Session V44: Focus Session: Interparticle Interactions in Polymer Nanocomposites - Surface Interactions

The synthesis of metal nanoparticulate catalysts within functional microgel particles

Electrostatically and sterically stabilized polymer microgel particles have been prepared containing either amino (poly(2-(diethylamino)ethyl methacrylate), PDEA) or carboxylic acid (poly(acrylic acid), PAA; poly(methacrylic acid), PMMA) functional groups. The PDEA, PAA and PMAA particles can be used for the incorporation of a large variety of metal nanoparticulate catalysts due to their functional amine and carboxylic acid groups; Pd, Ru and Ni nanoparticles have been synthesized. The more polar PAA microgels were designed as the nanocatalyst carrier system in aqueous reaction media while the less polar PMAA particles were prepared as the metal nanoparticle template for use in catalytic reactions that take place in organic solvents. The sterically and electrostatically stabilized microgel particles possess surface functional groups that can potentially interact with the microchannel walls of microfluidic catalytic reactors.   

The role of nanoparticle synergies in modifying the thermal properties of biodegradable polymer blends

Most of thermoplastic polymers are brittle, when sufficient amounts are added to get flame retardant properties.Furthermore, melt-blending starch with other biodegradable polymers is difficult since very few polymers are compatible with starches.We have developed a new nanoparticles where resorcinol diphenyl phosphates (RDP) is used to modify the surface energy, allowing the particles to be dispersed within polymer.When multiple types of particles share the same coating,they can be melt blended simultaneously and synergies can be achieved, imparting properties to the nanocomposite, which cannot be achieved by any single additive. Here we show that RDP modified starch, can be extruded together with the biodegradable polymers,Ecoflex and polylactic acid,to produce flame retardant nanocomposites which can pass the UL-94-V0 test.TEM images of the blend show that the RDP-coated starch particles were well dispersed within the polymer matrix providing the flame retardant properties,while the RDP clays are reducing the interfacial tension and contributing to compatibilization. Nanomechanical measurements of the chars remaining after cone calorimetric measurements indicate that maintaining flexibility of the chars may be an additional factor in achieving good flame retardant properties.   

Supramolecular Assembly in a Polymer Nanocomposite

Overcoming the inherent tendency of nanoparticles to aggregate is critical and has typically been the main focus of research on the fabrication of polymer nanocomposites. Developing specific or organized arrangements of nanoparticles is even more technically challenging, but also more appealing due to the possibility of creating nanocomposites with desired optical, electrical, or magnetic properties. Successful approaches in this area often rely on the polymer matrix to organize the nanoparticle additive, but are limited when one considers the high-throughput processing procedures used industrially. In this presentation, the preliminary findings of a research effort to use the strong, reversible, interparticle or intermolecular interactions found in supramolecular assembly to promote synergistic organization of the polymer matrix and dispersion of nanoparticles in a polymer nanocomposite will be described. Here, the effect of incorporating functionalized metal nanoparticles into a supramolecularly assembling metallopolymer based on reversible bonds formed between a metal atom (such as Zn$^{2+})$ and the ``mebip'' ligand (2,6-bis(19-methylbenzimidazolyl)pyridine) will be discussed.   

Stabilizing nanotube films with thin polymer layers: Mitigating van der Waals forces through excluded-volume interactions

Thin membranes of single-wall carbon nanotubes (SWCNTs) on elastic polymer substrates show considerable promise for flexible electronics applications, but the modulus and conductivity of these films decrease dramatically in response to applied strains. This softening arises from the strong van der Waals interactions between contacted nanotubes, which favor the parallel coarsening of SWCNT bundles in response to even very small external forces. By capping the SWCNT membranes with a thin layer of glassy polymer, we demonstrate a dramatic improvement in the mechanical response of the strained films. We link this behavior to the stabilizing influence of excluded-volume interactions mediated by the glassy polymer layer.   

SANS Studies of DNA-Templated Silver Nanoclusters

DNA-templated silver nanoclusters have received significant attention due to their useful properties, including high molar absorptivities, good quantum yields and photostability, and small size. Their potential use may range from biology to nanoscience. For example, they are promising biological fluorescence probes due to their fluorescence properties dependence to DNA template sequence. However, some basic features, like the structure of the DNA/Ag complex, are still unclear. We have conducted Small Angle Neutron Scattering (SANS) experiments to investigate the formation of the Nanoclusters. By comparing the SANS data from conjugated samples, pure DNA and DNA/Ag complex, we can characterize the size and position of the Ag clusters along the DNA strand. The time evolution of the DNA/Ag complex can also be studied since such aging process is kind slow. We found that the formation and aging of the Ag Nanoclusters are also strongly dependent on the DNA template sequence.   

Theory of the Structure and Miscibility of Soft Filler Polymer Nanocomposites

A new hybrid theory for soft filler polymer nanocomposites is constructed based on combining integral equation methods and small scale Monte Carlo simulation. The consequences of nanoparticle softness (surface fluctuations) and corrugation (discrete roughness) on the equilibrium behavior is investigated in the dilute filler limit. Under athermal (entropic) conditions, the monomer-particle pair correlations exhibit qualitatively different features relative to hard smooth spheres. Polymer-mediated depletion attractions in the particle potential-of-mean-force (PMF) are qualitatively modified by surface corrugation and/or fluctuations. With increasing particle softness, monomer-scale PMF oscillations are destroyed, and the interparticle separation and effective maximum attraction strength depend sensitively on surface fluctuation amplitude and monomer-nanoparticle size ratio. Second virial coefficient calculations are performed to estimate how particle softness/roughness modifies miscibility, and a mechanism is identified for entropically stabilizing rough nanofillers in the absence of a cohesive interface. Surface corrugation and softness is also found to significantly modify the bridging and sterically stabilized states associated with adsorbing polymers.   

Self-assembly of Nano-rods in Photosensitive Phase Separation

Computer simulations reveal how photo-induced chemical reactions in polymeric mixtures can be exploited to create long-range order in materials whose features range from the sub-micron to the nanoscale. The process is initiated by shining a spatially uniform light on a photosensitive AB binary blend, which thereby undergoes both a reversible chemical reaction and phase separation. When a well-collimated, higher intensity light is rastered over the sample, the system forms defect-free, spatially periodic structures. We now build on this approach by introducing nanorods that have a preferential affinity for one the phases in a binary mixture. By rastering over the sample with the higher intensity light, we can create ordered arrays of rods within periodically ordered materials in essentially one processing step.   

Dynamic mechanical properties of hydroxyapatite/polyethylene oxide nanocomposites: characterizing isotropic and post-processing microstructures

Compatible component interfaces in polymer nanocomposites can be used to facilitate a dispersed morphology and improved physical properties as has been shown extensively in experimental results concerning amorphous matrix nanocomposites. In this research, a block copolymer compatibilized interface is employed in a semi-crystalline matrix to prevent large scale nanoparticle clustering and enable microstructure construction with post-processing drawing. The specific materials used are hydroxyapatite nanoparticles coated with a polyethylene oxide-b-polymethacrylic acid block copolymer and a polyethylene oxide matrix. Two particle shapes are used: spherical and needle-shaped. Characterization of the dynamic mechanical properties indicated that the two nanoparticle systems provided similar levels of reinforcement to the matrix. For the needle-shaped nanoparticles, the post-processing step increased matrix crystallinity and changed the thermomechanical reinforcement trends. These results will be used to further refine the post-processing parameters to achieve a nanocomposite microstructure with triangulated arrays of nanoparticles.   

Thermodynamic Interactions in Polymer Nanocomposites towards Controlled Nanoparticle Dispersion

Thermodynamic interactions in polymer/nanoparticle blends can be used to control the dispersion of nanoparticles. Here we introduce the use of polymer blends comprising immiscible A and B homopolymers, an \mbox{A-B} diblock copolymer ``surfactant,'' and nanoparticles. Upon thermal annealing, the composites self-assemble into equilibrium morphologies with well-dispersed nanoparticles.Polystyrene (PS), polymethyl methacrylate (PMMA), and a PS-PMMA diblock were chosen as the model polymers; C$_{60}$ fullerene was the model nanoparticle. Blends were prepared for C$_{60}$ loadings from 0.1 to 2 mass\% for blends with symmetric homopolymers and a symmetric diblock copolymer. The molecular weight of the matched homopolymers was varied as 2, 20, and 40 kDa whilst the same 60-60 kDA diblock copolymer was used. Samples were studied using small angle neutron scattering, and the resulting morphologies were found to be lamellar for all C$_{60}$ loadings. Ongoing work is exploring the effects of polymer asymmetry on the nanostructure of the composites.   

Effect of competing monomer-monomer and monomer-particle interactions on the assembly of copolymer grafted nanoparticles

Functionalizing nanoparticles with copolymer ligands is an attractive method to tailor the assembly of the nanoparticles. In this talk we present simulation results that show the effect of competing monomer-monomer and monomer-particle interactions on assembly of nanoparticles grafted with AB copolymers with diblock or alternating sequence. We vary strengths of like-monomer (A-A and/or B-B) attractive interactions in the presence of strong or negligible monomer A- particle (A-P) attraction or monomer B- particle (B-P) interaction. At a constant particle size and graft length, the competing interactions and copolymer sequence dictate the amount of inter-grafted particle monomer aggregation, inter- and intra-graft monomer aggregation within the same grafted particle. The resulting monomer aggregation on/near the particle surface imparts an effective patchiness to the particle. For e.g., in case of particles grafted with AB diblock copolymer (with A block closer to the surface) the presence of strong B-P and B-B attractions leads to smaller attractive patches that extend from the surface, and in turn lead to anistropic assembly, while presence of strong A-P and A-A attractions lead to larger attractive patches closer to the particle surface and in turn isotropic assembly.   

Dielectric Breakdown in Filled Silicone Elastomers at the Particle/Matrix Interface

Silicone elastomers, widely used as electrical insulators and potting agents, are typically filled with large amounts of fumed silica nanoparticles to enhance their mechanical strength and toughness. It is well known that as the filler particle diameter decreases to the nanometer length scale, the interfacial surface area to volume ratio increases dramatically, consequently determining many of the physical properties of the macroscopic composite. Thus, to understand electrical breakdown in these materials, it is imperative to investigate the interface between the filler particle and the bulk elastomer. In order to eliminate some of the complexities of studying dielectric breakdown at the filler/matrix interface, it is necessary to remove particle-particle interactions. In this paper, the authors will present `creep' (surface) and `strike' (through-thickness) dielectric breakdown results at idealized oxide/polymer interfaces which closely emulate those found in filled elastomer systems. Weibull and I-V curve analysis will be used to describe the effects of silane coupling agent functionalization on dielectric breakdown, and in particular, how different chemical moieties play a role in high-field interfacial electronic transport.   

The Development of Filler Structure in Colloidal Silica -- Polymer Nanocomposites

The realization of the full potential for polymeric nanocomposites to manifest their entitled property improvements relies, for some properties, on the ability to achieve maximum particle-matrix interfacial area. Well-dispersed nanocomposites incorporating colloidal silica as the filler can be realized in both polystyrene and poly(methylmethacrylate) matrices by exploiting the charge stabilized nature of silica in nonaqueous solvents which act as Bronsted bases. We demonstrate that dispersions of colloidal silica in dimethylformamide are charge stabilized, regardless of organosilyl surface functionalization. When formulated with polymer solutions, the charge stabilized structure is maintained during drying until the charged double layer collapses. Although particles are free to diffuse and cluster after this neutralization, increased matrix viscosity retards the kinetics. We demonstrate how high molecular weight polymers assist in immobilizing the structure of the silica to produce well-dispersed composites. The glass transition temperatures of these composites do not vary, even at loadings up to 50 v{\%}.