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 aci d (ANBC-15) was investigated by differential scanning calorimetry, polarizi ng optical microscopy, and dynamic viscoelastic measurements. The phase seq uence 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 heat ing 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 'soli d-like' from the viscoelastic point of view, i.e. storage modulus (G')> los s modulus (G ") at 62.8 rad sec(-1) while the 'liquid-like' SmC phase is tr ansformed directly into the SmA phase without showing the D phase on heatin g. The isothermal frequency scans at a temperature in the D phase showed th e existence of a cross-over point, G' = G ", with G' < G " in the lower fre quency side, suggesting a structural fluctuation of the D phase. These resu lts are ascribed to the molecular structure of ANBC-15, which has a critica l alkoxy chain length in ANBC series.