纳米摩擦中极性有机分子超薄膜的结构、对称性及能量机理

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

采用分子动力学方法,模拟了由脂肪酸CnH2n+1COOH和C17H35COOH(n=12,13,14,15,16,17)按1:1比例组成的7种混合单层Langmuir-Blodgett(LB)膜和由C16H33COOH分子组成的单层膜的摩擦性质.结果显示:1)随着混合单层膜内的不同分子链长差的减小,其剪切压随之减小,摩擦力主要来自单层膜间的库伦作用;2)混合膜内的两种不同分子的尾基排列对其摩擦性能影响较大,当混合LB膜中所有分子尾基全同排列时剪切压较小.当分子链长差为1个C-C键长时,分子尾基排列对膜的摩擦性质影响较大.3)同种分子尾基全同排列组成的单层膜,当上下两单层膜的尾基呈镜面对称时,其剪切压随着分子链长的增加而减小,摩擦力主要来自膜间的库伦作用;当上下两单层膜的尾基呈中心对称时,膜间摩擦力主要来自膜间的范德华(VDW)作用.

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.