Studies on the alignment of nematic liquid crystals by polyimide layers

The liquid crystallinity of polyimide layers was thought to enhance the alignment of nematic liquid crystals (NLC) in the liquid crystal display (LCD). The asymmetric units introduced from trimellitic anhydride into the polyimide structure led to smectic or nematic phases of the resulting poly(ester imide)s (PESI). Dicarboxyimides, dicarboxyimides and dicarboxyimide chlorides were prepared by reacting trimellitic or selected tetracarboxylic acid anhydrides with omega-amino acids (Tables 1 and 2) by reacting (i) the synthesized carboxyimides with selected diols (Table 3), (ii) carboxyimide chlorides with diols (Scheme 2), (iii) transesterifying carboxyimides with hydroquinone diacetate to prepare about 30 polymers of varying chemical structures. The layer-forming ability of a 0.5% poly(ester imide) solution in DMF was evaluated in terms of specific viscosity (Ubbelohde instrument, 25 degrees C), eta = 0.1-0.3; M was rather low; good films were obtained, 5-10 mu m thick. TGA (Derivatograph MOM, Hungary) at 5 degrees C/min over 0-1000 degrees C, showed the length of the aliphatic chain between the carbonyl and the imide groups to affect the thermal resistance only slightly. Boetius hot-stage (polarized light) disclosed the mesophase range over 60-160 degrees C in eight polymers. DSC (Setaram DSC92, 5 degrees C/min, air) was used to establish the thermal and liquid-crystalline properties of the polyimides. The interference wedge method (Fig. 1) and the model nematic 6-CHBT, used to study the alignment depth (Table 4), showed the polyimides to align NCL quite well (Fig. 3), the alignment being the deeper the more mesogenic are the polymers used as the orienting films.