TRANSITION FROM EPITAXIAL TO NONEPITAXIAL ORDERED MONOLAYERS IN PYROLYZED 8CB STUDIED BY STM

To design long-lived liquid crystal devices, it is important to unders tand how liquid crystal properties change as the material degrades. Th e liquid crystal 4'-octyl-4-cyanobiphenyl (8CB) is known to form well- ordered, epitaxial monolayers on graphite. We have found that adding t wo oxidation products (to mimic the effects of air oxidation or pyroly sis) changes the surface ordering of 8CB, causing it to form monolayer s on graphite which possess good short-range order but only moderate l ong-range order, indicating the existence of many nonepitaxial domains . Although the unit cell dimensions of domains within these monolayers are identical to the unit cell dimensions of one of the oxidation pro ducts, both pure oxidation products were found to form epitaxial monol ayers on graphite, as did all binary mixtures of these materials. Thus the loss of epitaxy seen in the 8CB mixtures is a consequence of inte ractions between all three components (8CB and two oxidation products) . The observed behavior of these 8CB mixtures is consistent with segre gation of two components at domain walls separating domains of the pur e third component.