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

Abstract

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 quantities at the NI transition remain nearly insensitive to pressure. At atmospheric pressure, the magnitude of δH _NI amounts to about 10% of δH _CrN for the given samples. The transition enthalpies and entropies were also estimated from the Clapeyron relation, the volume changes required in this expression being taken from the PVT measurements previously reported. Somewhat larger values were obtained for both δH _tr and δS _tr in the latter estimation. The odd-even character with the spacer length n was clearly observed in the aforementioned thermodynamic quantities over the entire pressure range examined.