Studies on chitin-based liquid crystalline polymers - Studies of critical concentrations of N-phthaloyl critosan for the formation of lyotropic cholesteric phase using circular dichroism spectroscopy
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N-phthaloyl chitosan is able to form cholesteric liquid crystalline phase in concentrated solutions of some organic solvents, including DMF, DMSO, DMAc and pyridine.. Circular dichroism was applied to investigate the change of handedness for the above four systems. CD spectra suggested two kinds of absorption peaks when the concentration was higher than a critical concentration for forming liquid crystalline phase. One is a sharp peak at 330 nm, the other is a broad peak with a maximum at about 400 - 450 nm. However only sharp peak at 330 nm was observed for isotropic solutions. The sharp peak was induced by the helical conformation of molecular chains, and the broad peak was induced by the cholesteric helices of liquid crystal. Therefore the determination of the critical concentration for N-phthaloyl chitosan solutions by monitoring the very beginning of the appearance of the broad peak is feasible. The critical concentrations measured by CD were 1 wt % lower than those measured by polarized optical microscopy (POM) for all four solvents used. It means, as a method to measure the critical concentration, CD is more sensitive than POM. Two different ways of sample tilt were used in CD messurements to prove that the linear dichroism and birefringance had no effect on CD signal of cholesteric helices of the liquid crystal.