**4. Conclusions**

**Figure 13.** Thickness of PI-8 alignment layer

286 Optoelectronics - Advanced Materials and Devices

treatments [25], as illustrated in Figure 14.

**Figure 14.** Schematic diagram of LC alignment on PI surfaces

The cured PI-8 film was then rubbed with a nylon cloth-packed rubbing roller using the fol‐ lowing parameters: radius of rubbing roller: 50 mm; rotation speed: 1000 rpm; pile impres‐ sion: 0.32 mm and stage speed: 30 mm/s. Then, the FFS-LCD cells were assembled by two individual rubbed ITO glasses and the rubbing direction of the two glasses was anti-parallel to each other. For the pretilt angle measurements, the cell thickness was controlled to be about 50 μm by spraying spacers or glass fibers (50 μm) on PI-8 film surface; whereas for VHR and RDC measurements, the thickness was about 6 μm. The fabricated LC cells were sealed with a UV-curable epoxy sealant with two small filling holes left. LC molecules were then filled into the filling holes between the ITO plates via capillary action. The rod like LC molecules are thought to be oriented along the long alkyl chains pre-aligned by the rubbing

Table 5 tabulates the electrical properties of the fabricated LC cells. The LC cells exhibited an average pretilt angle of 2.7°, which could meet the demands of the FFS-LCD applications. Meanwhile, a VHR of 98.2% and a RDC of 230 mV were also obtained. This result indicated that PI-8 alignment layer could endow the LC cells with a high VHR level; however, a few As one of our continuous endeavors developing low-cost and high performance PI align‐ ment layers for TFT-LCDs, a systematic experiment was performed from monomer design and synthesis, PI preparation and characterization to final devices fabrication. First, several novel semi-alicyclic dianhydrides containing substituted-tetralin moiety were synthesized via a low-cost route with good yields. Then, a series of PIs were prepared from the dianhy‐ drides and aromatic diamines. The asymmetrical alicyclic structures in the dianhydride units destroyed the regularity of the PI molecule chains; thus enhances their solubility in common solvents. Meanwhile, this irregular packing of the PI chains is beneficial for the penetration of UV and visible light. Thus, a good optical transparency is achieved in the present PIs. Incorporation of the bulky alicyclic tetralin moiety in the dianhydride units did not deteriorate their thermal stability. More importantly, the current PIs showed good orien‐ tating ability to LC molecules. The LC cells with PI-8 as the alignment layer exhibited good optoelectrical properties.

Thus, the present semi-alicyclic PIs are considered to be promising alignment layers for TFT-LCDs. Research on the applications of the PIs in large area TFT-LCD monitors are in progress now.
