Papers by Author: Ick Soo Kim

Abstract: We report the electrospun poly(L-lactide) (PLA) nanofibers incorporating polyhedral oligosilsesquioxane (POSS)-modified multiwalled carbon nanotubes (MWNTs). FT-IR, transmission electron microscopy (TEM) and Raman analysis confirmed the existence of POSS macromers bonded to the MWNTs as an extra phase. The thermal and microstructure properties of the PLA hybrid nanofibers with POSS-modified MWNTs were investigated by thermogravimetric analysis (TGA) and Wide-angle X-ray diffraction (WAXD).

Abstract: Recently, we have reported that syndiotactic polypropylene (sPP) fibrous membrane could be successfully prepared from a multi component solvent system via electrospinning at room temperature. The base solvent of multi component solvent system was cyclohexane. As demonstrated SEM images, the formation of rough surface morphologies and the microholes on electrospun sPP fibers were observed. In this study, we successfully prepared sPP nanofiber webs from a different base solvent was decalin. SEM study demonstrated that the sPP nanofibers prepared from decalin based solvent had smoother surface morphology unlike cyclohexane based solvent, suggesting that the surface morphology of electrospun sPP nanofibers depended on the boiling point of each solvent used. Moreover, to investigate the crystal and molecular structures of electrospun sPP fibers from a different solvent system (i.e., decalin and cyclohexane based solvents), WAXD and FT-IR analysis were carried out. The results showed that the different solvent systems caused the different surface morphologies but the crystal and molecular structure of the electrospun sPP fibers didn’t change.

Abstract: We studied the effect of gap size on molecular orientation and crystalline structure of theuniaxially well-aligned nylon 6 nanofibers produced in the gap of the negatively charged metal plates. As evidenced by polarized FT-IR spectroscopy, relative intensity in several absorbance bands, including the N-H stretching, amide I, II, and III vibrations were found to be different in the parallel and perpendicular polarized FT-IR spectra. Moreover, X-ray analysis indicated that the metastable γ-form was predominant in the as-spun nylon 6 nanofiber, and was transformed into the thermodynamically stable α-form by increasing the cap size. These results suggested that the polymer chains were oriented perpendicular to the fiber direction. Molecular orientation to the fiber axis was enhanced as increasing the gap size.

Abstract: Researchers have paid much attention to small-scale natural fibers among the biological materials to seek innovative methods in order to create new high performance materials. Recently, spider dragline silk fibers are being studied because of their unique combination of high strength to weight ratio and high extensibility, which leads to a tough and lightweight fiber. Biomimetic fibers based on spider silk have been a focus of research for the past decade. However, there are still many unanswered questions about the mechanisms by which silk achieves its unique mechanical properties, as well as challenges in mechanical testing of electrospinning silk nanofibers which are often hindered by both small diameters and limited material availability. A method to characterize local mechanical behavior in small diameter nanofibers was developed to both improve understanding of structure-property in natural fibers and provide a method for comparing mechanical behavior in natural and electrospinning fibers.

Abstract: Recently ultrafine fibers of nanometer size and their fabrics are growingly interested not only in textile industry but also in non-textile industry, as they have excellent properties of lightness, high strength, comfort, function and etc as compared to those of conventional fibers. Evaluation of the mechanical properties is, then, important in connection with the reliability and durability of the products. Since commercialized machines are not always conventionally designed for mechanical property evaluation of nanofiber in any environments; air or fluids. We have, therefore, conducted to develop versatile testers including tension tester for strength measurement and friction tester for handle measurement. In this note, we present typical results for evaluation of mechanical properties of nanofiber/ web fabricated by means of an electrospining method.

Abstract: We report that the metallized single nanofibers could be successfully prepared by a combined technique of electrospinning and metallization. The mechanical properties of the metallized single nanofibers were investigated by using recently developed tensile test machine. It was found that the metallized single (polyurethane and polystyrene) nanofibers exhibited higher mechanical properties depending on the thickness of the deposited metal layers. For instance, compared with pure PU single nanofibers (Young`s modulus ca. 170 MPa), Young`s modulus for the metallized PU single nanofibers with Cu layers of 30 nm and 50 nm were increased to ca.610 MPa and ca.750 MPa, respectively. Furthermore, the tensile strength of 50 nm Cu-deposited PS single nanofibers (ca. 3.27 GPa) was clearly higher than those of pure PS (ca. 0.76 GPa) and 30 nm Cu-deposited PS (ca. 3.09 GPa) single nanofibers. The results may be attributed to the formation of metallic hard-coating layers onto the surface of single nanofibers.

Abstract: Training effect in the Fe-Mn-Si shape memory alloy is known as useful method to improve the shape memory effect. In this study, the training effects on damping capacity in Fe-20mass%Mn and Fe-20.5mass%Mn-12.5mass%Cr alloys have been investigated. As training treatments, the thermal training (only thermal cycling) and the thermo-mechanical training (thermal cycling with rolling deformation) are carried out. Internal friction was measured at room temperature using a free-decay method. Moreover, the behavior of dislocations was observed by TEM. Both training treatments improve the damping capacity of the Fe-Mn alloys with increasing the number of treatment. Strong training effect was found for the specimens trained by the thermo-mechanical training. The main training effect by thermal cycles is concluded to be due to size effects, while the size effects and volume fractional effects of martensite phase affect the damping capacity of the thermo-mechanically trained alloys. These training methods can improve both damping capacity and strength of Fe-Mn alloys.

Abstract: The conductive metallic nanofibers were prepared by using a combined technology of electrospinning and metallization. The electrospun polyurethane and poly(vinyl alcohol) (PVA) nanofibers are metallized with different thicknesses of metal (Cu, Ni) layer via a metallization. The thickness of the metallic layer, which is ranging from 50 nm to 200 nm, was controlled. The resultant metallized nanofibers are characterized using field emission scanning electron microscopy (FE-SEM), wide angle X-ray diffraction (WAXD), and thermogravimetric analysis (TGA). FE-SEM micrographs demonstrate that the nano-scaled metallic layers are well deposited onto the nanofibers. TGA result indicates that thermal stability of the metallized nanofibers was enhanced due to the barrier effects of the metallic thin layer. WAXD data also confirm that the metallic layers are well deposited onto the nanofibers. Remarkably, the fibrous morphologies were satisfactorily conserved after removal of the nanofiber template by heat treatment at ca. 400 oC for 24 hr., suggesting the successful deposition of metal layer onto the nanofiber template, and thereby resulted in the formation of metallic nanofibers and nanotubes depending on the diameter of electrospun nanofibers and the thickness of the deposited metallic layers in the conductive metallic nanofibers. In addition, it was observed that the metallized nanofibers exhibit higher conductive properties depending on the thickness of the deposited metallic layers.

Abstract: We have explored a straightforward approach for achieving water-resistant properties of the electrospun PVA nanofibers. The electrospun PVA nanofibers are post-treated with a hydrophobic polyhedral oligosilsesquioxane (POSS) hybrid macromer via a direct urethane reaction between the hydroxyl group of PVA and the isocyanate group of POSS macromers. The POSS-modified PVA nanofibers are characterized by fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and water resistant property. The morphologies of the electrospun PVA nanofibers before and after POSS post-treatments are regular and a narrow distribution of diameters was observed, indicating a uniform post-treatment of POSS macromers onto the PVA nanofibers. Thermal decomposition behavior of the POSS-modified PVA nanofibers was altered compared to the pure PVA nanofibers, suggesting the suppression of thermal decomposition due to the incorporation of POSS macromers. In addition, the pure PVA nanofiber mats immersed in pure water exhibited no characteristic morphology, whereas the POSS-modified PVA nanofiber mats showed the texture morphology, indicating an enhanced water-resistant property.

Abstract: Microstructure and texture evolution during equal channel angular pressing (ECAP) of Al-5 mass%Ti alloy are investigated for up to 8 passes via routes A and BC. Platelet-shaped Al3Ti particles in the Al-5mass%Ti alloy are cracked severely with repetitive ECAP passes, and the mean size of the Al3Ti particles is decreased with increasing the number of ECAP passes. Microstructural observation showed that an Al–Ti supersaturated solid solution is formed during the ECAP process. It is also found that the Al-Ti alloy after ECAP by route A and route Bc methods have very different microstructures. Namely, after ECAP by route Bc, the fine Al3Ti particles are homogeneously dispersed in Al matrix, while the microstructure has highly anisotropic distribution after ECAP by route A.