Phase behavior of mixtures of hard colloids and soft coarse-grained macromolecules.

Effective "soft" interactions between macromolecules such as polymers, amphiphilic dendrimers, and suitably designed DNA based dendritic molecules have been shown to be purely repulsive and non-diverging. We report the structure and phase behavior of a mixture of hard colloids and soft coarse-grained macromolecules. Through the use of Reference Interaction Site Model theory and molecular dynamics simulations we find that hard colloids and soft macromolecules act as depletants toward each other, generating a medium-induced effective attraction.

Polymer-mediated self-assembly, dispersion, and phase separation of Janus nanorods.

The challenge of stabilizing polymer nanocomposites lies in the fact that nanoparticles tend to phase separate from the polymer melt due to an entropic 'depletion attraction' between nanoparticles. Additionally, composites of polymer and nanorods show a decrease in miscibility with increasing nanorod aspect ratio [U. K.

Growth transitions and critical behavior in the non-equilibrium aggregation of short, patchy nanorods.

We have carried out Monte Carlo simulations to study the non-equilibrium aggregation of short patchy nanorods in two dimensions. Below a critical value of patch size ([Formula: see text]), the aggregates have finite sizes with small radii of gyration, [Formula: see text]. At [Formula: see text], the average radius of gyration shows a power law increase with time such that [Formula: see text], where [Formula: see text].

Phase behavior of polymer-nanorod composites: A comparative study using PRISM theory and molecular dynamics simulations.

Well-dispersed composites of polymer and nanorods have many emerging applications and, therefore, are an important area of research. Polymer reference interaction site model (PRISM) theory and molecular dynamics simulations have become powerful tools in the study of the structure and phase behavior of polymer nanocomposites. In this work, we employ both PRISM theory and molecular dynamics simulations to determine the structure and spinodal phase diagram of 1% volume fraction of nanorods in a polymer melt.

The effect of surface roughness on the phase behavior of colloidal particles.

Shape anisotropy of colloidal particles can give rise to complex intermolecular interactions that determine particle packing and phase behavior. The vapor-liquid coexistence curves of attractive rough particles display a shift when compared to attractive smooth spherical particles. We use Integral Equation Theory (IET) to determine the vapor-liquid spinodal phase diagram of smooth and rough colloidal particles interacting through square-well attraction.

Self-assembly of polymer-linked nanoparticles and scaling behavior in the assembled phase.

An entropic depletion-driven phase separation is known to be observed for mixtures of polymers and nanoparticles. While polymer-linked nanoparticles have been synthesized, their phase behavior has only been predicted for chemically specific interactions. We use integral equation theory to determine the structure and phase behavior of chemically isotropic polymer-linked nanoparticles at high densities.

Theoretical study of the structure and assembly of Janus rods.

Microscopic integral equation theory is applied to investigate the real and Fourier space structure and phase behavior of compositionally symmetric AB Janus rods that interact via hard core excluded volume interactions and competing repulsive and attractive tail potentials. If the spatial range of the latter are short and equal, chemical asymmetry results in attraction-driven assembly into a cylindrical micellar structure of spatial periodicity between one and two rods lengths. The apparent microphase spinodal ordering temperature increases with attraction range and rod length, and the microdomain coherence length strongly grows upon cooling.

Activated dynamics in dense fluids of attractive nonspherical particles. II. Elasticity, barriers, relaxation, fragility, and self-diffusion.

In paper II of this series we apply the center-of-mass version of Nonlinear Langevin Equation theory to study how short-range attractive interactions influence the elastic shear modulus, transient localization length, activated dynamics, and kinetic arrest of a variety of nonspherical particle dense fluids (and the spherical analog) as a function of volume fraction and attraction strength. The activation barrier (roughly the natural logarithm of the dimensionless relaxation time) is predicted to be a rich function of particle shape, volume fraction, and attraction strength, and the dynamic fragility varies significantly with particle shape. At fixed volume fraction, the barrier grows in a parabolic manner with inverse temperature nondimensionalized by an onset value, analogous to what has been established for thermal glass-forming liquids.

Activated dynamics in dense fluids of attractive nonspherical particles. I. Kinetic crossover, dynamic free energies, and the physical nature of glasses and gels.

We apply the center-of-mass versions of naïve mode coupling theory and nonlinear Langevin equation theory to study how short-range attractive interactions modify the onset of localization, activated single-particle dynamics, and the physical nature of the transiently arrested state of a variety of dense nonspherical particle fluids (and the spherical analog) as a function of volume fraction and attraction strength. The form of the dynamic crossover boundary depends on particle shape, but the reentrant glass-fluid-gel phenomenon and the repulsive glass-to-attractive glass crossover always occur. Diverse functional forms of the dynamic free energy are found for all shapes including glasslike, gel-like, a glass-gel form defined by the coexistence of two localization minima and two activation barriers, and a "mixed" attractive glass characterized by a single, very short localization length but an activation barrier located at a large displacement as in repulsive-force caged glasses.

Janus particle synthesis and assembly.

Janus particles, colloid-sized particles with two regions of different surface chemical composition, possess energetic interactions that depend not only on their separation but also on their orientation. Research on Janus and colloidal particles that are chemically patchy in even more complicated fashion has opened a new chapter in the colloid research field. This article highlights recent progress in both experiment and theory regarding synthesis and self-assembly of Janus particles, and tentatively outlines some areas of future opportunity.

The influence of shape on the glassy dynamics of hard nonspherical particle fluids. II. Barriers, relaxation, fragility, kinetic vitrification, and universality.

We extend and apply the nonlinear Langevin equation theory of activated barrier hopping dynamics in glassy fluids and colloidal suspensions to study broad families of one-, two-, and three-dimensional hard nonspherical particles. Beyond the ideal kinetic arrest volume fraction, entropic barriers emerge with heights (alpha relaxation times, inverse diffusion constants) that increase nonlinearly (nonexponentially) with volume fraction and in a manner that becomes stronger with particle dimensionality. Partial collapse of the volume fraction dependence of barrier heights and reduced relaxation times of different particle shapes within a fixed dimensionality class are achieved based on a difference volume fraction variable that quantifies the distance from the ideal mode coupling theory dynamic crossover.

The influence of shape on the glassy dynamics of hard nonspherical particle fluids. I. Dynamic crossover and elasticity.

We extend and apply the center-of-mass version of the microscopic naive mode coupling theory to study the ideal kinetic glass transition of dense fluids and suspensions composed of broad families of one-, two-, and three-dimensional hard nonspherical particles. A kinetic arrest diagram is constructed which indicates a dynamical crossover or onset of activated barrier hopping controlled transport. We find (quasi-) one-dimensional rods and rings form ideal glasses at the lowest volume fractions which decrease strongly with aspect ratio.