Victor Pryamitsyn

Mechanisms of steady-shear rheology in polymer-nanoparticle composites

We use coarse-grained computer simulations to delineate the mechanisms governing the steady-shear rheology of polymer-nanoparticle composites. Our studies specifically focus on the regimes where the particle sizes and the interparticle distances become comparable to the polymer sizes and where the interactions between polymer and particles become relevant in influencing the dynamical characteristics. Our results suggest the shear rheology of the composite is very similar to that of colloidal suspensions in a simple fluid when polymer rheology, the particle-induced changes in the polymer rheology and the polymer slip effects are accounted. At dilute and semidilute nanoparticle concentrations, the composite shear rheology is shown to be dominated by the shear thinning of the polymer chains which in turn is modified by the presence of the particles. For higher particle loads, the polymeric contribution to the rheology becomes much less important and the shear rheology is dominated by the particles stresses. ...

Noncontinuum effects in nanoparticle dynamics in polymers

We propose a continuum model for the dynamics of particles in polymer matrices which encompasses arbitrary size ratios of the polymer and particle. We present analytical and computer simulation results for the mobility of the particles and the viscosity of the suspension for the case of unentangled polymer melts. Our results indicate strong dependencies of the particle mobility upon the polymer-particle size ratios and much reduced intrinsic viscosities for the suspensions. These predictions rationalize some recent experimental observations on the dynamics of nanoparticles in polymer melts.

A coarse-grained explicit solvent simulation of rheology of colloidal suspensions

We use a simple extension of the dissipative particle dynamics (DPD) model to address the dynamical properties of macrosolutes immersed in complex fluid solvents. In this approach, the solvent particles are still represented as DPD particles, thereby retaining the time and length scale advantages offered by the DPD approach. In contrast, the solute particles are represented as hard particles of the appropriate size. We examine the applicability of this simulation approach to reproduce the correct hydrodynamical characteristics of the mixture. Our results focus on the equilibrium dynamics and the steady-state shear rheological behaviors for a range of volume fractions of the suspension, and demonstrate excellent agreement with many published experimental and theoretical results. Moreover, we are also able to track the glass transition of our suspension and the associated dynamical signatures in both the diffusivities and the rheological properties of our suspension. Our results suggest that the simulation approach can be used as a one-parameter model to examine quantitatively the rheological properties of colloidal suspensions in complex fluid solvents such as polymeric melts and solutions, as well as allied dynamical phenomena such as phase ordering in mixtures of block copolymers and particles.

Depletion and pair interactions of proteins in polymer solutions

We study the depletion, pair interaction, and phase behavioral characteristics of proteins in polymer solutions. We use a McMillan-Mayer-like approach [W. G. McMillan, Jr. and J. E. Mayer, J. Chem. Phys. 13, 276 (1945)] to suggest that the depletion characteristics should be studied at an effective polymer concentration which is a function of both the average polymer and the protein concentrations. In the protein limit, we show that the volume of the polymer depletion layers exceeds the size of the proteins, leading to effective polymer concentrations typically in the semidilute and concentrated regimes even when the average polymer concentrations are in the dilute regimes. We propose an approximate approach that accounts for the multibody depletion overlaps, and use an accurate numerical solution of polymer mean-field theory to address depletion characteristics in these regimes which are characterized by both the importance of polymer interactions as well as the curvature of the proteins relative to the correlation length of polymers. We show that the depletion characteristics of the protein-polymer mixture can be quite different when viewed in this framework, and this can have profound consequences for the phase behavior of the mixture. Our theoretical predictions for the phase diagram match semiquantitatively with published experimental results.

Dynamical mean-field theory for inhomogeneous polymeric systems

We propose and demonstrate a new computational approach which enables the simulation of the dynamics and rheology in inhomogeneous phases of multicomponent polymeric systems. Our approach generalizes Doi’s dynamical mean-field theory of rodlike polymers by combining single chain Brownian dynamics algorithms with phenomenological prescriptions for the dynamics of coarse-grained field variables. We provide a general overview of the technique and illustrate its applicability by our results in the context of a symmetric A+B polymer blend.

Many-body interactions and coarse-grained simulations of structure of nanoparticle-polymer melt mixtures

We present a computational approach for coarse-grained simulations of nanoparticle-polymer melt mixtures. We first examine the accuracy of an effective one-component approach based on a pair interaction approximation to polymer-mediated interactions, and demonstrate that even at low particle volume fractions, the polymer-mediated many-body interaction effects can prove significant in determining the structural characteristics of mixtures of nanoparticles and polymer melts. The origin of such effects is shown to arise from the extent of polymer perturbations resulting from the presence of the nanoparticles. To account for such effects, we propose a new simulation approach that employs a coarse-grained representation of the polymers to capture the many-body corrections to the polymer-mediated pair interaction potentials. The results of the coarse-grained simulations are shown to be in good quantitative agreement with the reference simulations. The method developed in this article is proposed as a tractable approach to coarse-grain and effect computer simulations of atomistic descriptions of polymer-nanoparticle systems.

Computer Simulations of Gas Diffusion in Polystyrene–C60 Fullerene Nanocomposites Using Trajectory Extending Kinetic Monte Carlo Method

The effects of nanoparticles on the rates of gas diffusion through glassy polymers were studied by a combination of molecular dynamics and kinetic Monte Carlo techniques designed to overcome the computational limitations in obtaining long-time trajectories in the diffusive regime of gas molecules in glassy polymer systems. Using such a methodology, we studied the effect of fullerene nanoparticles upon the diffusivities of N(2) and CO(2) in a polystyrene matrix. The addition of nanoparticles was found to cause a lowering of the diffusion coefficients of both N(2) and CO(2). However, the magnitudes of this lowering and their volume fraction dependencies are seen to depend explicitly on the nature of the penetrant and temperature. We discuss the possible physical mechanisms underlying such behavior.

Theory of monolayers of non-Gaussian polymer chains grafted onto a surface. Part 1.—General theory

A theory describing layers of polymer chains grafted to a flat surface (polymer ‘brush’) is developed. We consider a brush of chains with arbitrary extensibility and, within the framework of molecular field theory, we suggest a general scheme for calculating the self-consistent pseudo-potential, which determines the structure of the brush. This potential appears to be defined only by the mechanism of extensibility and is independent of the interactions between chains in the brush. A model potential for freely jointed chains (FJC) was calculated and used for describing an FJC polymer brush in good solvent conditions. We found good agreement between our results and the numerical calculations of Skvortsov et al.

Effect of confinement on polymer-induced depletion interactions between nanoparticles

Using a numerical implementation of polymer mean-field theory, we probe the effects of a structureless wall on the insertion free energies and the depletion interactions between nanoparticles in polymer solutions. Our results indicate that the insertion free energies and the polymer-induced interactions become mitigated in the presence of a wall. The range of influence of the walls is shown to correspond to the correlation length of the polymer solution. Surprisingly, our results demonstrate that even for particle sizes comparable to the correlation length of the polymer solution, the polymer depletion density profiles near the wall (in the absence of particles) can be used as a means to quantitatively predict the influence of the wall on both the insertion free energies and the depletion interactions.

Interactions and Aggregation of Charged Nanoparticles in Uncharged Polymer Solutions.

We employ an extension of the single chain in mean field simulation method to study mixtures of charged particles and uncharged polymers. We examine the effect of particle charge, polymer concentration, and particle volume fraction on the resulting particle aggregates. The structures of aggregates were characterized using particle-particle radial distribution functions and cluster size distributions. We observe that the level of aggregation between particles increases with increasing particle volume fraction and polymer concentration and decreasing particle charge. At intermediate regimes of particle volume fraction and polymer concentrations, we observe the formation of equilibrium clusters with a preferred size. We also examined the influence of manybody effects on the structure of a charged particle-polymer system. Our results indicate that the effective two-body approximation overpredicts the aggregation between particles even at dilute particle concentrations. Such effects are thought to be a consequence of the interplay between the respective manybody effects on the depletion and electrostatic interactions.