Showing papers by "Richard W. Siegel published in 2007"

Unfolding of Ribonuclease A on Silica Nanoparticle Surfaces

Abstract: This paper reports on the unfolding behavior of ribonuclease A (RNase A) on silica nanoparticle surfaces and quantitively demonstrates that nanoscale size and surface curvature play key roles in influencing the stability of adsorbed proteins. Urea denaturation analyses showed that the thermodynamic stability of RNase A decreased upon adsorption onto the nanoparticles, with greater decrease on larger nanoparticles. The stability changes of RNase A correlate well with the changes in the protein-nanoparticle interactions, which increase as the surface contact area and surface charge interaction increases. This study, therefore, provides fundamental information on the effect of nanoscale surfaces on protein structure and function.

Room-temperature assembly of germanium photonic crystals through colloidal crystal templating

Abstract: Using an evaporation-driven, room-temperature infiltration technique, we filled the interstitial space within an opal formed from 1 μm polystyrene spheres with preformed germanium nanoparticles. Subsequently, the Ge nanoparticles were bound together by backfilling with a photocuring adhesive, and the original polystyrene template was etched away to yield an inverse opal of air spheres in a matrix of Ge nanoparticles and photoadhesive. The reflectance spectra of these photonic materials were compared to computed band structures to deduce the refractive index of the air-germanium and air-photoadhesive composites; in conjunction with the Maxwell-Garnett model of dielectric mixing, this comparison also allowed the estimation of the Ge volume fraction within each composite material. On the basis of these methods, the interstitial space of the initial polystyrene opal template was filled to 49 vol % with Ge nanoparticles, acquiring an effective refractive index of 1.74. Backfilling with photoadhesive, followed by removal of the polystyrene opal, produced a Ge-in-polymer composite with a refractive index contrast of 2.05.

Crystal nucleation and growth in poly(ethylene terephthalate)/alumina‐nanoparticle composites

Abstract: The effect of nanoparticles on nonisothermal polymer crystallization was investigated using poly(ethylene terephthalate) (PET) nanocomposites with alumina (Al2O3) nanoparticles of average size 38 nm. The filler content in the nanocomposites was varied from 0 to 10 wt %. The interparticle spacing was observed to decrease (as expected) with an increase in loading of the nanoparticles. Contrary to previous reports in the literature on semicrystalline polymer-based composites with micron-size and macroscale particles, our differential scanning calorimetry, transmission electron microscopy, and X-ray studies showed that the addition of the nanoparticles did not cause heterogeneous nucleation of PET crystals in nanocomposites containing up to 3 wt % Al2O3. This is attributed to the nanoparticle curvature being comparable to the radius of gyration of the polymer. The addition of the nanoparticles was found to disrupt the spherulitic morphology of the PET because of their physical presence and their proximity to one another. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007

Crystal growth in alumina/poly(ethylene terephthalate) nanocomposite films

Abstract: The crystal growth and morphology in 150-nm-thick PET nanocomposite thin films with alumina (Al{sub 2}O{sub 3}) nanoparticle fillers (38 nm size) were investigated for nanoparticle loadings from 0 to 5 wt%. Transmission electron microscopy of the films showed that at 1 wt% Al{sub 2}O{sub 3}, the nanoparticles were well dispersed in the film and the average size was close to the reported 38 nm. Above 2 wt% Al{sub 2}O{sub 3}, the nanoparticles started to agglomerate. The crystal growth and morphological evolution in the PET nanocomposite films kept at an isothermal temperature of 217 C were monitored as a function of the holding time using in situ atomic force microscopy. It was found that the crystal nucleation and growth of PET was strongly dependent on the dispersed particles in the films. At 1 wt% Al{sub 2}O{sub 3}, the overall crystal growth rate of PET lamellae was slower than that of the PET homopolymer films. Above 2 wt% Al{sub 2}O{sub 3}, the crystal growth rate increased with nanoparticle loading because of heterogeneous nucleation. In addition, in these PET nanocomposite thin films, the Al{sub 2}O{sub 3} nanoparticles induced preferentially oriented edge-on lamellae with respect to the surface, which was not the case inmore » unfilled PET as determined by grazing-incidence X-ray diffraction.« less

High refractive index nanoparticle-loaded encapsulants for light-emitting diodes

Abstract: TiO 2 nanoparticle-loaded epoxy for light-emitting diode (LED) encapsulation is demonstrated to have a refractive index of n = 1.68, higher than that of pure epoxy (n = 1.53). The dispersion of surfactant-coated TiO 2 nanoparticles into epoxy is shown to reduce the number and size of TiO 2 agglomerates compared to uncoated TiO 2 nanoparticle-loaded epoxy. The scattering of nanoparticle-loaded media is highly dependent on nanoparticle size and loading factor. A multilayer graded refractive index structure for LED encapsulants with layer thicknesses less than the calculated mean optical scattering length is therefore proposed. The graded-index structure shows increasing optical transmittance as the number of layers increases.

The Role of Dislocations at the Catalyst−Wall Interface in Carbon Nanotube Growth

Abstract: Ex situ transmission electron microscopy (TEM) performed on catalytically grown multiwall carbon nanotubes identified two types of catalyst−nanotube wall interfaces. The interfaces consisted of crystalline domains with different orientations: twist and twin boundaries in correspondence with quasi-spherical particles closer to the nanotube base and tilt boundaries in correspondence with high aspect ratio, tapered particles further from the base. TEM suggests that the domain boundaries maintain a rather steady position coupled to the catalytic particles, whereas the carbon atoms precipitate along the nanotube axis away from the particles. It is concluded that the relative movement of the carbon atoms with respect to the dislocations comprising the nanotube domain boundary located at the catalyst−wall interface is a significant elementary process in nanotube crystal growth driven by surface diffusion. The results appear consistent with the concurrence of base and tip growth for the catalytic synthesis of ca...

Solubility- and temperature-driven thin film structures of polymeric thiophene derivatives for high performance OFET applications

Abstract: It has been shown that high charge mobility in solution-processible organic semiconductor-based field effect transistors is due in part to a highly parallel π-π stacking plane orientation of the semiconductors with respect to gate-dielectric. Fast solvent evaporation methods, generally, exacerbate kinetically random crystal orientations in the films deposited, specifically, from good solvents. We have investigated solubility-driven thin film structures of thiophene derivative polymers via spin- and drop-casting with volatile solvents of a low boiling point. Among volatile solvents examined, marginal solvents, which have temperature-dependent solubility for the semiconductors (e.g. methylene chloride for regioregular poly(3-alkylthiophene)s), can be used to direct the favorable crystal orientation regardless of solvent drying time, when the temperature of gate-dielectrics is held to relatively cooler than the warm solution. Grazing-incidence X-ray diffraction and atomic force microscopy strongly support that significant control of crystal orientation and mesoscale morphology using a "cold" substrate holds true for both drop and spin casting. The effects of physiochemical post-modificaiton on film crystal structures and morphologies of poly(9,9-dioctylfluorene-co-bithiophene) have also been investigated.

Method for preparing polyester nanocomposites

Abstract: A method for preparing a polyester nanocomposite is presented. The method comprises coating nanoparticles with a dicarboxylic acid. Combining the dicarboxylic acid coated nanoparticles with a coupling agent resulting in a first mixture. Then combining the first mixture with a polyester to form a polyester nanocomposite. The resulting polyester nanocomposite has among other properties a glass transition temperature greater than the polyester itself and also a crystallization temperature less than the polyester itself.