Thermodynamic scaling of the viscosity and the longitudinal relaxation time for three nematic n-alkyl-cyclohexyl-cyanophenyls

Abstract

Viscosity η ₂ and longitudinal relaxation time τ_|| of the nematogenic members of the n-alkyl-cyclohexyl-cyanophenyl (nPCH) series, studied under elevated pressure some time ago, are re-analysed using the scaling law TV^γ proposed originally for structural relaxation times in glass-forming liquids. The data obtained at isobaric, isothermal and isochoric thermodynamic conditions could be scaled to master curves with one adjustable parameter γ characterising a steepness of the intermolecular potential. These parameters, γ_η, γ_τ and Γ (obtained from the respective analyses of the T–V dependences along the clearing line) are compared and discussed. In spite of the fact that the two examined dynamical quantities, viscosity and relaxation time, characterise different aspects of the dynamic behaviour of molecules within the nematic phase, a good consistency of the potential parameters was achieved, especially when reduced quantities were analysed.

Keywords:

• nematic
• nPCH
• viscosity
• relaxation time
• thermodynamic scaling

Acknowledgement

The author thanks C.M. Roland for useful suggestions.