Static yield stress of a magnetorheological fluid containing Pickering emulsion polymerized Fe2O3/polystyrene composite particles.

The flow behaviors of magnetorheological (MR) suspensions containing Pickering emulsion polymerized Fe2O3/polystyrene (PS) composite particles were reanalyzed using the Seo-Seo model. The experimental shear stress data obtained experimentally from the magnetorheological fluid fit well to the Seo-Seo model, indicating that this model can describe the structural reformation process of the aligned fibers at various shear rates. Unlike the dynamic yield stress obtained from the Cho-Choi-Jhon (CCJ) model, the static yield stresses obtained from the Seo-Seo model exhibit the same quadratic dependence on the magnetic field strength for both pure Fe2O3 particle suspension and Fe2O3/PS particle suspensions, which is in agreement with the predictions of the polarization model. The static yield stress plausibly explains the difference in underlying mechanism of MR fluids.

Catalyst and doping methods for arc graphene.

Nitrogen-doped graphene synthesis with ∼g scale has been accomplished using the arc discharge method. The defects formed in the synthesis process were reduced by adding various metal catalysts, among which Bi2O3 was found to be the most effective. Adding dopants to the starting materials increased the electrical conductivity of the graphene product, and the doping concentration in graphene was tuned by adjusting the amount of nitrogen dopants. A step-wise technique to fabricate graphene thin films was developed, including dispersion, separation, and filtering processes. The arc graphene can also find its potential application in supercapacitors, taking advantage of its large surface area and improved conductivity by doping.

Physical properties of nanocomposites prepared by in situ polymerization of high-density polyethylene on multiwalled carbon nanotubes.

In situ metallocence polymerization was used to prepare nanocomposites of multiwalled carbon nanotubes (MWCNT) and high density polyethylene (HDPE). This polymerization method consists of attaching a metallocene catalyst complex onto the surface of MWCNT followed by surface-initiated polymerization to generate polymer brushes on the surface. All the procedures of polymerization made progress with one-pot process. The morphological observation of nanocomposites using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) showed that the nanotubes are uniformly dispersed throughout HDPE matrix. Physical properties of thermal and electrical conductivities and rheological response have been characterized. Since the carbon nanotubes are wrapped by PE molecules, the large interface provided by MWCNT's lead to strong phonon boundary scattering. Thus, the enhancement of thermal conductivity by the inclusion of nanotubes was quite restrictive. On the other hand, electrical conductivity and rheological properties show the property transition at the critical concentration of carbon nanotubes (percolation threshold). The DC conductivity increased with increasing weight fraction of MWCNT from 1.0 x 10(-13) S cm(-1) (neat HDPE) to 1.3 x 10(-2) S cm(-1) (HDPE/7.3 wt% of MWCNT) at room temperature and the electrical percolation threshold was ca. 7.3 wt%. The percolation threshold concentration of MWCNT for the rheological properties was ca. 8.7 wt%, similar to that of the electrical conductivity. Difference in the percolation behaviors between the MWCNT mixed nanocomposites and the PE-coated MWCNT nanocomposites is discussed in terms of the dispersion and the tube-tube distance of MWCNT.

Gas barrier film with a compositional gradient interface prepared by plasma modification of an organic/inorganic hybrid sol-gel coat.

A high-performance gas barrier film has been prepared using a simple oxygen plasma treatment of an organic/inorganic hybrid sol-gel coat on a poly(ether sulfone) substrate. The gas barrier layer formed from the plasma treatment was an inorganic silicon oxide (SiO(x)) having a compositional gradient interface with the sol-gel coat, where the chemical compositions changed gradually from inorganic to organic structure at the interface. The minimum attained oxygen transmission rate (OTR) of the gas barrier layer was 0.2 cm(3)/(m(2) day), which is superior or comparable to that of the gas barrier layer prepared by plasma-enhanced chemical vapor deposition (PECVD) or sputtering. Low-temperature oxygen plasma combustion of alkyl groups existing in the sol-gel coat is considered to be the major reaction for the formation of the SiO(x) gas barrier layer, where oxygenated carbon compounds and hydrogen were the main volatile components as verified by the analysis of optical emission spectroscopy. Plasma treatment time necessary to attain a high-performance SiO(x) gas barrier layer decreased with increasing RF plasma power. In a dynamic bending test, the gas barrier film preserved the initial gas barrier properties without any formation of film cracks or delamination. The compositional gradient at the interface between the gas barrier and the sol-gel coat is considered to provide a stress-relieving effect at the interface.