Venkat Ganesan

Curvature effects upon interactions of polymer-grafted nanoparticles in chemically identical polymer matrices

We study the interactions between polymer-grafted nanoparticles immersed in a chemically identical polymer melt using a numerical implementation of polymer mean-field theory. We focus on the interpenetration width between the grafted and free chains and its relationship to the polymer-mediated interparticle interactions. To this end, we quantify the interpenetration width as a function of particle curvature, grafting density, and the relative molecular weights of the grafted and free chains. We show the onset of wetting and dewetting as a function of these quantities and explain our results through simple scaling arguments to include the effects of curvature. Subsequently, we show that the interparticle potentials correlate quantitatively with the trends displayed by the interpenetration widths.

Modeling the anisotropic self-assembly of spherical polymer-grafted nanoparticles

Recent experimental results demonstrated that polymer grafted nanoparticles in solvents display self-assembly behavior similar to the microphase separation of block copolymers and other amphiphiles. We present a mean-field theory and complementary computer simulations to shed light on the parametric underpinnings of the experimental observations. Our theory suggests that such self-assembled structures occur most readily when the nanoparticle size is comparable to the radius of gyration of the polymer brush chains. Much smaller particle sizes are predicted to yield uniform particle dispersions, while larger particles are expected to agglomerate due to phase separation from the solvent. Selected aspects of our theoretical predictions are corroborated by computer simulations.

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. ...

Atomistic Simulations of Structure of Solvated Sulfonated Poly(ether ether ketone) Membranes and Their Comparisons to Nafion: I. Nanophase Segregation and Hydrophilic Domains

The results of extensive all-atom molecular dynamics (MD) simulations of water- and methanol-solvated SPEEK (sulfonated poly(ether ether ketone)) are reported. In this Part I of the two-part article, we present results elucidating the key structural features of hydrophilic domains with varying water, methanol content, and temperature. With increasing hydration, the membrane was observed to swell appreciably and transform from a state containing a large number of water clusters containing just few molecules at low water content to very large water clusters encompassing almost all molecules at the highest water contents. In comparison with the results reported for Nafion, SPEEK was observed to be characterized by more isolated and smaller sized water clusters and less pronounced percolation than Nafion at lower water contents, but the percolation characteristics became comparable to Nafion at higher water content. Water confined in SPEEK showed less internal structure than bulk water or water confined in Nafion. With increasing water content, solvation of sulfonic acid groups was noted. The average sulfur-sulfur separation in SPEEK was found to be higher compared with the results reported for Nafion. The backbone of SPEEK was found to be more rigid and more hydrophobic than that of Nafion. These observations suggest that the nanophase segregation in SPEEK is less pronounced than that in Nafion, which may contribute to the diminished crossover characteristics of SPEEK noted in experiments on direct methanol fuel cells (DMFC) and reported in Part II of this work.

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.

Translocation of a β-hairpin-forming peptide through a cylindrical tunnel

We use Langevin dynamics simulations of a minimalist off-lattice model to study the translocation of a beta hairpin forming peptide through a tunnel that mimics the exit tunnel in a ribosome. We have computed the free energy of the peptide as a function of its position relative to the tunnel exit and also studied the properties of the conformational ensemble, when the peptides position is restricted at different points along the tunnel. Confining the peptide within a sufficiently wide tunnel stabilizes the folded state. The protein then remains folded as it moves towards the tunnel exit. However, when the diameter D of the tunnel is below a certain critical value D(c), confinement destabilizes the folded state and forces the peptide to assume an extended configuration. In this case, as the peptide progresses towards the tunnel exit and eventually leaves the tunnel, it goes through a series of compact, misfolded conformations and eventually folds when it gets close to the exit. The critical tunnel diameter D(c) is comparable to the width of ribosomal tunnels. Our results suggest that co-translational folding is probably not universal, but rather a protein-specific phenomenon.

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.

Structural anomalies of fluids: origins in second and higher coordination shells.

Compressing or cooling a fluid typically enhances its static interparticle correlations. However, there are notable exceptions. Isothermal compression can reduce the translational order of fluids that exhibit anomalous waterlike trends in their thermodynamic and transport properties, while isochoric cooling (or strengthening of attractive interactions) can have a similar effect on fluids of particles with short-range attractions. Recent simulation studies by Yan [Phys. Rev. E 76, 051201 (2007)] on the former type of system and Krekelberg [J. Chem. Phys. 127, 044502 (2007)] on the latter provide examples where such structural anomalies can be related to specific changes in second and more distant coordination shells of the radial distribution function. Here, we confirm the generality of this microscopic picture through analysis, via molecular simulation and integral equation theory, of coordination shell contributions to the two-body excess entropy for several related model fluids which incorporate different levels of molecular resolution. The results suggest that integral equation theory can be an effective and computationally inexpensive tool for assessing, based on the pair potential alone, whether new model systems are good candidates for exhibiting structural (and hence thermodynamic and transport) anomalies.

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.