NONSYMMETRICAL DIMERIC LIQUID-CRYSTALS - THE PREPARATION AND PROPERTIES OF THE -OMEGA-(4-N-ALKYLANILINEBENZYLIDENE-4'-OXY)ALKANES

Six series of a family of non-symmetric dimers, the omega-(4-n-alkylan ilinebenzylidene-4'-oxy)alkanes, have been prepared. In three of the s eries, the number of methylene groups in the flexible alkyl spacer has been varied from 3 to 12 while the terminal alkyl chain is held const ant at either 2, 6 or 10. In the remaining series, the effect of incre asing the terminal chain length has been studied by holding the spacer length constant at 3,4 or 5 methylene units while varying the length of the terminal chain from 0 to 10 carbon atoms. The phase behaviour a nd transitional properties of these series have been investigated usin g optical microscopy and differential scanning calorimetry, while the structures of the mesophases have been studied using X-ray diffraction on both powder and aligned samples. This study has revealed novel pha se behaviour involving intercalated smectic phases stabilized by the m ixed mesogenic group interaction. We have identified the first example s of intercalated smectic C, smectic I, crystal B and crystal J phases and have assigned the layer stacking sequences in the crystal B phase s. An extension to the current nomenclature used to describe smectic p hases is proposed in order to include these new structural modificatio ns such that, for example, S(Ac), refers to the intercalated smectic A 'phase. The specific interaction between the two unlike groups has a much smaller influence on the nematic-isotropic transition temperature s than it has on the structures of the smectic phases. Quantitative es timates of the nematic-isotropic transition temperatures based on the Marcelja-Luckhurst theory provide further evidence for a specific inte raction between the unlike mesogenic groups. Finally, the thermal beha viour of binary mixtures of the analogous symmetric dimers is also fou nd to be governed by the mixed core interaction and intercalated smect ic phases are observed in these mixtures.