101 zy Liquid crystalline networks zyxw EM Terentjev The last 2 years have seen significant advances in synthesis, physical experiment and theory. Highlights include the experimental confirmation of the soft elasticity effect and the penetration of the ideas of random disorder into the field of polymer structure and rheology. Address Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge CB3 OHE, UK Current Opinion in Colloid & Interface Science 1999, 4:lOl-107 PII: S1359-0294(99)00017-5 zyxwvutsrqp 0 1999 Elsevier Science Ltd. All rights reserved. ISSN 1359-0294 Introduction Liquid crystalline elastomers and gels continue to fasci- nate scientists and engineers by their combination of physical properties that separates them from any other material. This uniqueness lies in the orientational symmetry breaking and the resulting coupling of rubber elasticity and liquid-crystalline degrees of freedom. In ordinary elastic solids the elastic deformation is created by relative movement of the same atoms (or molecules) that form the bonded low-symmetry lattice. Hence, when the deformation is small, the lattice symmetry is preserved and one obtains an ordinary elastic response (although often anisotropic); large deformations destroy the lattice integrity and simply break the material. In contrast, in elastomers and gels the macroscopic elastic response comes from the entropy change of polymer chains on the relative movement of their crosslinked end points which are quite far apart. What happens on a smaller length scale with chain segments is an indepen- dent matter. So, for instance, the nematic order can be established within these chains and its director can rotate, in principle independently of deformation of crosslinking points. Such an internal degree of freedom within, and coupled to, the elastic body constitutes what is known as the Cosserat medium: the relative movement of crosslinking points provides forces and elastic stresses, while the director rotation causes local torques and couple-stresses. However, liquid crystalline elastomers are richer even than Cosserat solids because (again due to the entropic nature of long polymer chains connecting the crosslinking points) rubbers are capable of very large deformations (being at the same time essentially incompressible). Hence, one expects a vari- ety of new physical properties, especially in the region of large deformations. Indeed, some such properties have been found in recent years. An unusual physical system, such as liquid crystalline elastomers (LCE) and gels, should be looked upon from the point of view of equally unique applications. LCE are poor for electrooptical display devices, a main thrust in conventional liquid crystals. Because of a necessary chemical complexity, they are not suitable for elastic bands and tennis balls (although high-tech rubber tyres are perhaps an open question). What LCE seem to be made for is the manipulation of optical axes of birefrin- gence by mechanical means. Unusual non-symmetric elasticity, with very low shear modulus and high impedance, is another characteristic physical property still waiting for an appropriate application. T h e third prospective area is rubbers with piezoelectric and/or non-linear optic properties which, again in contrast to traditional solid and ceramic materials, allow large de- formations and manipulation by mechanical means. After the concept of nematic elastomers was put for- ward by de Gennes in 1975 [l] and the first side-chain liquid crystalline polymer was crosslinked into an elas- tomer by Finkelmann in 1981 [Z], the initial research mainly focused on synthesis and characterisation. T h e reviews [3-51 give a comprehensive picture of that period. In recent years the emphasis has been gradually shifting towards studies of new physical properties. In the following sections we shall examine significant new developments in synthesis, physical experiment and theory. Synthesis For many years the prevailing type of materials forming LCE was the side-chain liquid crystalline polymer. Af- ter the initial work of Ringsdorf and Finkelmann, poly- acrylate backbones with a variety of mesogenic side- groups have been used by different groups [6-81 to produce a variety of LCE. However, it was quickly recognised that polyacrylate-based polymer chains have certain practical disadvantages, in particular a high glass transition zyxwvu T > 50°C and low backbone anisotropy. Side-chain liquid crystalline polymers based on siloxane backbones have shown more dramatic mechanical properties due to a much higher chain anisotropy and are conveniently liquid crystalline at room temperature (with Tg I SOC). In fact, the first ever nematic elastomer [Z] was already based on a polysiloxane side-chain mate- rial. Methods of crosslinking have varied from chemical, using copolymerisation with a small proportion of reac- tive groups on a chain and adding bi- or tri-functional crosslinking agents [9,lo], to radiational processes, using UV light with photoinitiators [ 111 or gamma-radiation z g : [12'1.