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 $\rm 2.6 \,\mu m$. The first curve ($\rm 7.2 \,\mu m$) is perfectly superimposable to the result of conoscopic measurements done with a $\rm 100\,\mu m$ 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 $\rm \mu m$ 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.