Pretransitional optical activity in a liquid-crystal system of high chirality

Abstract

Optical activity measurements in the isotropic phase just above the liquid-crystal transition temperature were performed for various concentrations of the chiral liquid crystal 4''-(2-methylbutylphenyl)-4'-(2-methylbutyl)-4-biphenylcarboxylate (CE2) in the nonchiral liquid crystal 4-n-pentylbenzenethio-4'-n-heptyloxybenzoate (7\ifmmode\bar\else\textasciimacron\fi{}S5). The mixtures investigated possessed extremely high chirality, with pitches varying from about 250 nm in the 40-wt. %-CE2 sample to approximately 100 nm in pure CE2. The pretransitional optical activity results clearly show that coupling between the structural modes of the ordered phase grow more and more important as the chirality increases. The agreement between these data and theory is quite good, thus permitting an estimation of the critical temperature for two of the structural modes.