TY - JOUR
T1 - Methylene- and ether-linked liquid crystal dimers II. Effects of mesogenic linking unit and terminal chain length
AU - Henderson, Peter A.
AU - Seddon, John M.
AU - Imrie, Corrie T.
PY - 2005/11
Y1 - 2005/11
N2 - 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.
AB - 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.
U2 - 10.1080/02678290500284983
DO - 10.1080/02678290500284983
M3 - Article
SN - 0267-8292
VL - 32
SP - 1499
EP - 1513
JO - Liquid Crystals
JF - Liquid Crystals
IS - 11
ER -