Hyejun Jung

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.

Control of molecular weight of polystyrene using the reverse iodine transfer polymerization (RITP) – Emulsion technique

The RITP-emulsion polymerization of styrene in the presence of molecular iodine has been successfully performed using potassium persulfate (KPS) as an initiator and 1-hexadecanesulfonate as an emulsifier under argon atmosphere at 80°C for 7 hrs in the absence of light. The effects of the iodine concentration, molar ratio between KPS and iodine, and solid contents on the molecular weight of polystyrene (PS) were studied. As the iodine concentration increased from 0.05 to 0.504 mmol under the fixed [KPS]/[I(2)] ratio at 4.5, the weight-average molecular weight of PS substantially decreased from 126,120 to 35,690 g/mol, the conversion increased from 85.0% to 95.2%, and the weight-average particle diameter decreased from 159 to 103 nm. In addition, as the ratio of [KPS]/[I(2)] increased from 0.5 to 6.0 at the fixed [I(2)] of 0.504 mmol, the weight-average molecular weight of PS decreased from 72,170 to 30,640 g/mol with high conversion between 81.7% and 96.5%. Moreover, when the styrene solid content increased from 10 to 40 wt.% at the fixed [KPS]/[I(2)] ratio of 4.5, the weight-average molecular weight of PS varied between 33,500 and 37,200 g/mol, the conversion varied between 94.9% and 89.7% and the weight-average diameter varied from 122 to 205 nm. Thus, the control of molecular weight of PS less than 100,000g/mol with high conversion (95%) and particle stability of up to 40 wt.% solid content were easily achieved through the usage of iodine with suitable ratio of [KPS]/[I(2)] in the RITP-emulsion polymerization technique, which is of great industrial importance.

Acetone vapor sensor made with cellulose-TiO2/MWCNTs hybridnanocomposite

A Titanium dioxide (TiO2)/multi wall carbon nanotubes (MWCNTs) nanocomposite was synthesized by hydrothermal process. X-ray diffractometry, transmittance electron microscopy observation revealed this nanocompsite has high nanocrystral structure. After blending with regenerated cellulose, this hybrid nanocomposite was used as a room temperature ammonia (NH3) gas sensor. The TiO2/MWCNTs hybrid nanocomposite exhibited much higher response and faster response-recovery to NH3 gas of concentration ranging from 3000 ppm to 9000 ppm. The TiO2/MWCNTs hybrid nanocomposite can be a flexible room temperature gas sensor. The gas sensing mechanism of this hybrid nanocomposite is also discussed in this paper.

Fabrication of macroporous carbon foam using glycol-derivatives as liquid templates

Porous carbon foams were fabricated using phenol and formaldehyde (PF) as carbon precursors. The pore size and porosity of the foams were controlled using glycol-derivatives as porogen during condensation polymerization. The morphology of the resulting carbon foam with macroporous porosity and well dispersed pore structures were verified by mercury porosimetry and scanning electron microscopy. The compressive stress of the resulting carbon foam increased with increasing molecular weights of glycol-derivatives due to the decrease in porosity. Furthermore, the reaction mechanism was identified to involve glycol-derivatives in the condensation reaction of PF, which generated the porosity of the resulting carbon foam by releasing volatile compounds according to thermogravimetric analysis and Fourier transform infrared spectroscopy. As a result, the mechanical properites of the carbon foam could be affected by the morphological properties of macroporous structures using different types and concentrations of glycol-derivatives.

Fabrication of IDT electrode onto cellulose electro-active paper by inkjet printing

This paper investigates a direct inkjet printing method for electrode patterning on cellulose Electro-Active Paper (EAPap). Flexibility and transparency of the EAPap are advantageous for a versatile substrate in flexible printable electronics. The effects of curing conditions are evaluated by electrical resistivity and morphological analysis. To fabricate EAPap device, inter-digital transducer (IDT) electrodes are printed on the EAPap with drop-on-demand inkjet printing method. Silver patterns are obtained from organometallic silver ink by jetting and heat treatment at 160°C in air. IDT patterns are made on cellulose for variety and extensive application of inkjet printing electronics.