Jae Kwan Lee

Enhancement of the crosslink density, glass transition temperature, and strength of epoxy resin by using functionalized graphene oxide co-curing agents

We synthesized diamine-functionalized graphene oxide, DDS–GO and HMDA–GO, by introducing 4,4′-diaminodiphenyl sulfone (DDS) or hexamethylenediamine (HMDA) into the carboxylic acid groups on graphene oxide (GO) via amide bonds. The introduction of diamines was confirmed by analytical methods such as FT-IR, TG-DTA, XPS, AFM, and optical microscopy. Then, we applied DDS–GO and HMDA–GO as co-curing agents for epoxy (EP) nanocomposites that were prepared by mixing bisphenol-A type EP and DDS curing agent (ca. 21 wt%). Interestingly, when 1.0 wt% of DDS–GO was added to the EP/DDS mixture, the crosslink density (CD) increased from 0.028 to 0.069 mol cm−3. Due to the higher CD, both the glass transition temperature and tensile strength of the EP/DDS/DDS–GO nanocomposite effectively improved from 160.7 °C to 183.4 °C and from 87.4 MPa to 110.3 MPa, respectively.

Effects of nitrogen doping from pyrolyzed ionic liquid in carbon nanotube fibers: enhanced mechanical and electrical properties

Nitrogen doping in carbon nanotube (CNT) fibers using pyrolyzed ionic liquid induced interfacial hydrogen bonding between individual CNTs, enhancing mechanical properties and electrical conductivity simultaneously. In particular, the nitrogen doped CNT fiber using the ionic liquid BMI-I exhibited about 104%, 714%, and 38% increased tensile strength (0.65 N/tex), elastic modulus (83 N/tex), and electrical conductivity (1350 S cm(-1)), respectively, compared to pristine CNT fiber.

All Sequential Dip-Coating Processed Perovskite Layers from an Aqueous Lead Precursor for High Efficiency Perovskite Solar Cells

A novel, sequential method of dip-coating a ZnO covered mesoporous TiO2 electrode was performed using a non-halide lead precursor in an aqueous system to form a nanoscale perovskite film. The introduction of a ZnO interfacial layer induced significant adsorption in the non-halide lead precursor system. An efficient successive solid-state ion exchange and reaction process improved the morphology, crystallinity, and stability of perovskite solar cells. Improved surface coverage was achieved using successive ionic layer adsorption and reaction processes. When all sequential dipping conditions were controlled, a notable power conversion efficiency of 12.41% under standard conditions (AM 1.5, 100 mW·cm−2) was achieved for the perovskite solar cells fabricated from an aqueous non-halide lead precursor solution without spin-casting, which is an environmentally benign and low-cost manufacturing processes.

Liquid Crystallinity of p-Aramid/Multi-walled Carbon Nanotube Composites

A systematic study regarding the liquid crystallinity of p-Aramid and MWCNTs composite at various p-Aramid and MWCNTs concentrations in sulphuric acid was investigated and optimized by solution viscosity measurement and opalescence observation. We observed a merged liquid crystalline phase consisting of both p-Aramid and MWCNTs, and we believe this is the first study to report this combined liquid crystalline phase in one suspension. In addition to providing fundamental insights, we envision this study could be useful to those developing a strong, light, and high-performance polymeric composite fibers.

Synthesis and characterization of poly(cyclohexylthioacrylate) (PCTA) with high refractive index and low birefringence for optical applications

AbstractIn this study, a new development in thermoplastics was examined: a poly(cyclohexylthioacrylate) containing a sulfur atom in a polymer chain with a high refractive index. The poly(cyclohexylthioacrylate) was synthesized by the free radical polymerization of cyclohexylthioacrylate. Transparent polymer films, fabricated by spin-coating from the polymer solution, exhibited a high refractive index of 1.55244 at 637 nm and low birefringence of 0.0001. The thermal decomposition temperature (Td) of the polymer was in the range of 355-363 °C, which indicates good thermal stability, while its glass transition temperature was 65 °C.