Dipolar and chain-linking effects on the rheology of grafted chains in a nanopore under shear at different grafting densities.

ABSTRACT

Nonequilibrium molecular dynamics simulations are applied to investigate the rheological properties of coplanar nanopore systems of amphiphilic chain molecules with the tails grafted to the walls of the nanopore and with the head-group ends immersed in a solvent inside the nanopore. In particular, the effects of modifying the interaction between the amphiphilic head-groups by repulsive dipolar interactions or directly covalently linking pairs of chains at the head-groups have been studied. Different grafting densities are considered. The chains are modeled by a harmonic bead-spring model, and all particles interact through the repulsive part of a shifted Lennard-Jones potential. Head-group linking is also governed by a bead-spring potential. A harmonic potential models the lattice vibrations of the atomic boundaries. The rheological properties are studied by a shearing process in which the heat generated is conducted away from the system through the walls by applying a Nosé-Hoover thermostat. Computed geometric parameters such as average chain length and average tilt angle indicate reduction in chain flexibility at large dipole moments. Dipolar repulsion is found to broaden the density profiles of the solvent. This effect is opposed by chain linking. For increasing head-group repulsion, the amphiphile-solvent interfaces become less diffusive that leads to systematic variations in viscosities with increasing dipole moments. Friction forces become stronger at large grafting density and for larger dipole moments. The changes in rheological properties for fixed grafting density are essentially governed by the change in the response of the normal pressure to the applied shear field. The velocity gradients depend strongly on the degree of grafting density but appear to be less sensitive to the strength of the interactions between the head groups.