Methylene- and ether-linked liquid crystal dimers II. Effects of mesogenic linking unit and terminal chain length |
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Authors: | Peter A Henderson John M Seddon Corrie T Imrie |
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Institution: |
a Department of Chemistry, School of Engineering and Physical Sciences, University of Aberdeen, Meston Walk, AB24 3UE, UK
b Department of Chemistry, Imperial College London, London SW7 2AZ, UK |
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Abstract: | Four series of liquid crystal dimers have been prepared containing either ether-linked or methylene-linked spacers. Changing the spacer from being ether-linked, i.e. O(CH2) n O, to methylene-linked, i.e. (CH2) n +2, results in decreased nematic-isotropic transition temperatures, and this reduction is more pronounced for odd-membered spacers. By contrast, the entropy change associated with the nematic-isotropic transition is higher for an even-membered methylene-linked dimer than for the corresponding ether-linked material. This trend is reversed for odd members. These observations are completely in accord with the predictions of a theoretical model developed by Luckhurst and co-workers in which the only difference between the dimers is their shape. For the highly non-linear pentamethylene-linked dimers, only those with a short terminal chain exhibited fluid smectic behaviour, specifically, a monotropic alternating SmC structure which allowed for the efficient packing of the bent molecules. Once the terminal chain reached a value of m = 9, a modulated ordered smectic phase was observed. For even-membered dimers, which exhibit only nematic phases upon melting for short terminal chain lengths, smectic phase behaviour was promoted with increasing terminal chain length, as is conventionally observed. Even-membered ether-linked dimers exhibited a SmC phase whereas even-membered methylene-linked dimers exhibited an ordered smectic G/J phase. Thus, it would appear that the differences in the transitional properties of ether- and methylene-linked dimers can be accounted for largely in terms of geometrical factors. |
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