Emergence of Smectic Mesophase in Binary Mixtures of Pure Nematogens

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Abstract
Binary mixtures both components of which are liquid crystals are known to form continuous mixed liquid crystals.1 Most of them do not undergo any change in their mesomorphic textures, though a few have been found to give rise to different textures.2 We report here a binary system consisting of (A) p-nitrophenyl-p'-n-amyloxycinnamate (90.5–120.5°) and (B) p-methoxyphenyl-p'-n-heptyloxycinnamate (91.0–120.5°) which transforms the original nematic texture into a mixed smectic A texture over a range of temperature and concentration. Both components individually are nematic liquid crystals. In their binary mixtures, a two dimensional more ordered mixed Smectic A mesophase emerges between 5.0–92.5 mol% of p-methoxyphenyl-p'-n-heptyloxycinnamate over a temperature range of about 50–105° at its maximum. The mixed smectic-nematic transition curve shows a marked rounded concavity, extrapolation of which on either side yields values of latent transition temperatures 77.0° and 66.5° for the components A and B respectively whch could be real in case they were to show polymesomorphism. These values are comparable with those obtained from the study of their homologous series.3 Besides this unique phenomenon of emergence of the mixed smectic A mesophase out of a mixture of two purely nematic substances, the nematic mesophase also persists continuously in the phase diagram over a range of temperature an concentration. The results are interpreted to account for the unique display of the molecular forces which make such an emergent mixed smectic phase possible.

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