Dichroic ultraviolet absorption of thin liquid crystal films

Cyanobiphenyl liquid crystals absorb strongly in the ultraviolet. These absorption bands are highly dichroic. The dichroic ratio changes as a function of order parameter and orientation of the liquid crystals. Plots of the dichroic ratio as a function of film thickness demonstrate that the polymer alignment layer not only imposes an orientation on the liquid crystal director but also lowers the order parameter of the liquid crystal near the polymer surface. A sharp decrease in dichroic ratio occurs at film thicknesses near 35 nm, suggesting a phase transition which may be caused by the appearance of a biaxial phase or smectic layering at the air interface.