Chong Min Koo

Synthesis and characterization of maleated polyethylene/clay nanocomposites

Maleic anhydride grafted polyethylene (maleated polyethylene)/clay nanocomposites were prepared by simple melt compounding. The exfoliation and intercalation behaviors depended on the hydrophilicity of polyethylene grafted with maleic anhydride and the chain length of organic modifier in the clay. When the number of methylene groups in alkylamine (organic modifier) was larger than 16, the exfoliated nanocomposite was obtained, and the maleic anhydride grafting level was higher than about 0.1 wt% for the exfoliated nanocomposite with the clay modified with dimethyl dihydrogenated tallow ammonium ion or octadecylammonium ion. The pure LLDPE showed only the intercalation, which does not depend on the initial spacing between clay layers.

Characteristics of polyvinylpyrrolidone-layered silicate nanocomposites prepared by attrition ball milling

Polyvinylpyrrolidone (PVP)/sodium montmorillonite (MMT) nanocomposites prepared via the solution intercalation method were investigated by UV/vis, SEM, X-ray diffraction, TEM, FT-IR and PLM (polarized light microscopy). PVP/MMT nanocomposites show exfoliation below 20 wt% MMT and intercalation above this concentration. Nanocomposites retain good optical clarity and increased thermal resistance with MMT content. The compatibility between PVP and MMT and their enhanced properties may be explained by hydrogen bonding interactions. In addition, the nanocomposites prepared under more rigorous mixing conditions show better transparency because the smaller particle sizes are induced. In addition, the study on optically clear PVP/MMT suspensions helps one to understand how optical anisotropy of MMT is affected by the existence of polymer in aqueous solution.

Chemical modification of carbon nanotubes and preparation of polystyrene/carbon nanotubes composites

Single-walled carbon nanotubes (SWNTs) have been chemically modified through the formation of carboxylic acid functionalities or by grafting octadecylamine and polystyrene onto them. We purified SWNTs with nitric acid to remove some remaining catalysts and amorphous carbon materials. After purification, we broke the carbon nanotubes and shortened their lengths by using a 3∶1 mixture of concentrated sulfuric acid and nitric acid. During these purification and cutting processes, carboxylic acid units formed at the open ends of the SWNTs. Octadecylamine and amino-terminated polystyrene were grafted onto the cut SWNTs by condensation reactions between the amine and carboxylic acid units. The cut SWNTs did not disperse in organic solvents, but the octadecylaminegrafted and polystyrene-grafted SWNTs dispersed well in dichloromethane and aromatic solvents (e.g., benzene, toluene). Composites were prepared by mixing polystyrene with the octadecylamine-grafted or polystyrene-grafted SWNTs. Each composite had a higher dynamic storage modulus than that of a pristine polystyrene. The composites exhibited enhanced storage moduli, complex viscosities, and unusual non-terminal behavior when compared with a monodisperse polystyrene matrix because of the good dispersion of carbon nanotubes in the polystyrene matrix.

Self-organized grafting of carbon nanotubes by end-functionalized polymers

A variety of end-functionalized polymers were grafted spontaneously onto multi-walled carbon nanotubes (MWNTs) using a solution mixing process. The end-functional groups of the polymers underwent noncovalent grafting to the defect sites at the surface of the purified MWNTs through zwitterionic interaction or hydrogen bonding. The physically grafted polymers including polystyrene (PS), poly(methyl methacrylate) (PMMA), polyethylene oxide (PEO), and polydimethylsiloxane (PDMS) provided sufficient compatibility with an organic solvent or polymer matrix, such that the nanotubes could be finely dispersed in various organic media. This approach is universally applicable to a broad range of polymer-solvent pairs, ensuring highly dispersed carbon nanotubes through simple solution mixing.