Test of the immiscibility concept for the induction of smectic layering in nematic liquid crystals.

Abstract

Chemical and structural features determine the mesophase behavior of both low molar mass liquid crystals (LMMLCs) and side-chain liquid crystalline polymers (SCLCPs). Chemical concepts are just emerging for converting the type of mesophase exhibited by both LNMMCs and SCLCPs. Coleen Pugh's group has developed one of the most promising chemical tools for these conversions: incorporation of immiscible components into liquid crystals, taking advantage of the segments' drive to microsegregate and form layered structures. In this way it is possible to switch a system exhibiting only nematic mesophases into one exhibiting smectic mesophases. Molecules containing saturated hydrocarbon and fluorocarbon segments of at least 6 to 8 carbons are known to organize into layers. Previously, when fluorocarbon segments of 6 to 8 carbons in length were incorporated into a hydrocarbon system that had exhibited only nematic mesophase behavior, both the LMMLCs and SCLCPs were switched to smectic mesophases. The microsegregation was so strong that the influence of the polymer backbone was minimal and the thermotropic behaviors of the polymers were similar to those of the low molar mass model compounds. This thesis further tests the concept that immiscible components can induce smectic layering. A series of fifteen 2,5-bis[(4<super>'</super>-(<italic>n</italic>-(perfluoroalkyl)alkyloxy)benzoyl)oxy]toluenes and their analogous polynorbornenes with fluorocarbon lengths of 2--4 have been synthesized and their liquid crystalline behavior studied in order to determine the minimum length of fluorocarbon segment necessary to induce smectic mesophases in both the LMMLCs and SCLCPs. Although smectic mesophases were observed for the LMMLCs, only four of the polymers exhibited smectic behavior suggesting the minimum fluoroalkyl length is greater than four to ensure microphase separation in the SCLCPs. 2,5-Bis[(4<super>'</super>-(<italic> n</italic>-(perfluoroheptyl)octyloxy)benzoyl)oxy]toluene was laterally substituted with twenty-four different linear or bulky substituents, as well as two additional polymer backbones, in order to test the hypothesis that microphase separation between the fluorocarbon and hydrocarbon segment was so strong that the polymer backbone was irrelevant in determining the mesophase behavior of the SCLCP. In all cases only smectic mesophases were observed for the compounds, confirming this hypothesis for fluorocarbon lengths of six or greater.