Yeong Suk Choi

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.

Synthesis and characterization of exfoliated poly(styrene-co-methyl methacrylate)/clay nanocomposites via emulsion polymerization with AMPS

Abstract A method was described for synthesis of exfoliated poly(styrene-co-methyl methacrylate)/clay nanocomposites through an emulsion polymerization with reactive surfactant, 2-acrylamido-2-methyl-1-propane sulfonic (AMPS) which made the polymer end-tethered on pristine Na-MMT. AMPS widened the gap between clay layers and facilitates comonomers penetrate into clay. Silicate layers affect the composition of comonomers, for example A0.3M10S10T5 showed the elevated composition of MMA end tethered on silicate when compared to the feed ratio and polar methyl methacrylate (MMA) was considered to have the stronger interaction with clay layers than styrene. The exfoliated structure of extracted nanocomposite was confirmed by XRD and transmission electron microscopy. The onset of thermal decomposition for nanocomposites shifted to a higher temperature than that for neat copolymer. The dynamic moduli of nanocomposites increase with clay content. Dynamic storage modulus and complex viscosity increased as the clay content increased. In low frequency region all prepared nanocomposites exhibited apparent low-frequency plateaus in the linear storage modulus. Complex viscosity showed shear-thinning behavior as the clay content increases.

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.

Poly(n-butyl acrylate-co-methyl methacrylate) and Poly(n-butyl acrylate-co-styrene)/Silicate Nanocomposites Prepared by Emulsion Polymerization

Two types of poly(n-butyl acrylate) copolymer/silicate nanocomposites have been produced: poly(n-butyl acrylate-co-methyl methacrylate) [P(BA-co-MMA)]/silicate nanocomposites and poly(n-butyl acrylate-co-styrene) [P(BA-co-ST)]/silicate nanocomposites. The P(BA-co-MMA)/silicate nanocomposites shows the exfoliated structures but a P(BA-co-ST)/silicate nanocomposites have intercalated structures, because the BA/MMA comonomer has a higher polarity (e-value in Q-e scheme) than the BA/ST comonomer. The BA/MMA comonomer expanded the interlayer space of the silicate wider than did the BA/ST comonomer. The thermal degradation onset point of the P(BA-co-MMA)/silicate nanocomposites was 43°C higher than that of pure P(BA-co-MMA). P(BA-co-MMA)T5%, P(BA-co-MMA)T10%, and P(BA-co-MMA)T20% exhibit 134, 302, and 195% increases, respectively, in their storage moduli at −20 °C relative to the pure copolymer.

Synthesis and Properties of Exfoliated Poly(methyl methacrylate-co- acrylonitrile)/Clay Nanocomposites via Emulsion Polymerization

Poly(methyl methacrylate-co-acrylonitrile) [P(MMA-co-AN)]/Na-MMT nanocomposites were synthesized through emulsion polymerization with pristine Na-MMT. The nanocomposites were exfoliated up to 20 wt% content of pristine Na-MMT relative to the amount of MMA and AN, and exhibited enhanced storage moduli,E′, relative to the neat copolymer. The exfoliated morphology of the nanocomposite was confirmed by XRD and TEM. 2-Acrylamido-2-methyl-1-propane sulfonic acid (AMPS) widened the galleries between the clay layers before polymerization and facilitated the comonomers, penetration into the clay to create the exfoliated nanocomposites. The onset of the thermal decomposition of the nanocomposites shifted to a higher temperature as the clay content increased. By calculating areas of tanδ?of the nanocomposites, we observed that the nanocomposites show more solid-like behavior as the clay content increases. The dynamic storage modulus and complex viscosity increased with clay content. The complex viscosity showed shear-thinning behavior as the clay content increased. The Young’s moduli of the nanocomposites are higher than that of the neat copolymer and they increase steadily as the silicate content increases, as a result of the exfoliated structure at high clay content.

Poly(methyl methacrylate-co-styrene)/silicate nanocomposites synthesized by multistep emulsion polymerization

Exfoliated poly(methyl methacrylate-co-styrene) [P(MMA-co-ST)]/silicate nanocomposites were synthesized through a multistep emulsion polymerization. The methyl methacrylate monomers were polymerized first and then the styrene monomers were polymerized. The nanocomposites had core-shell structures consisting of PMMA (core) and PS (shell); these structures were confirmed by1H NMR spectroscopy and TEM, respectively. P(MMA-co-ST) copolymers showed two molecular weight profiles and two glass transition temperatures (Tg) in GPC and DMA measurements. At 30 °C, the nanocomposites exhibited 83 and 91% increases in their storage moduli relative to the neat copolymer because the silicate layers were dispersed uniformly in the polymer matrix.

Alkylated sulfonated poly(arylene sulfone)s for proton exchange membranes

The attachment of flexible spacers into aromatic polymers is a molecular design approach that is used for improving processability of aromatic polymers. The concept is attractive because it enables not only the creation of aromatic polymers with improved processability but it is also possible to control phase morphology by simply introducing pendant side chains. Here we report new bisphenol A derivatives bearing alkyl chains of different lengths obtained by an addition reaction can readily make novel poly(arylene sulfone)s with aromatic dihalides and aromatic dioles. They were observed using two-dimension diffusion-ordered spectroscopy nuclear magnetic resonance (2D DOSY NMR) spectra. Attaching flexible alkyl groups into sulfonated poly(arylene sulfone)s allows for increased control of glass transition temperatures, Tg, of sulfonated poly(arylene sulfone)s. The alkylated sulfonated poly(arylene sulfone)s had flexibility, increased surface contact angle, improved methanol permeability, and high ion conductivity compared to the neat polymer. Due to the creation of aromatic polymers with improved processability by simply introducing pedant side chains, this novel alkylation method is expected to be applicable to other arylene based proton conductive polymers.