Doo Hyun Baik

Ionic interactions in polyacrylonitrile/polypyrrole conducting polymer composite

A conducting polymer composite was prepared by the postpolymerization of pyrrole in a polyacrylonitrile (PAN) matrix film. To enhance the electrostatic interaction between the two phases, a small amount of a sulfonate or a carboxylate (COO−) group was incorporated into the PAN structure. The presence of electrostatic interaction between the conducting polypyrrole and the anion-containing PAN copolymer was elucidated by examination of the morphology and the electrical properties of the composite. The aromatic sulfonate-containing matrix provided the composite with the best results in the electrical conductivity, the environmental stability of conductivity, and the morphological property.

Structures, electrical, and dielectric properties of PVDF-based nanocomposite films reinforced with neat multi-walled carbon nanotube

AbstractWe report, herein, on the structures, melting/crystallization, electrical, and dielectric properties of poly (vinylidene fluoride) (PVDF) nanocomposites reinforced with a neat multiwalled carbon nanotube (MWCNT). For our purposes, PVDF/MWCNT nanocomposite films with a wide range of MWCNT contents (0.0–20.0 wt%) are prepared via ultrasonicated solution-mixing and melt-compression methods. It is found that MWCNTs become well dispersed in nanocomposites by wrapping them with PVDF chains. The relative content of β-phase to α-phase crystals of a PVDF matrix is higher for the nanocomposite films with higher MWCNT content; although, the overall crystallinity of the nanocomposites is almost identical, irrespective of the MWCNT content. The electrical conductivity and dielectric permittivity of the nanocomposites as a function of frequency are strongly dependent on the MWCNT content. The electrical percolation threshold of PVDF/MWCNT nanocomposites is formed between 2.0 and 5.0 wt% MWCNT. The neat PVDF and nanocomposites with low MWCNT contents of 0.2 and 1.0 wt% are electrically insulating materials (∼10−9 S/cm at 102 Hz) with low dielectric permittivity of 9–28; while the nanocomposites with high MWCNT contents of 5.0–20.0 wt% have relatively high electrical conductivity values (10−4∼10−2 S/cm at 102 Hz). In contrast, the nanocomposite with 2.0 wt% MWCNT has a huge dielectric permittivity of ∼6520 at 102 Hz, although it has relatively low electrical conductivity of ∼10−8 S/cm at 102 Hz. The huge dielectric permittivity of the nanocomposite with 2.0 wt% MWCNT could be caused by charge accumulation at the interfacial layers between PVDF chains and MWCNTs in the vicinity of the electrical percolation threshold.

Synthesis and cyclization of aromatic polyhydroxyamides containing trifluoromethyl groups

Polyhydroxyamides derivatized with trifluoromethyl ether and trifluoromethyl ester groups were investigated as possible candidates for a new flame retardant polymer. Model compounds for these derivatized polyhydroxyamides were synthesized and their cyclization chemistry was investigated. The model compound study revealed that trifluoromethyl ester group containing model compounds can cyclize on heating, while trifluromethyl ether group containing model compounds cannot. The non-fluorinated ester and ether derivatives behaved similarly. The trifluoromethyl ester derivatized polyhydroxyamides were synthesized according to the procedures for the model compounds. TGA characterization revealed that the fluorinated polymers have nearly same thermal stability as the underivatized PHA after cyclization.

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.

Formation of Polypyrrole Copolymer in PSPMS Precursor Film by Electrochemical Polymerization

Abstract The electrochemical copolymerization process of polypyrrole (PPy) in the PSPMS precursor film has been discussed. Constant potential electrolysis and SEM experiments showed that pyrrolyl groups in the precursor were oxidized to form grafting centers and PPy grew through the film.

Preparation, structure and properties of poly(p-phenylene benzobisoxazole) composite fibers reinforced with graphene

Graphene-reinforced poly(p-phenylene benzobisoxazole) (PBO) composite fibers were manufactured via dry-jet wet-spinning of PBO/graphene mixtures in poly(phosphoric acid) (PPA), which were reaction products prepared by the in situ polymerization of 4,6-diaminoresorcinol dihydrochloride and terephthaloyl chloride in the presence of PPA solvent and exfoliated graphene sheets. The content of graphene sheets in the as-spun fibers was adjusted to 0.0∼2.0 wt%. The molecular structure, crystalline order, and morphology of the as-spun fibers of pristine PBO and PBO/graphene composites were identified using FTIR, X-ray diffraction, and electron/optical microscopy. The thermal stability and tensile mechanical properties of the composite fiber with 0.2 wt% graphene were found to be significantly improved compared to the pristine PBO fiber.

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.

Synthesis and characterization of copolyester sizing agents

The effect of main chain structure of anion-containing copolyesters on the properties of copolyester sizing agents was investigated. The copolyesters were prepared by conventional two step polymerization technique from DMT, DMI, DMS, EG, and DEG. The copolyesters synthesized were characterized by atomic absorption spectroscopy,1H-NMR Spectroscopy, GC, FTIR Spectroscopy, and DSC. The solubility decreased as the DMT content increased. The copolyesters having DMT:DMI=1:1 showed the minimum viscosity. The effect of EG content on the solution stability was not clear and the samples having high DMI content showed better solution stability. The water resistance was best when only DMI and EG were used, while it was worst when DMT:DMI was 1:0.