TEMPERATURE DEPENDENCE OF ACOUSTICAL RELAXATION TIMES INVOLVING THE VICINITY OF N-I PHASE TRANSITION POINT IN 5CB LIQUID CRYSTAL

Abstract

Acoustical properties of nematic liquid crystal 4-pentyl 4′-cyanobiphenyl (5CB) were investigated in temperature interval from 298 to 333 K involving the vicinity of nematic phase—isotropic phase transition temperature. Measurements of the velocity of sound propagation in liquid 5CB have been staged by pulse—phase method at frequencies 2.5; 5.0; 10; 15; 35; 50, and 500 MHz. Amplitude coefficient of sound attenuation α has been determined by pulse method in the frequency interval from 2.5 to 1800 MHz. Acoustic spectra of 5CB consist of two relaxation areas. The low-frequency region of acoustical relaxation in the vicinity of N-I phase transition is connected with order parameter fluctuations. Relaxation time of low-frequency region tends to infinity according to low τ - |t|−1/2. The high-frequency region cause by processes of hydrogen bonds rupture and formation between molecules of 5CB. Main contribution to sound attenuation in 5CB being made by volume effects. It was shown that, within limits of experimental error, relaxation times of shear and bulk viscosity are indistinguishable.