Chiral nematic state, cellulosics

Many cellulose derivatives form Hquid crystalline phases, both in solution (lyotropic mesophases) and in the melt (thermotropic mesophases). The first report (96) showed that aqueous solutions of 30% hydroxypropylceUulose [9004-64-2] (HPC) form lyotropic mesophases that display iridescent colors characteristic of the chiral nematic (cholesteric) state. The field has grown rapidly and has been reviewed from different perspectives (97—101). 

Liquid crystalline (LC) solutions of cellulose derivatives form chiral nematic (cholesteric) phases. Chiral nematic phases are formed when optically active molecules are incorporated into the nematic state. A fingerprint texture is generally observed under crossed polarizers for chiral nematic liquid crystals when the axis of the helicoidal structure is perpendicular to the incident light (Fig. 2). 

Chiral disubstituted PEDOTs have recently been prepared for the first time via transetherification of 3,4-dimethoxythiophene monomers with chiral glycols followed by potentiodynamic oxidation.174 An alternative approach to optically active PEDOTs has also been recently described, which involves the electrochemical polymerization of the EDOT monomer in aqueous hydroxypropyl cellulose (HPC) as a polymer lyotropic liquid crystal to give a chiral PEDOT/HPC hybrid.175 The PEDOT prepared in this chiral nematic liquid crystal exhibited optically active electrochromism in that it could be electrochemically switched between a dark blue reduced state and a sky blue oxidized form that exhibited a different CD spectrum. 

Lyotropic cellulosics mostly exhibit chiral nematic phases, although columnar phases have also been observed. The molecules in the thermotropic state also form chiral nematic order, but it is sometimes possible to align them in such a way that a helicoidal structure of a chiral nematic is excluded. Upon relaxation they show banded textures. Overviews on lyotropic LC cellulosics are... 

In this context, literature [90] states that at room temperature, acetoxypropyl cellulose exhibits both chiral nematic phases—the lyotropic and the termotropic one. When subjected to specific conditions of shear flow, the cellulose derivative cholesteric liquid crystal suffers transformations, such as cholesteric helix and cholesteric-to-nematic transition. The films prepared from anisotropic solutions of termotropic acetoxypropyl cellulose in an isotropic solvent exhibit anisotropic mechanical properties, generated by the molecular orientation of the solution under shear stress. Thus, liquid crystalline solutions give rise to films with anisotropic mechanical properties the films are brittle when stretched parallel to the shear direction and ductile when stretched perpendicular to it. 

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