Session B4: Hybrid Nanomaterials Assembly

``Hairy'' Nanoparticles in Block Copolymers and Homopolymers: Modeling using Hybrid Self-Consistent Field Theory

Today, dispersed nanoparticles play important role in various applications (toughened plastics, healthcare, personal care, etc.) Mesoscale simulations and theory are important in understanding what governs the morphology of nanoparticles under various conditions. In particular, for nanoparticle/block copolymer mixtures, two popular simulation methods are Self-Consistent Field/Density Functional Theory (SCF-DFT) (Thompson, Ginzburg, Matsen, and Balazs, Science 292, 2469 [2001]), and Hybrid Self-Consistent Field Theory (HSCFT) (Sides et al., Phys Rev Lett 96, 250601 [2006]). The two methods are shown to be very similar in their assumptions and end-results; the choice of the method to be used can depend on the specific problem. Here, we use modified HSCFT to explicitly account for the complicated role of short-chain ligands grafted onto nanoparticles to promote dispersion. In particular, we discuss the phase diagrams of such ``hairy'' nanoparticles in diblock copolymers as function of diblock composition, nanoparticle volume fraction, and ligand length. Depending on the particle size and ligand coverage, particles could segregate into favorable domain, stay close to the interface, or phase-separate from the block copolymer altogether. We also consider the dispersion of ``hairy'' nanoparticles in a homopolymer and analyze the morphologies of particle clusters as function of ligand length. The results could have interesting implications for the design of new nanocomposite materials.   

Direct hierarchical assemblies of nanoparticles in thin films

This abstract not available.   

Aqueous foams stabilised solely by nanoparticles

Particles are being increasingly used to stabilise foams and emulsions, the corresponding emulsions being known as ``Pickering'' emulsions. One of the peculiarities of these systems is the absence of Ostwald ripening: since the bubbles or drops do not grow (coalescence seems also suppressed) both foams and emulsions are stable over extremely long periods of time (months). These features make particles very interesting surface active agents as compared to standard surfactants or polymers/proteins. The origin of the suppression of ripening can be traced to the unusual behaviour of the interfacial layers made by these particles. The layers are solid-like and the usual characterisation methods (surface tension, surface rheology) are not straightforward to use. In this presentation, we will illustrate these difficulties with experiments made with partially hydrophobic silica nanoparticles. We will also discuss the relevance of foam characterisations methods such as multiple light scattering and X-ray tomography.   

Control of Nanoparticle Organization in Thin Homopolymer Films

The morphological structure of mixtures of homopolymers with chain-grafted nanoparticles is determined by competing interactions between the nanoparticle cores, the free host chains and the grafted chains. In the bulk, when the nanoparticle grafting density is low, the phase behavior is largely determined by a competition between attractive nanoparticle core-nanoparticle core interactions, mediated by the chains grafted to the surface. At high grafting densities, the entropic brush layer/free host chain interactions are dominant, leading to miscibility or to microscopic/macroscopic phase separation. Thin film mixtures are thermodynamically less stable than their bulk analogs due to the preferential attraction of grafted nanoparticles to the external interfaces. The preferential attraction of the nanoparticles to the interfaces is driven by factors that include: entropic gains of the grafted nanoparticles and linear host chains; van der Waals interactions between the nanoparticles and the interfaces. If the grafted chains and host chains are of dissimilar chemical structure, then the nanoparticles exhibit a tendency to segregate to the free surface, provided its grafts possess a lower surface energy than the host chains. Consequences of these interactions on the overall nanoparticle organization in thin homopolymer films will be discussed.   

Nanoparticle Self-Assembly in a Polymer Matrix and Its Impact on Phase Separation

The ubiquitous clustering of nanoparticles (NPs) in solutions and polymer melts depends sensitively on the strength and directionality of the effective NP-NP interactions, as well as on the molecular geometry and interactions of the dispersing fluid. Surface functionalization apparently can also lead to emergent anisotropic interactions that can influence NP dispersion. Since NP clustering can strongly influence the properties of polymer nanocomposites and NP solutions, we investigate the reversible self-assembly of model NPs into clusters under equilibrium conditions through a combination of simulation and analytic methods. First, we performed molecular dynamics simulations of polyhedral NPs in a coarse-grained dense bead--spring polymer melt and find a transition from a dispersed to clustered NP state, consistent with the thermodynamic models of equilibrium particle association such as equilibrium polymerization. We also describe the competition between self-assembly and phase separation in an analytic lattice model of a mixture of polymers and NPs. We then focus on the particularly interesting situation where the associating ``monomeric'' NP species form high molecular mass dynamic polymeric clusters and where the assembly process then transforms the phase boundary from a form typical of a polymer solution to one that more resembles a polymer blend with increasing association near the critical point for phase separation. The model calculations elucidate basic physical principles governing the coupling of self-assembly and phase behavior in these complex mixtures.