Linda S. Schadler

Load transfer in carbon nanotube epoxy composites

The mechanical behavior of multiwalled carbon nanotube/epoxy composites was studied in both tension and compression. It was found that the compression modulus is higher than the tensile modulus, indicating that load transfer to the nanotubes in the composite is much higher in compression. In addition, it was found that the Raman peak position, indicating the strain in the carbon bonds under loading, shifts significantly under compression but not in tension. It is proposed that during load transfer to multiwalled nanotubes, only the outer layers are stressed in tension whereas all the layers respond in compression.

Polymer nanocomposite dielectrics-the role of the interface

The incorporation of silica nanoparticles into polyethylene increased the breakdown strength and voltage endurance significantly compared to the incorporation of micron scale fillers. In addition, dielectric spectroscopy showed a decrease in dielectric permittivity for the nanocomposite over the base polymer, and changes in the space charge distribution and dynamics have been documented. The most significant difference between micron scale and nanoscale fillers is the tremendous increase in interfacial area in nanocomposites. Because the interfacial region (interaction zone) is likely to be pivotal in controlling properties, the bonding between the silica and polyethylene was characterized using Fourier transformed infrared (FTTR) spectroscopy, electron paramagnetic resonance (EPR), and x-ray photoelectron spectroscopy (XPS). The picture which is emerging suggests that the enhanced interfacial zone, in addition to particle-polymer bonding, plays a very important role in determining the dielectric behavior of nanocomposites.

Mechanisms of Enhanced Osteoblast Adhesion on Nanophase Alumina Involve Vitronectin

The role, including concentration, conformation, and bioactivity, of adsorbed vitronectin in enhancing osteoblast adhesion on nanophase alumina was investigated in the present study. Vitronectin adsorbed in a competitive environment in the highest concentration on nanophase alumina compared to conventional alumina. Enhanced adsorption of vitronectin on nanophase alumina was possibly due to decreased adsorption of apolipoprotein A-I and/or increased adsorption of calcium on nanophase alumina. In a novel manner, the present study utilized surface-enhanced Raman scattering (SERS) to determine the conformation of vitronectin adsorbed on nanophase alumina. These results provided the first evidence of increased unfolding of vitronectin adsorbed on nanophase alumina. Increased adsorption of calcium on nanophase alumina may affect the conformation of adsorbed vitronectin specifically to promote unfolding of the macromolecule to expose cell-adhesive epitopes recognized by specific cell-membrane receptors. Results of the present study also provided evidence of dose-dependent inhibition of osteoblast adhesion on nanophase alumina pretreated with vitronectin following preincubation (and thus blocking respective cell-membrane receptors) with either Arginine-Glycine-Aspartic Acid-Serine (RGDS) or Lysine-Arginine-Serine-Arginine (KRSR). These events, namely, enhanced vitronectin adsorption, comformation, and bioactivity, may explain the increased osteoblast adhesion on nanophase alumina.

Ultrafast optical switching properties of single-wall carbon nanotube polymer composites at 1.55 μm

Using a pump–probe method with a 150 fs laser at the wavelength of 1.55 μm, we have experimentally demonstrated that single-walled carbon nanotubes (SWNT) have an exciton decay time of less than 1 ps and a high third-order polarizability, which is reasonably interpreted as due to their azimuthal symmetry. These experimental results reveal that a SWNT polymer composite may be a candidate material for high-quality subpicosecond all-optical switches.

Glass transition behavior of alumina/polymethylmethacrylate nanocomposites

Abstract Alumina/polymethylmethacrylate (PMMA) nanocomposites were synthesized using 39-nm nanoparticles and in situ free-radical polymerization. At filler concentrations greater than 0.5 wt.%, the glass transition temperature, Tg, was observed to decrease precipitously by 25 °C compared to the neat polymer. At smaller weight fractions, there were no changes in the composite Tg. The abrupt changes seem to indicate a threshold at which a significant volume fraction of polymer has higher mobility that brings about the decrease in Tg. Consistent with this behavior, the Tg depression was suppressed by coating the nanoparticles to make them compatible with the matrix.

Protein immobilization on carbon nanotubes via a two-step process of diimide-activated amidation

Ferritin and bovine serum albumin (BSA) proteins are chemically bonded to nitrogen-doped multi-walled carbon nanotubes (CNx MWNTs) through a two-step process of diimide-activated amidation. First, carboxylated CNx MWNTs were activated by N-ethyl-N′-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDAC), forming a stable active ester in the presence of N-hydroxysuccinimide (NHS). Second, the active ester was reacted with the amine groups on the proteins of ferritin or BSA, forming an amide bond between the CNx MWNTs and proteins. This two-step process avoids the intermolecular conjugation of proteins, and guarantees the uniform attachment of proteins on carbon nanotubes. TEM and AFM measurements clearly confirmed the successful attachment. This approach provides a universal and efficient method to attach biomolecules to carbon nanotubes at ambient conditions.

Synthesis and mechanical properties of TiO2-epoxy nanocomposites

New developments in the synthesis of nanometer scale TiO2 particles have enabled the processing of exciting new nanoparticle/epoxy composites. An ultrasonic method was used to disperse the nanoparticles in epoxy, thus eliminating the need for solvent without sacrificing the ease of processing. Composites were processed at the 5, 10, 15 and 20 weight percent level and were characterized by SEM, tensile tests and scratch tests. These results at 10 percent loading were also compared with an equal loading of micron-size TiO2 fillers in epoxy to evaluate the strain-to-failure and scratch resistance.

Influence of nanoparticle surface modification on the electrical behaviour of polyethylene nanocomposites

In this study, we present the results of the influence of surface modification of TiO2 nanoparticles on the short-term breakdown strength and space charge distribution of low-density polyethylene (LDPE). A polar silane coupling agent N-(2-aminoethyl) 3-aminopropyl-trimethoxysilane (AEAPS) was used for the nanoparticle surface modification. Despite agglomeration and a poor interface compared to untreated nanoparticles, it was found that the incorporation of polar groups onto the nanoparticle surface improved both the dielectric breakdown strength and space charge distribution as compared to samples filled with untreated nanoparticles. Microstructure studies showed that the presence of polar groups on the TiO2 nanoparticle surface did not evidently affect the degree of crystallinity, crystalline morphology (except for internal spherulitic order), and chemical structure of the polymer matrix. The improved dielectric breakdown strength was therefore concluded to be directly due to beneficial effects related to the variation of the electrical features at the particle surface due to introduction of polar groups. For the same reason, with the use of surface modified nanoparticles, formation of space charge was suppressed.

On the tensile strength distribution of multiwalled carbon nanotubes

Individual multiwalled carbon nanotubes grown by chemical vapor deposition (CVD) were tensile tested within the chamber of an electron microscope using an atomic force microscope-based technique. Weibull–Poisson statistics could accurately model the nanotube tensile strength data. Weibull shape and scale parameters of 1.7 and 109GPa were obtained. The former reflects a wide variability in strength similar to that observed for high-modulus graphite fibers, while the latter indicates that the irregular CVD-grown tube wall structure requires, in some cases, higher breaking forces than more regular tube wall structures. This apparent strengthening mechanism is most likely caused by an enhanced interaction between the walls of the nanotube.

TiO2 nanocomposites with high refractive index and transparency

Transparent polymer nanocomposites with high refractive index were prepared by grafting polymer chains onto anatase TiO2 nanoparticlesvia a combination of phosphate ligand engineering and alkyne-azide “click” chemistry. Highly crystalline TiO2 nanoparticles with 5 nm diameter were synthesized by a solvothermal method and used as high refractive index filler. The synthesized phosphate-azide ligand anchors strongly onto the TiO2 nanoparticle surface and the azide end group allows for attachment of poly(glycidyl methacrylate) (PGMA) polymer chains through an alkyne-azide “click” reaction. The refractive index of the composite material increased linearly from 1.5 up to 1.8 by increasing the loading of TiO2 particles to 30 vol % (60 wt %). UV-vis spectra show that the nanocomposites exhibited a transparency around 90% throughout the visible light range. It was also found that the PGMA-grafted TiO2 nanoparticles can be well dispersed into a commercial epoxy resin, forming transparent high refractive index TiO2-epxoy nanocomposites.