A MECHANISTIC PICTURE OF THE EFFECTS OF RUBBING ON POLYIMIDE SURFACESAND LIQUID-CRYSTAL PRETILT ANGLES

A mechanistic picture of the effects of rubbing on polyimide alignment layer (AL) surfaces and nematic liquid crystal (LC) pretilt angles (T heta(P)), leading to establishing a mechanism of determination of Thet a(P)'s, was proposed on the basis of the study of the change in surfac e polarity of a polyimide AL with the rubbing force and density, and t he subsequent changes in the Theta(P) and the azimuthal anchoring ener gy. In the regime of relatively weak rubbing when inhomogeneous, patch y surface modification (the microscopic reorientation of polymer funct ional groups and the orientation of polymer backbones) occurs, the sur face polarity, the anchoring energy, and the Theta(P) all increase mon otonically with rubbing strength. These increases correlate to an incr ease in the area fraction of the reoriented/oriented AL surface which generates in-plane orientation of the first LC monolayer at the surfac e and subsequently bulk alignment. Beyond this region in which the AL surface gets modified sufficiently to induce fully developed in-plane orientation of the LC monolayer, the surface chemistry and roughness h ave dominant effects on the Theta(P) and the anchoring energy. The inc rease in surface polarity and the generation of a larger reoriented su rface area by high-force rubbing. which enhances the attractive intera ction between the LC monolayer and the rubbed surface, contribute to i ncreases in both the anchoring energy and the thermal stability of The ta(P) and to a decrease of Theta(P). This decrease of Theta(P) with in creasing AL surface polarity was verified by studying the relationship between Theta(P) and surface polarity under constant rubbing conditio ns by manipulating the surface polarity via the degree of imidization of a poly(amic acid) AL. (C) 1998 American Institute of Physics. [S002 1-8979(98)(04701-X].