CORRELATION BETWEEN SURFACE AND BULK ORIENTATIONS OF LIQUID-CRYSTALS ON RUBBED POLYMER SURFACE - ODD-EVEN EFFECTS OF POLYMER SPACER UNITS

Using surface optical second-harmonic generation, we have determined the molecular tilt angle and the in-plane orientational order of the liquid-crystal molecules 4'-n-octyl-4-cyanobiphenyl (8CB) on a series of polyimide surfaces. The polyimides differ in the length of the flexible spacers in between neighboring aromatic cores. Using birefringence measurements, we obtained the bulk tilt angle of liquid-crystal cells having the same series of surfaces. The surface molecular tilt angle, the in-plane orientational ordering near the surface, and the bulk tilt angle all exhibit an odd-even effect related to the odd or even number of spacer units in the polyimides. The results are explained by the fact that polyimide surfaces with even numbers of spacer units are smoother and capable of inducing higher surface ordering in the 8CB surface layer. The orientational order at the surface is shown to be intimately correlated with the bulk tilt angle.