Biaxial nematic and smectic-A boundaries in thin planar samples of 8OCB aligned by rubbed polyimide

The birefringence of planar 8OCB samples has been measured as a function of temperature at thicknesses decreasing from 7.2 to 2.6 μm. The first curve (7.2 μm) is perfectly superimposable to the result of conoscopic measurements done with a 100 μm homeotropic uniaxial sample, from a minimum of 0.08 close to the isotropic phase up to 0.19 in the smectic-A phase. For thinner planar samples, one gets an increasing birefringence (up to 35% more in both phases with a new plateau at 0.26). This means that the bulk uniaxial phase is surrounded by biaxial boundaries (a few μm thick) where the extreme indices of refraction lie parallel to the plates. A mean-field model of long-range interaction is proposed that takes into account the observed features.