A study of relationship between the pretilt angle and the polar anchoring strength in nematic liquid crystal on rubbed polyimide surfaces

We have investigated the effects of high pretilt angle on the polar (out-of-plane tilt) anchoring energy of the nematic liquid crystal (NLC), 4-cyano-4-n-penthylgiphenyl (5CB), on rubbed polyimide (PI) surfaces containing trifluoromethyl moieties. The pretilt angle of 5CB rapidly increases up to about 45 degrees with the rubbing strength, RS. Also, the polar anchoring energy of 5CB initially increases with the RS for weak rubbing and then decreases. From these results, we determined the relationship between the pretilt angle and the anchoring energy on rubbed PI surfaces. We suggest that the polar anchoring energy of 5CB with high pretilt angle is very small because of the combination of the micro-surface excluded volume effect and the van der Waals interaction between the LC molecules and the substrate surface on weakly rubbed PI surfaces containing trifluoromethyl moieties.