Chan Sol Kang

Synthesis and characterization of polyhydroxyamide copolymers as precursors of polybenzoxazoles

Polyhydroxyamide (PHA) and its copolymers were synthesized via low-temperature solution polycondensation of 3,3′-dihydroxybenzidine (DHB) with terephthaloyl chloride (TPC) and/or isophthaloyl chloride (IPC) in N,N-dimethylacetamide (DMAc) with the aid of lithium chloride (LiCl). The synthesized PHA copolymers were characterized by 1H-NMR spectroscopy, FT-IR spectroscopy, and thermogravimetric analyzer (TGA) in order to understand the relations between the chemical structures and the solubility and thermal properties of PHA copolymers. Thermal cyclization of PHA copolymers and the formation of polybenzoxazoles (PBOs) were investigated using FT-IR spectroscopy and TGA. The solubility test revealed that PHA copolymers containing IPC content over 20 mol% were readily dissolved in a variety of polar amide solvents with the aid of LiCl. TGA results showed that the thermal cyclization temperature, the thermal degradation temperatures of PHA copolymers and the thermal stabilities and the char yields of converted PBO copolymers were significantly reduced by the inclusion of meta-structure in the main chain especially when the IPC content was higher than 25 mol%.

Mechanical and Electrical Properties of Polyurethane Hybrid Nanocomposites Containing MWNT and Graphite as Conducting Nanoparticles

The synergistic effects of multi-walled carbon nanotubes (MWNT) and graphite on the structural features and mechanical and electrical properties of polyurethane (PU) nanocomposites were investigated as functions of filler content. SEM images of the PU/hybrid nanocomposite exhibit that the MWNTs and graphite are dispersed well in the PU matrix and form an interconnected network structure. Accordingly, the tensile strength and strain-at-break of the PU/hybrid nanocomposites were much higher than those of the PU/MWNT and PU/graphite nanocomposites at the same filler content. In addition, it is found that the PU/hybrid nanocomposite containing 1.25 wt% MWNTs and 1.25 wt% graphite shows the electrical conductivity of , which is higher by three orders than that of PU/graphite nanocomposite containing 20.0 wt% graphite. The highly improved mechanical and electrical properties of the PU/hybrid nanocomposite are thought to be due to the interconnected network structure of MWNTs and graphite in the PU matrix.

Synthesis and Characterization of para-Aramid Copolymers Containing Cyano Groups

Poly(p-phenylene terephthalamide) (PPTA) and poly(2-cyano-1,4-phenylene terephthalamide) (CNPPTA)- based copolymers were synthesized using a low temperature polycondensation reaction between terephthaloyl chloride, 2-cyano-1,4-phenylene diamine, and/or 1,4-phenylene diamine in a N-methylpyrrolidinone solvent containing 2% CaCl2. The molecular structures and thermal stabilities of PPTA, CNPPTA, and copolymers of the two with different cyano group contents were characterized using FT-IR spectroscopy and thermogravimetric analysis, respectively. The liquid crystalline states and polymer concentration-dependent viscosities of NMP/CaCl2 solutions containing different CNPPTA copolymer concentrations were investigated using polarized optical microscopy and viscometry, respectively. It was found that CNPPTA polymers with higher cyano group contents were more soluble in polar organic solvents, such as NMP, DMAc, and DMF, and that CNPPTA polymer solutions based on the NMP/CaCl2 system formed liquid crystalline structures above certain critical polymer concentrations. In addition, TGA results revealed that, although the initial degradation temperatures of CNPPTA copolymers were quite comparable to that of PPTA, their char yields at 800 o C were far higher.

Effect of purification method on the electrical properties of the carbon nanotube fibers

We purified as-received CNT fibers (CNTFs) with four different methods and systematically examined effects of various purifications on the morphology, structure, and electrical conductivity of the resultant CNTFs, respectively. The purified CNTFs were characterized by an optical microscope, transmission electron microscope (TEM) coupled with an energy dispersive X-ray spectrometer (EDS), Raman spectroscopy, and multiple source meters. Optical images showed that morphology of CNTFs did not largely change after purification. TEM images and EDS results showed that the Fe impurities, 21.9 wt%, in CNTFs were decreased to 0.17-1.20 wt% and were nearly eliminated by acid and alkali purifications, respectively. Raman results identified the ID/IG ratio of CNTFs was 0.71, while those of HCl treatment after steam with heat (HSCNTFs-HCl), NaOCl treatment after steam with heat (HSCNTFs-NaOCl), and NaOH treatment with heat without steam (NaOH-HCNTFs) were 0.45, 0.49, and 0.57, respectively, which means that purification methods of CNTFs performed in this study are thought to be satisfactory for manufacturing high-purity CNTFs. Electrical conductivity (1.4×104 S/m) of NaOH-HCNTFs (one-step procedure) was twice as high as that (7.3×103 S/m) of CNTFs, but lower than those (2.1-2.3×104 S/m) of HSCNTFs-HCl and HSCNTFs-NaOCl (two-step processes), which demonstrates that two-step processes rather than one-step procedure would have a positive effect on the electrical conductivity of the resultant CNTFs.

Synthesis of hydrophilic copolyesters and the characterization of their moisture-related cooling properties

A series of copolyesters (Co-PETs) containing poly(ethylene glycol) (PEG), 5-sodiumsulfodimethyl isophthalate (DMS), and dimethyl isophthalate (DMI) were synthesized via the conventional two-step melt-polycondensation method. The synthesized Co-PETs were characterized by 1H-NMR spectroscopy, FT-IR spectroscopy, differential scanning calorimeter (DSC), and thermogravimetric analyzer (TGA). The DSC results showed that the melting temperature (Tm) and the heats of fusion (ΔHm) of Co-PETs decreased with increasing the DMS content in Co-PET, while the inclusion of PEG did not affect their thermal properties significantly. The water absorption and the water contact angle of the Co-PET films were found to be significantly affected by the DMS content rather than PEG content. The moisture-related cooling properties of the fabric samples made of Co-PET 5 as well as PET were evaluated by using liquid moisture management tester (MMT) and Qmax measurements. The MMT and Qmax results indicated that Co-PET 5 fabric containing DMS 1.0 mol% and PEG 10.0 wt% in Co-PET seemed to be a good candidate for the fabric having durable cooling effects.

Effects of Drawing and Heat-Treatment Conditions on the Structure and Mechanical Properties of Polyhydroxyamide and Polybenzoxazole Fibers

We report the preparation of polybenzoxazole (PBO) fiber from polyhydroxyamide (PHA) precursor fiber which is free from strong acid such as polyphosphoric acid. We prepared the PHA fibers with different spin-draw ratios (SDRs) using a wet-spinning method and the PBO fibers with an SDR of 3.5 (SDR-3.5 PBO fibers) were prepared by various heat-treatment temperatures, and investigated their morphology, crystalline structure, and mechanical properties. The simultaneous thermogravimetric analysis-mass spectrometry (STA-MS) and field-emission scanning electron microscopy (FE-SEM) results confirmed that the diameter of the SDR-3.5 PBO fiber was much smaller than that of the SDR-3.5 PHA fiber, due to the release of water during the thermal cyclization reaction which forms the PBO structure. The wide-angle Xray diffraction (WAXD) pattern of the SDR-3.5 PBO fiber heat-treated at 350 °C (SDR-3.5 PBO 350 fiber) showed two peaks, at 2θ=14.83 ° and 24.38 °, and the diffraction angles dropped with increasing heat-treatment temperature. In addition, the initial modulus and tensile strength of the SDR-3.5 PBO fiber heat-treated at 550 °C (SDR-3.5 PBO 550 fiber) were found to be 19.1 GPa and 449.2 MPa, which were much higher than those of the SDR-3.5 PHA fiber, 9.3 GPa and 227.0 MPa, respectively.