Controlling the Location of Bare Nanoparticles in Polymer-Nanoparticle Blend Films by Adding Polymer-Grafted Nanoparticles

We present molecular dynamics simulations of polymer-nanoparticle blends in films containing both grafted and ungrafted nanoparticles where the particle cores are identical and grafted chains are similar to a matrix polymer. Our results indicate that it is possible to control the location of bare nanoparticles in the film by adding small amounts of polymer-grafted nanoparticles. In the presence of a substrate, bare particles are entropically pushed to the surface. We observed that the introduction of grafted particles to the blend prevents the migration of bare particles to the surface. This unusual behavior is caused by the formation of binary aspherical clusters due to the presence of grafted particles. Hence, parameters including grafting density and the length of the grafted polymer play a significant role in dictating the spatial arrangement of bare particles in the blend film. At higher values of these parameters, the grafted particle core is shielded from depletion attractions causing the density of bare particles to increase back near the surface.

Figure 1

Nanoparticle density profiles perpendicular to the surface for various (a) grafting densities, with $M_g=25$, and (b) grafted chain length, with ${\mathrm{\Sigma }}_g=0.36$, in systems with homogeneous particles. All particles are either grafted or ungrafted.

Figure 2

Bare particle density profiles perpendicular to the surface for various concentrations of grafted particles (${\mathrm{\Sigma }}_g=0.36$ and $M_g=25$) in systems with a mixture of bare and grafted particles. Inset: Excess adsorption of bare particles on the surface as a function of volume fraction of grafted particles.

Figure 3

(a) Asphericity of nanoparticle clusters formed within the system as a function of volume fraction of grafted particles (${\mathrm{\Sigma }}_g=0.36$ and $M_g=25$). Insets: Binary clusters (bare and grafted particles are represented by pink and green spheres, respectively) formed in the bulk for ${\varphi }_g=0.011$ (left), 0.043 (right) and average volume of clusters as a function of grafted particle concentration. (b) Normalized fraction of bare particles near the surface as a function of simulation time. Here, $f_{b_0}$ is the fraction of bare particles near the surface in randomly generated initial configuration.

Figure 4

Bare particle density profiles perpendicular to the surface for different (a) grafting densities, with $M_g=25$, and (b) grafted chain length, with ${\mathrm{\Sigma }}_g=0.36$, in systems with ${\varphi }_g=0.043$.