Surface Orientation of Main and Side Chains of Polyimide Alignment Layer Studied by Near-Edge X-ray Absorption Fine Structure Spectroscopy

The average near-surface orientation of the main and side chains of polyimide (PI) for liquid crystal (LC) alignment has been studied by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. A detailed analysis of the angular dependences of resonance intensities revealed that rubbing causes the inclination of the main chain with respect to the surface, the angle of which decreases with increasing rubbing strength. This variation was found to agree with the decrease of the LC pretilt angle in the cell fabricated with this PI surface. The relation between the surface tilt of the main chain and the pretilt of LC is almost the same for PI films with side chains. Thus, it is concluded that the main chain alignment is strongly correlated with the pretilt angle even in the side-chain PI alignment layer.