The structure and dynamics of the smectic B phase

Using X-ray diffraction we show that the unit cell in the smectic B phases of BPBAC and IBPBAC is tilted with respect to the layers by 5.6° and that there is considerable molecular disorder in a direction perpendicular to the layers. Incoherent, quasi elastic neutron scattering has shown that the molecules undergo a uniaxial rotation with diffusion constant Dr/1010 s–1 ~ 3 to 6 depending on temperature. On a similar time scale (10–11 s) there is also a molecular motion perpendicular to the layers which is much more restricted than translational diffusion. Its temperature dependent r. m. s. amplitude is 1 to 2 Å and it can also be described by the model of diffusion (Dt ~ 3 × 10 –8 cm2 s–1) in a cosine potential of maximum height about 28 kj mole–1.