High pressure optical studies of the chirality and phase behaviour of liquid crystalline blue phases

Abstract

The blue phase (BP) polymorphism of chiral S -(+)-4-(2-methylbutyl)phenyl-4-decyloxy benzoate (CE6) and S -(+)-4-(2-methylbutyl)phenyl-4-dodecyloxy benzoate (CE7) was studied at elevated transition pressures up to 280MPa using optical activity measurements. The increased density causes an increase in phase chirality which in the case of CE6 manifests itself in the disappearance of BP II (BP I-BP II-BP III triple point) and in the case of CE7 in the appearance of BP II. At atmospheric pressure, CE6 exhibits BP I, BP II and BP III, while CE7 only possesses BP I. This pressure behaviour of CE6 and CE7 is contrary to that of cholesteryl nonanoate, which has been investigated previously. This result is particularly interesting when discussed in connection with the effect of elongation of the terminal n alkylcarboxy chain of the cholesteryl n -alkanoates and of the terminal n -alkyloxy chain in the homologous series to which CE6 and CE7 belong. In the first case the phase chirality increases; in the second case it decreases. Since CE6 shows an obvious pretransitional optical activity in its isotropic liquid phase, pretransition and BP phase behaviour could be compared with each other at elevated transition pressures and temperatures. For those transition pressures or temperatures where anomalies in the BP phase behaviour occur, anomalies in the pretransition behaviour are also observed.