NOVEL MODULATED HEXATIC PHASES IN SYMMETRICAL LIQUID-CRYSTAL DIMERS

Homologues of the dimeric lpha,omega-bis(4-n-alkylanilinebenzylidene-4 '-oxy) alkanes (m.OnO.m) have been synthesised with spacer lengths n r anging from 9 to 12 methylene units and with terminal alkyl chain leng ths m of 10, 12 and 14. Characterisation of these materials has been c arried out by X-ray diffraction, differential scanning calorimetry and optical microscopy. In six of these compounds a novel modulated tilte d hexatic phase, denoted S-1, has been identified, in which the smecti c layers have a periodic modulation in their structure, analogous to t he S-(C over tilde) ribbon phase. This modulated hexatic phase is only observed for dimers with an odd parity spacer, suggesting that the av erage molecular shape, believed to be bent for the odd spacer dimers b ut linear for the even ones, is responsible for its formation. The com pound 10.O9O.10 exhibits a second modulated hexatic phase denoted S-2, which is monotropic, and which cannot be distinguished from the S-1 p hase by polarizing microscopy or by calorimetry. These long chain dime ric liquid crystals have been found to have a strong tendency to form tilted hexatic phases; indeed two of the compounds studied here are th e first dimers to undergo a transition directly from a tilted hexatic phase to the isotropic liquid.