An improved decoupled model for spin relaxation in 4-n-alkyloxy-4 '-cyanobiphenyls

An improved decoupled model has recently been proposed by us to account for the correlated internal rotations in flexible alkyl(oxy) chains of liquid crystals. In this model, the three-bond motions in the chain are disallowed , while both gauche migration and gauche pair production can now occur in t he chain. Preliminary results from the analysis of deuteron spin relaxation rates in the liquid crystal 4-n-hexyloxy-4'-cyanobiphenyl (6OCB) were enco uraging. We apply this motional model to interpret the existing relaxation data of 4-n-octyloxy-4'-cyanobiphenyl and 6OCB. Transition rates for the ch ain dynamics are obtained for both liquid crystals in conjunction with thei r overall rotational diffusion constants. The derived rotational diffusion constants are compared with those obtained from dielectric relaxation studi es. A consistent picture emerges in the present study for the two members o f a homologous series when the order director fluctuations are included in the nematic phases of these liquid crystals. (C) 2001 American Institute of Physics.