coils, similar to cooked spaghetti. This random sort of structure is called an amorphous glass. One important feature of glassy solids is that there is little or no motion of the molecules. An example of a plastic that is a glassy solid at room temperature is polymethylmethacrylate, more widely known as Perspex. When Perspex is heated above a certain temperature, it softens and becomes viscous liquid; in the liquid phase the arrangement of the polymer molecules is still a random tangle, but they are now more mobile and are able to slither past each other.
In some cases, polymers can form liquid crystal phases, where the molecules lose their positional order but retain their orientational order (see last week’s introductory article on liquid crystals). To form liquid crystals, the polymer molecules need to have a rigid structure so that they can adopt a definite orientation with respect to each other. Liquid crystal polymers can form nematic, chiral nematic and smectic phases in just the same way as conventional liquid crystals do (see previous feature). When the liquid crystal polymers solidify, the liquid crystal structure may become ‘frozen in’. This gives the material properties not normally associated with amorphous glassy plastics: high tensile strength, for example, or in some cases unusual electro-optical behaviour.
There are two ways of designing liquid crystal polymers. So-called main-chain polymers can be made from building blocks such as aromatic acids and amines (‘aromatic’ means the molecules contain rings of alternating single and double carbon-to-carbon bonds). These are rigid structures and they fuse to form a long stiff rod. You can also make main-chain liquid crystal polymers by using flexible links to join short, rigid units such as biphenyl groups. Alternatively, the rods can be hung like pendants from a flexible polymer chain. These are called side-chain polymers. Helmut Ringsdorf and colleagues at the University of Mainz in Germany also made liquid crystal structures from mixed main-chain and side-chain polymers. Figure 1 shows some of the possible structures.FIG-mg17685201.GIF
The best-known main-chain liquid crystal polymers are the aromatic polyamides, which go under the general name of aramids. A research team at Du Pont discovered them in the late 1960s. Because the polymer molecules in aramids are like long rigid rods, the forces between them are extremely strong. In fact, the attraction between the chains is so strong that heating does not loosen its grip. Aramids do not melt when heated but eventually decompose at a high temperature. This means that you cannot obtain the liquid crystalline phase by controlling the temperature, that is, by heating the solid phase or by cooling the liquid phase.
But suitable solvents can loosen these intermolecular forces. In the case of polyamides, the solvent is concentrated sulphuric acid. At the right concentrations, this solvent weakens the intermolecular forces between the polymer molecules just enough for a liquid crystal phase to form.
This is how Du Pont makes aramids commercially. It spins fibres from the liquid crystal phase and then washes away the sulphuric acid. The fibres retain the alignment of the stiff polymer chains with the result that they are stronger than steel. Du Pont sells this material as Kevlar, which is now used to replace steel wire, and to make plane and car bodies and bulletproof vests.
Obviously, concentrated sulphuric acid is not a particularly attractive material to use on a large scale. Chemicals companies have been trying to design new liquid crystal polymers in which the intermolecular forces are not so strong, so that a liquid crystal phase can be obtained simply by melting the solid polymer-a ‘melt-processable’ material. One way is to insert groups of atoms whose shape leads to a kink or sequence of kinks in the rod-like polymer. When the average density of the kinks per molecule is right, the melting points of these materials can be lowered so that the liquid crystalline phase can be obtained at reasonable temperatures, of less than 300 °C, say.
Using this and similar design strategies, industrial chemists have made some liquid crystalline polymers that do not have to be dissolved in solvents. These are very strong, very bendable and expand very little or not at all when heated. In addition, they are resistant to corrosive chemicals, do not burn easily and are excellent insulators. An example of a commercially available melt-processable, liquid crystalline, main-chain polymer is the polyester sold by Hoechst-Celanese under the trade name Vectra. Plastics such as Vectra can be used to mould complex parts: for example, fuel rails for car fuel-injection systems that would previously have required extensive machining of metal alloys.
Another way of designing melt-processable liquid crystal polymers is to make the main chain of the polymer less rigid by inserting flexible segments between the stiff aromatic cores. These flexible ‘spacers’ could be chains of carbon atoms, or chains of alternating silicon and oxygen atoms (siloxanes). Polymers containing siloxane chains have some of the lowest melting points recorded for main-chain liquid crystals.
Side-chain polymers have very different properties from main-chain polymers. They are much more like conventional plastics, being flexible, and easily processed using conventional technology into thin films and other forms. Heino Finkelmann and his colleagues in Germany and Valeri Shibaev with his colleagues in Moscow first made side-chain polymers in the late 1970s. From a structural point of view, liquid crystalline side-chain polymers can be regarded as derivatives of conventional plastics such as polystyrene or polymethylmethacrylate (Perspex). For example, you can obtain a liquid crystalline polymethacrylate from polymethylmethacrylate by replacing the methyl groups (CH3) with a ‘pendant’ consisting of four or five carbon atoms attached to a rod-like unit. Inserting these stiff rods into the molecule encourages the liquid crystal phase to form. The resulting liquid crystalline material has many physical properties similar to those of the parent plastic because the polymer backbone is unchanged. Just like Perspex, it is a glassy solid at room temperature.
In most respects, these polymers behave very much like conventional liquid crystals made of small molecules. For example, they respond to an electric field. There are, however, some important differences, which give liquid crystal side-chain polymers unique properties.
Normally, a thin film of a side-chain liquid crystal appears opaque. This is because the direction of alignment of the rod-like units (the director) is not the same throughout the material. In some domains it points in one direction; in others it points in a different direction. Where domains with different director orientations meet, so-called ‘defects’ form. These defects lead to scattering of light, which causes the opacity.
Applying an electric field to the film causes the directors to twist around so that the directors in all the domains point in same direction. The liquid crystal film then appears clear because light is no longer scattered by the domain structure. In conventional liquid crystals, switching off the electric field reverses the situation. But in side-chain polymers the film stays clear because the chains cannot move about easily (the liquids are very viscous) and they stay aligned. The only way to randomise the directors is to heat the material in the absence of an electric field until it melts into a true liquid.
You can exploit this behaviour to record information on an aligned liquid crystal using a laser that can heat tiny spots in the film about 10 micrometres across. This effect can be greatly enhanced by incorporating a dye, either as an additive or chemically bonded to the polymer backbone, which absorbs at the wavelength of the laser (usually in the infrared). The laser heats up and melts spots in the clear film.
When they cool down they form spots rich in defects which appear white against a clear background. When viewed between crossed polarisers, the spots stand out in high contrast as bright circles against a black background. You can then erase the information by illuminating the spot with an electric field switched on. This ensures that the liquid crystal phase formed as the area cools, has the same alignment as the rest of the film (see Figure 2).FIG-mg17685202.GIF
This property makes liquid crystal side-chain polymers ideal for developing erasable compact discs for storing information, or re-recordable discs for the hi-fi market. Ciaran McArdle and colleagues at GEC’s Hirst Research Centre in London have shown that it is possible to write and erase information 10,000 times over without the liquid crystal polymer films showing any appreciable degradation. The main factors affecting fatigue appear to be the effect of light on the dye molecules. Much of the research effort now is in trying to develop stable dyes that have maximum absorption of infrared light, and in trying to speed up the write/erase cycle.
Information from side-chain polymers
Carolyn Bowry and colleagues, also at GEC, have developed a system that is ideally suited for the archival storage of information-as WORM (write once read many times) media. Microfiche and 35-millimetre transparencies are WORM media. The researchers have removed the need for electric fields by designing a system based on the contrast between the natural opaque texture of the side-chain liquid crystal polymer film and the transparency of its glassy phase. The polymer sets to a glass at a temperature just below that at which the liquid crystalline phase melts into a normal liquid. A laser heats spots, or lines, in the opaque film so that they melt, becoming a clear liquid. When the laser beam is switched off, the spots cool rapidly. Because the liquid is very viscous, there is no time for the molecules to arrange themselves into the aligned domains of the opaque liquid crystal. Instead, the spots set as a clear glass.
This approach shows how accurately chemists can tailor the properties of liquid crystal polymers for specific applications. But liquid crystal side-chain polymers have even more remarkable uses. They can also be used to store holographic images, as Joachim Wendorff, Manfred Eich and Bernd Reck of the German Institute for Synthetic Materials in Darmstadt have shown. A hologram is a three-dimensional virtual image that is obtained by shining light on interference patterns that encode the image on a photosensitive film as a refractive index grating-in other words, a regular array of patterns produced by controlling the refractive index of the photosensitive film. Diffraction of the light by the grating produces the image.
To obtain the grating, you combine the light from a laser (which has all the light waves in phase) with light from the same laser reflected from the object. The interference patterns can be retained by a film of liquid crystalline side-chain polymer that responds to light. This is done by including light-sensitive molecular units as side chains, for example, units of stilbene. Stilbene can exist in two forms called cis-and trans-stilbene (see Figure 3). The trans form is rod-shaped and tends to promote the formation of the liquid crystalline phase, whereas the cis form is bent and disrupts the orientational order. Shining light on the material converts trans-stilbene to cis-stilbene. This alters the refractive index of the material in such a way as to produce an interference pattern. Holo-grams produced in this way are very clear, and liquid crystal polymers do not have to be developed like photographic emulsions. Nor do they need further treatment before the stored information can be read.FIG-mg17685203.GIF
Light can be used to transmit information as well as to store it. Optical fibres are rapidly replacing traditional cables because they can carry more information faster. Researchers are building devices for computers that work on light rather than electricity. This new technology requires materials that interact with light in a ‘nonlinear’ fashion when they are illuminated by high-intensity radiation, from a laser, for example. There is an enormous amount of effort going into the search for better nonlinear optical materials. As well as researchers in many university departments, several companies such as Du Pont, Eastman Kodak, Hoechst-Celanese, IBM and 3M in the US, Akzo and Philips in the Netherlands, Thomson CSF in France, and British Aerospace, British Telecom, GEC and ICI in Britain have been involved in this effort over the past seven years. This area of research and development is one of the most exciting and rapidly expanding areas in information technology.
In nonlinear materials, the oscillating electric field of an incident light beam causes electron clouds to move in response, forming an oscillating dipole. This creates a secondary field that oscillates not just at the original frequency but also at multiples of the frequency-the so-called second and third harmonics and so on. Normally, these higher order frequencies are not large enough to be seen. For radiation of high intensity, however, or if the electron clouds are symmetrical and easy to deform, the third harmonic may be detected. If the molecules also possess permanent dipoles-which means that the distribution of the electrons in the molecules is highly unsymmetrical-and if these dipoles are made to all point in the same direction, then the second harmonic frequency, which is double the original frequency, will be observed.
This kind of frequency doubling could be extremely useful for telecommuni-cations. The lasers currently used to send light signals down optical fibres operate in the near infrared. However, the density of information that you can launch down an optical fibre increases dramatically as the wavelength of light decreases. Unfortunately, lasers that emit in the ultraviolet and visible part of the spectrum are not suitable for use in telecommunications. So materials that can convert radiation at wavelengths in the near infrared to the visible or near ultraviolet would be very useful.
At the moment, inorganic materials such as lithium niobate are used for nonlinear optics. But in the past 15 years, chemists have discovered that organic materials can produce much larger and faster nonlinear optical responses. Nonlinear organic polymers are attractive because they are not so easily damaged by laser light as crystalline compounds. They can also be cast more easily into thin films and other shapes needed for the integrated optics that would be the heart of an optical computer.
Nonlinear polymers which are also liquid crystals are even better because the pendant groups in the side chains, which are responsible for the nonlinear effects, are aligned with respect to each other. This increases the nonlinear response several times over. However, for such polymers to be of use, the liquid crystal domains must be aligned and the permanent dipoles must all point in the same direction in order to obtain an overall asymmetrical distribution of charges. Applying an alternating electric or magnetic field to the polymer at high temperatures induces all the liquid crystal domains to align. A large direct current field will then reorientate the permanent dipoles in the molecules so that they point in the same direction. Cooling the liquid crystal to below the temperature at which the polymer sets into a glass ‘locks in’ this arrangement. This process is called ‘poling’.
In practice, however, there are problems. Over a few weeks, the alignment of dipoles decays. This is no good for optical devices that might have to last 10 years or more. Researchers are trying to find solutions, such as inserting pendant groups along the polymers to prevent the dipoles twisting around after they have been poled. Another way to lock in the dipole alignment is to cross-link the polymers chains after poling. Initial studies on non-liquid crystalline materials by Tobin Marks at Northwestern University in the US are encouraging. A further problem is that residual imperfections in the alignment of the directors in liquid crystal polymer films lead to scattering, which causes an unacceptable weakening of the light beam. If researchers succeed in solving this problem, nonlinear optical devices based on liquid crystal polymers could become a reality.
The molecular organisation achieved in liquid crystalline polymers, together with the high degree of control that can be exercised over their physical properties, has revolutionised polymer science and technology. These new materials make possible advanced technologies that would not have been accessible with conventional plastics. The shape of things to come could depend, quite literally, on the shape of these complex and fascinating materials.
George Attard is Courtaulds Research Fellow in the chemistry department at the University of Southampton. Corrie Imrie is a postdoctoral researcher in the Department of Polymer Science and Engineering at the University of Massachusetts.