Wansoo Huh

Phase separation behavior of cyanate ester resin/polysulfone blends

The composition of the blends and the curing temperature affect the morphology of the blends and the phase separation mechanism. The phase separation mechanism depends on the viscosity of medium at the initial stage of phase separation determined by the amount of thermoplastics and the curing temperature, and is closely related with the final morphology. When the homogeneous bisphenol A dicyanate (BADCy)/polysulfone (PSF) blends with low content of PSF (less than 10 wt %) were cured isothermally, the blends were phase separated by nucleation and growth (NG) mechanism to form the PSF particle structure. On the other hand, with more than 20 wt % of PSF content, the BADCy/PSF blends were phase separated by spinodal decomposition (SD) to form the BADCy particle structure. With about 15 wt % of PSF content, the blends were phase separated by SD and then NG to form a combined structure having both the PSF particle structure and the BADCy particle structure.

Effect of molecular weight between crosslinks on fracture behaviour of diallylterephthalate resins

The effect of molecular weight between crosslinks, Mc, on the fracture toughness and fracture behaviour of diallylterephthalate resin was investigated. As Mc increased, the fracture toughness increased rapidly up to a maximum value and then decreased gradually. Scanning electron micrographs showed that the fracture toughness increased with an increase in the length of the slow crack growth, i.e. plastic deformation zone. Also, fracture toughness was found to have a close relation to crack opening displacement. Such a correlation reveals that the difference in fracture toughness with Mc was due to the different sizes of the plastic zone. It is suggested that diallylterephthalate resin could undergo deformation via shear yielding at the crack tip with subsequent crack blunting. There was no evidence that crazes occurred in the fracture process.

Synergistic incorporation of hybrid heterobimetal–nitrogen atoms into carbon structures for superior oxygen electroreduction performance

Although Pt-based catalytic technology has led to significant advances in the development of electrocatalysts in fuel cells, Pt replacement with efficient and stable non-precious metal catalysts has a great technological significance for successful large-scale implementation of fuel cells. Here, we present the development of hybrid functional 1-dimensional carbon structures incorporated homogeneously with high contents of non-precious metal multi-dopants, consisting of iron, cobalt and nitrogen, as a promising alternative to Pt-based catalysts for the cathodic oxygen reduction reaction (ORR) through a modified electrospinning technique. These hybrid heterobimetal–nitrogen-incorporated carbon structures exhibit superior ORR electrocatalytic properties i.e., more positive reduction potential, high electroreduction current density, high electron transfer value (∼3.87) close to the perfect ORR and improved electrochemical stability with a very small decrease of ∼8 mV in half-wave potential. The observed enhancement in electrochemical performance can be ascribed to the increased amount of catalytically active sites with relatively high contents of heterometallic iron and cobalt atoms surrounded by nitrogen species and their homogeneous distribution on the catalyst surface.

Investigation of the surface rearrangement of poly(imide-siloxane) using dynamic contact angle measurements

Dynamic contact angle measurements and X-ray photoelectron spectroscopy (XPS) were used to investigate the surface compositions and surface rearrangement of poly(imide-siloxane) with various molecular weights and contents of amine-terminated poly(dimethyl siloxane) (ATPDMS). Four different water contact angles were measured to study the poly(imide-siloxane) surface: the initial advancing angle, the equilibrium advancing angle, the initial receding angle, and the equilibrium receding angle. Poly(imide-siloxane) with 2350 and 4300 g/mol of ATPDMS showed higher initial and equilibrium advancing contact angles than those of poly(imide-siloxane) with 433 g/mol of ATPDMS. Since the mobility of ATPDMS segments depended on the chain length of ATPDMS, the molecular weight of ATPDMS determined the surface composition of poly(imide-siloxane), the rate of surface rearrangement, and the contact angle hysteresis. Poly(imide-siloxane)s with 2850 and 4300 g/mol of ATPDMS were mostly covered with ATPDMS even if just 1 wt% of ATPDMS was incorporated, while poly(imide-siloxane) with 433 g/mol of ATPDMS was mostly covered with polyimide segments and partially with ATPDMS. The rate of surface rearrangement and the contact angle hysteresis decreased with the increasing molecular weight as well as content of ATPDMS. The actual ATPDMS-enriched layer thickness was also investigated by XPS. The actual thickness of the ATPDMS-enriched layer was about 15 nm for 2850 g/mol and 4300 g/mol of ATPDMS-modified poly(imide-siloxane) and about 7.5 nm for 433 g/mol of ATPDMS-modified poly(imide-siloxane)

Preparation of epoxy resin using n-hexadecane based shape stabilized PCM for applying wood-based flooring

Epoxy resin has excellent characteristics of moisture, low toughness, solvent and chemical resistance, low shrinkage on cure, superior electrical and mechanical resistance properties, and good adhesion to many substrates. In this experiment, we prepared epoxy resin with shape stabilized phase change material (SSPCM) to enhance the thermal properties of epoxy resin. The SSPCM was prepared through the vacuum impregnation method, and the SSPCM/epoxy resin composites were prepared through the shear stirring process and curing process. In the preparation process, the epoxy resin and hardener were mixed in a beaker at a one-to-one ratio. Then, 5, 10, 15, and 20 wt.% of the SSPCM was added to the mixture. The thermal properties and chemical properties of epoxy resin with SSPCM were analyzed from scanning electron microscopy, differential scanning calorimetry, thermal gravimetric analysis, and universal testing machine analyzer. From the analysis, we determined that the prepared epoxy resin with SSPCM has heat storage capacity and high thermal conductivity, compared with the epoxy resin.

Prediction of Delamination Resistance in Laminated Composites with Electrospun Nano -Interlayers using a Cohesive Zone Model

Nano -interlayers were fabricated with polycarbonate by using an electrospinning process to improve delamination resistance in laminated composites. The laminates were laid up with and without the nano -interlayers having the stacking sequence of (30/ -30/90 )s and tested under uniaxial tensile loading. Five nano -interlayers in the form of the electrospun nanofiber mats were placed at every ply interfaces. Thickness difference of the laminates with and without five nano -interlayers is less than 0.001 mm. First -ply -failure stress, delamination stress and ultim ate strength increased 8.4%, 8.1% and 9.8% with the addition of the nano -interlayers as compared with the pristine specimens. The number of microcracks at the delamination stress decreases significantly by 21.6% with the addition of the nano -interlayers. Therefore, the ultra thin electrospun nano -interlayer cannot only suppress the microcracking and delamination damage by reducing the interlaminar stress concentration at the free edges in the laminated composites, but increase the ultimate strength proper ty as well. A nonlinear finite element analysis was performed using interface elements based on a cohesive zone model to predict the delamination resistance of the laminates with the insertion of the electrospun nano -interlayers under the un idirectional tensile displacement loading . The laminated composite with the interface elements simulating the nano -interlayers was modeled with 3 -D eight -node hexahedral solid elements and interface decohesi on elements having a bilinear constitutive relation. Paramete r studies were carried out with various critical energy release rates and interfacial strengths .

Enhancing the flame-retardant performance of wood-based materials using carbon-based materials

AbstractWe sought to improve the flame-retardant performance of wood-based materials through the development of a coating material using carbon-based materials. The coating materials were applied to the surfaces of wood-based materials used for interior materials and furniture. We measured fire characteristics of the coated wood-based materials using a cone calorimeter. The coating materials were prepared by the mixing of carbon materials, such as natural graphite, expandable graphite, and exfoliated graphite nanoplatelets, in water-based coating materials. TG analysis revealed that water-based coating materials/carbon material-blended composites had good thermal durability in the working temperature ranges. The flame-retardant performance was confirmed through cone calorimeter experiments, and the result of the experiment satisfied the standard for flame-retardant performance in ISO 5600-1.