Odd-even effects of the polar anchoring strength for nematic liquid crystal on polyimide Langmuir-Blodgett surfaces with alkyl chain lengths

The odd-even effect of the alkyl chain length of rubbed polyimide (PI) Langmuir-Blodgett (LB) surfaces on the extrapolation length of 5CB has been successfully evaluated by measuring polar anchoring strength. The extrapolation length of 5CB for rubbed PI-LB surfaces with even numbers is small compared with odd numbers for alkyl chain lengths of greater than 7 carbons. The extrapolation length of 5CB on rubbed PI-LB surfaces with odd numbers increases gradually as the temperature increases but tends to diverge near the clearing temperature (T_c =35.3°C). The extrapolation length diverges because of rapidly decreasing surface order near Tc. It is suggested that the polar anchoring strength on rubbed PI-LB surfaces with an even number is strong because of relatively high surface ordering caused by more crystalline surfaces. Finally, it is concluded that the odd-even effects of the polar anchoring strength in nematic liquid crystals (NLCs) are strongly related to the character of the polymer and observed-clearly for long alkyl chain lengths