Effect of Cholesteryl Alkanoate Structure on the Pitch of the Cholesteric Mesophase
The pitches of a series of new cholesteryl alkanoates derived from branched alkanoic acids, some of them optically active, have been determined from extrapolation of pitches of binary mixtures of cholesteryl oleyl carbonate (COC). The pitches of 2-branched and linear cholesteryl alkanoates show a maximum as a function of the heats of fusion (ΔHf) of the pure crystalline materials. The pitches of the 3- and 4- branched alkanoates increase with AH_. In linear alkanoates the pitch is directly proportional to the mterfacial energy between the cholesteric mesophase and the crystal. These results relate crystal and mesophase structure.
Interpreted in terms of the volume requirements of the alkanoate chain, the dominant effect of branching with a single carbon is to make the alkanoate moiety more rigid through restriction of rotation about C-C single bonds. This decreases the volume requirement, allowing better crystal packing and lower angular displacements between adjacent molecules in the cholesteric helix, hence large pitches. A steric bulk increase is seen via decreased pitch when branches are lengthened or occur near the end of the alkanoate chain. These factors effect crystal and mesophase stability similarly up to a certain point, where chain folding may occur in the crystal, but not the mesophase. The structural generalizations are, however, subject to dramatic changes by alteration of configuration in chiral alkanoates. The dramatic effects of conformational and configurational factors on pitch are indicative of the potential of these systems for detailed study of these factors once the nature of the cholesteric mesophase is more fully understood.
KeywordsBinary Mixture Angular Displacement Volume Requirement Intermolecular Distance Adjacent Molecule
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