Relationship between tribological performance of liquid crystals and their molecular structure

The tribological behaviour of nematic and smectic liquid crystals was observed using a two-roller friction tester under the conditions of 735 N load and 220 rpm rotational speed. Each liquid crystal has two typical structures: a flexible structure (alkyl chain) and a rigid structure (cyanophenyl group). Structural features of the liquid crystals were (1) alkyl cyanobiphenyl (CB), (2) alkoxyl cyanobiphenyl (ECB) and (3) alkyl cyanophenyl cyclohexane (CPC). The frictional coefficient of the test samples was lower and the estimated film thickness was larger than those of synthetic oils. The friction coefficient was not affected by the carbon number of the alkyl chain, but was closely dependent on the molecular structure of the rigid part of the liquid crystal module. The frictional coefficient of CB which has a biphenyl group was 0.035, but CPC had a higher friction coefficient of 0.045. The molecular orientation of the rigid group was observed under shearing conditions. It is concluded that the flat structure of the biphenyl group plays an important role on the tribological behaviour.