Influence of order in thin smectic polymer films on the structure at the surface

Abstract

Vertical and lateral structures of thin films (6–200 nm thick) of a combined liquid crystalline polymer with mesogenic groups in the main and side chains were investigated by X-ray reflectometry and scanning tunneling microscopy. The liquid crystalline polymer exhibits a cholesteric, a smectic C * and a smectic A mesophase. Films prepared by spin-coating onto glass substrates were investigated as a function of temperature in the different mesophases. The main interest was focussed on the smectic C * phase, in which the polymer chains show different orientations depending on the film thickness. In films thicker than about 10 nm, chains are oriented perpendicular to the substrate due to interface effects. Film thickness constraints in thinner films force chains to lie predominatly flat on the substrate and films partially show dewetting. X-ray reflection was used for the determination of structural parameters like layer thickness, chain orientation, phase sequence, perturbations in the layered structure and laterally averaged surface roughnesses, which partially depend on film thickness. Direct information about lateral surface structures on the nanometer length scale was obtained from scanning tunneling microscopy. In films thinner than about 10 nm after annealing holes of height of the initial film thickness are observed. In thicker films extended terraces are observed separated by steps of height of single layers. Defects in the layered structure are assumed to be responsible for the formation of such a surface structure. The surface profile could quantitatively be analyzed by a recent theoretical treatment of edge dislocation in thin liquid crystalline films.