Winey, Karen I
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Publication Single Wall Carbon Nanotube Fibers Extruded from Super-Acid Suspensions: Preferred Orientation, Electrical and Thermal Transport(2004-01-15) Zhou, Wei; Vavro, Juraj; Winey, Karen I; Guthy, K.; Fischer, John E; Ericson, Lars M; Ramesh, Sivarajan; Saini, Rajesh K; Davis, Virginia A; Kittrell, Carter; Pasquali, M.; Hauge, Robert H; Smalley, Richard EFibers of single wall carbon nanotubes extruded from super-acid suspensions exhibit preferred orientation along their axes. We characterize the alignment by x-ray fiber diagrams and polarized Raman scattering, using a model which allows for a completely unaligned fraction. This fraction ranges from 0.17 to 0.05±0.02 for three fibers extruded under different conditions, with corresponding Gaussian full widths at half-maximum (FWHM) from 64o to 44o±2o. FWHM, aligned fraction, electrical and thermal transport all improve with decreasing extrusion orifice diameter. Resistivity, thermoelectric power and resonant-enhanced Raman scattering indicate that the neat fibers are strongly p-doped; the lowest observed ρ is 0.25mΩcm at 300 K. High temperature annealing increases ρ by more than 1 order of magnitude and restores the Raman resonance associated with low-energy van Hove transitions, without affecting the nanotube alignment.Publication Relationship Between Dispersion Metric and Properties of PMMA/SWNT Nanocomposites(2007-07-27) Kashiwagi, Takashi; Fagan, Jeffrey; Douglas, Jack F; Yamamoto, Kazuya; Heckert, Alan N; Leigh, Stefan D; Obrzut, Jan; Du, Fangming; Lin-Gibson, Sheng; Mu, Minfang; Winey, Karen I; Haggenmueller, RetoParticle spatial dispersion is a crucial characteristic of polymer composite materials and this property is recognized as especially important in nanocomposite materials due to the general tendency of nanoparticles to aggregate under processing conditions. We introduce dispersion metrics along with a specified dispersion scale over which material homogeneity is measured and consider how the dispersion metrics correlate quantitatively with the variation of basic nanocomposite properties. We then address the general problem of quantifying nanoparticle spatial dispersion in model nanocomposites of single wall carbon nanotubes (SWNT) dispersed in poly(methyl methacrylate) (PMMA) at a fixed SWNT concentration of 0.5 % using a 'coagulation' fabrication method. Two methods are utilized to measure dispersion, UV-Vis spectroscopy and optical confocal microscopy. Quantitative spatial dispersion levels were obtained through image analysis to obtain a 'relative dispersion index' (RDI) representing the uniformity of the dispersion of SWNTs in the samples and through absorbance. We find that the storage modulus, electrical conductivity, and flammability containing the same amount of SWNTs, the relationships between the quantified dispersion levels and physical properties show about four orders of magnitude variation in storage modulus, almost eight orders of magnitude variation in electric conductivity, and about 70 % reduction in peak mass loss rate at the highest dispersion level used in this study. The observation of such a profound effect of SWNT dispersion indicates the need for objective dispersion metrics for correlating and understanding how the properties of nanocomposites are determined by the concentration, shape and size of the nanotubes.Publication Small angle neutron scattering from single-wall carbon nanotube suspensions: evidence for isolated rigid rods and rod networks(2004-01-19) Zhou, Wei; Islam, M. F; Wang, H.; Ho, D. L; Winey, Karen I; Yodh, A. G; Fischer, John EWe report small angle neutron scattering (SANS) from dilute suspensions of purified individual single wall carbon nanotubes (SWNTs) in D2O with added sodium dodecylbenzene sulfonate (NaDDBS) ionic surfactant. The scattered intensity scales as Q-1 for scattered wave vector, Q, in the range 0.005 < Q < 0.02 Å-1. The Q-1 behavior is characteristic of isolated rigid rods. A crossover of the scattered intensity power law dependence from Q-1 to Q-2 is observed at ~0.004 Å-1, suggesting the SWNTs form a loose network at 0.1 wt% with a mesh size of ~160 nm. SANS profiles from several other dispersions of SWNTs do not exhibit isolated rigid rod behavior; evidently the SWNTs in these systems are not isolated and form aggregates.Publication Cellular Structures of Carbon Nanotubes in a Polymer Matrix Improve Properties Relative to Composites with Dispersed Nanotubes(2008-03-03) Mu, Minfang; Walker, Amanda M; Torkelson, John M; Winey, Karen IA new processing method has been developed to combine a polymer and single wall carbon nanotubes (SWCNTs) to form electrically conductive composites with desirable rheological and mechanical properties. The process involves coating polystyrene (PS) pellets with SWCNTs and then hot pressing to make a contiguous, cellular SWCNT structure. By this method, the electrical percolation threshold decreases and the electrical conductivity increases significantly as compared to composites with a well-dispersed SWCNTs. For example, a SWCNT / PS composite with 0.5 wt% nanotubes and made by this coated particle process (CPP) has an electrical conductivity of ~ 3 x 10-4 S/cm, while a well-dispersed composite made by a coagulation method with the same SWCNT amount has an electrical conductivity of only ~ 10-8 S/cm. The rheological properties of the composite with a macroscopic cellular SWCNT structure are comparable to PS, while the well-dispersed composite exhibits a solid-like behavior, indicating that composites made by this new CPP method are more processable. In addition, the mechanical properties of the CPP-made composite decrease only slightly, as compared with PS. Relative to the common appoach of seeking better dispersion, this new fabrication method provides an important alternative means to higher electrical conductivity in SWCNT / polymer composites. Our straightforward particle coating and pressing method avoids organic solvents and is suitable for large-scale, inexpensive processing using a wide variety of polymer and nanoparticles.Publication Out-of-plane mosaic of single-wall carbon nanotube films(2004-03-22) Winey, Karen I; Zhou, Wei; Fischer, John E; Sreekumar, T. V.; Kumar, S.; Kataura, H.For single-wall carbon nanotube (SWNT) films deposited from suspension onto filter membranes, or by drop casting or spin coating onto flat substrates, the tube axes lie preferentially in the film plane. Using x-ray scattering and a two-dimensional detector, we show that this out-of-plane mosaic spread can be easily and accurately quantified. It varies significantly with deposition conditions, and the aligning effects of deposition and external force in the film plane (e.g., magnetic field) are additive. Films from well-dispersed tubes show better alignment than from poor dispersions. The finite out-of-plane mosaic in C60@SWNT films enables quantitative separation of one-dimensional diffraction (chains of C60 peas) from the 2D rope lattice diffraction.Publication Temperature Dependence of Thermal Conductivity Enhancement in Single-walled Carbon Nanotube/polystyrene Composites(2010-02-23) Jakubinek, Michael B; White, Mary Anne; Mu, Minfang; Winey, Karen IThe thermal conductivity of single-walled carbon nanotube (SWCNT)/polystyrene composites, prepared by a method known to produce a uniform distribution of SWCNT bundles on the micrometer length scale, was measured in the temperature range from approximately 140 to 360 K. The thermal conductivity enhancement (50% for 1 mass % at 300 K) is reasonably constant above room temperature but is reduced at the lower temperatures. This result is consistent with the expected, large contribution of interfacial thermal resistance in SWCNT/polymer composites. Enhancements in electrical conductivity show that 1 mass % loading is in the region of the electrical percolation threshold.Publication Controlling the In Vitro Release Profiles for a System of Haloperidol-Loaded PLGA(2007-06-11) Siegel, Steven J; Budhian, Avinash; Winey, Karen IWe have used a systematic methodology to tailor the in vitro drug release profiles for a system of PLGA/PLA nanoparticles encapsulating a hydrophobic drug, haloperidol. We applied our previously developed sonication and homogenization methods to produce haloperidol-loaded PLGA/PLA nanoparticles with 200–1000 nm diameters and 0.2–2.5% drug content. The three important properties affecting release behavior were identified as: polymer hydrophobicity, particle size and particle coating. Increasing the polymer hydrophobicity reduces the initial burst and extends the period of release. Increasing the particle size reduces the initial burst and increases the rate of release. It was also shown that coating the particles with chitosan significantly reduces the initial burst without affecting other parts of the release profile. Various combinations of the above three properties were used to achieve in vitro release of drug over a period of 8, 25 and >40 days, with initial burst <25% and a steady release rate over the entire period of release. Polymer molecular weight and particle drug content were inconsequential for drug release in this system. Experimental in vitro drug release data were fitted with available mathematical models in literature to establish that the mechanism of drug release is predominantly diffusion controlled. The average value of drug diffusivities for PLGA and PLA nanoparticles was calculated and its variation with particle size was established.Publication A Coagulation Method to Prepare Single-Walled Carbon Nanotube/PMMA Composites and Their Modulus, Electrical Conductivity, and Thermal Stability(2003-12-15) Fischer, John E; Du, Fangming; Winey, Karen IA coagulation method that provides better dispersion of SWNT in the polymer matrix has been used to produce single-walled carbon nanotube (SWNT)/poly(methyl methacrylate) (PMMA) composites. Optical microscopy and SEM show improved dispersion of SWNT in the PMMA matrix, which is a key factor in composite performance. Aligned and unaligned composites were made with purified SWNT with different SWNT loadings, from 0.1 to 7 wt%. Comprehensive testing shows improved elastic modulus, electrical conductivity, and thermal stability with addition of SWNT. The electrical conductivity of a 2 wt% SWNT composite decreases significantly (>105), when the SWNT is aligned and this result is discussed in terms of percolation.Publication Increased Flexural Modulus and Strength in SWNT / Epoxy Composites by a New Fabrication Method(2006-01-03) Moniruzzaman, Mohammad; Du, Fangming; Romero, Naiffer; Winey, Karen IA new method for preparing SWNT/epoxy nanocomposites has been developed which involves high shear mixing of the epoxy resin and SWNT and heat treating the mixture prior to introducing the hardener. The glass transition temperature of the epoxy resin is unaffected by the presence of nanotubes. An improvement of 17% in flexural modulus and 10% in flexural strength has been achieved at 0.05 wt% of nanotubes. These improvements in flexural modulus and strength are attributed to good dispersion of the nanotubes and grafting of epoxy resin to SWNT by an esterification reaction.Publication Nanoparticle Networks Reduce the Flammability of Polymer Nanocomposites(2005-12-01) Kashiwagi, Takashi; Du, Fangming; Douglas, Jack F.; Winey, Karen I; Harris, Richard H.; Shields, John R.Synthetic polymer materials are rapidly replacing more traditional inorganic materials such as metals and natural polymeric materials such as wood. Since these novel materials are flammable, they require modifications to decrease their flammability through the addition of flame-retardant (FR) compounds. Recently, environmental regulation has restricted the use of some halogenated FR additives, initiating a search for alternative FR additives. Nanoparticle fillers are highly attractive for this purpose since they can simultaneously improve both the physical and flammability properties of the polymer nanocomposite. We show that carbon nanotubes can surpass nano-clays as effective FR additives if they form a jammed network structure within the polymer matrix, such that the material as a whole behaves rheologically like a gel. We find this kind of network formation for a variety of highly extended carbon-based nanoparticles: single and multi-walled nanotubes, as well as carbon nanofibers.