A QUASI-ELASTIC NEUTRON-SCATTERING STUDY OF MOLECULAR-DYNAMICS IN A COLUMNAR LIQUID-CRYSTAL

The incoherent quasi-elastic neutron scattering from the heptyl and oc tyl derivatives of the acta-alkylphthalocyanines, CnPcH(2), in their c rystalline and liquid crystalline (discotic columnar) phases has been measured. Diffusive motion is observed on the 10(-10) to 10(-12) s tim escale. This is interpreted as alkyl chain motion. A. detailed compari son of the mesophase data for C7PcH(2) has been made with the predicti ons of the rotational isomeric state model, and an extra motion on thi s timescale has been invoked, which probably is a localized translatio nal motion. A small discontinuity in the elastic fraction of the quasi -elastic scattering is consistent with a change in the molecular tilt at the transition between the two D-hd phases.