Proton magnetic relaxation study of pretransitional phenomena in the isotropic phase of a nematic liquid crystal II. Presence of inner magnetic field gradients as revealed by self-diffusion study

Abstract

Above the clearing point of a nematic LC self-diffusion was studied by the Hahn pulse sequence (90-t-180) in the presence of a weak (up to 4 Gs cm-1) permanent external magnetic field gradient G = ∂H/∂z. The non-linearity of spin-echo decays was interpreted as a result of the inner magnetic field gradient Gamma, indicating the magnetically non-homogeneous nature of the pretransitional zone of the LC. The pretransitional zone was considered to be biphasic (locally ordered clusters ⇔ isotropic surrounding). Due to the orientation of clusters in the magnetic field and the anisotropy of the diamagnetic susceptibility of their molecules, the diamagnetic susceptibility of clusters in the z direction should be different from that of their disordered surroundings. Therefore, clusters behave as specific filled particles, their diamagnetic response in the presence of G being different from that of their isotropic surroundings, i.e. clusters can experience a translational motion in the external field gradient. This leads to the peculiar diamagnetic separation in space, accompanied by an increase of the field gradient. The inner field gradient was shown to be proportional to G2t3 and to decrease with temperature.