Determination of the translational order parameter for smectic liquid crystals using small-angle neutron scattering

A simple method to determine the translational order parameter of a smectic liquid crystal is presented. It is based on a measurement of the absolute intensity of the first-order layer reflection from an unaligned (powder) sample using small-angle neutron scattering. The Lorentz factor and molecular form factor are used to derive the order parameter from the intensity. The method is demonstrated for 4-octyl-4'-cyanobiphenyl (8CB), using two different isotopomers which give a consistent translational order parameter value of 0.5 in the smectic A phase.