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The addition of transient networks to polymer composites marks a new direction toward the design of novel materials, with numerous biomedical and industrial applications. The network structure connected by transient cross-links (CLs) relaxes as time evolves, which results in the stretching release of polymer strands between transient CLs during strain. Using molecular dynamics simulations, we measure directly the stress-strain curves of double polymer networks (DPNs), containing both transient and permanent components, at different strain rates. Lifetime and density of transient CLs control the relaxation spectrum of transient networks and determine the mechanical properties of DPNs. A Rouse mode analysis reveals that at high strain rates the mechanical strength of DPNs is defined jointly by the cross-linking structures of permanent and transient networks. At low strain rates, the cross-linking structure of transient network relaxes, leaving the permanent component of the network as a sole contributor to the mechanical strength of DPNs. The transient network is shown to facilitate a dissipation of energy at higher strain rates and prevents a rupture of the network, while the permanent network preserves the structural integrity of the composite at low strain rates. This study provides computational and theoretical foundations for designing polymer composites with desirable mechanical strength and toughness by means of tuning transient networks.

Recent advances in technologies such as nanomanufacturing and nanorobotics have opened new pathways for the design of active nanoparticles (NPs) capable of penetrating biolayers for biomedical applications, e.g., for drug delivery. The coupling and feedback between active NP motility (with large stochastic increments relative to passive NPs) and the induced nonequilibrium deformation and relaxation responses of the polymer network, spanning scales from the NP to the local structure of the network, remain to be clarified. Using molecular dynamics simulations, combined with a Rouse mode analysis of network chains and position and velocity autocorrelation functions of the NPs, we demonstrate that the mobility of active NPs within cross-linked, concentrated polymer networks is a monotonically increasing function of chain stiffness, contrary to passive NPs, for which chain stiffness suppresses mobility. In flexible networks, active NPs exhibit a behavior similar to passive NPs, with a boost in mobility proportional to the self-propulsion force. These results are suggestive of design strategies for active NP penetration of stiff biopolymer matrices.

Humic acids (HAs) have important environmental and geochemical effects on soil, water environments and sediment. HAs strongly complex some metal ions, which affects the migration of metal ions and the colloidal aggregation of HA. Here, the complexation of Ca2+ and Mg2+ with HA in aqueous solution under neutral conditions has been systematically studied by molecular dynamics (MD) simulation. The results show that the aggregation of HA is caused by the complexation of HA and metal ions, mainly due to the intermolecular bridging between Ca2+, Mg2+ and COO&minus, groups. Monodentate and bidentate coordinations have been found between Ca2+ and COO&minus, groups of different HA molecules in the same simulation system. Mg2+ only has a monodentate coordination with COO&minus, group.

Using molecular dynamics simulations and scaling theory, we present a systematic study of the function of cylindrical nanopores which are decorated with polymer brushes. Our focus is on the regimes...

A continuum-space mean-field model for brushes made of starlike polymers in Θ-solvent is developed. The model accounts for the segregation of molecules into two vertical layers and predicts the population fractions of these layers. It reproduces recently reported deviations of the brush heights from scaling theory at moderate and high grafting densities. In the regime of very low monomer densities, the scaling of its brush heights is crossing over to the classical scaling prediction. During our analysis, we modify the elastic properties of the molecules, as well as their volumetric interactions, in order to study the impact of these model parameters on the resulting brush properties. Comparisons with another, recently published mean-field model of brushes on a lattice and with molecular dynamic simulations are carried out. It is demonstrated that most variants of the mean-field models are generating brush heights in reasonable agreement with the simulation data. The partition of molecules into the two populations, however, exhibits a significant model dependence.

Despite a large number of researches to understand the aggregation behavior and adsorption mechanism of asphaltene, the effects of asphaltene molecular structure on the adsorption/desorption behavior of asphaltene on rock surfaces remain scarce, which limits the understanding of the interaction mechanism under complex conditions and seriously restricts the development of heavy oil recovery technology. Here, in-situ adsorption/desorption experiments combined with molecular dynamics simulations were used to fill in the gap. The results show: 1) on hydrophilic surfaces, the heteroatoms or polar groups present in the symmetrical components of asphaltene mainly determine the adsorption behavior; 2) on intermediate wettability surfaces, the adsorption of asphaltene is driven primarily by π-π stacking and hydrogen bonding interactions; 3) on hydrophobic surfaces, it is difficult to adsorb asphaltene due to the lack of polar sites and steric effects, but once adsorption occurs, it is difficult to be stripped off by low salinity water. Our results provide a simple strategy to investigate the adsorption behavior of asphaltene on rock surfaces using laboratory experiments and theoretical simulations, which not only contributes to fundamental understanding but also helps to develop a new guide for practical applications.

In recent publications, a couple of theoretical models, ranging from simple scaling approaches and box-like mean-field models to self-consistent field techniques, have been proposed to describe the structure of brushes that are composed of dendrimers. In this work, we carry out molecular dynamics simulations in various density regimes, and compare the resulting data with predictions of the latest theoretical models. This study enables us to access the ranges of validity and accuracy of the respective models, as well as their shortcomings. Significant differences in the predicted conformations of the dendrimers are observed and analyzed in the context of their elastic properties in the different models. This work offers guidelines for future research directions, which may lead to further progress regarding the understanding of branched polymer brushes.

The properties of polymer-nanoparticle (NP) mixtures significantly depend on the dispersion of the NPs. Using molecular dynamics simulations, we demonstrate that, in the presence of polymer-NP attraction, the dispersion of NPs in semidilute and concentrated polymers can be stabilized by increasing the polymer concentration. A lower polymer concentration facilitates the aggregation of NPs bridged by polymer chains, as well as a further increase of the polymer-NP attraction. Evaluating the binding of NPs through shared polymer segments in an adsorption blob, we derive a linear relationship between the polymer concentration and the polymer-NP attraction at the phase boundary between dispersed and aggregated NPs. Our theoretical findings are directly relevant for understanding and controlling many self-assembly processes that use either dispersion or aggregation of NPs to yield the desired materials.

Controlling the nanoparticle (NP) diffusion in polymers is a prerequisite to obtain polymer nanocomposites (PNCs) with desired dynamical and rheological properties and to achieve targeted delivery of nanomedicine in biological systems. Here we determine the suppression mechanism of direct NP–polymer attraction to hamper the NP mobility in adsorbing polymers and then quantify the dependence of the effective viscosity ηeff felt by the NP on the adsorption duration τads of polymers on the NP using scaling theory analysis and molecular dynamics simulations. We propose and confirm that participation of adsorbed chains in the NP motion break up at time intervals beyond τads due to the rearrangement of polymer segments at the NP surface, which accounts for the onset of Fickian NP diffusion on a time scale of t ≈ τads. We develop a power law, ηeff ∼ (τads)ν, where ν is the scaling exponent of the dependence of polymer coil size on the chain length, which leads to a theoretical basis for the design of PNCs and nan...

We present a setup in which passive colloids inside a solvent are moved to the boundaries of the container. The directional transport is facilitated by self-propelling microparticles (“walkers”) with an activity gradient, which reduces their propulsion in the vicinity of bounding walls. An attractive interaction leads to the adsorption of walkers onto the colloid-surfaces in regions of low walker activity. It is shown that the activity gradient generates a free energy gradient which in turn acts as a driving force on the passive colloids. We carry out molecular dynamics simulations and present approaches to a theoretical description of the involved processes. Although the simulation data are not reproduced on a fully quantitative level, their qualitative features are covered by the model. The effect described here may be applied to facilitate a directional transport of drugs or to eliminate pollutants.

We analyze the scaling properties of grafted polyrotaxanes and compare the resulting scaling laws with molecular dynamics simulations. These molecules display a resemblance to bottle brushes, but the mobility of the rotors yields a backbone tension that is uniform along its contour. Properties like brush extension, backbone tension, and rotor mobility as functions of rotor density and degree of polymerization of grafted chains are well covered by a simple scaling model. The dynamics of rotor release after a breakup of the polyrotaxane—a process recently observed in laboratory experiments—is studied in detail. It consists of three stages: a rapid backbone retraction, a driven rotor expulsion, and a diffusive transport. The complexity of this process, which may be exploited for drug delivery systems, is asking for further in-depth simulation studies.

The properties of starlike polymer brushes in Θ-solvent are investigated using molecular dynamics simulations, scaling- and mean-field theory. Deviations of layer thickness and monomer density from predictions of scaling theory are analyzed and explained. Contrary to common believes, we conclude that the Alexander-deGennes scaling theory is not valid for starlike polymer brushes in Θ-solvent, unless the grafting density is extremely low. A comparison with structural and dynamical properties of starlike polymer brushes in good solvent is carried out. Previous studies in good solvent reported about the dual-population structure of these branched polymer brushes, with coexisting phases of stretched and retracted molecules; the same separation emerges in Θ-solvent, though at higher grafting densities. Molecular tension and population fractions are shown to be solvent-dependent, and a reduced dynamical flip-rate between the two populations is observed for the case of the Θ-brush.

We present a systematic approach to characterize the stretching scenarios for dendrimer brushes. We compare mean-field theory for second-generation dendrimer brushes in good solvent, taking into account finite extensibility of chains, with direct molecular dynamics simulations. We analyze the stretching scenario of the dendrimers and find very broad distributions of forces acting on the free branches of individual molecules. Our results question the accuracy of a recently introduced method to determine the stretching scenario in terms of a simple scaling analysis of the monomer concentrations. Instead, the resulting scaling exponents appear to respond to model-dependent implementations of the molecular elasticity—an observation which may open paths toward an improved theoretical modeling of polymers in numerical procedures. As a function of grafting density, dendrimer brushes are scaling in a similar manner as linear chain brushes, but systematic deviations exist which increase with the functionality and ...

We investigate the structural properties of binary polymer brushes, composed of functional 4-armed star polymers and chemically identical linear polymers of different molecular weights. The molecular dynamics simulations confirm recent self-consistent field studies, in which a considerable potential of these systems for the design of switchable surfaces has been claimed. The length of the linear chains serves as a control parameter, which, while passing over a critical value, induces a sharp transition of the molecular conformation. We investigate these transitions at different grafting densities and summarize our findings in a phase diagram. The temperature dependence of the brush structure is investigated in a non-selective solvent, and non-trivial variations of the surface composition are observed. The quantity of these latter effects would be insufficient to build switchable systems, and we argue that a minor quantity of solvent selectivity would suffice to enable the desired feature of an environment-responsive coating.

Using molecular dynamics simulations, we study the properties of liquid state polymer-nanoparticle composites confined between two parallel substrates, with an attractive polymer-substrate interaction. Polymers are in the semidilute regime at concentrations far above the overlap point, and nanoparticles are in good solvent and without enthalpic attraction to the substrates. An increase of temperature then triggers the crystallization of nanoparticles on one of the two substrate surfaces-a surprising phenomenon, which is explained in terms of scaling theory, such as through competing effects of adsorption-and correlation blobs. Moreover, we show that the first, closely packed layer of nanoparticles on the substrate increases the depletion attraction of additional nanoparticles from the bulk, thereby enhancing and stabilizing the formation of a crystalline phase on the substrate. Within the time frame accessible to our numerical simulations, the crystallization of nanoparticles was irreversible; that is, their crystalline phase, once created, remained undamaged after a decrease of the temperature. Our study leads to a class of thermoreactive nanomaterials, in which the transition between a homogeneous state with dissolved nanoparticles and a surface-crystallized state is triggered by a temperature jump.

Using molecular dynamics simulations, the forces acting on nanoparticles inside polymer brushes are computed. Vertical force profiles are obtained under variation of grafting density, nanoparticle (NP) size, solvent quality, and degree of polydispersity. The force profiles are integrated to obtain the inclusion free energies. If the NP size is fixed, this energy scales with the osmotic pressure, consistent with current theoretical models. These models also predict a scaling of the free energy proportional to the volume of the NP, which is verified in good solvent and at high densities. Otherwise, the power exponent remains lower, and surface tension as a possible cause for the observed deviation is discussed. Polydispersity is shown to reduce the inclusion free energy, while the power law scaling as a function of NP size remains unchanged. Finally, polymer brushes in Θ-solvent are shown to violate simple scaling predictions within the density regime covered in this work. Using a Flory–Huggins mean-field m...

We investigate the properties of polymer brushes made of symmetric and asymmetric starlike polymers in good solvent using Langevin dynamics simulations. Two types of populations coexist in such a brush, one of them being made of highly stretched polymers and a second one made of stars that are retracted inside the lower brush regions. Scaling properties of molecular tension, brush layer thickness, and brush density are in reasonable agreement with those of linear chain brushes. The dynamics of stars switching between both the retracted and the stretched states is analyzed, and an exponential scaling with the stretching energy difference is proposed. Most of our results are found in good agreement with recent SCF calculations of Polotsky et al., with one notable exception that their proposed transition from a longest path stretching mode to a uniform stretching mode, the latter being dominant at high densities, has not been supported by our data. Instead, the longest path stretching dominates at high densi...

Off-lattice computer simulations of polyelectrolyte brushes are carried out using the Debye approximation with explicit counterionsbutimplicitsalt.Theresultsarecomparedwithexplicitsalt ion simulations and self-consistent field theory. We demonstrate that the data generated with these different techniques are in excellent agreement,therebyconfirmingthevalidityoftheDebyeapproximation in the context of polyelectrolyte brushes. The efficiency of the Debye approximation is verified through benchmark computations. Further on,wedevelopanimprovedFlory-typemean-fieldmodel,basedonthe original argument by Pincus, but taking into account both the excluded volume and finite extensibility of chains. On the basis of this model, we demonstrate how the interplay of counterion pressure and excludedvolumerepulsionexplainstheobservedeffectofsaltontheswellingofthebrushundergoodsolventconditions.Thebrush height as a function of grafting density is investigated, and it is argued that the resulting power law scaling of -1/3 is incapable of distinguishing between excluded volume and electrostatic effects.

Off-lattice molecular dynamics simulations are carried out to study fully charged polyelectrolyte brushes with salt, and a comparison with an off-lattice self-consistent field (SCF) theory including finite stretching and volume effects [Biesheuvel et al. Macromolecules 2008, 41, 6254] is presented. The SCF approach is able to reproduce the brush heights at different grafting densities, salt concentrations, and chain lengths on a semiquantitative level. At high grafting densities, the density profiles obtained with both techniques exhibit a particularly close agreement, while at low densities systematic deviations between their shapes are observed. The approximation of local electroneutrality, which the SCF approach is based on, is studied, and its implications are discussed. In this context, the strengths and limitations of the SCF model are analyzed in detail.

Control of ion transport and fluid flow through nanofluidic devices is of primary importance for energy storage and conversion, drug delivery and a wide range of biological processes. Recent development of nanotechnology, synthesis techniques, purification technologies, and experiment have led to rapid advances in simulation and modeling studies on ion transport properties. In this review, the applications of Poisson–Nernst–Plank (PNP) equations in analyzing transport properties are presented. The molecular dynamics (MD) studies of transport properties of ion and fluidic flow through nanofluidic devices are reported as well.

The microphase separation of binary polymer brushes at different temperatures is studied via Langevin molecular dynamics simulations. The order−disorder transition (ODT) from a homogeneous state to...

Binary polymer brushes with hydrophobic/hydrophilic behavior are forming a two-layer system with a collapsed hydrophobic and a swollen hydrophilic phase. The process of switching upon sudden change of the solvent quality is analyzed in detail for various solvent selectivities, chain lengths, and grafting densities. This process is highly reversible since after a microphase separation the chains are moving collectively inside their phase domains so that the interactions between chains of different species are diminished. The thickness of the collapsed layer does not scale linearly with the chain length N, an unexpected result which is discussed in the paper. The switching relaxation times display a scaling of N2 like Rouse relaxation and not of N3 like vertical relaxation times in equilibrium.

Using molecular dynamics (MD) simulation and evolutionary game theory, we incorporate the spacial structure of individuals into the study of the behaviors of cooperation, by adopting the prisoner's dilemma and snowdrift game as metaphors of cooperation between unrelated individuals. The results show that the introduction of spacial structure enhances cooperation using the strategy of prisoner's dilemma while does not make much changes to the cooperation if the strategy of snowdrift game is used. It is also found that our model is a meta-phase between regular ring graph model and complex network model. And the “activity of players” T * we introduced makes our simulation much more closer to real world problems.

We study static and dynamic properties of polymer brushes of moderate and high grafting densities using molecular dynamics simulations. The self-consistent-field theory is able to reproduce the observed density profiles once finite extensibility of the chains is included. A similar approach, using a Langevin-type model to account for a coupling between the vertical and lateral chain fluctuations, is successfully implemented to explain the lateral chain fluctuations at moderate-to-high grafting densities. The corresponding fluctuation dynamics in the direction lateral to the surface is well described by Rouse scaling. The dependence of the dynamics on grafting density (concentration) is very well described by the dynamic behavior of thermal blobs for the fluctuations in the direction both perpendicular and parallel to the surface. We find an anisotropic fluctuation dynamics related with two characteristic dynamic length scales in the both directions. We have further investigated the pulling forces acting on the polymer bonds. The force acting on the first bond directly connected to the surface is reduced at higher grafting densities, which is explained with an entropic contribution to the bond tension. A nonvanishing end-monomer tension is observed at high grafting densities. The integrated force (excess free energy) follows the prediction of the thermal blob model of the brush.

A mean-field model is developed to predict the layer-thickness of sparse and salt-free polyelectrolyte brushes, exposed to an external electric field, which attracts the polyelectrolytes to the substrate. In molecular dynamics simulations, it is shown that a fraction of polymers collapses entirely to screen the charge of the substrate. The remaining brushes are then treated as field-free brushes at reduced grafting density. The mean-field model may thus be applied to field-free brushes, both in their osmotic and their weak charge regimes. It yields simple, closed equations for the brush height and for the partition of counterions in- and outside the brushes, and accurately reproduces simulation data of the collapse of the brushes during the crossover between both charge regimes.

We present molecular dynamics simulations of the interaction of fullerene-like, inhomogeneously charged proteins with polyelectrolyte brushes. A motivation of this work is the experimental observation that proteins, carrying an integral charge, may enter like-charged polymer brushes. Simulations of varying charge distributions on the protein surfaces are performed to unravel the physical mechanism of the adsorption. Our results prove that an overall neutral protein can be strongly driven into polyelectrolyte brush whenever the protein features patches of positive and negative charge. The findings reported here give further evidence that the strong adsorption of proteins is also driven by entropic forces due to counterion release, since charged patches on the surface of the proteins can act as multivalent counterions of the oppositely charged polyelectrolyte chains. A corresponding number of mobile co- and counterions is released from the brush and the vicinity of the proteins so that the entropy of the total system increases.

In order to study nanostructured materials, a fundamental framework of the theory and the computer-experimental studies is established. The essential characteristics of the mesoscopic phase transitions and critical phenomena in these materials are evaluated by means of this approach. For nanostructured materials consisting of inert gas atoms, we study mesoscopic phase transitions and critical phenomena by generalizing the renormalization theory and the Metropolis Monte Carlo method. The results obtained by the both methods are reported in two papers: computational results in the present paper and the theoretical results in the paper which follows.

By defining a topological constraint value (rn), the static and dynamic properties of a polymer brush composed of moderate or short chains with different topological ring structures are studied using molecular dynamics simulation, and a comparison with those of linear polymer brush is also made. For the center-of-mass height of the ring polymer brush scaled by chain length h ~ N ν , there is no significant difference of exponent from that of a linear brush in the small topological constraint regime. However, as the topological constraint becomes stronger, one obtains a smaller exponent. It is found that there exists a master scaling power law of the total stretching energy scaled by chain length N for moderate chain length regime, F ene ~ Nρ ν , for ring polymer brushes, but with a larger exponent ν than 5/6, indicating an influence of topological constraint to the dynamic properties of the system. A topological invariant of free energy scaled by 〈c〉5/4 is found.

We study the effective interactions between nanoparticles immersed in an athermal polymer solution using Molecular dynamics. The directly measured polymer-induced depletion forces are well described with a scaling model in which the attraction between particles is caused by the depletion of concentration blobs and thus independent of the length of the polymer chains. We find strong evidence for a repulsive barrier which arises when the distance between the particles is of the order of the correlation length of the solution and which can be interpreted as a packing effect of concentration blobs. Interestingly, the scaling picture can be extended into the regime in which higher virial coefficients of the polymer solution become relevant. We derive a universal relation between the attraction force at the particle contact, f(0), and the osmotic pressure Π as f(0)∼Π(2/3), demonstrating its validity over a wide range of concentrations of the polymer solution.

Via computer simulations, we demonstrate how a densely grafted layer of polymers, a {\it brush}, could be turned into an efficient switch through chemical modification of some of its end-monomers. In this way, a surface coating with reversibly switchable properties can be constructed. We analyze the fundamental physical principle behind its function, a recently discovered surface instability, and demonstrate that the combination of a high grafting density, an inflated end-group size and a high degree of monodispersity are conditions for an optimal functionality of the switch.

In this paper, polymer brushes are studied via molecular-dynamics simulations at very high grafting densities, where the crossover between the brush regime and the polymer-crystal regime is taking place. This crossover is directly observed with the structure factor and pair-correlation function. With increasing grafting density, this crystallization is progressing from the core layer of the brush towards the surface layer. The same process is analyzed using the lateral fluctuations of the monomers as a signature of their diminishing mobility. Additionally, bond forces and the chain excess free energy indicate a transition from the brush regime to the overstretched regime, which is in agreement with predictions of a modified self-consistent field theory.

The existence of a free energy barrier, which prohibits the upward motion of retracted molecules into the surface region of starlike polymer brushes, is analyzed through molecular dynamics simulations in good solvent. This barrier emerges at moderate and high grafting densities, as a result of a density-discontinuity at the branching points of the highly stretched starlike molecules. The vertical force profiles of brushes of varying densities are taken with the help of a probe-particle that is gradually moved into the brush, and the results are compared with the density profiles and their negative gradients which generate the local osmotic pressures. Chain expulsion simulations, supported by scaling theory, are conducted to understand the dynamics of individual molecules inside the brushes. We prove that the flip-rates between retracted and extended states, being of relevance for the generation of efficiently switchable, environment-responsive brush layers, are determined by the elastic tension of the stretched molecules.

Using molecular dynamics (MD) simulations and density functional theory (DFT) calculations, we systematically study the effective pair potential between two particles induced by unconnected monomers and by polymers at various polymer concentrations (above the overlap), particle sizes, and polymer–particle interactions. In the case of athermal interactions, we verify that the entropic depletion forces between two nanoparticles inside a solvent of unconnected monomers oscillate in accordance with the radial distribution of monomers around one nanoparticle, and that the strength of polymer-induced entropic depletion forces rises linearly with the increase of nanoparticle size. These results are quite consistent with previously obtained experimental and theoretical results. When introducing attractive interactions between nanoparticles and polymers, the adsorption of polymer segments on the surface of each nanoparticle induces repulsive forces between the nanoparticles which can eliminate the depletion attractions. Enhancing the attraction between monomers and nanoparticles leads to the formation of thin polymer-layers on the surfaces of nanoparticles. As a consequence, the depletion attraction reappears at a somewhat increased particle distance. The observed phenomena become increasingly pronounced at higher polymer concentrations. Throughout this work we systematically compare computer simulation results with predictions from density functional theory and show that the data obtained with both approaches are quite consistent with each other.

The static and dynamic properties of a system of end-grafted flexible ring polymer chains grafted to a flat substrate and exposed to a good solvent are studied by using a molecular dynamics method. The monomers are described by a coarse-grained bead-spring model. Varying the grafting density ρ and the degree of polymerization or chain length N, we obtain the density profiles of monomers, study the structural properties of the chain (radius of gyration, bond orientational parameters, etc.), and also present the dynamic characteristics such as chain energy and bond force. Compared with a linear polymer brush, the ring polymer brush exhibits different static and dynamic properties for moderate or short chain length, while it behaves like linear polymer brush in the regime of long chain length.