Jae Woo Chung

Despite their potential as reinforcing agents for poly(lactic acid) (PLA), cellulose nanocrystals (CNCs) tend to self-aggregate, limiting both optical transparency and mechanical reinforcement. In this study, incorporating poly(L-lactic acid)-grafted CNCs (LCNCs) into PLA simultaneously enhanced both optical transparency and mechanical reinforcement compared to neat CNCs. Three types of LCNCs—LCNC1 (apparent molar substitution, MS_app = 3.36), LCNC2 (MS_app = 25.6), and LCNC3 (MS_app = 56.9)—were synthesized. Fourier transform infrared (FT-IR) and ¹H nuclear magnetic resonance (¹H NMR) spectra revealed poly(L-lactic acid) (PLLA) grafting onto the CNC surface, and the MS_app values were estimated from the ¹H NMR peak integrals. X-ray diffraction patterns (XRD) showed that the native crystalline structure of the CNCs was preserved even after surface modification. LCNC3 (MS_app = 56.9) exhibited a smaller hydrodynamic size in an organic solvent, consistent with reduced aggregation, whereas LCNC1 (MS_app = 3.36) remained severely aggregated like neat CNCs. The PLA/LCNC3 nanocomposite retained transparency comparable to neat PLA and achieved the highest tensile strength among the nanocomposites considered here. These results indicate that a higher MS_app, reflecting greater PLLA graft development on CNCs, is associated with improved dispersion, optical transparency, and mechanical reinforcement in PLA nanocomposites.

Polymer electrolyte membranes based on perfluorosulfonic acid (PFSA) exhibit excellent proton conductivity and thermal stability. However, PFSA membranes remain vulnerable to radical‐induced degradation during prolonged fuel cell operation, which compromises long‐term membrane integrity. To address this limitation, we investigated a stabilization approach combining thermal annealing and cerium oxide (CeO ₂ ) nanoparticle incorporation as a route to improve membrane durability under harsh conditions. Oxidative degradation was simulated using Fenton’s test, and membrane durability was assessed through structural and functional analyses before and after oxidative stress exposure. The PFSA membranes examined in this study included Nafion, thermally annealed Nafion, and thermally annealed Nafion incorporating CeO ₂ nanoparticles. Fluoride ion emission rate analyses, scanning electron microscopy, atomic force microscopy, water uptake measurements, and gas permeability analyses demonstrated that thermal annealing and the incorporation of CeO ₂ nanoparticles effectively suppressed radical‐induced chemical degradation and improved the physical stability of the PFSA membranes. The pore structure, mechanical properties, and proton conductivity of the thermally annealed PFSA membrane containing CeO ₂ nanoparticles were similar to those before Fenton’s test. The results from this study demonstrate that the combination of thermal annealing and incorporation of CeO ₂ nanoparticles enhances the chemical, structural, and functional stability of PFSA membranes.

This study presents the synthesis and characterization of hexadecyl-modified SiO₂ (HD-SiO₂) nanoparticles and their application in the fabrication of a multilayered elastomer hybrid sheet to enhance water resistance in implantable biomedical devices. The surface modification of SiO₂ nanoparticles was confirmed via FT-IR and TGA analyses, showing the successful grafting of hydrophobic hexadecyl groups. FE-SEM and DLS analyses revealed spherical HD-SiO₂ nanoparticles with an average size of 360 nm. A multilayered elastomer hybrid sheet, consisting of a PDMS-based circuit-protecting body, a water resistance layer of HD-SiO₂, a planarization layer, and a biocompatible layer of polydopamine, was fabricated and characterized. The water resistance layer exhibited superhydrophobic properties, with a water contact angle of 154.7° and a 27% reduction in water vapor transmission rate (WVTR) compared to the circuit-protecting body alone. The device packaged with both the circuit-protecting body and water resistance layer demonstrated a tenfold increase in operational lifespan in water medium compared to the device without the water resistance layer. Cytotoxicity and cell proliferation tests on human dermal fibroblast cells (HDFn) confirmed the biocompatibility of the multilayered sheet, with no significant cytotoxicity observed over 48 h.

The complementary design of a multiphasic supramolecular nanostructure constructed via sequence-biased statistical copolymerization can provide a super-tough supramolecular plastic with a facile healing capability.

As the penetration of distributed energy resources (DER) has increased, research on direct current (DC) power transmission and distribution has been actively performed. The DC system has the advantage of high-power transmission efficiency. However, it has a very large and rapid increase in fault current in the DC system directly after a fault occurs. As one of the countermeasures, studies on the application of the superconducting fault current limiter (SFCL) into the DC system have been conducted to protect major facilities from DC fault current, which is expected to alleviate the power burden on the DC circuit breaker through its quench operation. Among the studied DC SFCLs, the trigger-type DC SFCL using a transformer, which can achieve the peak DC fault current-limiting operation, has been suggested. However, the DC fault current-limiting operation, in the case of the DC SFCL with a current-limiting reactor (CLR), was analyzed to not be effectively executed in the steady state since the transient state directly follows the fault occurrence. In this paper, the DC fault current-limiting operation of a twice-quench trigger type SFCL using a transformer considering magnetizing current and its CLR was analyzed. Through DC fault current-limiting experiments according to the inductance of its current-limiting reactor (CLR), the effective current-limiting design of twice-quench trigger type SFCL using a transformer was described.

To use implantable biomedical devices such as electrocardiograms and neurostimulators in the human body, it is necessary to package them with biocompatible materials that protect the internal electronic circuits from the body’s internal electrolytes and moisture without causing foreign body reactions. Herein, we describe a hydrogel surface-modified polyurethane copolymer film with concurrent water permeation resistance and biocompatibility properties for application to an implantable biomedical device. To achieve this, hydrophobic polyurethane copolymers comprising hydrogenated poly(ethylene-co-butylene) (HPEB) and aliphatic poly(carbonate) (PC) were synthesized and their hydrophobicity degree and mechanical properties were adjusted by controlling the copolymer composition ratio. When 10 wt% PC was introduced, the polyurethane copolymer exhibited hydrophobicity and water permeation resistance similar to those of HPEB; however, with improved mechanical properties. Subsequently, a hydrophilic poly(vinyl pyrrolidone) (PVP) hydrogel layer was formed on the surface of the polyurethane copolymer film by Fenton reaction using an initiator and crosslinking agent and the effect of the initiator and crosslinking agent immobilization time, PVP concentration and crosslinking agent concentration on the hydrogel properties were investigated. Finally, MTT assay showed that the hydrogel surface-modified polyurethane copolymer film displays excellent biocompatibility.

The mechanically-enhanced urea-formaldehyde (UF) microcapsules are developed through a multi-step in situ polymerization method. Optical microscope (OM) and field emission scanning electron microscope (FE-SEM) prove that the microcapsules, 147.4 μm in diameter with a shell thickness of 600 nm, are well-formed. From ¹H-nuclear magnetic resonance (¹H-NMR) analysis, we found that dicyclopentadiene (DCPD), a self-healing agent encapsulated by the microcapsules, occupies ca. 40.3 %(v/v) of the internal volume of a single capsule. These microcapsules are mixed with EPDM (ethylene-propylene-diene-monomer) and Grubbs’ catalyst via a solution mixing method, and universal testing machine (UTM) tests show that the composites with mechanically-enhanced microcapsules has ca. 47% higher toughness than the composites with conventionally prepared UF microcapsules, which is attributed to the improved mechanical stability of the microcapsule. When the EPDM/microcapsule rubber composites are notched, Fourier-transform infrared (FT-IR) spectroscopy shows that DCPD leaks from the broken microcapsule to the damaged site and flows to fill the notched valley, and self-heals as it is cured by Grubbs’ catalyst. The self-healing efficiency depends on the capsule concentration in the EPDM matrix. However, the self-healed EPDM/microcapsule rubber composite with over 15 wt% microcapsule shows an almost full recovery of the mechanical strength and 100% healing efficiency.