Liquid crystal elastomers (LCEs) are lightly cross-linked polymer networks with mesogenic order [1]. Being lightly linked they are capable, as classical elastomers are, of huge deformations. They can be elongated by factors of six or more, that is, of 600% strain, or sheared by of order one or more. Their deformability reflects the fact that elastomers are really polymeric liquids with high molecular mobility but where the cross-links between chains that hold the rubber network together do not allow macroscopic flow. The linking is light in the case of elastomers where deformability is great. Such linking leads to low mechanical moduli of typically 105−106 Pa. Since these networks have conventional, liquid-like values for their bulk moduli that are

much higher than the shear moduli, it follows that they change shape at constant volume. For small distortions, the sum of their Poisson ratios governing their size change in the two perpendicular directionsmust accordingly be 1. Classical (isotropic) elastomers are the same, and their Poisson ratios are both 1/2. We shall find smectic elastomers also conserve volume, but their Poisson ratios are most complex because their shape changes are dominated by the underlying layering of the smectic phase.