Characterization and alignment properties on polyimide surface using polymer films for flexible liquid crystal displays

We have investigated the generation of pretilt angle for nematic liquid crystal (NLC) alignment by the rubbing alignment method in two types of polyimide (PI) surfaces using polymer substrates. The NLC pretilt angle generated by the rubbing alignment method on a polymer substrate is about 3 deg, and it tends to decrease as rubbing strength increases. The pretilt angle measured is about 1.7 deg smaller on the glass substrate than on the polymer substrate. We obtain an atomic force microscope (AFM) image of the rubbed PI surface with a polymer film that has formed the micro-groove structure at a low curing temperature (120°C). However, no grooves were obtained on the glass substrate at the same temperature. It is considered that this alignment may be attributed to the roughness of the micro-groove substrate. Therefore, LC alignment is affected by the type of topographical structure. The pretilt angle was larger on the rubbed soluble-type PI surface with a higher imidization rate using the polymer film; the pretilt angle on the polyimide surface is attributed to high imidization rate of the PI. Consequently, flexible LCDs required at the low-temperature processing should use the soluble-type PI as the alignment layer. Also, good eletrooptical (EO) curves were observed for the TN-LCD using the polymer substrate at a low curing temperature.