B.J. Park

Preparation and Characterization of Poly(Methyl Methacrylate) Coated TiO2 Nanoparticles

Titanium dioxide (TiO2) nanoparticles were modified with poly(methyl methacrylate) (PMMA) to improve the dispersion stability of the nanoparticles in a dielectric medium and to reduce the density mismatch between TiO2 and a dielectric medium for a microcapsule‐type electrophoretic display application. Nanoparticles were coated with PMMA by in situ dispersion polymerization. The PMMA‐coated TiO2 nanoparticles were characterized by fourier transform‐infrared spectrometrey (FT‐IR), electrophoretic light scattering (ELS), and scanning electron microscopy (SEM). Density of PMMA‐coated TiO2 nanoparticles was found to be dependent on the thickness of the PMMA coating on the nanoparticles. An increase of thermal stability of the PMMA layer and the contents of PMMA on the surface of the nanoparticles were measured via thermogravimetric analysis (TGA).

Pentacene TFT With Reduced Threshold Voltage Using PMMA-co-MAA/Sol-Gel-Derived

We report improvements in the characteristics of pentacene thin-film transistors (TFTs) achieved by using a poly(methylmethacrylate-co-methacrylic acid) (PMMA-co-MAA)/sol-gel-derived TiO2 composite insulator. The gate-leakage current of TFTs containing this insulator is comparable to that of devices composed of bare PMMA-co-MAA. The reduction in the threshold voltage is the most pronounced improvement observed herein (compared to the variations of other characteristic parameters). This can be explained by the negative surface potential of the sol-gel-derived TiO2 film.

\hbox{TiO}_{2}

Monodisperse polystyrene (PS) seed latex was initially prepared by a dispersion polymerization, and then polyaniline (PANI)-coated PS composite particles were synthesized by diffusion and polymerization of aniline at the interface. Characteristics of the obtained PS/PANI composite particles such as morphology and thermal property were examined using SEM and TGA, respectively. In addition, we employed a rotational rheometer to investigate rheological behaviour of the ER fluids based on both PANI particles and PS/PANI composites. Yield stress and flow response were investigated under electric field strengths showing typical ER behaviours. Further, the Cole-Cole plot and the dielectric spectra gave relaxation times of the ER systems, confirming the correlation of dielectric properties with ER performance.

Composite Insulator

We report the effects of moisture on the electrical characteristics of pentacene field-effect transistors (FETs) with the polyvinylpyrrolidone (PVPy) gate insulator. For the condition of relative humidity below 40%, the pentacene FET exhibited a stable operation without a shift in the threshold voltage upon a gate voltage sweep direction. With increasing the relative humidity above 50%, the threshold voltage shifted critically toward the positive direction, accompanied by significant degradations in the field-effect mobility and the subthreshold slope. These moisture-induced characteristic degradations could be substantially recovered by the low-temperature annealing process under a base pressure of 2 × 10−3 Torr.

Preparation of polystyrene/polyaniline composite particles and their electrorheology

In this study, a sol-gel process is introduced to fabricate the composite film of polystyrene and titania, using titanium tetraisoproxide as a titania precursor. Pentacene-based organic thin-film transistor (OTFTs) with the composite film as a gate insulator operates at a low voltage below −4 V and exhibits high performances, such as threshold voltage of −0.9 V, subthreshold slope of 0.2 V/decade, and mobility of 4.7 cm2/Vs, which are mainly attributed to a high dielectric property of the fabricated composite insulator. These results demonstrate that sol-gel derived composite insulators are essentially promising for low-voltage OTFTs with high performances.

Effects of Moisture on Pentacene Field-Effect Transistors with Polyvinylpyrrolidone Gate Insulator

Polystyrene/clay nanocomposite with organically modified montmorillonite was synthesized via emulsion polymerization. Organic styrene monomer was first intercalated into intergalley of the clay hosts, followed by a typical emulsion polymerization with surfactant and initiator. To investigate the effect of clay loading, we also prepared PS/clay nanocomposites with different clay contents. Wide angle X-ray diffraction (XRD) and transmission electron microscopy (TEM) showed that the emulsion polymerization of PS in the presence of clay produces partially exfoliated and intercalated nanocomposites. In order to study their rheological properties, the melt pallets of composite materials were prepared and measured via a rotational rheometer with a parallel plate geometry.

Electrical Characteristics of Organic Thin-Film Transistors with Polystyrene/Sol-Gel Derived Titania Composite Insulator

To enhance dispersion stability of magnetorheological (MR) fluids, hybrid magnetic particles of carbonyl iron (CI)/ poly(vinyl butyral) (PVB) with core/shell microstrcutre (CI-PVB) were prepared, since pure magnetic CI based MR fluid systems show severe sedimentation of the CI particles due to the large density mismatch with the carrier liquid and difficulties in redispersion after caking. The composite particles of CI-PVB have a lower density than that of the pure CI particles, while exhibiting almost original magnetic property of the CI. Both CI and CI-PVB particles were dispersed in mineral oil (20 vol%) and their MR characteristics were examined via a rotational rheometer with a magnetic field supplier. Various characterizations of the CI-PVB particles were performed via SEM, TEM and FT-IR. Both yield stress and flow curve of shear stress as a function of shear rate of the MR fluids were investigated under applied magnetic field strengths.