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 mu m. The first curve (7.2 mu m) is perfectly superimposable to the result of conoscopic measurements done with a 100 mu m 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 mu m 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.