Bon-Cheol Ku

In Situ Synthesis of Thermochemically Reduced Graphene Oxide Conducting Nanocomposites

Highly conductive reduced graphene oxide (GO) polymer nanocomposites are synthesized by a well-organized in situ thermochemical synthesis technique. The surface functionalization of GO was carried out with aryl diazonium salt including 4-iodoaniline to form phenyl functionalized GO (I-Ph-GO). The thermochemically developed reduced GO (R-I-Ph-GO) has five times higher electrical conductivity (42,000 S/m) than typical reduced GO (R-GO). We also demonstrate a R-I-Ph-GO/polyimide (PI) composites having more than 10(4) times higher conductivity (~1 S/m) compared to a R-GO/PI composites. The electrical resistances of PI composites with R-I-Ph-GO were dramatically dropped under ~3% tensile strain. The R-I-Ph-GO/PI composites with electrically sensitive response caused by mechanical strain are expected to have broad implications for nanoelectromechanical systems.

Cross‐linked Multilayer Polymer‐Clay Nanocomposites and Permeability Properties

Abstract Electrostatically layered aluminosilicate nanocomposites have been prepared by the sequential deposition of poly(allylamine hydrochloride)/poly(acrylic acid)/poly(allylamine hydrochloride)/saponite (PAH/PAA/PAH/saponite)10 on poly(ethylene terephtalate) (PET) film. Exfoliated saponite nanoplatelets were obtained by extensive shaking, sonication, and centrifugation of a water suspension. To minimize permeability and improve the mechanical integrity, cross‐linking of composite films was carried out at different temperatures. The formation of amide linkage induced through heating was observed by Fourier Transform Infrared (FT‐IR) and x‐ray photoelectron spectroscopy (XPS). The cross‐linking of nanocomposites (PAH/PAA/PAH/saponite)10 showed 60% decrease in permeability of oxygen when compared with the pristine PET substrate film. In contrast, water permeability of the nanocomposite membrane was not affected by heating temperature and deposition cycles.

Layer-by-layer assembled graphene oxide films and barrier properties of thermally reduced graphene oxide membranes

In this study, we present a facile method of fabricating graphene oxide (GO) films on the surface of polyimide (PI) via layer-by-layer (LBL) assembly of charged GO. The positively charged amino-phenyl functionalized GO (APGO) is alternatively complexed with the negatively charged GO through an electrostatic LBL assembly process. Furthermore, we investigated the water vapor transmission rate and oxygen transmission rate of the prepared (reduced GO [rGO]/rAPGO)10 deposited PI film (rGO/rAPGO/PI) and pure PI film. The water vapor transmission rate of the GO and APGO-coated PI composite film was increased due to the intrinsically hydrophilic property of the charged composite films. However, the oxygen trans mission rate was decreased from 220 to 78 cm 3 /m 2 ·day·atm, due to the barrier effect of the graphene films on the PI surface. Since the proposed method allows for large-scale production of graphene films, it is considered to have potential for utilization in various applications.

SYNTHESIS AND ELECTROSPINNING OF A NOVEL FLUORESCENT POLYMER PMMA-PM FOR QUENCHING-BASED OPTICAL SENSING

ABSTRACT This work presents the synthesis of a new fluorescent polymer and its applicability for optical sensing using the electrospinning technique for the fabrication of nanofibrous membrane sensors. A new fluorescent monomer was first synthesized by coupling reactions between methacryloyl chloride and a pyrene derivative, 1-pyrene butanol. Fluorescent polymers containing different ratios of pyrene were then obtained by the copolymerization of this monomer with methylmethacrylate using 2,2′-azobisisobutyronitrile as the initiator. These polymers show distinct and well-defined fluorescence that is characteristic of the pyrene chromophores. Quenching-based optical chemical sensors were then fabricated by the electrospinning technique. The preliminary results show that these sensors have an order of magnitude higher sensitivity to the target analyte 2, 4-dinitro toluene than sensors formed from continuous thin films. This is believed to be due to the higher surface area to volume ratio of the electrospun nanofibrous membranes. The quenching behavior follows Stern-Volmer bimolecular quenching kinetics. The synthesis, characterization, electrospinning fabrication, and sensing capability of these polymers are discussed.

Carbon nanotube core graphitic shell hybrid fibers.

A carbon nanotube yarn core graphitic shell hybrid fiber was fabricated via facile heat treatment of epoxy-based negative photoresist (SU-8) on carbon nanotube yarn. The effective encapsulation of carbon nanotube yarn in carbon fiber and a glassy carbon outer shell determines their physical properties. The higher electrical conductivity (than carbon fiber) of the carbon nanotube yarn overcomes the drawbacks of carbon fiber/glassy carbon, and the better properties (than carbon nanotubes) of the carbon fiber/glassy carbon make up for the lower thermal and mechanical properties of the carbon nanotube yarn via synergistic hybridization without any chemical doping and additional processes.

A new approach to catalyze template polymerization of aniline using electrostatically multilayered hematin assemblies

Abstract In this work, we describe a new approach to catalyze the template polymerization of conducting polyaniline (Pani). Electrostatic layer‐by‐layer (ELBL) self‐assembly of a polyelectrolyte and a biomimetic catalyst, hematin has been utilized to construct a nanocomposite film catalyst. Poly(dimethyl diallylammonium chloride) (PDAC) and hematin have been used as polycation and counter anions, respectively. The absorption spectra by UV‐VIS‐NIR spectroscopy showed that a conductive form Pani was formed, not only as a coating on the surface of the ELBL composites, but was also formed in solution. Furthermore, it was found that the reaction rate was affected by pH and concentration of hematin in the multilayers. The feasibility of controlled desorption of hematin molecules from the LBL assembly was explored and demonstrated by changing pH and hematin concentration. It is believed that hematin sandwiched between positively charged polyelectrolytes in the composite films was slowly released into the solutions and then used to catalyze the template polymerization of aniline with SPS, resulting in a water soluble form of Pani. The polymerization rate of aniline in solution was enhanced with decreasing pH of the solutions due to increased desorption of hematin nanoparticles from the multilayers. These ELBL hematin assemblies demonstrated both a way to functionalize surfaces with conductive Pani and a potential method of reusability of the catalyst for improved cost effectiveness.

Tribological properties of carbon fiber-reinforced aluminum composites processed by spark plasma sintering

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Synthesis and characterization of polybenzoxazole/graphene oxide composites via in situ polymerization

In this study, poly(amic acid) was prepared via a polycondensation reaction of 3,3’-dihydroxybenzidine and pyromellitic dianhydride in an N-methyl-2-pyrrolidone solution; reduced graphene oxide/polybenzoxazole (r-GO/PBO) composite films, which significantly increased the electrical conductivity, were successfully fabricated. GO was prepared from graphite using Brodie’s method. The GO was used as nanofillers for the preparation of r-GO/PBO com posites through an in situ polymerization. The addition of 50 wt% GO led to a significant increase in the electrical conductivity of the composite films by more than sixteen orders of magnitude compared with that of pure PBO films as a result of the electrical percolation networks in the r-GO during the thermal treatment at various temperatures within the films.

Synthesis of Polyaniline Using Electrostatically Layered Hematin Assemblies

The enzyme horseradish peroxidase has been recently used for the polymerization of water-soluble conductive polyaniline. Hematin, an ironporphyrin molecule that catalyzes the oxidative polymerization of aniline and phenol, is a robust and inexpensive biomimetic alternative to horseradish peroxidase. Composite thin film assemblies were fabricated by electrostatic layer-by-layer (ELBL) alternate deposition of a polyelectrolyte, poly(dimethyl diallylammonium chloride), and hematin. These multilayer assemblies were then used for the polymerization of aniline in the presence of a polystyrene sulfonate as a template. It was found that the polymerization rate depends on both pH and the concentration of hematin deposited and the polyaniline synthesis was possible both onto the ELBL surfaces and in solution. It was also discovered that it is possible to reuse the ELBL assembled hematin substrates for multiple aniline polymerizations.

Thermal Behavior in Stabilization of Large Tow PAN-based Carbon Fiber

In this study, we investigated stepwise stabilization process of 48 k filaments PAN precursor to observe thermal behavior of PAN fibers. We also controlled parameters such as oven temperature, air flow direction, velocity, thermal residence time, and tow size to optimize stabilization process for large tow carbon fibers. FTIR, elemental analyzer, density column, X-ray diffractometer were used to evaluate stabilization degree and chemical structural evolution during thermal stabilization. The oxidation process of PAN fibers makes cross-linking reaction more easier between intermolecular chains and enduces cyclization reaction of acrylonitrile. In addition, the degree of air diffusion into fibers affects the mechanical properties of the final carbon fiber. The carbon fiber with ca. 10% of oxygen content and 1.40 g/cm of density showed the best mechanical properties with 2.5 GPa tensile strength and 214 GPa tensile modulus.