Glancing-Angle Neutron Diffraction from Smectic C Liquid-Crystal Surfaces

Glancing-angle neutron diffraction has been used to study the variation in smectic layer tilt in high- and low-pretilt smectic C cells with distance from the surface. Evidence for varying tilt regions within high-pretilt smectic C cells exists; however, complete analysis is not possible without the appropriate depth-resolved information. Cells containing a liquid crystal and either rubbed polyimide or 5° obliquely evaporated silicon monoxide alignment layers were studied. A partially deuterated liquid crystal was used so that total external reflection could be achieved. Using the D17 diffractometer at the ILL, measurements were made in and around the regime of total external reflection. The tilt angle of the smectic layers was determined from the azimuthal angle of their Bragg reflection on the area detector. Below the critical edge, the penetration depth of the transmitted evanescent wave could be limited to ~ 300 Å, thus ensuring surface specificity. Varying penetration depths were achieved by alteration of the angle of incidence. Analysis of the Bragg reflection has shown the surface of the SiO cell to have a layer tilt of about 3° higher than that of the bulk. Negligible difference between surface and bulk chevron arm tilts was encountered for the polyimide cell. In both cases, the degree of surface specificity and the accuracy of the layer tilts obtained is discussed.