Pressure-temperature phase diagrams of smectogenic 4′-alkyl-4-cyanobiphenyls (9CB, 10CB, 11CB and 12CB)

Abstract

The pressure-temperature (p - T) phase diagrams for four smectogenic members of the 4′-alkyl-4-cyanobiphenyl homologous series (nCB, n = 9, 10, 11 and 12) over the temperature range 320–410 K and pressure range 0.1–300 MPa (3 kbar) were constructed using DTA. At 1 atm 9CB exhibits nematic and smectic A_d phases, while the other members show only the smectic A_d phase. However, at elevated pressures the clearing line splits in the case of 10CB and 11CB which indicates the induction of a nematic phase. It was found that the triple point, where the isotropic, nematic and smectic phases coexist, is strongly shifted to higher pressures with increasing chain length. This was interpreted as being caused by a loss of the rod-like shape of the molecules containing longer alkyl tails which explore a range of conformations. The slope of the clearing line, dT /dp, depends strongly on the length of the alkyl chain for the nCB series, but does not show a step-wise change between the nematogenic and smectogenic members.