Methods for the Detection of Cellulosic Mesophases

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 

At the selective reflection wavelength, a component of linearly polarized light, e.g., right-handed or left-handed circularly polarized light, is totally reflected without a phase reversal by a twisted helicoidal structure at a wavelength correlated to the pitch P. An oblique incident of light is presumed onto the substrate plane or nematic sheet (see Fig. 3) 

The width of the reflection band is given by AA=AnP, where An is the birefringence 

If X Xo the plane of polarization of incident light is rotated by an angle and converted to elliptically polarized light. De Vries [13] applied Maxwell s equations to twisted sheets and derived for 0, the optical rotatory power 

Equations (1) and (2) contain the pitch P besides experimentally available quantities, i.e., the selective reflection wavelength A , the optical rotation 0, and the refractive indices Mo and of a nematic sheet. The usually determined cholesteric birefringence can be converted to the nematic one by A nem= 2Awci,. By fitting Eq. (2) to the actual measured data, the pitch is obtained and the validity of Eq. (2) tested. In the limit of A Eq. (2) leads to 

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