Induced Cholesteric Systems: Pitch Depdendence on Temperature and Concentration

Temperature dependences of the helical pitch have been studied for different induced cholesteric systems. It is shown that the sign of dP/dT in the same nematic solvent, may depend upon the nature of optically active dopants. For example, typical non-smectogenic nematics exhibit dP/dT values which are positive with l-menthol derivatives, and negative if the induced chirality is due to tygogenyn derivatives. A general theory of cholesteric pitch is developed providing an explanation of the experimental data in question as well as a description of pitch vs. concentration relationships in nematic—cholesteric mixtures. Also presented are some data for induced cholesteric systems with a smectogenic nematic component, where pretransitional phenomena with P → ∞ are observed.