Proton Spin Relaxation Study of Order Fluctuations Above the Nematic–Isotropic Transition in the Liquid Crystal MBBA: II. Coherent and Incoherent Scattering

Abstract

The proton spin–lattice relaxation time T₁ was measured at 18 different Larmor frequencies in the isotropic phase of MBBA at a fixed temperature of 51 °C. The refined theory of Ghosh in the framework of the deGennes theory is used to give a consistent picture for interpreting data over the frequency range of 4 to 60 MHz. At both low and high Larmor frequencies, the critical order fluctuations in the isotropic phase of MBBA arise mainly from a coherent scattering term which is observed in light scattering experiments. At a frequency of around 10 MHz, the critical order fluctuations come from both coherent and incoherent scattering. The smaller incoherent scattering can be studied by NMR under suitable experimental conditions. The correlation times for the fluctuations in coherent and incoherent scattering are different. The present study also gives the spin relaxation rate due to molecular diffusion as a function of frequency at 51 °C. An alternative to the Harmon–Muller frequency dependence of (T₁)_diff. is suggested for MBBA.