Jae Ryoun Youn

Ultraviolet nanoimprinted polymer nanostructure for organic light emitting diode application

Light extraction efficiency of a conventional organic light emitting diode (OLED) remains limited to approximately 20% as most of the emission is trapped in the waveguide and glass modes. An etchless simple method was developed to fabricate two-dimensional nanostructures on glass substrate directly by using ultraviolet (UV) curable polymer resin and UV nanoimprint lithography in order to improve output coupling efficiency of OLEDs. The enhancement of the light extraction was predicted by the three-dimensional finite difference time domain method. OLEDs integrated on nanoimprinted substrates enhanced electroluminance intensity by up to 50% compared to the conventional device.

Fabric Surface Roughness Evaluation Using Wavelet-Fractal Method Part I: Wrinkle, Smoothness and Seam Pucker

A wavelet-fractal method to calculate the fractal dimension is proposed to objectively evaluate the surface roughness of fabric wrinkle, smoothness appearance and seam pucker. The proposed method was validated using the fractal surfaces produced from the mathematical functions and compared with the box and cube counting methods. The more accurate three-dimensional mesh grid data points of wrinkle replicas, smoothness appearance replicas and seam pucker samples were obtained using a three-dimensional, noncontact scanning system. As a supplementary reference the standard roughness parameters, which differentiate the degree of fabric surface roughness, were also investigated. The results show that the fractal dimension measured by the wavelet-fractal method as well as the surface average mean curvature show the power to clearly discern the grades of wrinkle, smoothness appearance as well as seam pucker, and thus can evaluate the fabric surface roughness objectively and quantitatively

Ultra-high dispersion of graphene in polymer composite via solvent freefabrication and functionalization

The drying process of graphene-polymer composites fabricated by solution-processing for excellent dispersion is time consuming and suffers from a restacking problem. Here, we have developed an innovative method to fabricate polymer composites with well dispersed graphene particles in the matrix resin by using solvent free powder mixing and in-situ polymerization of a low viscosity oligomer resin. We also prepared composites filled with up to 20 wt% of graphene particles by the solvent free process while maintaining a high degree of dispersion. The electrical conductivity of the composite, one of the most significant properties affected by the dispersion, was consistent with the theoretically obtained effective electrical conductivity based on the mean field micromechanical analysis with the Mori-Tanaka model assuming ideal dispersion. It can be confirmed by looking at the statistical results of the filler-to-filler distance obtained from the digital processing of the fracture surface images that the various oxygenated functional groups of graphene oxide can help improve the dispersion of the filler and that the introduction of large phenyl groups to the graphene basal plane has a positive effect on the dispersion.

ENVIRONMENTALLY FRIENDLY PROCESSING OF POLYURETHANE FOAM FOR THERMAL INSULATION

An environmentally friendly processing method of polyurethane foam was proposed and evaluated for the application of thermal insulation. For the production of polyurethane foam, chlorofluorocarbon (CFC) gases were eliminated to minimize environmental destruction. Carbon dioxide gas was used as a blowing agent instead of the CFC gas. Ultrasonic excitation was also applied to the mixture of polyol and isocyanate to increase the rate of nucleation and decrease the thermal conductivity. Nucleation and growth of bubbles were studied theoretically and experimentally. Water was used as a chemical blowing agent and carbon dioxide gas as the physical blowing agent. Experiments were carried out with different saturation pressures, and experimental results were evaluated to determine the best foaming conditions. Rate of bubble nucleation, final bubble sizes, conversion during polymerization reaction, and other parameters were predicted theoretically with the assumption that negative pressure is generated by the ultrasonic field and the bubble growth is controlled by diffusion of the gas from the resin into the bubble. The best results such as low thermal conductivity and small size of bubbles were obtained when the polyol which has been mixed with water was saturated with carbon dioxide gas and the ultrasonic excitation was applied to the mixture of polyol and isocyanate right after the impingement mixing.

Characterization of Fiber Orientation in Short Fiber Reinforced Composites with an Image Processing Technique

Abstract.An experimental method is developed to measure the three-dimensional fiber orientation in short fiber reinforced composites by utilizing an image processing technique. The second order orientation tensor can be calculated with geometrical data that were obtained from two parallel planar cross-sections. The orientation state of individual fibers is determined from the geometry of the elliptical cross-sectional shape on the polished surface. The basic concept in determining the three-dimensional fiber orientation tensor is to slice the composite sample twice in the same direction within a small distance. The tensor is determined by using a digital image processing technique and a computational code which calculates the tensor from the geometrical characteristics obtained for the elliptical fiber cross-sections. Experiments are performed to measure the three-dimensional orientation tensor of composite specimens and good results are obtained by using the method proposed in this study

Effect of compressibility on flow field and fiber orientation during the filling stage of injection molding

Abstract The anisotropy caused by the fiber orientation that is inevitably generated by the flow during the injection molding of short fiber reinforced polymers, greatly influences the dimensional accuracy, the mechanical properties and other qualities of the final product. Since the filling stage of the injection molding process plays a vital role in determining the orientation of the fiber, an accurate analysis of the flow field for the filling stage becomes a necessity. Unbalanced filling occurs when a complex or a multi-cavity mold is used, leading to the development of regions where the fiber suspension is under compression. It is impossible to make an accurate calculation of the flow field during filling within analysis assuming an incompressible fluid. In this study, a FEM/FDM hybrid scheme with consideration of compressibility was developed to calculate the flow field. At the moment of complete filling, the three-dimensional fiber orientation field was estimated by solving the equation of orientation change for the second-order orientation tensor with the fourth-order Runge-Kutta method. A mold with four cavities with different filling times was produced to compare the results of numerical analysis with experimental data. There was good agreement between the experimental and theoretical results when the compressibility of the polymer melt was considered for the numerical simulation. Also, qualitative and quantitative comparisons of fiber-orientation states for compressible and incompressible fluids were made.

Processing of microcellular nanocomposite foams by using a supercritical fluid

Polystyrene/layered silicate nanocomposites were prepared by melt intercalation. To examine the distribution of the clay in polymer matrix, small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) were used. Intercalated nanocomposites were obtained and their rheological properties were investigated. Microcellular nanocomposite foams were produced by using a supercritical fluid. As clay contents increased, the cell size decreased and the cell density increased. It was found that layered silicates could operate as heterogeneous nucleation sites. As the saturation pressure increased and the saturation temperature decreased, the cell size decreased and the cell density increased. Microcellular foams have different morphology depending upon the dispersion state of nanoclays.

Degradation and rheological properties of biodegradable nanocomposites prepared by melt intercalation method

Biodegradable nanocomposites were prepared by mixing a polymer resin and layered silicates by the melt intercalation method. Internal structure of the nanocomposite was characterized by using the small angle X-ray scattering (SAXS) and transmission electron microscope (TEM). Nanocomposites having exfoliated and intercalated structures were obtained by employing two different organically modified nanoclays. Rheological properties in shear and extensional flows and biodegradability of nanocomposites were measured. In shear flow, shear thinning behavior and increased storage modulus were observed as the clay loading increased. In extensional flow, strain hardening behavior was observed in well dispersed system. Nanocomposites with the exfoliated structure had better biodegradability than nanocomposites with the intercalated structure or pure polymer.

Incorporation of sheet-forming effects in crash simulations using ideal forming theory and hybrid membrane and shell method

In order to achieve reliable but cost-effective crash simulations of stamped parts, sheet-forming process effects were incorporated in simulations using the ideal forming theory mixed with the three-dimensional hybrid membrane and shell method, while the subsequent crash simulations were carried out using a dynamic explicit finite element code. Example solutions performed for forming and crash simulations of I- and S-shaped rails verified that the proposed approach is cost effective without sacrificing accuracy. The method required a significantly small amount of additional computation time, less than 3% for the specific examples, to incorporate sheet-forming effects into crash simulations. As for the constitutive equation, the combined isotropic-kinematic hardening law and the nonquadratic anisotropic yield stress potential as well as its conjugate strain-rate potential were used to describe the anisotropy of AA6111-T4 aluminum alloy sheets.

Measurements of anisotropic yielding, bauschinger and transient behavior of automotive dual-phase steel sheets

In order to present better prediction capability in computational analysis, mechanical properties of the dualphase high strength steel have been characterized especially for anisotropy as well as the Bauschinger and transient behavior. As for the anisotropy, the non-quadratic anisotropic yield function Yld2000-2d has been utilized and its material parameters have been obtained using the uni-axial tension tests as well as the hydraulic bulge test. To measure the hardening behavior including the Bauschinger and transient behavior, a newly developed test method has been applied for the uni-axial tension/compression and compression/tension tests, in which solid blocks along the both sides of the sheet specimen prevent buckling. From the tension/compression curves, the equations to describe isotropic and kinematic hardening behavior have been obtained. The modified Chaboche model has been confirmed to well represent the hardening behavior including the Bauschinger and transient behavior.