Thermodynamic scaling and the characteristic relaxation time at the phase transition of liquid crystals Available to Purchase

The longitudinal relaxation time $τ$ of a series of alkyl-isothiocyanato-biphenyls $(nBT)$ liquid crystals in the smectic E phase was measured as a function of temperature $T$ and pressure $P$ using dielectric spectroscopy. This relaxation time was found to become essentially constant, independent of $T$ and $P$⁠, at both the clearing point and the lower temperature crystalline transition. $τ(T,P)$ could also be superposed as a function of the product $TVγ$⁠, where $V$ is the specific volume and $γ$ is a material constant. It then follows from the invariance of the relaxation time at the transition that the exponent $γ$ superposing $τ(T,V)$ can be identified with the thermodynamic ratio $Γ=−∂log(Tc)∕∂log(Vc)$⁠, where the subscript $c$ denotes the value at the phase transition. Analysis of literature data on other liquid crystals shows that they likewise exhibit a constant $τ$ at their phase transitions. Thus, there is a surprising relationship between the thermodynamic conditions defining the stability limits of a liquid crystalline phase and the dynamic properties reflected in the magnitude of the longitudinal relaxation time.