Session P19: Focus Session: Physics of Polymer Nanocomposites I

Liquid Crystalline Ordering of Nano-rods in Polymer Nanocomposites

Polymer functionalization, i.e. attachment at the end polymer of functional groups with affinity for nanoparticles has been shown to be a very successful route for designing polymer nanocomposites. In this talk, we discuss the self-assembly of nano-rods with functionalized polymers and show a rich phase diagram with different ordered phases. We also show that there is a critical value of affinity (the functionalized end block-nanoparticle energy interaction) where the nanorods exhibit liquid crystalline ordering. We discuss experimental implications and further work.   

Confinement, Ordering, and Optical Properties of Au Nanorods in Polymer Thin Films

Gold nanorods (Au NRs) possess interesting optical properties in terms of absorbance and polarizaton of light that can be further manipulated by controlling their long range order in polymer thin films. We systematically investigate orientational correlations for low aspect ratio (L/D = 3.3) Au NRs functionalized with poly(ethylene glycol) (PEG) confined within a poly(methyl methacrylate) (PMMA) film as a function of nanorod volume fraction and PMMA molecular weight. The NR spacing decreases as $\phi _{rod}^{-0.5} $ whereas the local 2D orientational order parameter increases linearly with $\phi _{rod} $, approaching a maximum value of 0.4 at high volume fractions of rods. We compare these results to Au NRs confined in a poly(ethylene oxide) (PEO) thin film as well as simulations of non-interacting rods.   

Effect of matrix chemical heterogeneity on effective filler interactions in model polymer nanocomposites

The microscopic Polymer Reference Interaction Site Model theory has been applied to spherical and rodlike fillers dissolved in three types of chemically heterogeneous polymer melts: alternating AB copolymer, random AB copolymers, and an equimolar blend of two homopolymers. In each case, one monomer species adsorbs more strongly on the filler mimicking a specific attraction, while all inter-monomer potentials are hard core which precludes macrophase or microphase separation. Qualitative differences in the filler potential-of-mean force are predicted relative to the homopolymer case. The adsorbed bound layer for alternating copolymers exhibits a spatial moduluation or layering effect but is otherwise similar to that of the homopolymer system. Random copolymers and the polymer blend mediate a novel strong, long-range bridging interaction between fillers at moderate to high adsorption strengths. The bridging strength is a non-monotonic function of random copolymer composition, reflecting subtle competing enthalpic and entropic considerations.   

Controlled Formation of Multicomponent Nanoparticle Assemblies

Nanoparticle assemblies are hierarchical material structures that have the potential to combine materials properties and well-defined geometries at multiple length scales to yield advanced properties and functions. We have developed a novel ~method to assemble inorganic~nanoparticles that are tailored with ligands into~line patterns with controlled spacing between neighboring lines. The width of the line can vary over a wide range at micron length scales, while the length can reach multi-centimeter lengths. The method of assembly is based on a ~flow-coating method, where a~dilute~nanoparticle solution is held by capillary force under a stationary knife blade as a~substrate is translated beneath. Convective nanoparticle assemblies are formed spontaneously at the contact line of the meniscus. The spacing, width, and thickness of deposited lines is~controlled by programming velocity profiles of the translating stage. Building upon the tailored chemistry of the nanoparticles, complex assemblies with precise arrangements of CdSe quantum dots with different sizes are achieved.~This method provides an easy, robust, and lithography-free method for the formation of hierarchical~assemblies, which will have advantageous effects on properties ranging from mechanics to optoelectronics.   

Solubility and Mixing Enthalpy of Ideal Nanoparticles in a Polymer Melt

Polymer nanocomposites, where the nanoparticles are smaller than the radius of gyration of the polymer, exhibit unusual mechanical properties such as a Non-Einstein-Like viscosity drop. The dispersion and solubility of the nanoparticles play a critical role which we study with small angle neutron scattering of cross-linked polystyrene nanoparticles in deuterated linear polystyrene chains. This system has a positive Second Virial Coefficient indicating favorable solubility, as well as a negative enthalpy of mixing, which is contrary to blends of protonated and deuterated linear polystyrene. This indicates a stable suspension of well dispersed nanoparticles which is apparently required for a decrease in the melt viscosity, contrary to theoretical predictions and many experimental observations with larger particles.   

Phase behavior of polymer/nanoparticle blends with attractions near a substrate

We use a fluids density functional theory with mean-field attractions to investigate the phase behavior of polymer/nanoparticle blends near a substrate. The blends are modeled as a mixture of spherical particles and freely jointed chains near a planar wall. The attractive interaction between the nanoparticles and the polymer is given by an exponential form. Earlier studies have shown that for an athermal system, there is a first order transition, where the nanoparticles expel the polymer from the surface to form a monolayer at a certain nanoparticle concentration. This transition has been justified by nanoparticle segregation to the substrate observed in experiments. In the presence of attractions, the first order transition moves to higher nanoparticle concentrations as the strength of attraction is increased. Our preliminary results show that there is a smooth transition at very high attractions. We present results for different nanoparticle concentrations at constant pressure to mimic experimental conditions. This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. Department of Energy, Office of Basic Energy Sciences user facility at Los Alamos National Laboratory (Contract DE-AC52-06NA25396) and Sandia National Laboratories (Contract DE-AC04-94AL85000).   

Graphene/Polymer Nanocomposites

Graphite has attracted large attention as a reinforcement for polymers due to its ability to modify electrical conductivity, mechanical and gas barrier properties of host polymers and its potentially lower cost than carbon nanotubes. If graphite can be exfoliated into atomically thin graphene sheets, it is possible to achieve the highest property enhancements at the lowest loading. However, small spacing and strong van der Waals forces between graphene layers make exfoliation of graphite via conventional composite manufacturing strategies challenging. Recently, two different approaches to obtain exfoliated graphite prior to blending were reported: thermal treatment (Schniepp et al., JACS 2006) and chemical modification (Stankovich et al., J Mat Chem 2006). Both start from graphite oxide. We will describe and evaluate these exfoliation approaches and the methods used to produce graphene reinforced thermoplastics, particularly polyester, polycarbonate and polyurethane nanocomposites. Three different dispersion methods - melt blending, solution mixing and in-situ polymerization -- are compared. Characterization of dispersion quality is illustrated with TEM, rheology and in electrical conductivity, tensile modulus and gas barrier property improvement.   

Thermodynamics of Polymer-Clay Nanocomposites Revisited: Compressible Self-Consistent Field Theory Modeling of Melt-Intercalated Organoclays

We formulate a ``compressible'' version of lattice self-consistent field theory (SCFT) to describe thermodynamic behavior of organically modified clays in polymer melt. The melt consists of the homopolymer matrix and a fraction of end-functionalized ``active'' chains, each chain having a single ``sticker'' end-group with strong affinity to the clay surface. We calculate the phase map for this system as function of the melt composition and the strength of the ``sticker'' adhesion to the clay. It is shown that within the compressible SCFT model, intercalated morphologies are favored in a significantly broader parameter range than was expected based on the incompressible SCFT analysis. We provide a qualitative analysis of this result and discuss implications for the physics of nanocomposites in general.   

Self-assembly of Nanoparticles at Polymer Surfaces Induced by Supercritical Carbon Dioxide

The surface segregation (i.e., preferential segregation of one component to the surface in multicomponent systems) is common to all material classes and is typically driven by a reduction in surface energy which more than compensates for the entropy loss and/or energy gain associated with the demixing of the components. However, the conventional surface segregation requires high temperatures, typically close to 200$^{\circ}$C, and long annealing time, in order to ensure enough polymer mobility. Here we show a low-temperature and environmentally green method to preferentially migrate inorganic nanoparticles to the polymer surface regions using supercritical carbon dioxide near the critical point. In addition, the scCO$_{2}$-based surface segregation may be a general phenomenon regardless of a choice of nanoparticles and polymers. In this talk, we will discuss the mechanism and detailed characterization of the surface structures using scanning probe microscope, neutron/X-ray reflectivity, and X-ray photoelectron spectroscopy.   

Theory and Simulation Studies of Copolymer Functionalized Nanoparticles in Polymer Nanocomposites

Significant interest has grown around the ability to create polymer nanocomposites with controlled spatial arrangement of nanoparticles mediated by a polymer matrix. By functionalizing or grafting polymers on to nanoparticle surfaces and systematically tuning the composition, chemistry, molecular weight and grafting density of the functionalized polymers we can tailor the inter-particle interactions and precisely control the assembly/dispersion of the particles in the polymer matrix. While prior experimental and theoretical work in this area has mostly been on homopolymer grafted particles at high brush-like grafting densities, we study copolymer grafted nanoparticles at low grafting densities in a homopolymer matrix. Using an integrated approach involving Polymer Reference Interaction Site Model (PRISM) theory and Monte Carlo simulations we will present the effect of monomer sequence and molecular weight of the grafted copolymer, compatibility of the graft and matrix polymers, and nanoparticle size on the conformations of the grafted polymers, and the effective interactions between the grafted nanoparticles in the matrix.   

Polymer-mediated structure of nanoparticles in dense melts: transferability and an effective one-component approach

Two component PRISM theory is used to investigate the dependence of polymer-mediated interactions between the sites of complex nanoparticles on filler shape. Dilute limit potential-of-mean force (PMF) calculations for spheres, diatomics, 4-site rods and disks, and a tetrahedron are compared under depletion, steric stabilization, and bridging conditions. The PMFs are only weakly dependent on filler shape (good transferability), and scale nearly linearly with filler to monomer site diameter ratio. The question of a whether a reliable one component nanoparticle model can be constructed based on the dilute limit polymer-mediated PMF as input to the HNC integral equation theory is explored for spherical nanoparticles. Compared to the full 2-component theory results, stronger depletion and steric stabilization, but weaker bridging, is predicted, trends due to the qualitatively different spatial organization of polymers in the three regimes of interfacial cohesion. However, overall the structural behaviors predicted by the one-component approach are qualitatively reliable.   

Controlling the Self-Assembly of Semiconducting Nanocrystals within Conjugated Rod-Coil Block Copolymers

Blends of conjugated polymers and inorganic nanoparticles have been investigated for numerous optoelectronic applications however optimization relies on precise control over the nanoscale morphologies. Here, we show that conjugated rod-coil block copolymers can be designed to self assemble into controllable morphologies with the coil block templating nanocrystal location. We have constructed a model system where nanocrystals are blended with poly(alkoxy-phenylene vinylene-b-2-vinylpyridine) (PPV-b-P2VP), which self assembles into tunable morphologies. Semiconducting nanocrystals reside within the P2VP domain, due to the favorable interactions between P2VP and the nanoparticle surface as well as the exclusionary effects of the liquid crystalline PPV. The placement of the nanoparticles can be tuned by altering domain size, nanocrystal diameter and nanocrystal surface chemistry. These findings are used to develop a comprehensive understanding of the self assembly processes in conjugated rod-coil block copolymer nanocomposites.