Cholesteric Elastomers with Mechanical Control of Optical Spectra

Abstract

Elastomers are elastic media which mixes as no one, also, they have three important properties: orientational order of large range in amorphous soft materials, macroscopic susceptibility to the molecular shape, and quenching to the topological constraints. Classical liquid crystals are fluids typically composed by rigid molecules, which with a continuous model, are represented by bars and exhibit an orientational order of large range. The simplest order displayed by these systems is the nematic, for which, all the molecules are aligned in average. Complementary, the polymeric long chains embodying anisotropic rigid units can be nematically aligned and may form polymeric liquid crystals. However, the long chains are elongated when their rigid monomeric components are oriented, giving rise to an anisotropic material. If additionally, the polymeric chains are joint to a backbone in such way that their topology is restrained, hence the melt condenses in a very elastic solid or rubber. It is convenient to mention that in general, within the rubbers, the nematic monomeric molecules retain the same mobility as in a liquid phase. These soft constrictions make the resulting material, which is then a solid very extensible. Rubbers resist mechanic deformations since the polymeric chains reach their maximum entropy when they stay in their natural state without deformation. The polymerization of these compounds creates links between the chains which joint to the backbone formed collectively among themselves. It is to be expected that in this process, the anisotropic rigid units of nematic character, for instance (nematogens) which lie in the inner of the medium, form spontaneously domains distributed in all the rubber, whose preferred orientation is to be in different directions. This variety of domains causes light scattering giving rise to a macroscopic turbid appearance to the material. One very important advance in the design of these materials was managed by Finkelman, by developing a procedure for obtaining samples which form a single domain. The basic idea consists in applied electric field to the melt substance in order to align the anisotropic monomeric units while the polimerization is taking place and/or the temperature