A quasielastic neutron scattering study of molecular reorientation in the nematic phases of PAP and POAB

We present the results of Q.N.S. studies for two members of the homologous series of alkoxyazoxybenzenes, C_nH_2n+1O-ϕ-N₂O-ϕ-OC_nH_2n+1; PAP (n = 2) and POAB (n = 3). The Q.N.S. measurements were performed on the non-deuteriated (d₀-PAP and d₀-POAB) and the chain deuteriated samples, d₁₀-PAP and d₁₄-POAB. Three models were fitted to the experimental data: (1) uniaxial rotational diffusion of the molecule around the axis with the smallest moment of inertia, (2) uniaxial rotational diffusion of the two moieties of the molecule around the N-ϕ bonds, (3) 180° instantaneous jumps of the two moieties of the molecule around N-ϕ bonds. We have assumed the molecule to exist in the trans conformation. The translational diffusion of the molecules and the methyl groups' reorientation were neglected. It turned out that model (3) does not describe the experimental data well. Models (1) and (2) describe the experimental data equally well, giving no preference for the axis of rotation. However, comparison of our results with those obtained from dielectric relaxation suggests the choice of model (2) as responsible for the Q.N.S. data. The correlation times determined by fitting to both rotational diffusion models are of the order of several picoseconds. However, the correlation times determined for d₁₀-PAP and d₁₄-POAB are two or three times longer than for d₀-PAP and d₀-POAB, respectively, which indicates the existence of additional motion of the end chains.