Relations between the structure, symmetry and the energy mechanism of the polar-organic molecule ultra-films during the tribology

Abstract

The molecular dynamics simulation has been used to study the tribological properties of the seven kinds mixed Langmuir-Blodgett (LB) monolayers, which composed of the fatty acids CnH2n+1COOH and C17H31COOH (n=12,13,14,15,16,17) by the ratio of 1:1, and the monolayers compoesd by C16H33COOH. The results showed that: (1) the shear pressure of mixed LB monolayers decreased as the chain-length of molecules Cn increased. The friction force main come from the coulombic interaction between the monolayers. (2) the arrangement of the two kinds of tails of the molocules influence the friction character. The shear pressure is smaller when all the tails are isotactic-arrangement than anisotactic-arrangemnet, espacilly when the differrence of the chain-length is one C-C bondlength. (3) The shear pressure decreseased as the chainlength decreased, and the friction froce main come from the coulombic interaction, when tails of the up and down isotactic monolayers which are composed of one kind molecule are mirror symmetry, or the friction froce main come from the van der wasls (VDW) interaction when tails are central symmtry.