Evaluation of Out-of-Plane Anchoring Strength of Nematic Liquid Crystal at Liquid Crystal-Isotropic Polymer Surface.

A (polymer/liquid crystal) composite film in 3-dimensionally bicontinuous phase-separated structure is expected to become available as a novel liquid crystal display material. The decay response behavior of the composite film is regulated by the magnitude of the polar anchoring strength for orientation of nematic liquid crystal molecules. When a nematic liquid crystal is sandwiched between isotropic polymer surfaces with a certain pretilt angle to the substrate layer, the Schlieren texture and singular surface disclination (Bloch wall) are observed under a polarizing optical microscope. The magnitude of the polar anchoring strength, A_polar was evaluated by measuring the width of the Bloch wall, since the width of this Bloch wall is governed by an equilibrium between the magnitude of A_polar and the elastic energy for the molecular orientation. This evaluation method of the magnitude of A_polar is designated as the “surface disclination method”. Three kinds poly (alkyl fumarate)s with different side chain lengths were used as the polymer substrates. The magnitude of A_polar decreased with an increse of side chain length, in other words, with an increase of surface thermal molecular motion. This result suggests that the evalution method had the advantage that one can evaluate the magnitude of the polar anchoring strength of a (isotropic polymer/nematic liquid crystal) interface without any external field, such as an eletric field or magnetic field.