Isotropic cubic phase of 4′-n-pentadecyloxy-3′-nitrobiphenyl-4-carboxylic acid (ANBC-15):: DSC, microscopic and dynamic viscoelastic studies

The phase behaviour of 4'-n-pentadecyloxy-3'-nitrobiphenyl-4-carboxylic acid (ANBC-15) was investigated by differential scanning calorimetry, polarizing optical microscopy, and dynamic viscoelastic measurements. The phase sequence for the virgin sample of ANBC-15 is crystal-smectic C (SmC)-cubic (D)-smectic A (SmA)-'structured liquid' (I-1)-isotropic liquid (I-2) on first heating. It was found that the appearance of the D phase on the second heating depends on the top temperature of the first heating. It was also shown that the D phase is formed on heating when the preceding SmC phase is 'solid-like' from the viscoelastic point of view, i.e. storage modulus (G')> loss modulus (G ") at 62.8 rad sec(-1) while the 'liquid-like' SmC phase is transformed directly into the SmA phase without showing the D phase on heating. The isothermal frequency scans at a temperature in the D phase showed the existence of a cross-over point, G' = G ", with G' < G " in the lower frequency side, suggesting a structural fluctuation of the D phase. These results are ascribed to the molecular structure of ANBC-15, which has a critical alkoxy chain length in ANBC series.