Polymer-induced Interactions Research Articles

Effects of Polymer Molecular Weight on Structure and Dynamics of Colloid-Polymer Bridging Systems.

We investigate the effects of polymer molecular weight on the structure and dynamics of a model colloid-polymer bridging system using confocal microscopy. Polymer-induced bridging interactions between trifluoroethyl methacrylate-co-tert-butyl methacrylate (TtMA) copolymer particles and poly(acrylic acid) (PAA) polymers of molecular weight Mw of 130, 450, 3000, or 4000 kDa and normalized concentrations c/c* ranging from 0.05 to 2 are driven by hydrogen bonding of PAA to one of the particle stabilizers. At a constant particle volume fraction ϕ = 0.05, the particles form clusters or networks of maximal size at an intermediate polymer concentration and become more dispersed upon further addition of polymer. Increasing the polymer Mw at a fixed normalized concentration c/c* increases the cluster size: suspensions with 130 kDa polymer contain small clusters that remain diffusive, and those with 4000 kDa polymer form larger, dynamically arrested clusters. Biphasic suspensions with distinct populations of disperse and arrested particles form at low c/c*, where there is insufficient polymer to bridge all particles, or high c/c*, where some particles are sterically stabilized by the added polymer. Thus, the microstructure and dynamics in these mixtures can be tuned through the size and concentration of the bridging polymer.

Effect of adsorbed polyelectrolytes on the interactions and elasticity of charged surfactant bilayers

We present studies on the structure of complexes of the cationic, bilayer-forming surfactant, didodecyldimethylammonium bromide (DDAB), and the anionic polyelectrolyte sodium polyacrylate (PAANa). In the presence of uncomplexed polyelectrolyte in the coexisting aqueous solution, these complexes are found to exhibit a swelling transition followed by a deswelling transition on increasing the salt concentration. Lamellar structures with low periodicities occur at both low and high salt concentrations, which are stabilized by polymer bridging and van der Waals attraction, respectively. The swollen complex found at intermediate salt concentrations forms the sponge phase. Our results reveal that polyelectrolyte adsorption on bilayers has a profound effect on inter-bilayer interactions. The polymer-induced interaction changes from being attractive to repulsive as the surface coverage increases on increasing the salt concentration. Our results also confirm that polymer adsorption alters the elastic moduli of the bilayer, in agreement with earlier theoretical predictions.

Modeling Intercolloidal Interactions Induced by Adsorption of Mobile Telechelic Polymers onto Particle Surfaces

We combine the self-consistent field theory and the Derjaguin approximation to predict the polymer-induced colloidal interactions and the nonuniform distributions of the loops and bridges of telech...

Tailored Colloidal Stability and Rheological Properties of Graphene Oxide Liquid Crystals with Polymer-Induced Depletion Attractions.

Graphene oxide liquid crystallinity (GO LC) has been widely exploited for high-performance graphene-based applications. In this regard, colloidal stability of GO LC suspension is a crucial requirement, particularly while polymers are often added to the GO LC. Unfortunately, current level of knowledge on how polymers influence the structure and properties of GO LC is not sufficient to systematically guide the development of applications. Here, we investigate the microstructure and rheological properties of GO LC suspensions in the presence of polymer additives with varying molecular weights and concentrations. Similar to conventional colloidal systems, non-negligible polymer-induced interactions are found in GO LC suspensions, which can effectively modulate the interaction among GO platelets and the relevant physical properties. On the basis of extensive small-angle X-ray scattering and rheological measurements, we demonstrate that, contrary to the general perception, polymer-induced depletion attraction can increase the colloidal stability of GO, while also preventing the vitrification of GO LC. In addition, a proper level of polymer additive can reduce the viscosity of GO LC suspensions by orders of magnitude, providing an effective route to GO LC-based solution processing. After all, the colloidal stability and rheological properties of GO can significantly impact the quality of GO. Therefore, we believe that our finding will be of great interest in the field of graphene-based applications, as it presents effective strategies for improving properties.

Influence of dielectric inhomogeneities on the structure of charged nanoparticles in neutral polymer solutions.

We study the structural characteristics of a system of charged nanoparticles in a neutral polymer solution while accounting for the differences in the dielectric constant between the particles, polymer and the solvent. We use a hybrid computational methodology involving a combination of single chain in mean-field simulations and the solution of the Poisson's equation for the electrostatic field. We quantify the resulting particle structural features in terms of radial distribution function among particles as a function of the dielectric contrast, particle charge, particle volume fraction and polymer concentration. In the absence of polymers, charged macroions experience increased repulsion with a lowering of the ratio of particle to solvent dielectric constant. The influence of the dielectric contrast between the particle and the solvent however diminishes with an increase in the particle volume fraction and/or its charge. In the presence of neutral polymers, similar effects manifest, but with the additional physics arising from the fact that the polymer-induced interactions are influenced by the dielectric contrast of the particle and solvent.

Aggregation-Driven Controllable Plasmonic Transition of Silica-Coated Gold Nanoparticles with Temperature-Dependent Polymer-Nanoparticle Interactions for Potential Applications in Optoelectronic Devices.

Localized surface plasmon resonance (LSPR) effect relies on the shape, size, and dispersion state of metal nanoparticles and can potentially be employed in many applications such as chemical/biological sensor, optoelectronics, and photocatalyst. While complicated synthetic approaches changing shape and size of nanoparticles can control the intrinsic LSPR effect, here we show that controlling interparticle interactions with silica-coated gold nanoparticles (Au@SiO2 NPs) is a powerful approach, permitting wide range of optical bandwidth of gold nanoparticles with great stability. The interparticle interactions of Au@SiO2 NPs are controlled through concentration-, temperature-, and time-dependent polymer-induced interactions. The polymer-induced interactions modulate the state of particle dispersion, resulting an effective plasmonic shift by more than 200 nm. We further explore the microstructure of particle aggregation and explain mechanisms of plasmonic shift based on the results of small-angle X-ray scattering (SAXS) and discrete dipole approximation (DDA) calculation. We show that an effective control of LSPR behavior is now available through trapped aggregation of Au@SiO2 NPs with temperature variation. We anticipate that the suggested strategy can be employed in many practical applications such as optical bioimaging and optoelectronic devices.

Theoretical Predictions of Temperature-Induced Gelation in Aqueous Dispersions Containing PEO-Grafted Particles.

In this work, we utilize classical polymer density functional theory (DFT) to study gelation in systems containing colloidal particles onto which polymers are grafted. The solution conditions are such that the corresponding bulk system displays a lower critical solution temperature (LCST). We specifically compare our predictions with experimental results by Shay et al. (J. Rheol. 2001, 45, 913-927), who investigated temperature response in aqueous dispersions containing polystyrene particles (PS), with grafted 45-mer poly(ethylene oxide) (PEO) chains. Our DFT treatment is based on a model for aqueous PEO solutions that was originally developed by Karlström for bulk solutions. In this model, monomers are assumed to be in either of two classes of states, labeled A and B, where B is more solvophobic than A. On the other hand, the degeneracy of B exceeds that of A, causing the population of solvophobic monomers to increase with temperature. In agreement with experimental findings by Shay et al., we locate gelation at temperatures considerably below TΘ, and far below the LCST for such chain lengths. This gelation occurs also without any dispersion interactions between the PS particles. Interestingly, the polymer-induced interaction free energy displays a nonmonotonic dependence on the grafting density. At high grafting densities, bridging attractions between grafted layers take place (considerably below TΘ). At low grafting densities, on the other hand, the polymers are able to bridge across to the other particle surface. Shay et al. conducted their experiments at very low ionic strength, using deionized water as a solvent. We demonstrate that even minute amounts of adsorbed charge on the surface of the particles, can lead to dramatic changes of the gelation temperature, especially at high grafting densities. Another interesting prediction is the existence of elongated (chainlike) equilibrium structures, at low particle concentrations. We emphasize that our model does not rely upon any temperature-dependent interactions.

Nanoparticle Organization in Sandwiched Polymer Brushes

The organization of nanoparticles inside grafted polymer layers is governed by the interplay of polymer-induced entropic interactions and the action of externally applied fields. Earlier work had shown that strong external forces can drive the formation of colloidal structures in polymer brushes. Here we show that external fields are not essential to obtain such colloidal patterns: we report Monte Carlo and molecular dynamics simulations that demonstrate that ordered structures can be achieved by compressing a "sandwich" of two grafted polymer layers, or by squeezing a coated nanotube, with nanoparticles in between. We show that the pattern formation can be efficiently controlled by the applied pressure, while the characteristic length-scale, that is, the typical width of the patterns, is sensitive to the length of the polymers. Based on the results of the simulations, we derive an approximate equation of state for nanosandwiches.

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.

Polymer-mediated spatial organization of nanoparticles in dense melts: Transferability and an effective one-component approach

We study two problems in the framework of the integral equation theory of polymer-mediated spatial organization of nanoparticles in dense melts motivated by multiscale simulation and many body physics issues. How nonspherical nanoparticle shape modifies polymer-induced interactions under dilute nanoparticle conditions is investigated over a range of primary particle sizes and interfacial cohesion strengths. Nonuniversal consequences of nonspherical shape are found for the pair-correlation function on local scales and some qualitative differences on larger scales due primarily to intraparticle connectivity constraints. For a large enough nanoparticle site diameter, the potentials of mean force (PMF) for all shapes studied (sphere, rod, disk, compact tetrahedral cluster) exhibit linear scaling with the size ratio of nanoparticle to polymer monomer site diameter and quite good "transferability." The ability of a simple effective one-component approach, based on the dilute nanoparticle PMF as an effective pair-decomposable potential, to describe interparticle structure at nonzero volume fractions is also studied. Although not generally quantitatively accurate due to neglect of many body correlation effects, especially at high nanoparticle loadings and near contact separations, the simple approach captures rather well many aspects of the real space structure. The errors incurred depend systematically on whether interfacial cohesion strength results in contact aggregation, steric stabilization, or bridging. For the filler collective static structure factor, many body effects are weakest for local cage scale correlations and grow significantly at smaller wavevectors under depletion or bridging conditions.

Modeling polymer-induced interactions between two grafted surfaces: Comparison between interfacial statistical associating fluid theory and self-consistent field theory

The interaction between two polymer grafted surfaces is important in many applications, such as nanocomposites, colloid stabilization, and polymer alloys. In our previous work [Jain et al., J. Chem. Phys. 128, 154910 (2008)], we showed that interfacial statistical associating fluid density theory (iSAFT) successfully calculates the structure of grafted polymer chains in the absence/presence of a free polymer. In the current work, we have applied this density functional theory to calculate the force of interaction between two such grafted monolayers in implicit good solvent conditions. In particular, we have considered the case where the segment sizes of the free (sigma(f)) and grafted (sigma(g)) polymers are different. The interactions between the two monolayers in the absence of the free polymer are always repulsive. However, in the presence of the free polymer, the force either can be purely repulsive or can have an attractive minimum depending upon the relative chain lengths of the free (N(f)) and grafted polymers (N(g)). The attractive minimum is observed only when the ratio alpha = N(f)/N(g) is greater than a critical value. We find that these critical values of alpha satisfy the following scaling relation: rho(g) square root(N(g)) beta(3) proportional to alpha(-lambda), where beta = sigma(f)/sigma(g) and lambda is the scaling exponent. For beta = 1 or the same segment sizes of the free and grafted polymers, this scaling relation is in agreement with those from previous theoretical studies using self-consistent field theory (SCFT). Detailed comparisons between iSAFT and SCFT are made for the structures of the monolayers and their forces of interaction. These comparisons lead to interesting implications for the modeling of nanocomposite thermodynamics.

Potential of mean force between two nanometer-scale particles in a polymer solution

Expanded ensemble density-of-states simulations and a connectivity altering algorithm are used to investigate the effective interactions that arise between nanoparticles suspended in polymer solutions. Our calculations with systems of long polymeric chains reveal oscillations in the effective polymer-induced interactions between the particles, even at low concentrations. The range of these interactions is considerably longer than originally anticipated, and their origin is traced back to the chain-end effects and density fluctuations that were absent in previous treatments of these systems.

Fluctuation-induced long-range interactions in polymer systems

We discover a new universal long-range interaction between solid objects in polymermedia. This polymer-induced interaction is directly opposite to the van der Waalsattraction. The predicted effect is deeply related to the classical Casimir interactions,providing a unique example of universal fluctuation-induced repulsion ratherthan normal attraction. This universal repulsion comes from the subtracted softfluctuation modes in the ideal counterpart of the real polymer system. The effect canalso be interpreted in terms of subtracted (ghost) large-scale polymer loops. Weestablish the general expressions for the energy of polymer-induced interactionsfor arbitrary solid particles in a concentrated polymer system. We find that thecorrelation function of the polymer density in a concentrated solution of very longchains follows a scaling law rather than an exponential decay at large distances.These novel universal long-range interactions can be of importance in variouspolymer systems. We discuss the ways to observe/simulate these fluctuation-inducedeffects.

Polymer Polydispersity Effect on Depletion Interaction between Colloidal Particles

The effect of polymer polydispersity on the polymer-induced interaction between colloidal particles due to non-adsorbing ideal chains is investigated. An analytical theory is developed for the polymer-segment density between two plates and in the space surrounding two spheres by extending a recently proposed superposition approximation to include polymer polydispersity. Monte Carlo computer simulations were made to test the validity of the analytical theory. The polymer densities predicted by the superposition approximation are in reasonable agreement with simulation results for the polydisperse case. The simulations show that depletion leads to a size fractionation of the polymers. It is shown that size polydispersity has a small effect on the interaction between two parallel plates but a more significant effect on the interaction between two spheres. The range of the potential increases and the contact potential drops with increasing polydispersity. Polymer-segment density as a function of y for three values of x, as indicated, in the space surrounding two colloidal spheres with radius R = Rg0 and h = 0.48Rg0. Symbols are the MC results: polydisperse polymer (○; z = 1) and monodisperse polymer (•) samples. Curves are the predictions of the product-function approximation for monodisperse polymer (solid lines) and polydisperse polymer (z = 1, dashed lines).

Polymer depletion interaction between parallel walls--a Monte Carlo study.

An off-lattice bead-spring model of self-assembling equilibrium ("living") polymers is used to study the polymer-induced interaction between parallel walls immersed in polydisperse solutions of different concentration by means of Monte Carlo simulation. The two walls form an open slit in contact with an external reservoir so that the confined system may exchange monomers with the surrounding phase and adapt its polydispersity in order to relax the confinement constraint. We find that the properties of the polymers in the constrained system as well as the net force deltaF acting on the walls depend essentially on the polymer concentration in the reservoir which leads to qualitative differences in their behavior with changing inter-planar distance H: In a dilute polymer solution at concentration phi below the semi-dilute threshold phi* the force between the walls is attractive and decreases steadily with growing wall separation H, so that deltaF approximately 0 at H/ Rg> or =3 if H is measured in gyration radii Rg of the unperturbed polymers. The total monomer concentration within the slit is smaller than the concentration in the reservoir and decreases monotonically with H/Rg-->0. The ratio Nin/Nout of mean chain length Nin in the slit to that in the reservoir, Nout, decreases from unity at H-->infinity, goes through a minimum at H/Rg approximately 1, and then rises again to Nin/Nout>1 for wall separations H/Rg<1. In contrast, in a dense solution of equilibrium polymers at phi>phi* one detects no indirect wall-wall interaction, deltaF approximately 0, for H larger than the monomer size. Thus, earlier speculations about the existence of possible depletion interaction between parallel walls even in a dense polymer system cannot be confirmed. Inside the slit the monomer density is found to be always larger than in the reservoir while Nin/Nout<1 and decreases steadily as H/Rg-->0. The depletion force between parallel plates has been determined also in a monodisperse solution of conventional polymers. Qualitatively the force behavior does not differ from that of living polymers.

Small mesoscopic particles in dilute and semidilute solutions of nonadsorbing polymers

Polymer-induced interactions between two small mesoscopic particles of radius R and between a particle and a wall are investigated for dilute or semidilute embedding solutions of long flexible nonadsorbing polymer chains with radius of gyration Rg. Asymptotically exact predictions are obtained using a “small radius expansion,” to express the interactions in terms of properties of the polymer solution without particles. The nonmonotonic dependence of the second virial coefficient B2CC of a dilute suspension of particles on the interchain overlap is discussed in detail. The magnitude of the minimum of B2CC/R3 increases as a power law in Rg/R. The exponent and amplitude are quite different from the earlier prediction of an integral-equation approach. For dilute polymers in two dimensions outside two circular disks in contact, exact results are given for the monomer-density depletion profile, the pressure variation along the perimeter of, and the polymer-induced force between the two disks.

Polymer-Induced Repulsive Forces: Exponential Scaling

The polymer-induced repulsive interactions between the surfaces of a liquid film are measured for two systems using two force measurement techniques, in the case where the polymer is in a good solvent and adsorbs at the liquid surface. The force decays exponentially with the film thickness with a decay length proportional to the radius of gyration. The data are compared with a recent theoretical work which predicts a repulsive exponentially decaying force at large thicknesses. [S0031-9007(98)05373-3]

Computer simulation of colloid-polymer mixtures

We present results of a computer simulation study of a simple model for a colloid dispersed in a dilute polymer solution. The study aims to investigate the role of entropy in such mixtures. We have computed the polymer-induced interaction, the phase behaviour, and the local structure of the colloidal component, and determined the dependence of these properties on the polymer-colloid size ratio.

Colloids dispersed in polymer solutions. A computer simulation study

We report a computer-simulation study of a simple model for a colloid dispersed in a polymer solution. The simulations were performed using a computational scheme that allows simulation at constant osmotic pressure of the polymers. We present results for the polymer-induced interaction, the equation-of-state, and the phase diagram. The simulations show that when the radius of gyration of the polymers Rg is sufficiently large compared to the diameter of the colloidal particles σcol (2Rg/σcol≥0.45), addition of polymer induces a colloidal ‘‘liquid–vapor’’ phase separation. For shorter polymers, only the solid–fluid transition is observed. In addition, we find that the nonpairwise additivity of the polymer-induced attraction between the colloidal particles has a pronounced effect on the equation-of-state and the phase behavior of a colloid–polymer mixture. The perturbation theory of Lekkerkerker et al. [Europhys. Lett. 20, 559 (1992)] is found to perform well, except at low densities.

Polymer-induced interaction and quasi-micellar molecular organization in dynamic equilibrium

Polymer-induced interaction and quasi-micellar molecular organization in dynamic equilibrium