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Understanding the transport properties of water in self-assembled block copolymer morphologies is important for furthering the use of such materials as water-purifying membranes. In this study, we used coarse-grained dissipative particle dynamics simulations to clarify the influence of pore morphology on the self-diffusion of water in linear-triblock-copolymer membranes. We considered representative lamellar, cylindrical, and gyroid morphologies and present results for both the global and local diffusivities of water in the pores. Our results suggest that the diffusivity of water in the confined, polymer-coated pores differs from that in the unconfined bulk. Explicitly, in confinement, the mobility of water is reduced by the hydrodynamic friction arising from the hydrophilic blocks coating the pore walls. We demonstrate that in lamella and cylindrical morphologies, the latter effects can be rendered as a universal function of the pore size relative to the brush height of the hydrophilic blocks.

We use atomistic computer simulations to examine ion-transport phenomena for backbone polymerized cationic liquids with bistrifluoromethylesulfonylimide (TFSI-) counterions. We consider a system in which the polymerized cation moiety is the imidazolium ring and study the structural characteristics and ion mobilities for cases in which the cations are separated by four, six, and eight methylene units on the backbone. A pendant polymerized ionic liquid, 1-butyl-3-vinylimidazolium, is compared to the backbone series across ion coordination and hopping features. The anion diffusivity in backbone polymerized cationic liquids is found to decrease with increasing spacer length, which is shown to result from a decrease in intramolecular and intermolecular hopping frequencies due to an increasing distance separating imidazolium moieties. In comparison with pendant polymerized ionic liquids, we observe that the participation rates of intermolecular hopping events in the backbone polymers far exceed that of the pendant, and the intrapolymeric ionic coordination profile shows the TFSI- of the pendant polymer with a high propensity for coordination by multiple imidazolium, compared with one monomer from a given polymer for the backbone series. Despite these differences, backbone polymerized ionic liquids are seen to possess correlated diffusivity and ion-association relaxation times, in a manner similar to the results observed in past studies for pendant variants.

We develop a simple model to probe the ion transport and mechanical properties of low volume fraction colloidal nanoparticle gels. Specifically, we study the influence of the morphology of gels on ion diffusion and the corresponding roles of affinity to and enhanced ion transport along nanoparticle surfaces. We employ kinetic Monte Carlo simulations to simulate ion transport in the colloidal gels, and we perform nonequilibrium molecular dynamics to study their viscoelastic behavior. Our results indicate that in the presence of enhanced diffusion pathways for ions along the particle surface, morphology has a significant influence on the diffusivity of ions. We demonstrate that some gel morphologies can exhibit simultaneously enhanced ion transport and mechanical properties, thus illustrating a strategy to decouple ion transport and mechanical strength in electrolytes.

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 peptide's 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.

We present a self-consistent field theory model for the self-assembly behavior of rod-coil block copolymers. The orientational interactions between the rods were modeled through a Maier-Saupe interaction, while the enthalpic interactions between rods and coils were modeled through a standard Flory-Huggins approach. We outline a "real-space" numerical approach to solve the self-consistent field equations for such rod-coil block copolymers. A major focus of our work is upon the nonlamellar phases observed in the experiments on such polymers. To develop a physical understanding of these phases and their regimes of occurrence, we compute the two-dimensional phase diagram for our model. The latter shows significant departures from the one-dimensional phase diagram, but matches qualitatively with the existing experimental results. We also present scaling arguments that rationalize the numerical results for the self-assembly behavior.

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.

In this brief article, we present results from coarse-grained molecular dynamics simulations which probed the relationship between the local segmental dynamics and the tagged monomer dynamics in lamellar phases of diblock copolymers. Our results demonstrate that monomer relaxation times do not provide directly a quantitatively accurate measure of the spatial variations in segmental dynamics. However, a convolution of the monomer density distributions with their corresponding relaxation times is shown to provide an approximate, but accurate, quantitative characterization of the average local segmental dynamics.

In this paper we address the generic effects arising from the interplay of thermal fluctuations and reactions. This is accomplished by considering specifically the kinetics of reactions effected in microemulsion media. In the first part of this paper we consider the kinetics of the reaction A+B→O/ in bicontinuous microemulsion media, wherein the solutes A and B are assumed to be preferentially attracted to water and oil, respectively, and O/ constitutes an inert product. We formulate the diffusion and reaction of these solutes in a field-theoretical framework within which the fluctuations of the background microemulsion are embedded. We then employ mean-field arguments and a perturbative Wilson-type renormalization group (RG) approach to discern the relevance, at long length scales, of the background fluctuations. Our analysis indicates that the dynamic fluctuations of the microemulsion prove irrelevant in impacting the asymptotic kinetics of the reaction. In view of the fact that our field-theoretic appr...

Thermal Taylor dispersion theory for time-periodic systems was used to study the extent of chaotic laminar heat transfer enhancement and axial thermal dispersion occurring during combined transverse and axial annular flow between two nonconcentric circular cylinders undergoing alternate rotations. A local Newton’s “law of cooling” heat transfer boundary condition was used on the outer cylinder, whereas the inner cylinder was supposed insulated. The effective heat transfer coefficient H* describing the global rate of heat loss from the system (differing in general from the true microscale Newton’s law heat transfer coefficient h on the outer cylinder) was calculated as a function of the system parameters, thereby serving to quantify the extent of chaotic heat transfer enhancement. The axial thermal Taylor dispersivity provided an independent measure of the effects of chaotic mixing, as too did the axial thermal velocity. Calculations were performed for three different cases: (i) concentric cylinder rotati...

We use atomistic simulations to probe the ion conductivities and mechanical properties of polyethylene oxide electrolytes containing Al2O3nanoparticles. We specifically study the influence of repulsive polymer-nanoparticle and ion-nanoparticle interactions and compare the results with those reported for electrolytes containing the polymorph β-Al2O3nanoparticles. We observe that incorporating repulsive nanoparticle interactions generally results in increased ionic mobilities and decreased elastic moduli for the electrolyte. Our results indicate that both ion transport and mechanical properties are influenced by the polymer segmental dynamics in the interfacial zones of the nanoparticle in the ion-doped systems. Such effects were seen to be determined by an interplay between the nanoparticle-polymer,nanoparticle-ion, and ion-polymer interactions. In addition, such interactions were also observed to influence the number of dissociated ions and the resulting conductivities. Within the perspective of the influence of nanoparticles on the polymer relaxation times in ion-doped systems, our results in the context of viscoelastic properties were consistent with the ionic mobilities. Overall, our results serve to highlight some issues that confront the efforts to use nanoparticle dispersions to simultaneously enhance the conductivity and the mechanical strength of polymer electrolyte.

We study the effective pair interactions between two charged spherical particles in polyelectrolyte solutions using polymer self-consistent field theory. In a recent study [V. Pryamitsyn and V. Ganesan, Macromolecules 47, 6095 (2015)], we considered a model in which the particles possess fixed charge density, the polymers contain a prespecified amount of dissociated charges and, the dielectric constant of the solution was assumed to be homogeneous in space and independent of the polymer concentration. In this article, we present results extending our earlier model to study situations in which either or both the particle and the polymers possess partially dissociable groups. Additionally, we also consider the case when the dielectric constant of the solution depends on the local concentration of the polymers and when the particle's dielectric constant is lower than that of the solvent. For each case, we quantify the polymer-mediated interactions between the particles as a function of the polymer concentrations and the degree of dissociation of the polymer and particles. Consistent with the results of our previous study, we observe that the polymer-mediated interparticle interactions consist of a short-range attraction and a long-range repulsion. The partial dissociablity of the polymer and particles was seen to have a strong influence on the strength of the repulsive portion of the interactions. Rendering the dielectric permittivity to be inhomogeneous has an even stronger effect on the repulsive interactions and results in changes to the qualitative nature of interactions in some parametric ranges.

We develop a kinetic Monte Carlo model for photocurrent generation in organic solar cells that demonstrates improved agreement with experimental illuminated and dark current-voltage curves. In our model, we introduce a charge injection rate prefactor to correct for the electrode grid-size and electrode charge density biases apparent in the coarse-grained approximation of the electrode as a grid of single occupancy, charge-injecting reservoirs. We use the charge injection rate prefactor to control the portion of dark current attributed to each of four kinds of charge injection. By shifting the dark current between electrode-polymer pairs, we align the injection timescales and expand the applicability of the method to accommodate ohmic energy barriers. We consider the device characteristics of the ITO/PEDOT/PSS:PPDI:PBTT:Al system and demonstrate the manner in which our model captures the device charge densities unique to systems with small injection energy barriers. To elucidate the defining characteristics of our model, we first demonstrate the manner in which charge accumulation and band bending affect the shape and placement of the various current-voltage regimes. We then discuss the influence of various model parameters upon the current-voltage characteristics.

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.

We apply the methodology of self-consistent Brownian dynamics simulations to study the self-assembly behavior in melts of semiflexible-flexible diblock copolymers as a function of the persistence length of the semiflexible block. Our results reveal a novel progression of morphologies in transitioning from the case of flexible-coil to rod-coil copolymers. At even moderate persistence lengths, the morphologies in the semiflexible-block rich region of the phase diagram transform to liquid crystalline phases. In contrast, the phases in the flexible-block rich region of the phase diagram persist up to much larger persistence lengths. Our analysis suggests that the development of orientational order in the semiflexible block to be a critical factor influencing the morphologies of self-assembly.

We use polymer self-consistent field theory to quantify the interfacial properties of random copolymer brushes (AB) in contact with a homopolymer melt chemically identical to one of the blocks (A). We calculate the interfacial widths and interfacial energies between the melt and the brush as a function of the relative chain sizes, grafting densities, compositions of the random copolymer in the brush, and degree of chemical incompatibility between the A and B species. Our results indicate that the interfacial energies between the melt and the brush increase (signifying expulsion of the free chains from the brush) with increasing grafting density, chemical incompatibility between A and B components, and size of the free chains relative to the grafted chains. We also compare the interfacial energies of random copolymers of different sequence characteristics and find that, except for the case of very blocky or proteinlike chains, blockiness of the copolymer has only little effect on interfacial properties. Our results for interfacial energies are rationalized based on the concept of an "effective volume fraction" of the brush copolymers, f(eff), which quantifies the chemical composition of the brush segments in the interfacial zone between the brush and melt copolymers. Using this concept, we modify the strong-stretching theory of brush-melt interfaces to arrive at a simple model whose results qualitatively agree with our results from self-consistent field theory. We discuss the ramifications of our results for the design of neutral surfaces.

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.

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.

We present a model for photovoltaic device characteristics based on drift-diffusion ideas which allows for prescription of arbitrary morphologies of donor and acceptor phases while simultaneously incorporating the role of anisotropic charge transport of holes and excitons. We use such a model to address the interplay between anisotropic charge transport and the orientational and/or crystalline ordering of donor molecules. We presents results illustrating the influence of the degree of orientational ordering, the impact of ordering parallel and perpendicular to the electrodes and the role of device thickness.

We present the details and results of a simulation study addressing the dynamics and rheology of rod suspensions over a wide regime of concentrations ranging from dilute to concentrated systems. Our study compares the results of two complementary simulation methods. The first method adapts a recently proposed explicit solvent simulation strategy and incorporates both hydrodynamical effects and steric interactions between the rod units. We compare the results of such a method with those obtained from a Brownian dynamics simulation approach which retains the steric interactions but neglects the effects of hydrodynamic interactions. Overall, our results in the context of the translational and rotational diffusivities are in agreement with the hydrodynamical predictions in the dilute regime and the corresponding results of the tube model and its extensions thereof in the semidilute regimes. The latter results suggest that effects of hydrodynamic interactions on the translational and rotational diffusivities are secondary relative to the steric interactions and at best lead only to a small correction to the results of the classical tube model. Our results in the context of linear viscoelasticity also broadly confirms the predictions of the tube model for the storage and loss moduli and allows us to extract for the first time the independent hydrodynamic and Brownian contributions to the zero shear viscosity. While the relative magnitudes of these contributions are consistent with the theoretical predictions, the quantitative magnitudes are quite different from the theoretical predictions. Overall, these results confirm the validity of the hydrodynamic "screening" hypothesis and ratify the neglect of hydrodynamical stresses in quantifying the linear rheology of Brownian rod suspensions.

We use a combination of polymer mean field theory and Monte Carlo simulations to study the polymer-bridged gelation, clustering behavior, and elastic moduli of polymer-nanoparticle mixtures. Polymer self-consistent field theory is first numerically implemented to quantify both the polymer induced interparticle interaction potentials and the conformational statistics of polymer chains between two spherical particles. Subsequently, the formation and structure of polymer-bridged nanoparticle gels are examined using Monte Carlo simulations. Our results indicate a universality in the fractal structure for the polymer-bridged networks over a wide range of parametric conditions. Explicitly, near the gelation transition, the fractal dimension d(f) ranges between 2.2 and 2.5, and above the gelation thresholds, the elastic moduli are found to follow a universal power law G(') proportional, variant(eta-eta(c))(nu(eta) ) with a critical exponent nu(eta) approximately 1.82. The latter suggests strong similarities between polymer-bridging induced percolation and classical elastic resistor network percolation. Our results show a very good agreement with the experimental results for polymer-particle mixtures and suggest a possible framework for experimentally distinguishing the origins of gelation phenomena observed in polymer-particle mixtures.