It is easy to understand the self-assembly of particles with anisotropic shapes or interactions (for example, cobalt nanoparticles or proteins) into highly extended structures. However, there is no experimentally established strategy for creating a range of anisotropic structures from common spherical nanoparticles. We demonstrate that spherical nanoparticles uniformly grafted with macromolecules ('nanoparticle amphiphiles') robustly self-assemble into a variety of anisotropic superstructures when they are dispersed in the corresponding homopolymer matrix. Theory and simulations suggest that this self-assembly reflects a balance between the energy gain when particle cores approach and the entropy of distorting the grafted polymers. The effectively directional nature of the particle interactions is thus a many-body emergent property. Our experiments demonstrate that this approach to nanoparticle self-assembly enables considerable control for the creation of polymer nanocomposites with enhanced mechanical properties. Grafted nanoparticles are thus versatile building blocks for creating tunable and functional particle superstructures with significant practical applications.

No abstract

We present a self-consistent field theory model for the self-assembly behavior of rod-coil block copolymers. The orientational interactions between the rods were modeled through a Maier-Saupe interaction, while the enthalpic interactions between rods and coils were modeled through a standard Flory-Huggins approach. We outline a "real-space" numerical approach to solve the self-consistent field equations for such rod-coil block copolymers. A major focus of our work is upon the nonlamellar phases observed in the experiments on such polymers. To develop a physical understanding of these phases and their regimes of occurrence, we compute the two-dimensional phase diagram for our model. The latter shows significant departures from the one-dimensional phase diagram, but matches qualitatively with the existing experimental results. We also present scaling arguments that rationalize the numerical results for the self-assembly behavior.

No abstract

We use computer simulations to study the mechanisms governing the linear viscoelasticity behavior of composites of spherical nanofillers dispersed in polymer melt matrices. Our results suggest that particles can influence the viscoelastic properties of the system by a variety of different mechanisms. On one hand, the particleinduced effects on the dynamics of polymer segments modify the relaxation spectrum of the polymers. Second, particle jamming effects lead to slow relaxations and substantial enhancements in elasticity. Finally, our results suggest that the strain field distortion caused by the presence of rigid inclusions also affects the overall modulus of the composite. For our model system, we delineate the regimes and frequencies at which the different effects manifest and also suggest how the picture can be generalized for parametric conditions different from our simulations.

No abstract

The theory of the coil-globule transition is developed based on the Flory method and with the use of various distribution functions for the radius of gyration of the chain for the regions of chain swelling and contraction. The theory coincides with experimental data for polystyrene and with results of computer simulation for long chains.

A novel Brownian dynamics method has been developed to simulate the properties of polymer brushes under shear. Simulations of 100 chains with a chain length of 100 segments have been carried out for a range of shear rates. Compared to previous methods there is a substantial saving in computational time as the self-consistent molecular field method has been chosen to calculate the volume interactions between polymer segments. An important criterion for observing significant deformations of the brush is that the chains must be stretched to beyond the Gaussian threshold. Density profiles and end segment distribution functions for the grafted chains have been determined and show a collapse of the brush under shear in a way similar to that in which a brush contracts in a poor solvent. In particular, the free ends of the chains become concentrated in a narrow region at the periphery of the brush. The number of chains that are affected by shear has also been calculated and shows that there is a progressive transmission of shear into the brush.