No abstract

Deuterium NMR is used to examine the molecular order exhibited by an organosiloxane tetrapode giving the first experimental evidence, using a bulk sample, for the existence of a biaxial nematic phase in this type of compounds. The temperature dependence of the averaged quadrupolar coupling constant and asymmetry parameter was determined in the compound's nematic phase. Two distinct regimes could be identified, one with a vanishing asymmetry parameter corresponding to a uniaxial nematic phase and another with a significant temperature dependent asymmetry parameter, corresponding to a biaxial nematic phase. The high values obtained for the asymmetry parameter at the lower end of the nematic range are well above experimental error and constitute a definite proof of the biaxial nature of the nematic phase exhibited by the studied compound for those temperatures.

FT-IR and 2D correlation spectroscopy were employed to study the microstructural changes ocurring during phase transitions of a liquid crystal poly(amidoamine) codendrimer (PAMAM (L1)16(L2)16) generation 3, functionalized on the terminal groups by one-chain promesogenic calamitic units (4-(4'-decyloxybenzoyloxy)salicylaldehyde (L1)) and two-chain promesogenic calamitic units (4-(3',4'-didecyloxybenzoyloxy)salicylaldehyde (L2)). Spectral modifications associated with molecular conformation rearangements allowing for molecular shape change on going from a liquid−crystalline organization to another were found. The transition temperatures were calculated, and they are in good agreement with the DSC data. Spectral analysis gives evidence of the LC phase transitions and to an additional transition associated with the existence of conformers. Various types of hydrogen bonding have been established.

The mesomorphic properties of 27 homologues of the N-alkylpyridinium alkylsulphate series were studied by X-ray diOE raction and dilatometry. All of these compounds exhibit single smectic A phases whose layer spacings are between 0.64 and 0.8 times the molecular length. Polarizing optical microscopy observation shows that these peculiar layer spacings are not due to the tilting of the molecules; a new packing model is proposed to explain the particular structure of these thermotropic ionic liquid crystal phases. Introductionfor this diOE erence of bulkiness [1]. Among the diOE erent It has previously been shown [1] that the homologous models, one was retained in which the layer is formed series of N-alkylpyridinium dodecylsulphates (nPySO 4 m, by two, single, ionic sublayers where the pyridinium see gure 1), with m 5 12 (the number of carbon atoms rings sandwich the ionic part of the dodecylsulphate of the aliphatic chain connected to the sulphate group), anion; these two sublayers are separated by the aliphatic consists of thermotropic ionic liquid crystals.chains of the anion, which are, on average, standing Each of these compounds exhibit a single smectic A upright. The two ionic sublayers are covered by the phase, whose temperature range and clearing point are disordered aliphatic chains of the pyridinium rings, reasonably independent of the chain length, and whose which show large random tilting uctuations to ensure layer spacing is about 0.8 times the molecular length a compatible molecular area between the ionic and calculated as the simple sum of the lengths of the cation aliphatic parts. However, these conclusions were drawn and the anion. As polarizing optical microscopy on the purely from knowledge of the layer spacings without phases shows undoubtedl y that they are orthogonal any direct experimental information on the molecular smectic phases, tilting of the molecules cannot be invoked area, which is clearly a key parameter to describe the as a reason for the peculiar layer spacing, and so it was molecular arrangement within smectic layers [2]. In supposed that this behaviour would be related to the that previous work, moreover, only the length of the cross-sectional area of the ionic species which greatly chain attached to the pyridinium ring was changed exceeds that of the extended aliphatic tails. From the (n was varied between 8 and 15) while the length of the analysis of powder X-ray diOE raction patterns, various counteranion chain was kept constant (m 5 12). A precise models were then discussed, which considered that the discussion of the eOE ect of chain length was therefore not alkyl chains would be compelled either to possess large, possible. random tilt or to interdigitate in order to compensateIn this paper we present new results associated with the study of 27 new homologues, with (n 1 m) varying between 20 and 40 and (n Õ m) varying between Õ 10 and 1 8 (see the table). For the symmetric compounds (n 5 m), dilatometric measurements complement the powder X-ray diOE raction measurements. ...