CHOLESTERIC MESOPHASES

In the past few years, there have been important developments concerning the role and the use of chirality in liquid crystals. An example which is now relatively well known is the chiral smectic C* low molecular weight liquid crystal (LMWLC) which lowers the response time of displays by a factor between one hundred and one thousand [1-3]. In chiral liquid crystal polymers (cholesteric or twisted nematic and smectic C*), it is necessary to understand the relation between the chemical structure and the properties of mesophases in liquid and solid polymer. Furthermore, chirality is also found in abundance in biopolymers and may be important for life processes. Applications in industry (high modulus fibers, sensors, non linear optics . . .) have already been realised or are being pursued. The aim of this paper is to emphasize the polymer character of these liquid crystals. What is the influence of the molecular weight, the degree of substitution, the viscoelasticity and the existence of a glass transition? How to use the polymer character to obtain new informations on the liquid crystal state or to make new devices? This rapid overview roughly covers the following parts: (1) Formation and general properties of cholesteric mesophases: we give a rapid discussion of thermotropics and lyotropics, of the influence of degree of polymerisation, degree of substitution, concentration, temperature . . ., on the cholesteric-isotropic transition and on the order observed by textures or as described more quantitatively by the order parameter. (2) Optical properties: we give a brief presentation of a theoretical work concerning the pitch prediction taking into account the asymmetric term in the interaction potential and the comparison with experimental results. (3) Textures and defects which are mainly observed in these mesophases. (4) Flow properties and rheological behaviour which are fundamental for processing. (5) Blue phases near the isotropic-cholesteric transition which are discussed with emphasis on the specificity of the polymer state. It is usual to distinguish main chain and side chain liquid crystal polymers [4, 5], which may be modelled as indicated in Fig. 1. Cellulosic derivatives are good examples of main chain cholesteric polymers [6]. Cellulose is a very abundant polymer: each year, biosynthesis produces about 10(11) tons of cellulose. It readily accepts grafting of substituents, thus producing derivatives like cellulose acetate of hydroxypropylcellulose. Examples of side chain liquid crystal polymers are those obtained with homo- or copolymers of polyacrilate, polymethacrylate or polysiloxane.