The study of orientational relaxation of rodlike molecules in the presence of an adsorbing surface by the molecular dynamics method

Abstract

Coefficients of translational and rotational self-diffusion of rigid-chain rodlike molecules formed from four spherical particles are determined by the molecular dynamics method. Simulations are performed for a three-dimensional canonical ensemble of 4096 Lennard-Jones particles within the range of chain concentration in its monomer varying from 2 to 100 mol % with allowance for the adsorption of chains on two parallel walls confining the system. Changes in the concentration profiles of chains and solvent particles over the normal to walls during variations in adsorption energy are considered. It is shown that the dependences of translational and rotational self-diffusion coefficients on the concentration and adsorption energy govern the changes in the characteristic times of the orientation-disorientation processes of molecules. All specific features of the establishment of orientational order and its relaxation are determined mainly by the degree of coverage of the adsorption monolayer. The contributions of the second and third monolayers to the weighted-mean mobility of chains begin to be pronounced with an increase in concentration. The exchange of chains between the adsorption monolayer and bulk solution is suppressed with an increase in the adsorption energy, and the monolayer is transformed into a set of two-dimensional “crystallites.” These crystallites form a typical domain structure on the adsorbing surface. The orientation and, hence, the ordering of domains by the external field occur a little more slowly than the orientation of molecules in solution. The disorientation requiring asynchronous rotations of chains is impeded, thus resulting in noticeable retardation of this process relative to the orientation, and upon achievement of a certain value of adsorption energy, the orientation of chains induced in the first adsorption monolayer becomes stable.