Monte Carlo simulations of the static friction between two grafted polymer brushes.

A configurational bias Monte Carlo method has been developed to study the static friction between grafted polymers immersed in a good solvent. Simple models using the soft quadratic potential from a dissipative particle dynamics study have been used to model polyzwitterionic brushes at physiological pressures (up to 7.5 MPa). Three models of decreasing rigidity have been used to model the friction between the brushes by calculating the tangential component of the pressure induced by a mismatch in the registry of the two grafting surfaces. The static friction coefficient can be calculated for three model systems and the slip between the layers occurs at a much lower values of shear force for the more flexible polymer layer. A moderate increase in the flexibility of the chains reduces the friction coefficient by a factor of ca. 20. Tilting the layer directors of the brushes also increases the static friction between the layer when the top, tilted layer is displaced in the direction away from the tilt. Non-equilibrium dynamics techniques for the same model were performed using dissipative particle dynamics and the limiting extremes of the Stribeck curve corresponding to the boundary lubrication regime and the hydrodynamic lubrication regime were observed for these flat surfaces. As expected, µk is significantly lower than µs for the same system. The dynamical friction coefficients in the model are in good agreement with those observed in the experiment and the ratio of µk/µs of between 0.11 and 0.5 observed in the simulations is in reasonable agreement with the value of 0.5 normally observed for these seen for these systems.

Structure and rheology of star polymers in confined geometries: a mesoscopic simulation study.

Mesoscopic simulations of star polymer melts adsorbed onto solid surfaces are performed using the dissipative particle dynamics (DPD) method. A set of parameters is developed to study the low functionality star polymers under shear. The use of a new bond-angle potential between the arms of the star creates more rigid chains and discriminates between different functionalities at equilibrium, but still allows the polymers to deform appropriately under shear. The rheology of the polymer melts is studied by calculating the kinetic friction and viscosity and there is good agreement with experimental properties of these systems. The study is completed with predictive simulations of star polymer solutions in an athermal solvent.

Nonequilibrium Molecular Simulations of New Ionic Lubricants at Metallic Surfaces: Prediction of the Friction.

We report nonequilibrium molecular dynamics of ionic liquids interacting with metallic surfaces. A specific set of interaction parameters for ionic liquids composed of alkylammonium cations and alkylsulfonate anions with an iron surface, which has been previously developed (J. Chem. Theory Comput.2012, 8, 3348) is used here. We develop a procedure for a quantitative prediction of the friction coefficient at different loads and shear rates. The simulated friction coefficient agrees very well with the available experimental ones. The dependence of friction on the load, shear velocity, surface topology, and length of alkyl side chains in the ionic liquid is also investigated. The changes in the frictional forces are explained in terms of the specific arrangements and orientations of groups forming the ionic liquid at the vicinity of the surface.

Interactions and Ordering of Ionic Liquids at a Metal Surface.

An atomistic force field for ionic liquids interacting with a metal surface is built on the basis of quantum methods. Density functional calculations of alkylammonium cations and alkylsulfonate anions interacting with a cluster of iron atoms were performed, at a series of distances and orientations, using the M06 functional that represents noncovalent interactions. A site-site potential function was then adjusted to the BSSE-corrected DFT interaction energies. Finally, the polarization of the metal by the ions was taken into account using induced dipoles to reproduce the interaction energy between charges and a conductor surface. When combined with a molecular force field for the ionic liquid and a suitable potential for metals, our model allows the computer simulation of heterogeneous systems containing metal surfaces or nanoparticles in the presence of ionic liquids. Our aim is to study tribological systems with ionic lubricants. We report molecular dynamics results on the structure of the interfacial layer of several alkylammonium alkylsulfonate ionic liquids at a flat iron surface, including analyses of the positional and orientational ordering of the ions near the surface, and charge density profiles. Both anions and cations are found in the first ordered layer of ions near the surface, with the oxygen atoms of the sulfonyl groups interacting more strongly with the metal. The interfacial layer is essentially one ion thick, except for very short chain ionic liquids in which a second layer is observed. The effects of different lengths of the nonpolar alkyl side chains on the cation and the anion are different: whereas butyl chains on the sulfonate anions tend to be directed away from the surface, those on ammonium cations lie more parallel to the surface.