No abstract

The interactions between mesophase-forming copolymers and nanoscopic particles can lead to highly organized hybrid materials. The morphology of such composites depends not only on the characteristics of the copolymers, but also on the features of the nanoparticles. To explore this vast parameter space and predict the mesophases of the hybrids, we have developed a mean field theory for mixtures of soft, flexible chains and hard spheres. Applied to diblock-nanoparticle mixtures, the theory predicts ordered phases where particles and diblocks self-assemble into spatially periodic structures. The method can be applied to other copolymer-particle mixtures and can be used to design novel composite architectures.

We investigate the influence of hard nanoparticles on the phase behavior of diblock copolymers. Using Monte Carlo simulations, we obtain phase diagrams as a function of the nanoparticle size and concentration. When the size of the nanoparticles becomes comparable to the radius of gyration of the minority (A) block, we observe the formation of new superstructures, where the particles selfassemble inside the copolymer micelles. We develop a theoretical model, based on the strong segregation limit approximation, and show that these self-assembled structures can be either stable or metastable, depending on the particle size and volume fraction. The formation of such phases is due to the interplay between the particle-particle excluded-volume interactions, preferential particle/block-A interactions, and the enthalpic and stretching interactions within the diblock.

Mixtures of diblock copolymers and nanoscopic spherical particles can yield well-ordered hybrid materials, which can be used for separation processes, catalysis, and optoelectronic applications. Predicting the morphologies of these systems is difficult because the final structures depend not only on the characteristics of the copolymer but also on the features of the particles. Combining self-consistent field and density functional theories, we develop a model that allows us to determine the equilibrium or metastable phases of diblock copolymer/spherical nanoparticle composites, without making a priori assumptions about the structure of the system. Using this model, we illustrate various examples where mixtures of diblocks and nanoparticles self-assemble into mesoscopically ordered phases. The model can be generalized to other types of copolymers and particles and can be modified to include homopolymers or solvent molecules. Thus, the technique constitutes a useful tool for determining the structures of a large class of nanocomposites.

Interactions between nanoparticles and cell membranes may play a crucial role in determining the cytotoxicity of nanoparticles as well as their potential application as drug delivery vehicles or therapeutic agents. It has been shown that such interactions are often determined not by biochemical but by physicochemical factors (e.g., nanoparticle size, hydrophobicity, and surface charge density). Here, we propose a mesoscale thermodynamic model describing the transitions in membrane morphology observed after exposure to various types of nanoparticles. Our simulations demonstrate under which conditions (determined by particle size and hydrophilic/hydrophobic interactions) the particles can adsorb into the membrane or compromise the membrane integrity to result in the formation of nanosized holes. The model could be refined to include a more accurate description of various phospholipid membranes, and its results could be applied in the design of specific nanoparticles for various biomedical applications.

We propose a simple theory describing the influence of nanoparticles on thermodynamics of binary polymer mixture. In particular, we consider the case in which nanoparticles preferentially segregate into one of the polymeric components. Depending on the particle radius R p and the polymer degree of polymerization N, addition of nanoparticles can either promote or hinder mixing of the polymers. We calculate how the addition of nanoparticles shifts the spinodal of the polymer blend. These results help to improve understanding of recent simulations on the dynamics of polymer/particle mixtures.

Simulations show that when low-volume fractions of nanoscale rods are immersed in a binary, phase-separating blend, the rods self-assemble into needle-like, percolating networks. The interconnected network arises through the dynamic interplay of phase-separation between the fluids, through preferential adsorption of the minority component onto the mobile rods, and through rod-rod repulsion. Such cooperative effects provide a means of manipulating the motion of nanoscopic objects and directing their association into supramolecular structures. Increasing the rod concentration beyond the effective percolation threshold drives the system to self-assemble into a lamellar morphology, with layers of wetted rods alternating with layers of the majority-component fluid. This approach can potentially yield organic/inorganic composites that are ordered on nanometer scales and exhibit electrical or structural integrity.

We combine a density functional theory (DFT) with a self-consistent field model (SCF) to calculate the phase behavior of thin, oblate colloidal particles that are coated with surfactants and dispersed in a polymer melt. These coated particles represent organically modified clay sheets. By integrating the two methods, we can investigate the effect of the surfactants' characteristics (grafting density F gr and length Ngr) and the polymer-surfactant interaction energy on the polymer-clay phase diagram. Depending on the values of these critical parameters and the clay volume fraction, φ, the system can be in an isotropic or nematic phase (which corresponds to an exfoliated composite). The system can also form a smectic, crystal, columnar, or "house-of-cards" plastic solid, as well as a two-phase (immiscible) mixture. Using this model, we isolate conditions that lead to the stabilization of the homogeneous, exfoliated phases (the isotropic and nematic regions) and to the narrowing of the immiscible two-phase regions.