Dominic J. Tildesley

Molecular dynamics simulation of the orthobaric densities and surface tension of water

Molecular dynamics simulations have been performed to study the liquid–vapor equilibrium of water as a function of temperature. The orthobaric densities and the surface tension of water are reported for temperatures from 316 K until 573 K. The extended simple point charge (SPC/E) interaction potential for water molecules is used with full Ewald summation. The normal and tangential components of the pressure tensor were calculated and are presented at 328 K. The nature of the long‐range contribution to the surface tension has been studied in detail. At 328 K the calculated surface tension is 66.0±3.0 mN m−1 in comparison with the experimental value of 67 mN m−1. The simulated surface tensions between 316 K and 573 K are in good agreement with experiment. The orthobaric densities are in better agreement with experimental values than those obtained from the Gibbs ensemble calculation for the SPC model of water.

On the role of hydrodynamic interactions in block copolymer microphase separation

A melt of linear diblock copolymers (AnBm) can form a diverse range of microphase separated structures. The detailed morphology of the microstructure depends on the length of the polymer blocks An and Bm and their mutual solubility. In this paper, the role of hydrodynamic forces in microphase formation is studied. The microphase separation of block copolymer melts is simulated using two continuum methods: dissipative particle dynamics (DPD) and Brownian dynamics (BD). Although both methods produce the correct equilibrium distribution of polymer chains, the BD simulation does not include hydrodynamic interactions, whereas the DPD method correctly simulates the (compressible) Navier Stokes behavior of the melt. To quantify the mesophase structure, we introduce a new order parameter that goes beyond the usual local segregation parameter and is sensitive to the morphology of the system. In the DPD simulation, a melt of asymmetric block copolymers rapidly evolves towards the hexagonal structure that is predict...

Electrostatic Interactions in Dissipative Particle Dynamics: Toward a Mesoscale Modeling of the Polyelectrolyte Brushes

We report mesoscopic simulations of bulk electrolytes and polyelectrolyte brushes using the dissipative particle dynamics (DPD) method. The calculation of the electrostatic interactions is carried out using both the Ewald summation method and the particle-particle particle-mesh technique with charges distributed over the particles. The local components of the pressure tensor are calculated using the Irving and Kirkwood, and the method of planes and mechanical equilibrium is demonstrated. The profiles of the normal component of the pressure tensor are shown to be similar for both the Ewald and particle-particle particle-mesh methods for a single polyelectrolyte brush. We show that the PPPM method with the MOP technique is the appropriate choice for simulations of this type. The mesoscale modeling of a strongly stretched polylectrolyte brush formed by strong charged polymer chains at a high grafting density shows that the polyelectrolyte follows the nonlinear osmotic regime, as expected from the calculation of the Gouy-Chapman length and the dimensionless Manning ratio.

Mesoscopic simulation of entanglements using dissipative particle dynamics : Application to polymer brushes

We use a simple spring-spring repulsion to model entanglements between polymers in dissipative particle dynamics (DPD) simulations. The model is applied to a polymer brushes system to study lubrication. We demonstrate that this method leads to mechanical equilibrium in polymer brushes using the normal DPD time step. The number of bond crossings is calculated to provide a quantitative description of the entanglement. We demonstrate that it is possible to avoid 99% of the bond crossings with the values of spring-spring repulsion that can be used without significantly decreasing the time step. A shear force is applied to the system to study the effect of the decrease in the bond crossings on the structure and rheological properties of the brushes. In particular, we show how the friction coefficient increases with the decrease in the bond crossings of the polymers.

Mesoscale modeling of polyelectrolyte brushes with salt.

We report dissipative particle dynamics (DPD) simulations of a polyelectrolyte brush under athermal solvent conditions. The electrostatic interactions are calculated using the particle-particle particle-mesh (PPPM) method with charges distributed over the particles. The polymer beads, counterions, co-ions, and solvent particles are modeled explicitly. The DPD simulations show a dependence of the brush height on the grafting density and the charge fraction that is typical of the nonlinear osmotic brush regime. We report the effect of the addition of salt on the structural properties of the brush. In the case of a polyelectrolyte brush with a high surface coverage, the simulations reproduce the transition between the nonlinear osmotic brush regime where the thickness of the brush is independent of the salt concentration and the salted regime where the brush height decreases weakly with the salt concentration.

Dissipative particle dynamics simulation of grafted polymer brushes under shear

The dissipative particle dynamics technique has been used to study grafted polymer chains under shear in solvents of different quality. We show that width of the grafted polymer chains decreases with increasing shear rate for an adsorbed monolayer and bilayer in an athermal solvent but that the layer shrinks in a poor solvent and there is no further decrease in the width on shearing. The polymer chains are preferentially aligned along the direction of the shear and tilted away from the surface normal with even the smallest shear rates. The orientational ordering with respect to the layer director increases with increasing shear. Reducing the quality of the solvent dramatically reduces the width of the grafted layers and the friction between them.

Fluid phase equilibria using molecular dynamics: the surface tension of chlorine and hexane

In this paper we demonstrate that the direct molecular dynamics method can be used to predict accurate fluid phase equilibria for molecular fluids. The method is applied to chlorine and n-hexane to calculate the coexisting densities, vapour pressure, and surface tension as a function of temperature. Chlorine is modelled as a rigid diatomic molecule, and n-hexane as an isotropic united-atom model. For hexane we use two sets of parameters for the intermolecular potential. The main difference in the parameters is the strength of the repulsion-dispersion interaction of the terminal methyl group eCH3 /k = 90·44 K (model I) and = 114 K (model II); systematic differences in the calculated properties are found for the models. For chlorine, the liquid-vapour densities and vapour pressures are in excellent agreement with experimental results, and with those previously calculated using the Gibbs ensemble Monte Carlo method (GEMC). Good agreement with the experimental surface tensions is obtained. For hexane, the cal...

The compression of polymer brushes under shear: the friction coefficient as a function of compression, shear rate and the properties of the solvent

We present a study of the compression of polymer-grafted surfaces using the dissipative particle dynamics (DPD) method at constant chemical potential. We demonstrate the importance of performing simulations of compression at fixed chemical potential of the solvent by comparing the simulated force-compression curves at constant chemical potential and density with the experimental profile determined for poly(ethylene-propylene) chains grafted onto mica surfaces in a cyclohexane solvent. The simulated force-distance and friction profiles are presented as a function of the polymer grafting density, the shear rate and the nature of the solvent. We also study the influence of the steepness of conservative potential between polymer segments and the size of the solvent elements (particles) on the form of the force-compression and friction-compression profiles.

Frictional forces in polyelectrolyte brushes: effects of sliding velocity, solvent quality and salt

We report dissipative particle dynamics (DPD) simulations of grafted polymer brushes under shear at three separation distances. We investigate the impact of the shear on single monolayers and compressed bilayers formed by neutral and charged polymer chains. The dependence of the friction on the solvent quality is studied for different charge fractions in two compressed polymer brushes. The change in the frictional forces upon the addition of the salt is analyzed in terms of change in the interpenetration coefficient. We complete this study by a description of the brush structure including ordering, composition, tilt and local electroneutrality at different separation distances, salt concentrations, charge fractions and shear rates. Interestingly, some heterogeneities in the monomer density profiles of polyelectrolyte brushes appear under shear and vanish upon addition of salt.

Interactions between polymer brushes and a polymer solution: mesoscale modelling of the structural and frictional properties

We report mesoscopic simulations of polymer brushes in the presence of free polymer chains. We have used the dissipative particle dynamics method to model entangled polymer chains in a good solvent. We have studied the structure of the grafted and free chains under shear, as well as the dynamics of the detechment of polymer chains from the grafted surface. A comparison between entangled and nonentangled systems is presented. Entanglements are modeled using a bond repulsion potential. The rheology has been studied at different polymer grafting densities, in the presence of a varying number of free chains. We have shown that the detachment of grafted chains creates a central zone of free chains which exhibits a bulk like behaviour. The viscosity of this region is significantly increased compared to that of a polymer brush with the same grafting density and no detached chains. The presence of free polymer chains dramatically changes the friction coefficient. We have established that a fraction of detached chains of only 15% induces a decrease in the friction of 50%. This behaviour is correlated to the decrease of the interpenetration between the two opposing brushes. The detachment of chains from the surface, that may well occur at high sliding velocity in the surface force apparatus, is likely to have a significant effect on the observed experimental friction.