Origin of particle clustering in a simulated polymer nanocomposite and its impact on rheology

Abstract

Many nanoparticles have short-range interactions relative to their size, and these interactions tend to be ''patchy'' since the interatomic spacing is comparable to the nanoparticle size. For a dispersion of such particles, it is not a priori obvious what mechanism will control the clustering of the nanoparticles, and how the clustering will be affected by tuning various control parameters. To gain insight into these questions, we perform molecular dynamics simulations of polyhedral nanoparticles in a dense bead-spring polymer melt under both quiescent and steady shear conditions. We explore the mechanism that controls nanoparticle clustering and find that the crossover from dispersed to clustered states is consistent with the predictions for equilibrium particle association or equilibrium polymerization, and that the crossover does not appear to match the expectations for first-order phase separation typical for binary mixtures in the region of the phase diagram where we can equilibrate the system. At the same time, we cannot rule out the possibility of phase separation at a lower temperature. Utilizing the existing framework for dynamic clustering transitions offers the possibility of more rationally controlling the dispersion and properties of nanocomposite materials. Finally, we examine how nanocomposite rheology depends on the state of equilibrium clustering. We find that the shear viscosity for dispersed configurations is larger than that for clustered configurations, in contrast to expectations based on macroscopic colloidal dispersions. We explain this result by the alteration of the polymer matrix properties in the vicinity of the nanoparticles. We also show that shear tends to disperse clustered nanoparticle configurations in our system, an effect particularly important for processing.

Model for reversible nanoparticle assembly in a polymer matrix

“…[10][11][12][13] The mechanism of cluster assembly is particularly relevant to polymer-NP composites, but has potential relevance for many fields, such as biological systems where molecular assembly often occurs in the "crowded" intracellular and extracellular environments. 14,15 Our study builds upon the observations made in an earlier study 16 on the factors controlling NP dispersion in polymer melts. From Ref.…”

“…From Ref. 16 it was found that the NP underwent a thermally reversible transition to a clustered state upon cooling, in lieu of ordinary phase separation from the polymer matrix. The clustering had characteristics similar to those of equilibrium polymerization [10][11][12][13]17,18 and micelle formation.…”

Rheology of Polymers with Nanofillers

“…Although there were some differences in scale between the simulated composite and the experimental data, qualitative similarities were noted in the shear behavior. Starr et al [2003] investigated the effect of particle clustering on the PNC rheological behavior. Systems with well-dispersed particles showed higher η, especially at lowγ, which increased with concentration (φ ≥ 0.15).…”

Some issues in polymer nanocomposites: Theoretical and modeling opportunities for polymer physics

“…Most such theories and models use a coarse-grained perspective where simple micromechanical representations are used for the polymers and fillers, with their interactions represented by few coarse-grained parameters. Using such representations, theories and simulations based on PRISM, [16][17][18] self-consistent field approaches, [19][20][21][22] field-theoretic, 23 and molecular dynamics simulations [24][25][26] have addressed the equilibrium structure and phase behavior of mixtures of (mostly) spherical nanofiller units dispersed in homopolymers, polymer blends, and block copolymers. Despite these advances, a number of open issues relevant to PNCs still remain:…”