Liquid-crystal-anchoring transitions at surfaces created by polymerization-induced phase separation

Abstract

A surface anchoring transition of a nematic at polymer surfaces created by polymerization-induced phase separation is presented. This transition is unusual in that it occurs far from bulk nematic phase transitions and it is tunable across nearly the entire nematic temperature range by modification of the polymer side group. Anchoring behavior is qualitatively understood by considerating enthalpic and entropic contributions to surface energy. Interesting behavior of some polymer-dispersed liquid-crystal films is explained, and observations provide a pathway to control properties. @S1063-651X~98!50308-X# PACS number~s!: 64.70.Md, 64.75.1g Surface energetics of liquid crystals is intriguing and crucial for technologies that use liquid crystals, such as flatpanel displays and optical components. The fact that liquid crystals exhibit preferential orientation at interfaces, a phenomenon called surface anchoring, has been known for over 80 years, but its molecular origins are still elusive. Whether a liquid-crystal material prefers to orient with the director n normal to the interface ~homeotropic anchoring!, in the plane of the interface ~homogeneous anchoring!, or along a tilted direction or directions depends upon subtle interactions that are not well understood. Unidirectional homogeneous alignment can be produced by rubbing a substrate @20# and homeotropic alignment by treating the bounding surfaces with amphiphilic alkyl compounds such as surfactants @1‐4#. The less common tilted anchoring has been seen at a glass surface exposed to smoke soot @19# or coated with a commercial fluorinated surfactant @5#. Transitions between surface anchoring states are particularly exciting because they arise from a competition between opposing components of surface anchoring. A number of nematic surface anchoring transitions have been observed. Most of them occur close ~within ;10 K! to bulk phase transitions of the nematic to isotropic, smectic, or crystalline phases. Noting the sensitivity of several contributions to surface anchoring on the bulk nematic order parameter S, surface anchoring transitions near the nematic-isotropic transition have been attributed to the rapid decrease in S upon heating close to the nematic-isotropic transition @6#. Near the transition to smectic or crystalline phases, pretransitional layering near surfaces has been cited to explain transitions to homeotropic anchoring @7#. Surface anchoring transitions very far from bulk phase transitions are rare. Volovik and Lavrentovich @8# reported a transition at a surface composed of two fluid components known to induce opposing anchoring tendencies. This could perhaps arise because of a changing surface composition or because of different temperature dependencies of the anchoring strength of the two components. Recently, Drzaic @9# reported a surface anchoring transition far from bulk liquidcrystal transitions at poly~vinyl alcohol! surfaces modified by