Session H18: Polymer Composites

Resistive Switching Behavior in Silver Nanowire-Polystyrene Composites

Thin-film resistive switching devices have been extensively studied for use as memory elements, while the resistive switching behaviors of bulk composites have not been widely investigated to date. We report resistive switching in bulk, isotropic silver nanowire-polystyrene composites. Composites with nanowire concentrations close to and greater than the percolation threshold show resistive switching behaviors consistent with the formation of reversible, field-induced metal filaments between silver nanowires. In contrast, at nanowire concentrations significantly below the electrical percolation threshold, the composites exhibit no switching. Comparable experiments performed on single-walled carbon nanotube-polystyrene composites, both above and below the electrical percolation threshold, produced no resistive switching. The fields at which switching occurs in our silver nanowire composites are significantly lower than those previously reported for thin-film, polymer-based resistive switching elements, and on/off cycling experiments indicate that the switching process is stable over approximately 20,000 cycles.   

Dependence of Electrical Percolation Behavior on Filler Aspect Ratio in Isotropic Silver Nanowire-Polystyrene Composites

Electrical percolation behaviors in networks of conductive cylindrical particles in insulating matrices have been studied extensively by analytical models, simulations and experiments. Most analytical techniques assume that the particles have effectively infinite aspect ratios; however, many nanoparticle processing approaches result in cylindrical fillers with finite aspect ratios. We report percolation behavior in isotropic silver nanowire/polystyrene composites in which the nanowires have finite, controlled aspect ratios (L/D=31.0 \underline {+} 3.7, 16.4 \underline {+} 1.5, and 8.23 \underline {+} 0.57). We then compare the critical volume fractions, $\phi _{c}$, obtained from our experiments to (1) simulations of percolation behavior of soft-core rods with finite aspect ratio, and analytical percolation models for (2) soft-core and (3) core-shell rods, to determine the contributions of the soft-core and infinite-aspect-ratio assumptions to the discrepancies between experimental and analytical percolation behaviors. Our experimental values of $\phi _{c}$ are in better agreement with our soft-core simulations than with both core-shell and soft-core analytical models. Thus, we conclude that the infinite-aspect-ratio assumption contributes more significantly to errors in predicting percolation onset than the soft-core assumption.   

Ionic and Electronic Conductivity of Poly(Ionic Liquid) - Gold-Nanoparticle Composites

In this work we describe the single-step synthesis of a robust, self-supporting gold nanoparticle - ionic liquid-derived polymer composite and evaluate its transport properties. Specifically, a gold nanoparticle-containing ionic liquid-derived polymer is synthesized in a single step by UV irradiation of a metal ion precursor-doped, self-assembled d$i$alkylimidazolium-based ionic liquid. The as-synthesized gold nanoparticle-IL composite prepared with 16 {\%} (w/w) water adopts a hexagonal perforated lamellar structure. The conductivity of self-supporting no-gold and gold nanoparticle-ionic liquid-derived polymer composites was characterized using electrochemical impedance spectroscopy (EIS). The dispersion curves shifts towards the low frequency region with increasing Au nanoparticle content. The near overlap of the no-Au nanoparticle composition and the lowest Au nanoparticle containing polymer suggest that at low Au nanoparticle content the transport remains unchanged and is primarily ionic conduction and matrix mediated. Samples prepared at the highest Au nanoparticle show two component transport. The electronic transport may be attributed to the increased number density and filling fraction of the gold nanoparticles within the hexagonally arranged pores of the composite.   

Thermal Features and Glass Transition in Polystyrene-Nanodiamond Composites

Polystyrene-Nanodiamond composites were obtained by dissolving the polymeric matrix into a theta solvent (cyclohexane) followed by the addition of diamond nanoparticles from Aldrich (with a particle size ranging between 3 and 8 nm) and subsequent sonication for about 100 minutes by using a Hielscher high power (1 kW) sonicator. The homogeneous solution was poured onto microscope slides and the solvent has been removed by heating in an oven at 125 $^{o}$C for about 3 hours. Composites containing various amounts (from 0 {\%} to 25 {\%} nanodiamonds within polystyrene) have been investigated. The physical properties of the as obtained nanocomposites were investigated by DSC, TGA, Raman, and WAXS. Glass transition temperature was shifted to higher temperatures and the thermal stability was enhanced by the addition of nanodiamonds. A phenomenological model for the observed changes is proposed (within the free volume approximation) and discussed in detail. Acknowledgements: This research was supported by US Army Research Laboratory (W911NF-08-1-0353) and LSAMP -UTPA.   

Glass Transition in Polystyrene -- TiO2 Nanocomposites

Polystyrene is a typical amorphous polymer with almost no crystallinity degree. The addition of very small particles to polystyrene is expected to change significantly the mechanical and thermal features of this polymer due to the competition between the macromolecular chains and the nanofiller for the same cluster of free volumes. The shift of the glass transition temperature and of the associated mechanical properties is expected to increase as the particle size is reduced. Anatase nanoparticles from Aldrich, with an average diameter of 15 nm have been dispersed within PS. To achieve a homogeneous distribution of nanoparticle the polymer was dissolved within a theta solvent (cyclohexene), the solution has been sonicated for about 100 minutes, and the solvent has been evaporated by heating at 125 $^{\circ}$C for few hours. Thermal characteristics of composites containing various fractions of anatase were measured by TGA, DSC, and DMA. The research was focused on the shift in the glass transition temperature of polystyrene due to the loading with nanoparticles. Acknowledgements: The research was supported by the Welch Foundation, Air Force Research Laboratory (FA8650-07-2-5061), and US Army Research Laboratory/Office (W911NF-08-1-0353).   

Enhancement of nanoparticle dispersion in crosslinked network composites through surface modification

Nanoparticles (NPs) have been investigated as an effective method for the toughening of coatings. Challenges in complete dispersion represent the major barrier to wider application. A grafting-to approach was utilized to tether poly(ethylene glycol) (PEG) chains to the surface of silica NPs. This process was shown to offer a simple and effective means for dispersion of NPs in a crosslinked EPON-Jeffamine network. PEG layer thickness and T$_{g}$ were dependent on the feed ratio of PEG to nanoparticles, investigated by DLS and DSC, respectively. TEM, AFM and optical imaging confirmed the improved dispersion of PEG-ylated NPs versus bare silica whereas mechanical analysis demonstrated the effects on film toughening. Tethering of PEG chains to silica NPs using Diels-Alder chemistry allows for reversible surface modification. This method has possible uses in self-healing materials and coatings.   

Negative Permittivity and Permeability Metamaterials of Polymer-based Nanocomposite

Metamaterials with negative permittivity and negative permeability are a novel class of artificial materials. For large scale, light weight and deformable applications, it is compelling to develop flexible polymetric metamaterials. Accordingly, double negative metamaterials are created by incorporating Fe/Fe$_{3}$O$_{4}$ core-shell magnetic nanoparticles (MNPs) and conductive carbon nanofillers into the polyetherimide matrices. Dielectric tests and transmission-line perturbation method detect the negative permittivity and the negative permeability of the nanocomposites, respectively. This is attributed to the interaction between MNPs, the nanofillers and the polymer. By adjusting the component structures, the frequency range of the negative permittivity or permeability can be controlled, which is promising for the development of left-handed metamaterials.   

Bimodal-Sized Magnetic Nanoparticles in Polymers

Functionalization of iron oxide nanoparticles with polymer chain attachments offers the formation of equilibrium self-assembled structures. These structures find profound applications where reversible mechanism of self-assembly can be examined in a new model system. Here, we will present our initial results from the bottom-top synthetic approach of magnetic nanoparticles. Particles with varying sizes (bimodal size distribution) that are grafted with poly(styrene) chains are mixed in poly(styrene) in solution and their aggregation level and the effect of particle size ratio on the structures and mechanical behavior will be presented. The effects of collinear dipolar interaction and repulsion from grafted chains on the organization of particles within polymers are going to be discussed.   

Isotactic polypropylene carbon nanotube composites -- crystallization and ordering behavior

The field of Polymer Nanocomposites (PNCs) is growing steadily in recent years. We use carbon nanotubes (CNTs) to affect the crystallization behavior of the polymers. Isotactic Polypropylene (iPP) is very widely used and is a good model system to understand the physics of other similar polymers. iPP/CNT PNCs form $\alpha $, $\beta $, and $\gamma $ crystallographic phases under a variety of crystallization conditions: non-isothermal and isothermal melt crystallization, shear, stress, fiber extrusion, etc. The crystal growth is altered from spherulitic to $\alpha $-fibrillar upon the nucleation effect of CNTs. We are studying the effect of different temperature treatment schemes and different isothermal crystallization conditions. We found also that the smectic ordering in iPP is improved by the introduction of CNTs. We use Differential Scanning Calorimetry, Wide Angle X-ray scattering, Microscopic Transmission Ellipsometry and Avrami analysis. Research supported by: Assumption College Faculty Development Grant, funding for students' stipends, instrumentation and supplies, the NSF Polymers Program of the DMR, grant (DMR-0602473) and NASA grant (NAG8-1167).   

Radiation Effects on Polypropylene Carbon Nanofibers Composites: Spectroscopic Investigations

Dispersion of carbon nanostructures within polymeric matrices affects their physical and chemical properties (increased Young modulus, improved thermal stability, faster crystallization rates, higher equilibrium degree of crystallinity, modified glass, melting, and crystallization temperatures, enhanced thermal and electrical conductivity). Nevertheless, little is known about the radiation stability of such nanocomposites. The research is focused on spectroscopic investigations of radiation-induced modifications in isotactic polypropylene (iPP)-vapor grown nanofiber (VGCNF) composites. VGCNF were dispersed within iPP by extrusion at 180$^{o}$C. Composites containing various amounts of VGCNFs ranging from 0 to 20 {\%} wt. were prepared and subjected to gamma irradiation, at room temperature, at various integral doses (10 MGy, 20 MGy, and 30 MGy). Raman spectroscopy, ATR, and WAXS were used to assess the radiation-induced modifications in these nanocomposites. Acknowledgements: This research was supported by the Welch Foundation (Department of Chemistry at UTPA), by Air Force Research Laboratory (FA8650-07-2-5061) and by US Army Research Laboratory/Office (W911NF-08-1-0353).   

Radiation-Induced Changes of Thermal Properties of Polypropylene Carbon Nanofibers Composites

Dispersion of nanostructures within polymeric matrices affects their thermal properties and stability. Shifts, convolutions, and splitting of the main transitions (glass transition, melting transition, and crystallization temperature) were reported. In most cases, the thermal decomposition of the polymeric matrix is delayed or shifted towards higher temperatures. Nevertheless, little is known about the effect of ionizing radiation on the thermal stability and phase transitions in such nanocomposites. Spectroscopic investigations of radiation-induced modifications in isotactic polypropylene (iPP)-vapor grown nanofiber composites (VGCNF) are reported. VGCNFs were dispersed within iPP by extrusion at 180$^{o}$C. Composites containing various amounts of VGCNFs ranging from 0 to 20 {\%} were prepared and subjected to gamma irradiation, at room temperature, at various integral doses (10 MGy, 20 MGy, and 30 MGy). Thermal characteristics were of iPP-VGCNF composites were measured by TGA, DSC, and DMA. Acknowledgements: This research was supported by the Welch Foundation, Air Force Research Laboratory (FA8650-07-2-5061), and US Army Research Laboratory/Office (W911NF-08-1-0353).   

Computational Modeling of Polystyrene-b-(ethylene-co-butylene)-b-styrene and Mineral Oil Gels and Nanocomposites

There has been substantial interest in thermoplastic elastomer (TPE) gels composed of poly(styrene-b-(ethylene-co-butylene)-b-styrene)(SEBS) and hydrocarbon oils. Although the effects of adding nanoparticles on TPE gels is relatively unexplored, research in polymer nanocomposites have shown that the addition of nanoparticles enhanced physical properties. The microstructure of such a system is dependent on a variety of parameters such as block copolymer and nanoparticle concentrations, temperature, nanoparticle size, and nanoparticle interaction; thus to perform an extensive study of phase space, mesoscale modeling should be used in conjunction with the experimentation. To complement our experimental system, mesoscale modeling of this TPE gel and the corresponding nanocomposite are preformed using dynamic density functional theory and self consistent field theory through Materials Studio, where morphology of the system is studied as a function of various parameters and conditions. The validity of the computational methods has been confirmed for a number of experimental results, and subsequently has been used to explore a larger design space than is accessible solely through experimental methods.