Moo Hyun Kwon

Rheological properties and stability of magnetorheological fluids using viscoelastic medium and nanoadditives

In order to improve the stability of magnetorheological (MR) fluids, viscoelastic medium having 2.2 Pa yield stress has been used as a continuous phase and nanosized CrO2 particles are added too. The rheological properties as well as the dispersion stability of MR fluids have been studied by using a stress-controlled rheometer and sedimentation test. The steady-shear MR response was independent of the continuous and nano additives and the fieldinduced yield stress increased subquadratically with the flux density. Since the constant stress is generated within the limit of zero shear rate, the plateau in the flow curve corresponds to the Bingham yield stress. Under an external field, the yield stress varied as B3/2. The yield stress has an approximately linear relation with the particle volume fraction.

Reorientation of Colloidal Crystalline Domains by a Thinning Meniscus

When water is evaporated quickly from a water-based colloidal suspension, colloidal particles protrude from the water surface, distorting it and generating lateral capillary forces between the colloidal particles. The protruded colloidal particles are then assembled into ordered colloidal crystalline domains that float on the water surface on account of their having a lower effective density than water. These colloidal crystal domains then assemble together by lateral capillary force and convective flow; the generated colloidal crystal has grain boundaries. The single domain size of the colloidal crystal could be controlled, to some extent, by changing the rate of water evaporation, but it seems very difficult to fabricate a single crystal over a large area of the water’s surface without reorienting each colloidal crystal domain. To reorient such colloidal crystal domains, a glass plate was dipped into the colloidal suspension at a tilted angle because the meniscus (airwaterglass plate interface) is pinned and thinned by further water evaporation. The thinning meniscus generated a shear force and reoriented the colloidal crystalline domains into a single domain.

Optical Properties and Characteristics of the CdSe Nanoparticles Synthesized at Room Temperature

CdSe nanoparticles were synthesized by using colloidal methods at room temperature. Nanoparticle size was controlled by the amount of stabilizer, pH, and stabilizer type and was characterized by TEM and XRD. All the synthesized CdSe nanoparticles showed the quantum confinement effect. With increased amounts of mercaptoacetic acid as a stabilizer, the size of nanoparticles decreased. The UV-VIS absorption and photoluminescence (PL) properties could also be tailored by controlling particle size. The solubility in organic solvent and the PL characteristics were enhanced through surface capping by an organic passivator.

Electrohydrodynamics of Rigid Macromolecules with Permanent and Induced Dipole Moments

The orientation distribution of rigid macromolecules dissolved at low concentration in a dielectric Newtonian fluid is solved for the case in which simultaneous hydrodynamic and electric fields are applied. Macromolecules are taken to be Brownian rigid spheroids of arbitrary aspect ratio and both permanent and induced dipole moments are included. To solve the problem with arbitrary strength of fields, the Galerkin method based on spherical harmonics is adopted. The converged solutions are used in the prediction of the steady state birefringence and extinction angle over very wide ranges of field strengths. Due to the coupled effects of flow and external field, somewhat complicated responses are predicted and saturation features are revealed at the strong flow limit. External field dependence due to the induced dipole is similar to that of the permanent case for most field conditions. Finally, it is shown, taking poly‐γ‐benzyl‐L‐glutamate solutions with various molecular weights as an example, how results ...

Multiple-Layered Colloidal Assemblies via Dipping Method with an External Electric Field

When using the dipping method for crystal formation, mono-layered colloidal crystal structures depend upon the lift-up rate of a glass substrate. The mono-layered colloidal crystals showed the highest quality when the glass substrate was raised at a rate of 3 mm/min at 25 °C in a 1 wt% polystyrene colloidal suspension (ethanol medium). In addition, in order to obtain multiple-layered colloidal crystals, an external electric field was introduced. Multiple-layered colloidal crystals were successfully obtained via this method. The colloidal particles were well ordered over large areas and assembled into a homogeneous structure.

Effects of Nucleating Agent on Nonisothermal Crystallization of Syndiotactic Polystyrene

The effects of nucleating agent on nonisothermal crystallization were examined for syndiotactic polystyrene (SPS) using Differential Scanning Calorimetry (DSC). The crystallization peak temperature Tc, the crystallization rate parameter (CRP), the enthalpy of crystallization and melting, and the recrystallization behavior were compared between organic (DMBS, 1,3,2,4-dis-5-(3,4-dimethyl benzylidene) sorbitol) and inorganic (talc) nucleating agents. Both of the nucleation agents promoted the crystallization rate of SPS until some critical concentration of about 3,000 ppm. DMBS worked more effectively as a nucleating agent than talc, although the Tc showed lower level in SPS/DMBS than SPS/talc.

Generation of Graded Index Profile of Poly(methyl methacrylate) by a Photochemical Reaction

Fabrication of a graded index profile was possible via photochemical reaction of cinnamoyl groups with 350 nm wavelength UV light to form crosslinked structures. Such structural change may induce the change in the refractive index. In order to generate graded index profile in the PMMA polymer optical fiber (POF) with cinnamoyl groups by photochemistry, a methyl methacrylate monomer containing a cinnamoyl functional group in the side chain were prepared. This monomer was then copolymerized with methylmethacrylate with various compositions not only to utilize advantages of poly(methyl methacrylate) but also to overcome the drawbacks of the cinnamate homopolymer. Changes of refractive indices were investigated with various contents of cinnamoyl group and varying irradiation time. Large change in the refractive index (Δ≈0.01)|and its proper profile shape (g≈?2.2)|can be obtained by changing irradiation time.

Fabrication of Polymeric Hollow Spheres Having Macropores by a Quenching and Sublimation Process

We fabricated polymeric hollow spheres having macropores, which combine the advantageous properties of porous materials and hollow spheres. To fabricate such spheres, a polystyrene/methylmethacrylate solution was dispersed in water by vigorously stirring and then the suspension was quenched using liquid nitrogen. Water and methyl methacrylate present in the quenched suspension were readily sublimated by freeze-drying. Conclusively, the hollow-sphere structure and the macropores of its shell were created by the processes of liquid nitrogen-quenching and sublimation of methyl methacrylate domains within the shell, respectively.

Morphology evolution in PS/LDPE blends in a twin screw extruder: Effects of compatibilizer

In order to understand the mixing in polymer extrusion, the morphology of PS/LDPE blends compatibilized with SEBS was simulated based on the Lee and Park model. Experiments were performed in a co-rotating twin screw extruder and a Haake mixer, and the morphology of the compatibilized blends of various compositions was compared with the non-compatibilized ones. The size of the dispersed phase of the compatibilized blend was smaller than the non-compatibilized one, and the blend of the PE matrix had a smaller size in the dispersed phase than the non-compatibilized one. Simulation results were agreed well with experiments, and the Lee and Park model could be applied in the extrusion analysis of compatibilized polymer blends.

Experimental electrohydrodynamics of poly(γ-Benzyl-L-Glutamate) in M-Cresol solution subjected to shear flow and electric fields

Electrohydrodynamics of a dilute solution of rigid macromolecules was experimentally studied in a continuation of previous theoretical work. We used poly(γ-benzyl-L-glutamate), having 4 different molecular weights ranging from 15,000 to 236,000, dissolved in m-cresol. Poly(γ-benzyl-L-glutamate) solutions were subjected to combinations of simple shear flow field and uniform electric field perpendicular to the shear direction. Transient birefringence and extinction angle were simultaneously measured using the phase-modulated birefringence method. Steady state results were compared with the theoretical prediction from previous works and rotational difiusivity and permanent dipole strength of PBLG were obtained from multiple parameter fitting. Consequently, the optical state of PBLG solution could be explained to a certain extent by the dimensionless field parameters established in the previous theory.