Phase Diagram and Dynamics of the Liquid Crystal Isopentylcyanobiphenyl (5CB)

From measurements of the specific volume as a function of temperature and pressure, the phase diagram for the liquid crystal forming isopentylcyanobiphenyl (5*CB) was determined. There are a number of phases (isotropic liquid, glass, cholesteric, and crystalline), and we show that the phase boundaries differ from previous reports, reflecting the slow crystallization kinetics of the system. Using dielectric spectroscopy at ambient and elevated pressure, we identify the relaxation processes in the isotropic and cholesteric phases. From application of density scaling to the dynamics, we obtain scaling exponents, which were found to vary with the type of motion. Moreover, unlike previous results for many other liquid crystals, the scaling exponent for the isotropic state differs from the thermodynamic potential parameter, and hence the relaxation time along the clearing line varies significantly with pressure. The distinctive properties of 5*CB reflect the very different volume dependences of the thermodynamics and the dynamics.