Odd-even effects of the surface anchoring strength in NLC on polyimide langmuir-blodgett surfaces with alkyl chain lengths

The liquid-crystal (LC) alignments on the pretilt angle, the induced optical retardation, the anchoring strength, and the surface ordering in 4-n-penthy-4-cyanobiphenyl (5CB) on a rubbed polyimide (PI) Langmuir-Blodgett (LB) surface with alkyl chain lengths have been investigated. Large pretilt angles are observed for 5CB on a rubbed PI-LB surface with alkly chain lengths of 7, 8, and 11 carbons. Also, the induced optical retardation on the PI-LB surface is increased by the rubbing. For alkyl chain lengths of more than 7 carbons, the extrapolation length, d _e, of 5CB on rubbed PI-LB surfaces with an even numbers of carbons is smaller than that of 5CB on rubbed PI-LB surfaces with odd numbers of carbons. The extrapolation length of 5CB with an odd numbers of carbons increases gradually as the temperature increases, but it tends to diverge near the clearing temperature T _c. The extrapolation length diverges because of the rapidly decreasing surface order near the clearing temperature. We suggest that the polar anchoring strength with an even number is strong because of the relatively high surface ordering caused by more surfaces which are crystalline. The anchoring energy of 5CB with an alkyl chain-length of 7 is approximately 1×10 ^-3 (J/m ²), indicating a relatively strong anchoring strength.