Surface Wetting Characteristics of Rubbed Polyimide Thin Films

Amide and imide based polymers are a catalogue of versatile materials that have a wide range of applications from scientific interests to commercial products because of their great thermal stability, excellent electric properties, highly mechanical strength, and superior chemical resistance (Sroog, 1976). In a thin film form, polyimides have been found to have many important uses in optoelectronic and photonic applications. One of the most successful examples in industrial applications is the use of polyimide thin films as molecular alignment layers in liquid crystal displays. By unidirectionally rubbing a thin layer of polyimide coated on a substrate a template with some form of anisotropy can be created. When a liquid crystal is put in contact with the rubbed polyimide film, the interactions between the surface and the liquid crystal molecules degenerate into actions with orientational features. As a result, liquid crystal molecules are driven to orient in a desired direction. Because of its outstanding ability and reliability in molecular alignment, the easiness in processing and cost effective, rubbing polyimide becomes the standard liquid crystal alignment technique, and rubbed polyimide thin films as efficient alignment layers are, up to date, still irreplaceable components in modern LCDs. On the other hand, a surface process will cause changes in chemicophysical and physcochemical properties at outmost surface of a polymer, and these changes, in turn, will induce many interesting surface phenomena, and impose a number of interesting aspects for scientific research and may lead to engineering applications. In many circumstances, a comprehensive knowledge of surface properties of polyimide thin films is of prime importance for elucidating mechanisms behind surface phenomena. A number of experimental results have shown that rubbing causes polymer chains to become oriented unidirectionally along the rubbing direction (Sawa et al., 1994; Sakamot et al., 1994; Hirosawa, 1996; Arafune, 1997), and the anisotropy in the distribution of the polymer chains is considered to be the main factor responsible for liquid crystal alignment. Wetting characteristics of a polymer surface are remarkably sensitive to chemical compositions and morphology of the outmost surface, and can provide a wide range of information on physical properties of the surface. The changes in surface characteristics of polyimide thin films due to mechanical rubbing must be reflected by surface wettability of the polymer 10