Proton Spin Relaxation in the Liquid Crystal PAA

The proton spin–lattice relaxation time of the liquid crystal para-azoxyanisole (PAA) was studied at 30 MHz in the temperature range 125–445°K. The rotating frame experiment was performed at 15 and 30 MHz in a rotating field of 6 G over the temperature interval 125–425°K, and the rotating field strength dependence of T1ρ was studied in the range 1.6–16 G at three different temperatures. These observations show that in the nematic phase three processes are contributing to the spin–lattice relaxation: a slow cooperative mode, ωs ∼ 10 sec−1, the translational diffusion, and the order parameter fluctuation as proposed by Pincus. In the isotropic phase the order parameter fluctuations contribute much less because of the small size of the order clusters. The frequency and temperature dependence of various mechanisms are discussed and it is shown that the time autocorrelation function of the collective order fluctuations decays nonexponentially in the nematic phase.