High pressure differential thermal analysis of dimer liquid crystals: α, ω-Bis [(4,4′-cyanobiphenylyl) oxy] alkanes

The phase behaviour of dimer liquid crystals (DLC), α, ω-bis[(4,4′-cyanobiphenylyl) oxy]alkanes (CBA-n with n = 9,10) has been studied by differential thermal analysis (DTA) over a pressure range from 0.1 to 150 MPa. Both samples exhibit crystal (Cr)↔nematic (N)↔isotropic (I) transitions under all experimental conditions. The slopes of the phase boundary curve (dp/dt)tr were determined from the P tr vs. T tr phase diagram, where the subscript tr designates CrN or NI. Both transition temperatures T CrN and T NI were found to increase almost linearly as a function of pressure; CBA-9: (dp/dt)CrN = 3.92, (dp/dt)NI = 2.03; CBA-10: (dp/dt)CrN = 3.66, (dp/dt)NI = 2.17, the units being MPaK−1. As a consequence, the nematic region defined by the interval between the CrN and NI transitions becomes broader as the applied pressure increases. While the transition enthalpies δS CrN and the associated entropies δS CrN at the CrN transition decrease substantially with increasing pressure, the corresponding quant...