Soonjong Kwak

POLYETHYLENE/CLAY NANOCOMPOSITE BY IN SITU EXFOLIATION OF MONTMORILLONITE DURING ZIEGLER–NATTA POLYMERIZATION OF ETHYLENE

Complete exfoliation of montmorillonite during Ti-based Ziegler-Natta polymerization of ethylene has been successfully carried out by using montmorillonite (MMT-OH) modified with intercalation agents containing hydroxy groups. Hydroxyl groups in intercalation agents offer facile reactive sites for anchor ing catalysts in between silicate layers. Comparison of exfoliation characteristics between MMT-OH and non-intercalated montmorillonite showed that the feasibility of exfoliation during ethylene polymerization was highly dependent on the catalyst fixation method.

Synthesis of 4-tert-butyldimethylsilyloxystyrene and its copolymerization with styrene using (η5-indenyl)trichlorotitanium in the presence of methylaluminoxane

Poly(styrene-co-4-tert-butyldimethylsilyloxystyrene) as a precursor of hydroxyl-functionalized syndiotactic polystyrene was successfully synthesized via (η5-indenyl)trichlorotitanium (IndTiCl3)-catalyzed copolymerization of styrene with 4-tert-butyldimethylsilyloxystyrene in toluene at 25°C in the presence of methylaluminoxane (MAO) ([Al]/[Ti] = 2 000). The amount of styrene derivative incorporated into the polymeric chain for a 20,7 : 1 mole feed ratio of styrene to 4-tert-butyldimethylsilyloxystyrene was found to be 1,8 mol-% from a 1H NMR analysis. The styrene derivative was successfully prepared from 4-hydroxybenzaldehyde via first protecting the hydroxyl group using tert-butyldimethylchlorosilane followed by the ‘Wittig-type’ reaction with the ‘Tebbe’ reagent. The yield was about 82 wt.-% on the basis of the initial amount of 4-hydroxybenzaldehyde used.

Fabrication and electrical properties of platinum nanofibres by electrostatic spinning

One-dimensional (1D) platinum nanofibres (NFs) were prepared via electrospinning of Pt precursor/polymer composite fibres followed by subsequent thermal annealing. Pt-NFs several hundred micrometres long and ~50?nm in diameter were obtained. The internal morphology of the Pt-NFs was largely governed by thermal annealing conditions. X-ray diffraction and high-resolution transmission electron microscopy revealed a strong correlation between thermal annealing conditions and the internal grain structure of the Pt-NFs. The current?voltage (I?V) characteristics of single Pt-NF indicated that the internal grain structure of Pt-NFs plays an important role in the electrical properties of nanoscale Pt fibres.

Wear behavior of in situ polymerized carbon nanotube/ultra high molecular weight polyethylene composites

AbstractA carbon nanotube (CNT)/ultra high molecular weight polyethylene (UHMWPE) composite has been prepared through in situ polymerization of ethylene using Ti-based Ziegler-Natta catalysts fixed on the surface of CNT. The in situ polymerization of ethylene produced CNTs regularly encapsulated with UHMWPE, which showed very uniform dispersion of CNTs in the UHMWPE matrix after direct molding. In tensile and ring-on-block wear tests, the in situ polymerized composites showed mechanical and wear properties superior to mechanically blended composites. In particular, the polymerized composite displayed a remarkable suppression of abrasive wear, which was the wear mechanism observed in unfilled UHMWPE and mechanically blended composites; the in situ polymerized composite containing about 10 wt% of CNT had an approximately 2.5 times lower wear rate than unfilled UHMWPE. Moreover, the polymerized composite showed higher thermal conductivity with CNT content when compared to the blended composites, which suggests an easier transfer of heat generated during a severe wear operation.

Comparison of activities of homogeneous Ti‐based catalysts in the presence of methylaluminoxane (MAO) for synthesis of syndiotactic polystyrene by UV/visible spectroscopic study

The catalytic activities in syndiospecific polymerization of styrene in hydrocarbon using homogeneous Ti-based catalysts in the presence of methylaluminoxane (MAO) were investigated through UV/visible spectroscopic analysis. A strong UV absorption band of CpTiCl3, itself, incipiently appeared at λmax = 400 nm in toluene, followed by a bathochromic shift with its remarkable decrease by the addition of MAO. The absorption band intensity at λmax = 400 nm arising from delocalization of π-electrons on the cyclopentadienyl ring decreased by methylation in the presence of MAO with regard to the mechanism for production of an active center (“cation-like”), for example, the change of the ionic nature. The intensity decrease at λmax = 400 nm was suppressed over 2000 of the [Al]/[Ti] ratio. In the case of Ti(OC4H9)4 having a σ-ligand, new and broad UV absorption bands were developed at λmax = 340 nm and 410 nm in the presence of MAO in contrast with the CpTiCl3/MAO system. Comparison between the relative absorption intensities at λmax = 340 nm and 410 nm led to the determination of a maximum catalytic activity of Ti(OC4H9)4 in the presence of MAO related to the polymerization yield. The maximum polymerization yield was observed with regard to the relative maximum value of the absorption intensity at λmax = 410 nm with the [Al]/[Ti] ratio (500). From observation for polymorphism of the final products via differential scanning calorimetric analysis (DSC), the thermally unstable β-form seemed to be produced by the CpTiCl3/MAO system independent of the MAO concentration, the Ti(OC4H9)4/MAO system produced a thermally stable α-form in the low MAO concentration (up to 100 of the [Al]/[Ti] ratio), and a mixture of α- and/or β-forms over 200 of the [Al]/[Ti] ratio under our experimental conditions.

Static yield stress of a magnetorheological fluid containing Pickering emulsion polymerized Fe2O3/polystyrene composite particles.

The flow behaviors of magnetorheological (MR) suspensions containing Pickering emulsion polymerized Fe2O3/polystyrene (PS) composite particles were reanalyzed using the Seo-Seo model. The experimental shear stress data obtained experimentally from the magnetorheological fluid fit well to the Seo-Seo model, indicating that this model can describe the structural reformation process of the aligned fibers at various shear rates. Unlike the dynamic yield stress obtained from the Cho-Choi-Jhon (CCJ) model, the static yield stresses obtained from the Seo-Seo model exhibit the same quadratic dependence on the magnetic field strength for both pure Fe2O3 particle suspension and Fe2O3/PS particle suspensions, which is in agreement with the predictions of the polarization model. The static yield stress plausibly explains the difference in underlying mechanism of MR fluids.

Catalyst and doping methods for arc graphene

Nitrogen-doped graphene synthesis with ∼g scale has been accomplished using the arc discharge method. The defects formed in the synthesis process were reduced by adding various metal catalysts, among which Bi2O3 was found to be the most effective. Adding dopants to the starting materials increased the electrical conductivity of the graphene product, and the doping concentration in graphene was tuned by adjusting the amount of nitrogen dopants. A step-wise technique to fabricate graphene thin films was developed, including dispersion, separation, and filtering processes. The arc graphene can also find its potential application in supercapacitors, taking advantage of its large surface area and improved conductivity by doping.

Surface modification of poly(ethylene-2,6-naphthalate) using NH3 plasma

We have performed NH3 plasma treatment of poly(ethylene-2,6-naphthalate) (PEN) surfaces with the purpose of incorporating nitrogen functional groups. X-Ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion spectroscopy (ToF-SIMS) were used to determine changes in the chemical structure of the PEN surfaces in response to different levels of plasma power. Plasma power had a significant impact on the type of nitrogen functional groups as well as the level of nitrogen incorporation. Considerable degradation and oxidation occurred with an excessive plasma power, rather than nitrogen incorporation. Optical emission spectroscopy (OES) confirmed that the type of chemical species present in the plasma discharge primarily determines the surface functionality with the plasma treatment. Partially decomposed ammonia species (NH and NH2) are considered to react with the PEN surface and form primarily amine groups. However, stable N2 species produced by bimolecular combination of fully decomposed atomic species led to the formation of imine groups via atomic rearrangement. Moreover, ion bombardment by N2+ species adversely affected the chemical structure of the surface, resulting in degradation of ester and carbon-carbon bonds and surface etching of carbon atoms.

Nonisothermal crystallization behaviors of nanocomposites prepared by in situ polymerization of high-density polyethylene on tungsten oxide particles

Morphology development and thermal properties of polyolefin composites (tungsten oxide (WO3)/high density polyethylene (HDPE)) were characterized by differential scanning calorimetry, polarized optical microscopy, and nonisothermal crystallization kinetics. Tungsten oxide (WO3) and HDPE composites were prepared by the in situ metallocene polymerization method which consisted of attaching a metallocene catalyst complex onto the surface of the nuclei (WO3) and followed by the surface-initiated polymerization. A kinetic equation proposed by Seo was employed to analyze the nonisothermal crystallization characteristics of the composites. The polarized optical microscopy and the Avrami exponent verified the importance of the interaction between the HDPE molecules and the nuclei surface for the HDPE molecular ordering around the metal powder as well as the morphological development in the early stage. The Avrami exponent, n, determined from the nonisothermal crystallization kinetics analysis indicates random 3-dimentsional morphology development for the WO3/HDPE composites. Dispersed WO3 accelerated the crystallization rate due to heterogeneous nucleation effect as indicated by the shift in the crystallization peaks to higher temperatures. The obtained crystalline structures were compared to those for the neat polyethylene resins and carbon nanotube (CNT)/HDPE composites.