Study of dipole dynamics and pre-transitional effects at isotropic to nematic phase transition by low-frequency dielectric relaxation measurements

Thermal microscopy (TM) as well as low-frequency (LF) dielectric relaxation studies are carried out on a nematic liquid crystal (NLC), viz E7 (Merck Ltd, UK). The isotropic to nematic (IN) transition temperature TIN determined by TM and LF-dielectric permittivity measurements agrees with the available data. Dielectric loss studies in the frequency region of 5-10 MHz indicate a relaxation (in the kHz region) akin to Debye type off-centered dispersion. The observed nematic relaxation is found to correspond to reorientation (about the short axis) of the nematic dipole to the external field. The temperature variation of the nematic relaxation frequency f_R is found to follow an Arrhenius shift, with an activation energy of 1.7eV. Temperature variation of the dielectric strength (Δε = ε₀ -ε_∞) and the distribution parameter α in the nematic phase are discussed. The dynamic response of the nematic dipoles and growth of pre-transitional fluctuations are found to be nonlinear in the vicinity of the IN transition. The value of the exponent α_eff= 0.072 indicates weak growth of transitional fluctuations across the IN transition.