Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session P30: Polymers - Inorganic Composites III |
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Sponsoring Units: DPOLY Chair: Mark Dadmun, University of Tennessee, Knoxsville Room: LACC 505 |
Wednesday, March 23, 2005 11:15AM - 11:27AM |
P30.00001: Novel Route to Mesoporous silica with perpendicular nanochannels from polymer/inorganic nanocomposite films Sivakumar Nagarajan, Mingqi Li, Rajaram Pai, Craig Weinman, Christopher Ober, Thomas Russell, James Watkins Mesoporous metal oxide films have generated intense interest due to their potential scientific and technological importance. For applications such as sensors, separations and detection devices, structures having cylindrical channels oriented normal to the surface are highly desirable, but have remained elusive. Recently we reported a new approach to mesoporous materials that involves the infusion and selective condensation of metal oxide precursors within one phase domain of a highly ordered, preformed block copolymer template dilated with supercritical carbon dioxide to yield a polymer/inorganic nanocomposite film. The organic component of the nanocomposite is then removed to produce the mesoporous oxide. To date ordered spherical and randomly oriented cylindrical morphologies have been replicated to yield silica/organosilicate mesostructures in films over micron thick while maintaining all the structural details of the sacrificial copolymer template. The preparation of phase-segregated block copolymer films with cylindrical domains oriented normal to the surface using controlled solvent evaporation has recently been realized. Here we report the replication of these templates to yield the corresponding silicate mesostructures containing arrays of perpendicular channels. [Preview Abstract] |
Wednesday, March 23, 2005 11:27AM - 11:39AM |
P30.00002: A Fracture Resisting Molecular Interaction in Trabecular Bone: Sacrificial Bonds and Hidden Length Dissipate Energy as Mineralized Fibrils Separate Georg E. Fantner, Tue Hassenkam, Johannes H. Kindt, James C. Weaver, Henrik Birkedal, Leonid Pechenik, Jacqueline A. Cutroni, Laura S. Golde, Marquesa M. Finch, Philipp Thurner, Geraldo A.G. Cidade, Galen D. Stucky, Danie E. Morse, Paul K. Hansma A molecular energy dissipation mechanism in the form of sacrificial bonds and hidden length was previously found in bone constituent molecules of which the efficiency increased with the presence of Ca$^{2}$+ ions in the experimental solution. Here we present evidence for how this sacrificial bond-hidden length mechanism contributes to the mechanical properties of the bone composite. From investigations into the nanoscale arrangement of the bone constituents in combination with pico-Newton adhesion force measurements between mineralized collagen fibrils, based on single molecule force spectroscopy, we find evidence that bone consists of mineralized collagen fibrils and a non fibrillar organic matrix which acts as a ``glue'' that holds the mineralized fibrils together. We propose that this ``glue'' resists the separation of mineralized collagen fibrils. Like in the case of the sacrificial bonds in single molecules, the effectiveness of this ``glue'' increases with the presence of Ca$^{2+}$ ions. We further investigate how this molecular scale strengthening mechanism increases the fracture toughness of the macroscopic material. [Preview Abstract] |
Wednesday, March 23, 2005 11:39AM - 11:51AM |
P30.00003: Self-assembled anisotropic polymer particles by polycondensation in lyotropic surfactant mesophases Guruswamy Kumaraswamy, Mohan Wadekar We report the formation of crosslinked polysiloxane particles whose morphologies are directed by the nonionic surfactant phase in which they self-assemble. Under conditions that allow slow condensation of silanol monomers, the polymer particles formed have a geometry similar to the parent mesophase: rod-like particles form in a hexagonal mesophase and sheet-like in a lamellar phase. The characteristic diffraction pattern obtained from the liquid crystalline surfactant assembly is preserved during polycondensation. Interestingly, while the geometry of the particle is similar to that of the mesophase, the particles are microns in size, three orders of magnitude larger than the characteristic size of the surfactant mesophase. Our observed morpholgies differ from those predicted by theories of ordering of colloidal particles in liquid crystalline phases. In our system too, ordering of polymer colloid particles is a result of minimization of elastic free energy due to distortion of the nematic matrix. However, the polymer particles in our experiments are not formed at once, but grow slowly as polycondensation proceeds. We speculate that slow condensation and cross-linking kinetics, gradual build-up of molecular weight and the non-linear architecture of the polysiloxane molecules might allow the dynamic organization of the particles by the liquid crystalline mesophase, leading to the formation of the observed particle geometries. [Preview Abstract] |
Wednesday, March 23, 2005 11:51AM - 12:03PM |
P30.00004: Electron spin resonance on carbon nanotubes-polymer composites Mircea Chipara, Zaleski Jeffrey, David Hui, Ning Pan Electron spin resonance (ESR) is used to assess the quality of carbon nanotubes. However, few studies were done on carbon nanotubes dispersed in polymeric matrices. Not annealed carbon nanotubes exhibit three electron spin resonance (ESR) line; a wide resonance line located at g values larger than g=2.0023 assigned to catalyst residues and two lines located close to g=2.0023 assigned to paramagnetic impurities and electrons delocalized over the conducting domains of carbon nanotubes. The annealing process reduces dramatically the intensity of the wide line. Qualitatively, same resonance lines were observed in carbon nanotubes-polymer composites. Samples of polystyrene loaded with various amounts of carbon nanotubes ranging from 0.1{\%} to 10.0{\%} (wt) were prepared. The effect of nanotube dispersion on the parameters of the resonance line is presented. The effect of temperature on the resonance line parameters was investigated. A matrix effect was observed within the glass transition range. This has been assigned to the adhesion of nanotubes to the polymeric matrix. [Preview Abstract] |
Wednesday, March 23, 2005 12:03PM - 12:15PM |
P30.00005: Flow Based Control of Conductivity in Nanotube Composites Kalman Migler, Sam Kharchenko, Jan Obrzut, Jack Douglas Nanotube composites are finding applications due to their ability to enhance the electrical conductivity of polymeric materials. They exhibit a percolation threshold in both rheological and electrical properties at mass fractions less than 0.01. We study the interrelationship between these two coupled transport properties by simultaneous dielectric spectroscopy and rheology. We find that the frequency dependent electrical conductivity is quite sensitive to shear flow near the percolation threshold; it can reversibly vary by six orders of magnitude and can become highly anisotropic. Interestingly, the shear dependence of the viscosity and the conductivity show distinct behaviors, indicating that different aspects of the nanotube network are probed by these two transport coefficients. [Preview Abstract] |
Wednesday, March 23, 2005 12:15PM - 12:27PM |
P30.00006: Effect of Carbon Nanotube Alignment in Polymer Nanocomposites on the Electrical Conductivity Fangming Du, John E. Fischer, Karen I. Winey We studied the effects of nanotube alignment on the electrical conductivity of the nanotube / polymer nanocomposites. Nanocomposites with single-walled carbon nanotubes (SWNTs) and poly(methyl methacrylate) (PMMA) were prepared via our coagulation method. The nanotubes were subsequently aligned to various extents by melt fiber spinning and the degree of alignment was quantified by x-ray scattering. Electrical percolation in the nanocomposites with isotropic nanotubes occurs at $\sim $0.39wt{\%} SWNT. At all nanotube loadings investigated (0.5 to 3 wt{\%}), the electrical conductivity exhibits percolation behavior with decreasing nanotube alignment. At nanotube loadings just above the concentration threshold for percolation, a maximum electrical conductivity is observed at intermediate level of alignment. We attribute the existence of this optimal nanotube alignment to the competition between the number of tube-tube contacts and the distance between these contacts. A two-dimensional Monte Carlo simulation of sticks in a unit square was developed to mimic these nanotube/polymer nanocomposites and shows similar trends. In sum, we observed percolation behavior with respect to nanotube alignment and obtained the maximum electrical conductivity by partially aligning the nanotubes. [Preview Abstract] |
Wednesday, March 23, 2005 12:27PM - 12:39PM |
P30.00007: Thermal Conductivity of Single-Walled Carbon Nanotube / Polyethylene Nanocomposites Reto Haggenmueller, John E. Fischer, Karen I. Winey, Jesse J. Cugliotta, Jennifer R. Lukes The thermal conductivity of nanocomposites with single walled carbon nanotubes (SWNTs) and polyethylene are being investigated with attention to the effect of the degree of PE crystallinity and the alignment of both the PE and SWNT. The nanocomposites were prepared via the hot-coagulation method, resulting in a good dispersion of the SWNTs in the polymer matrix. Characterization methods include the comparative and modulated thermo-reflectance method to measure thermal conductivity, x-ray scattering to quantify SWNT and PE alignment, and SEM and AFM to determine SWNT dispersion. SWNTs act as nucleation sites for the PE and this crystalline interface might enhance heat transfer between the SWNT and matrix. At 30 wt{\%} SWNT, isotropic composites made with low density and high density PE have thermal conductivities of 1.8 and 3.5 W/m-K, respectively, clearly demonstrating the importance of degree of PE crystallinity. Alignment of both the SWNT and PE was produced by melt fiber spinning and results show that the thermal conductivity increase with orientation. [Preview Abstract] |
Wednesday, March 23, 2005 12:39PM - 12:51PM |
P30.00008: Controlling the Dispersion and Properties of Single-Walled Carbon Nanotube-Polymer Nanocomposite Asif Rasheed, Mark Dadmun, Phillip Britt, David Geohegan, Ilia Ivanov Carbon nanotubes possess extraordinary electrical and mechanical properties. Dispersing nanotubes in a polymer matrix provides an effective way to exploit these extraordinary properties, however this has been difficult to achieve due to strong inter tube interaction. Previous work in our lab has shown that optimized hydrogen bonding between a copolymer and an anisotropic filler enhances miscibility of the mixture. Controlling the extent of hydrogen bonding between a copolymer and carbon nanotube gives a well-dispersed nanocomposite for both single and multi-wall carbon nanontubes as indicated by Raman spectroscopy, dynamic mechanical analysis, electrical conductivity, optical microscopy and SEM. The amount of hydrogen bond interactions in the nanocomposite is controlled by varying the copolymer composition. The results are critical in understanding interfacial phenomenon in polymer and nanocomposites and provide a mechanism to design materials with tunable properties. [Preview Abstract] |
Wednesday, March 23, 2005 12:51PM - 1:03PM |
P30.00009: From Carbon Nanotube Dispersion to Composite Nanofibers Yachin Cohen, Yael Dror, Wael Salalha, Alexander L. Yarin, Eyal Zussman, Wim Pyckhout-Hintzen Composite polymer nanofibers containing single-walled carbon nanotubes (SWCNT) are fabricated by electrospinning. We describe the path from dispersing individual SWCNTs or thin bundles in water using amphiphilic polymers, through a structural characterization of the polymer conformation in the SWCNT/polymer hybrid to the characteristics of the electrospun composite nanofibers. An alternating copolymer of styrene and sodium maleate (PSSty) and gum arabic (GA)-a highly branched natural polysaccharide were successfully used to produce stable aqueous dispersions. Measurements of small angle neutron scattering (SANS) show that both polymers form a thick corona of adsorbed coils on the nanotubes. The large coils introduce a significant steric barrier stabilizing the dispersions, in addition to electrostatic repulsion by charged groups. The composite nanofibers showed good distribution and alignment of the SWCNTs in the poly(ethylene oxide) (PEO) nanfubers, as revealed by transmission electron microscopy. X-ray diffraction demonstrated a high degree of orientation of the PEO crystals in the electrospun nanofibers. Enhanced tensile properties were achieved due to the high degree of alignment of both nanotubes and polymer crystals, and a strong interface, especially with PSSty. The morphology and possible applications of these composite nanofibers will be discussed. [Preview Abstract] |
Wednesday, March 23, 2005 1:03PM - 1:15PM |
P30.00010: Enhanced alignment of Multi-Walled Carbon Nanotubes in Electrospun PS/PMMA Polymer Blends Jaemin Kim, Kwanwoo Shin Electrospinning has been recognized as an efficient technique to obtain ultrafine polymeric nanofibers. A variety of polymers have been successfully electospun into continuous polymeric fibers having micron to submicron diameters. In this work, multiwalled carbon nanotubes are incorporated into electrospun nanofibers which consist of PS/PMMA polymer blends during the electrospinning process. It was observed that multiwalled carbon nanotubes(MWNT) are linearly oriented along the fiber axis in which internally co-continuous phase morphology of the PMMA is formed in the PS matrix This highly oriented MWNT structures in the fiber are characterized by transmission electron microscopy. The internal morphology of polymer blends are determined by selectively etching the PMMA with a good solvent using scanning electron microscopy, and staining the PS portion with osmium tetra teroxide. [Preview Abstract] |
Wednesday, March 23, 2005 1:15PM - 1:27PM |
P30.00011: Processing Phase Diagram of Polymer Carbon-Nanotube Composites Erik K. Hobbie, Dan Fry, Howard Wang The `phase diagram' of a model polymer/carbon-nanotube melt composite is measured as a function of concentration, shear stress and geometrical confinement. We observe a hierarchy of flow-induced structure, including dispersed (para) nematics, a variety of aggregates, and `jammed' fractal networks. By applying simple scaling arguments from polymer physics to rigid-rod gels, our data suggest that the portion of the network responsible for the arrest of flow is more diffuse than the full elastic network, akin to `force chains' in granular media. [Preview Abstract] |
Wednesday, March 23, 2005 1:27PM - 1:39PM |
P30.00012: Measurements of particle orientation in simple shear and channel flows of polypropylene/clay nanocomposites Laura Dykes, Wesley Burghardt, Kosmas Kasimatis, John Torkelson We report studies of flow-induced orientation in dispersions of organically modified montmorillonite clay in polypropylene. The nanocomposite samples were prepared using two methods. Melt blending in a twin-screw extruder led to intercalated samples in which the layered structure of the clay remains intact. An additional step of solid-state shear pulverization leads to samples with a higher degree of exfoliation of individual clay sheets. In situ x-ray scattering was used to probe particle orientation in steady shear using an annular cone and plate shear cell which provides information about particle orientation in the flow-gradient plane. The more highly exfoliated pulverized sample shows significantly lower orientation than the intercalated melt-blended sample. Both samples were also studied in extrusion-fed channel flows. In slit-channel geometries, the dominant shear rate direction is parallel to the x-ray beam, allowing information about orientation in the flow- vorticity plane to be obtained. In fact, little scattering was observed in these configurations, confirming the tendency of clay particles to `lie down' in the shear flow. Superposition of extension via contractions or expansions in slit-channel flows did not reorient particles sufficiently to bring them `into view' in these geometries. [Preview Abstract] |
Wednesday, March 23, 2005 1:39PM - 1:51PM |
P30.00013: Rheology of Non-dilute Polystyrene/Cloisite/Toluene Solutions Jun Li, Vladimir Zaitsev, Steven Schwarz, Jonathan Sokolov, Miriam Rafailovich We have previously described a simple model of spin casting for polymer/clay nanocomposite films in which the viscosity of the polymer solution at low solvent concentration is a critical parameter. We have therefore examined the shear dependent viscosity of polystyrene/Cloisite-6A/toluene solutions over a wide range of weight fractions, and for various molecular weights. The data is well described by the Carreau model $\eta -\eta _\infty =(\eta -\eta _0 )(1+\lambda ^2\dot {\gamma }^2)^{-N}$, where the parameters in the model show a clear dependence on the clay/PS ratio. We will discuss the trends observed in the viscosity data, and their impact on the uniformity of spin cast films. [Preview Abstract] |
Wednesday, March 23, 2005 1:51PM - 2:03PM |
P30.00014: Melt rheology studies of polymer chain dynamics in the presence of nanofillers Sudeepto Sen, Sanat Kumar We will present results from recent and ongoing melt rheology runs on a series of nanocomposites with polystyrene as the matrix and silica nanoparticles (10-15 nm in diameter) as the fillers. A number of combinations of the matrix molecular weight and nanofiller concentrations are being studied to elucidate the trends in the system dynamics for a number of different concentrations of the nanofillers in a given matrix molecular weight and for a given nanofiller concentration over a range of matrix molecular weights. Melt rheology runs are being performed also on the pure polymers for comparison purposes. This elucidates the long term relaxation dynamics of a matrix polymer in the presence of nanofillers; when compared with the corresponding pure polymer in the terminal region, the nanocomposite shows a lower G$'$ slope indicating a slowing down of the relaxation dynamics in the presence of the nanoparticles. [Preview Abstract] |
Wednesday, March 23, 2005 2:03PM - 2:15PM |
P30.00015: Polymer nanocomposites: permeability, chain dynamics, mechanical properties Laxmi Sahu, Nandika D'Souza Polymer nanocomposites based on dispersion of surfactant treated expandable smectite clays such as montmorillonite layered silicates (MLS) have shown promise as organic-inorganic hybrids with the potential to improve barrier properties. Separately, flexible displays based on plastic substrates have reduced lifetimes tied to the low barrier properties. While there has been a general attribution of improved barrier properties to the tortuous path, this does not consider the influence the introduction of a secondary filler has on the morphology of the host polymer. Here we examine the influence of MLS nanoplatelets on the barrier properties and chain dynamics of polymers. We investigate the potential for host polymer modification by comparing two crystallizable polymers nylon and PET and resulting well dispersed nanocomposites. We study mechanical, cyclic fatigue and permeability of films. Permeability of the biaxially stretched film and when the film undergoes fatigue of 50 and 10000 cycles are also measured. Chain dynamics were modeled based on the Burger model fit to creep-recovery data. A systematic approach to predict the permeability considering amorphous, crystalline and MLS content and comparison with experimental values were done. We also conducted water absorption measurements to highlight the water absorption differences in the two polymers. Dimensional stability of PET was studied by measuring coefficient of thermal expansion of thin film on Si substrate by ellipsometry method. [Preview Abstract] |
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