Viscoelastic behaviour of a disk-like thermotropic nematic phase (ND) versus temperature

We have investigated 'hexa-n-alcanoyloxytruxene' HATXC(9)H(19) which exhibits a disk-like thermotropic nematic phase between 65 and 84 degrees C. Using a light-beating technique we have measured the orientational diffusivities D-twist and D-bend for several temperatures, which correspond respectively to twist and bend deformations. D-twist increases monotonically as temperature increases, while D-bend remains constant. This result means that the curvature elastic constant K-22 changes more rapidly than the viscosity eta(twist), while K-33 and eta(bend) change in the same ratio. This means also that in the discotic thermotropic nematic phase, when temperature increases the effective hydrodynamic object, in orientational fluctuation, changes shape to be disk-like rather than rod-like.