Min Ho Jee

Influences of tensile drawing on structures, mechanical, and electrical properties of wet-spun multi-walled carbon nanotube composite fiber

A composite fiber based on multi-walled carbon nanotube (MWCNT) and poly(vinyl alcohol) (PVA) is prepared by wet-spinning and then stretched uniaxially with various draw ratios. Structural features and the mechanical and electrical performances of the MWCNT/PVA composite fiber are investigated as a function of draw ratio. It is identified from polarized Raman spectroscopy that the composite fiber exhibits an enhanced alignment of MWCNTs along the fiber axis with increasing the draw ratio. Accordingly, initial moduli and tensile strengths of the drawn composite fibers are much higher than those of undrawn composite fiber because of the efficient stress-transfer between MWCNTs. In addition, electrical conductivity of the composite fiber is significantly increased with the increase of the draw ratio, which is explained by the fact that the alignment of MWCNTs in drawn composite fibers enhances the interaction between MWCNTs, eventually leading to the generation of more efficient paths for electrical conduction.

Synthesis and Thermal Cyclization of Aromatic Polyhydroxyamides(I) -Effect of the Benzene Ring Substitution Structure-

We have synthesized polyhydroxyamides (PHAs), a possible precursor which could be converted to polybenzoxazole (PBO) through a thermal cyclization reaction, by low temperature solution polymerization of 3,3`-dihydroxybenzidine with terephthaloyl chloride or isophthaloyl acid. Structural characteristics, solubility, thermal cyclization, and thermal decomposition of the PHAs were investigated by the FT-IR, DSC and TGA in order to understand the effect of chemical structure of acyl chlorides on the thermal properties of PHAs. The FT-IR study reveals that two types of PHAs can cyclize on heating and be transformed into PBOs. The meta-type PHA (m-PHA) shows better solubility in N,N-dimethylformamide and dimethyl sulfoxide than the para-type PHA (m-PHA). DSC and TGA results demonstrate that the m-PHA can cyclize at lower temperature than p-PHA, which is due to the difference in activation energy of thermal cyclization between m-PHA and p-PHA. TGA results reveal that the p-PHA has better thermal stability than m-PHA while in flame.

Effects of wet-spinning conditions on structures, mechanical and electrical properties of multi-walled carbon nanotube composite fibers

A series of composite fibers composed of multi-walled carbon nanotube (MWCNT) and poly(vinyl alcohol) (PVA) are prepared by varying co-flowing wet-spinning conditions such as spinning geometry and PVA concentration, which affect aligning shear stress for MWCNTs during the wet-spinning. Then, structural features, mechanical and electrical performances of MWCNT/PVA composite fibers are investigated as a function of the aligning shear stress of the wet-spinning process. SEM images of the composite fibers exhibit that MWCNTs are wetted effectively with PVA chains. Polarized Raman spectra confirm that the alignment of MWCNTs is enhanced along the composite fiber axis with increasing the aligning shear stress of the spinning process. Accordingly, initial moduli and tensile strengths of the composite fibers are significantly increased with the increment of the aligning shear stress. In addition, it is found that electrical conductivities of MWCNT/PVA composite fibers increase slightly with the aligning shear stress, which is associated with the formation of efficient electrical conduction paths caused by well-aligned MWCNTs along the composite fiber axis.

Electrical Properties and Heating Performance of Polyurethane Hybrid Nanocomposite Films Containing Graphite and MWNTs

Polyurethane(PU) hybrid nanocomposite films containing graphite and multiwalled carbon nanotubes (MWNTs) were prepared by a solution-casting method, and their electrical properties and heating performance were investigated as a function of conducting hybrid filler content. The electrical resistivity of the PU/hybrid nanocomposite films decreased slightly from 2.40×10 2 Ωcm to 2.79×10 1 Ωcm with increasing the hybrid filler content (5.0~9.0 wt%). The current-voltage (I-V) characteristics of these nanocomposite films indicate a significant increase in current level with 9.0 wt% hybrid filler content, which reflects the fact that above the percolation threshold, the conduction mech- anism in the nanocomposite films changes from tunneling conduction (nonlinear) to ohmic conduction due to the direct contact between the graphite and the MWNTs (linear). As a result, the PU/hybrid nanocomposite films containing 9.0 wt% hybrid filler with 4.5 wt% graphite and 4.5 wt% MWNTs can be quickly heated from room temperature to 48.8 o C within 80 s by applying a DC voltage of 30 V, whereas the PU nanocomposite films containing 30.0 wt% graphite or 5.0 wt% MWNTs could only be heated to 39.2 o C and 30.6 o C, respectively.

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.

Effects of Molecular Weight of Poly(ethylene glycol) on the Thermal Properties and the Alkaline Hydrolysis of Copolyesters

Abstract: Copolyesters containing sodium sulfonate group and poly(ethylene glycol) (PEG) were synthesized with var-ious molecular weight of PEG. The selected molecular weights of PEG were 1000, 2000, 4000, 6000, and 16000. Thethermal properties and the crystallization behaviors of the copolyesters were investigated by using differential scanningcalorimeter (DSC). Alkaline hydrolysis of the copolyesters was studied as a function of the treatment time. The meltingtemperatures and the heats of fusion of the copolyesters increased with an increase in the PEG molecular weight, whilethe weight reduction rate of copolyesters in an alkaline solution decreased with an increase in the PEG molecular weight.At the same PEG content in copolyesters, hard segment length increases with the PEG molecular weight, which wasthought to be the most important factor that determines the thermal properties and the alkaline hydrolysis behavior. Keywords: copolyesters, molecular weight of PEG, hard segment length, thermal properties, hydrolysis in alkaline solution

Effects of Chemical Functionalization of MWCNTs on the Structural and Physical Properties of Elastomeric Copolyetherester-based Composite Fibers

Elastomeric copolyetherester (CPEE)-based composite fibers incorporating various neat and functionalized multiwalled carbon nanotubes (MWCNTs) were prepared through a conventional wet-spinning and coagulation process. The influence of functionalized MWCNTs on the morphological features, and the thermal, mechanical properties and electrical conductivity of CPEE/MWCNT (80/20, w/w) composite fibers were investigated. FE-SEM images show that a composite fiber containing poly(ethylene glycol)-functionalized MWCNTs (MWCNT-PEG) has a relatively smooth surface owing to the good dispersion of MWCNT-PEGs within the fiber, whereas composite fibers including pristine MWCNTs (p-MWCNT), acid-functionalized MWCNTs (a-MWCNT), and ethylene glycol-modified MWCNTs (MWCNT-EG) have quite a rough surface morphology owing to the presence of MWCNT aggregates. As a result, the CPEE/MWCNT-PEG composite fiber exhibits noticeably increased thermal and tensile mechanical properties as well as a faster crystallization behavior, which stems from an enhanced interfacial interaction between the CPEE matrix and MWCNT-PEGs.

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.

Thermal Properties and Alkaline Weight Reduction of Anionic Copolyesters/Dodecylbenzenesulfonate Blend Films

Abstract: A series of anionic copolyester/dodecylbenzenesulfonate (DBS) blend films were prepared by a melt-com-pounding method and then annealed under a range of conditions. To eliminate possible experimental errors in measur-ing the alkaline reduction rate of the samples, the crystallinity should be uniform. We used Fourier transform infrared(FT-IR) spectroscopy to measure and control the relative crystallinity of the annealed samples. The relationshipbetween the crystallinity and reduction rate, as well as the annealing conditions of the annealed blend samples, wasderived. FT-IR analysis showed that the crystallization rate of copolyester/DBS blend films is lower than that of purecopolyester. However, the alkaline weight reduction rate of the blend films, which have similar crystallinities, increasesdramatically with increasing DBS content, even at a low DBS content of 1 to 3 wt%, although the effect of the DBSbecomes negligible above 5 wt%. However, unlike the result for annealed films, the alkaline weight reduction rate foramorphous films exhibited a linear relationship between the alkaline reduction rate and the DBS content for all of theexamined DBS content, demonstrating that the effects of the DBS on the alkaline weight reduction of the copolysestercould be affected by processing conditions such as post-drawing and annealing. Keywords: anionic copolyesters, DBS, annealing, alkaline weight reduction, FT-IR

A Study on the Terpolyamides Based on the Isomorphic-Replacement-Type Copolyamide 66/6T −Effect of Comonomer Structure on the Properties of Terpolyamides−

Two kinds of semi-aromatic polyamide terpolymers based on the isomorphic replacement copolymer system of poly(hexamethylene adipamide-co-hexamethylene terephthalamide) (PA 66/6T), that is, poly(hexamethylene adipamide-co-hexamethylene terephthalamide-co-hexamethylene sebacamide) (PA 66/6T/610) and poly(hexamethylene adipamide-co-hexamethylene terephthalamide-co-hexamethylene isophthalamide) (PA 66/6T/6I), were synthesized by melt polycondensation in order to study the effect of comonomer structure on the properties of terpolyamides. Although the incorporation of the 610 or 6I in the PA terpolymers led to overall depression in the thermal properties of terpolymers, PA 66/6T/610 exhibited noticeably higher thermal properties such as melting point (), heat of fusion (), and tensile mechanical performances as well as lower water absorption than those of PA 66/6T/6I. The lower water absorption might be originated from the enhanced chain mobility and the lower density of amide bond owing to the long aliphatic chain of the sebacic acid. On the other hand, the glass transition temperatures () of PA 66/6T/6I copolymers increased with the increment of the 6I content, which was resulted from the reduced chain mobility due to the rigid aromatic structure of the comonomer.