期刊
LIQUID CRYSTALS
卷 32, 期 11-12, 页码 1499-1513出版社
TAYLOR & FRANCIS LTD
DOI: 10.1080/02678290500284983
关键词
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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 evenmembered 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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