Sang Eun Shim

Measurement of the dispersion stability of pristine and surface-modified multiwalled carbon nanotubes in various nonpolar and polar solvents

A qualitative and rapid measurement technique based on multiple light scattering was employed to analyze the dispersion stability of black multiwalled carbon nanotube (CNT) suspensions. Pristine and chemically oxidized CNTs were dispersed in various polar and nonpolar solvents. The change in the transmission of near-infrared light from the suspensions was periodically measured along the height of a sample cell at room temperature. Using this method, it was possible to obtain the variation of the dispersion stability within only a day. Pristine and surface-modified CNTs dispersed in nonpolar media aggregated within 2 h and sedimentation progressively proceeded with time. As the polar component of the solubility parameter and the solubility in water decreased, faster aggregation and severe sedimentation occurred and vice versa. When the CNTs were modified with carboxylic anion groups, the dispersibility in polar solvents was significantly enhanced due to the combination of polar–polar affinity and electrostatic repulsion, with the result that the transmission flux remained unchanged. The origin of electrostatic repulsion can be found from the increased zeta potential and conductivity of CNTs with carboxylic anion groups.

Fabrication of silica nanotubes using silica coated multi-walled carbon nanotubes as the template.

Silica nanotubes were synthesized using the multi-walled carbon nanotubes (MWCNTs) as the template material. First, we prepared silica coated MWCNT composites by surface oxidation of MWCNTs using KMnO(4) in the presence of a phase transfer catalyst and followed by grafting of 2-aminoethyl 3-aminopropyl trimethoxy silane, AEAPS. The amine groups in grafted AEAPS on MWCNTs could activate the silica shell formation by acid-base interaction. The synthesized silica was formed a uniform layer on MWCNTs with a controllable thickness and possessed sturdy 3-dimensional stability. After calcinations at 800 degrees C, the inner MWCNTs of the composite were completely decomposed and the outer silica shell layer maintained without distortion of its original shape. Finally, we could obtain the silica nanotubes having 13.0 nm of average layer thickness.

Living radical dispersion photopolymerization of styrene by a reversible addition - fragmentation chain transfer (RAFT) agent

In this study, an addition – fragmentation chain transfer agent bearing dithioester group is synthesized and applied to conventional dispersion photopolymerization of styrene in ethanol medium in the presence of poly(N-vinylpyrrolidone) stabilizer with varying amounts of the RAFT agent and optionally with conventional initiator, azobisisobutyronitril (AIBN) at various temperatures. Monomer conversion, molecular weight evolution, polydispersity index (PDI), and final particle sizes are measured. The PDI of the formed polymer is between 1.5 and 2.5 in the presence of RAFT agent. Higher concentration of RAFT agent or elevated temperature leads to the acceleration of the polymerization rate resulting in fast conversion, and reducing molecular weight and PDI. Stable polystyrene beads above 1 mm in diameter are successfully prepared by means of RAFT method applied in dispersion polymerization. The weight average particle sizes are between 1.08 and 2.04 mm, and the uniformity ðDw=DnÞ is ranged from 1.26 to 2.51. q 2003 Elsevier Ltd. All rights reserved.

A Review on Thermal Conductivity of Polymer Composites Using Carbon-Based Fillers : Carbon Nanotubes and Carbon Fibers

Recently, the use of thermal conductive polymeric composites is growing up, where the polymers filled with the thermally conductive fillers effectively dissipate heat generated from electronic components. Therefore, the management of heat is directly related to the lifetime of electronic devices. For the purpose of the improvement of thermal conductivity of composites, fillers with excellent thermally conductive behavior are commonly used. Thermally conductive particles filled polymer composites have advantages due to their easy processibility, low cost, and durability to the corrosion. Especially, carbon-based 1-dimensional nanomaterials such as carbon nanotube (CNT) and carbon nanofiber (CNF) have gained much attention for their excellent thermal conductivity, corrosion resistance and low thermal expansion coefficient than the metals. This paper aims to review the research trends in the improvement of thermal conductivity of the carbon-based materials filled polymer composites.

Electrical, thermal, and rheological properties of carbon black and carbon nanotube dual filler-incorporated poly(dimethylsiloxane) nanocomposites

AbstractConductive nano-sized carbon black (CB) and carbon nanotube (CNT) are widely used in the polymer industry due to their excellent electrical and thermal conduction behavior and reinforcing ability. Herein, poly(dimethylsiloxane) (PDMS) nanocomposites filled with both conductive CB and CNT were fabricated using a planetary mixer and two-roll mill. Due to excellent thermal, electrical, and physical properties of CB and CNT, both the thermal and electrical properties of the composites and the dynamic mechanical properties were improved by the incorporation of CB and CNT into PDMS. The thermal conductivities of the composites linearly increased with filler concentration, and the electrical threshold was determined to be low. The rheological properties of the PDMS/CB and PDMS/CB/CNT nanocomposites were significantly influenced by filler incorporation.

Polyelectrolyte-assisted synthesis of polystyrene microspheres by dispersion polymerization and the subsequent formation of silica shell

Polystyrene (PS) microspheres were synthesized via dispersion polymerization in alcoholic media. A cationic polyelectrolyte, polyethyleneimine (PEI) was successfully used as a steric stabilizer. The concentration of initiator, monomer, and the solubility parameter of medium showed typical phenomena observed in dispersion polymerization. However, the sensitivity to the change in the particles size was almost twofold greater than conventional stabilizers. Spherical PS particles were synthesized with the PEI concentrations ranging from 5 to 20 wt.% to styrene, but conversion over 95% was achieved over 10 wt.% PEI. As-prepared PEI-stabilized PS microspheres were used as the template for the subsequent formation of a silica shell. As a result, a robust silica layer was fabricated on PS microspheres due to the increased interaction between PEI and tetraethyl orthosilicate (TEOS).

Reversible addition-fragmentation chain transfer (RAFT) bulk polymerization of styrene : Effect of R-group structures of carboxyl acid group functionalized RAFT agents

Three dithioester-derived carboxyl acid functionalized RAFT(reversible addition-fragmentation chain transfer) agents, viz. acetic acid dithiobenzoate, butanoic acid dithiobenzoate and 4-toluic acid dithiobenzoate, were used in the RAFT bulk polymerization of styrene, in order to study the effects of the R-group structure on the living nature of the polymerization. By conducting the polymerization with various concentrations of the RAFT agents and at different temperatures, it was found that the R-group structure of the RAFT agents plays an important role in the RAFT polymerization; the bulky structure and radical stabilizing property of the R-group enhances the living nature of the polymerization and allows the polymerization characteristics to be well controlled.

Size and uniformity variation of poly(MMA-co-DVB) particles upon precipitation polymerization

Stable poly(methyl methacrylate-co-divinylbenzene) (poly(MMA-co-DVB)) microspheres were prepared by precipitation polymerization using acetonitrile as the main medium under various polymerization conditions, including modifications of the agitation speed, monomer and initiator concentrations, DVB content in the monomer mixture, and the use of various cosolvents. Gentle agitation was required to obtain smooth spherical particles. The individually stable microspheres were obtained at monomer concentrations of up to 15 vol% in an acetonitrile medium. The number-average diameter increased linearly with respect to increases in the monomer and initiator concentrations. We found, however, that the uniformity of the microspheres was independent of the variation of the polymerization ingredients because nuclei formation was solely influenced by the crosslinking reaction of the monomers. We obtained higher yields for the polymerization at higher concentrations of monomer and initiator. The concentration of DVB in the monomer mixture composition played an important role in determining not only the size of the microspheres but also the yield of the polymerization. In addition, although we employed various cosolvents as the polymerization medium, we found that acetonitrile/2-methoxyethanol was the only system that provided spherical particles without coagulation. This finding indicates that the precipitation polymerization is strongly dependent on the solvent used as the medium.

Preparation of macroporous carbon foams using a polyurethane foam template replica method without curing step

AbstractMacroporous carbon foams were synthesized without a curing step using polyurethane (PU) foam as the sacrificial template and resorcinol-formaldehyde (RF), phenol-formaldehyde (PF), and phenol-resorcinol-formaldehyde (PRF) resins as the carbon precursors. PU foam was impregnated with these three aqueous resin solutions and carbonized directly at 830 °C in an inert atmosphere for 3 h without a pre-curing step. During heating to carbonization temperature, the impregnated resins were cured and subsequently carbonized. The curing, degradation of PU foam and carbonization mechanism were examined by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The pore structure, electrical conductivity, and compressive strength of the prepared carbon foams using the RF, PF, and PRF resin carbon precursors were measured. Among them, the carbon foam fabricated from PF resin had a surface area, electric conductivity and compressive strength of 63 m2/g, 3.409 S/m and 0.25 MPa, respectively.

In-Situ Synthesis of PS/(-)Silica Composite Particles in Dispersion Polymerization Using An (±) Amphoteric Initiator

Core/shell (±)PS/(−)silica nanocomposite particles were synthesized by dispersion polymerization using an amphoteric initiator, 2,2′-azobis [N-(2-carboxyethyl)-2,2-methylpropionamidine](HOOC(CH2)2HN(HN=) C(CH3)2CN=NC (CH3)2C(=NH)NH (CH2)2COOH), VA-057. Negatively charged (−6.9 mV) silica was used as the stabilizer. The effects of silica addition time and silica and initiator concentrations were investigated in terms of polymerization kinetics, ultimate particle morphology, and size/size distribution. Uniform hybrid microspheres with a well-defined, core-shell structure were obtained at the following conditions: silica content = 10−15 wt% to styrene, VA-057 content=above 2 wt% to styrene and silica addition time=0 min after initiation. The delay in silica addition time retarded the polymerization kinetics and broadened the particle size distribution. The rate of polymerization was strongly affected by the silica content: it increased up to 15 wt% silica but then decreased with further increase in silica content. However, the particle size was only marginally influenced by the silica content. The zeta potential of the composite particles slightly decreased with increasing silica content. With increasing VA-057 concentration, the PS microspheres were entirely coated with silica sol above 1.0 wt% initiator.