Donghyun Cho

Characterization of polystyrene and polyisoprene by normal-phase temperature gradient interaction chromatography

Temperature gradient interaction chromatography (TGIC) is applied to the characterization of polyisoprene (PI) and polystyrene (PS) using normal-phase (NP) stationary phase--bare silica or diol bonded silica. Tetrahydrofuran-isooctane mixtures are used as a mobile phase. PI and linear and star shaped PS samples are successfully fractionated in terms of the molecular mass with a high resolution comparable to that of reversed-phase (RP) HPLC. Temperature dependence of the retention shows that the enthalpy of adsorption of PS to the stationary phase is exothermic. In addition, some characteristic features of the NP-TGIC system relative to those of RP-TGIC are presented, which include a high sensitivity on the polar end group and the simultaneous size-exclusion chromatographic and TGIC characterization of PS and PI mixtures.

Retention mechanism of poly(ethylene oxide) in reversed-phase and normal-phase liquid chromatography.

The retention behavior of low- and high-molecular-mass poly(ethylene oxide) (PEO) in reversed-phase (RP) and normal-phase (NP) liquid chromatography was investigated. In RPLC using a C18 bonded silica stationary phase and an acetonitrile-water mixture mobile phase, the sorption process of PEO to the stationary phase showed deltaH(o) > 0 and deltaS(o) > 0. Therefore, PEO retention in RPLC separation is an energetically unfavorable, entropy-driven process, which results in an increase of PEO retention as the temperature increases. In addition, at the enthalpy-entropy compensation point the elution volume of PEO was very different from the column void volume. These observations are quite different from the RPLC retention behavior of many organic polymers. The peculiar retention behavior of PEO in RPLC separation can be understood in terms of the hydrophobic interaction of this class of typical amphiphilic compounds with the non-polar stationary phase, on the one hand, and with the aqueous mobile phase, on the other. The entropy gain due to the release of the solvated water molecules from the PEO chain and the stationary phase is believed to be responsible for the entropy-driven separation process. On the other hand, in NPLC using an amino-bonded silica stationary phase and an acetonitrile-water mixture mobile phase, PEO showed normal enthalpy-driven retention behavior: deltaH(o) < 0 and deltaS(o) < 0, with the retention decreasing with increasing temperature and PEO eluting near the column void volume at the enthalpy-entropy compensation point. Therefore, high-resolution temperature gradient NPLC separation of high-molecular-mass PEO samples can be achieved with relative ease. The molecular mass distribution of high-molecular-mass PEO was found to be much narrower than that measured by size-exclusion chromatography.

Characterization of a 4-miktoarm star copolymer of the (PS-b-PI)3 PS type by temperature gradient interaction chromatography

Temperature gradient interaction chromatography (TGIC) was applied for the separation of a complex miktoarm star copolymer which has one polystyrene (PS) arm and three polystyrene-b-polyisoprene (PS-b-PI) diblock copolymer arms. Such miktoarm star polymers are much more difficult to characterize than branched homopolymers since the byproduct, typically polymers with missing arm(s) or coupled products, have not only different molecular weights but also different compositions. TGIC was able to fully separate the byproducts, and the composition of the molecular species corresponding to the different separated elution peaks was determined by two methods, fractionation/NMR and multiple detection (UV and RI). A reasonable agreement between the results of the two methods was obtained. By using the composition found, the corresponding molecular weights were determined by multi-angle light scattering detection. Based on the composition and the molecular weight we were able to identify the structure of the different molecular species.

Preparation and properties of semifluorinated block copolymers of 2-(dimethylamino)ethyl methacrylate and fluorooctyl methacrylates

Semifluorinated block copolymers of poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) and poly(fluorooctyl methacrylates) (PFOMA) were prepared using group transfer polymerisation via sequential monomer addition. Wide ranges of copolymers were obtained with good control over both molecular weight and composition by adjusting the monomers/initiator ratio. The micellar characteristics of the copolymers in water and chloroform were investigated by quasi-elastic light scattering and transmission electron microscopy. The size and morphologies of micelles were greatly influenced by copolymer composition, pH, and temperature. In addition, the solubility of copolymers and the formation of water-in-carbon dioxide (W/C) microemulsions were described in terms of the cloud points. The block copolymers exhibited the excellent ability of stabilizing W/C microemulsions.

Temperature gradient interaction chromatography and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry analysis of air terminated polystyryllithium

The reaction products of polystyryllithium with air were characterized by size-exclusion chromatography, temperature gradient interaction chromatography and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Polystyryllithium was prepared by anionic polymerization of styrene initiated with sec-butyllithium in cyclohexane under an Ar atmosphere. It was confirmed that polystyryl ketone, polystyryl alcohol, and directly coupled polystyrene were the major products in addition to the normally terminated polystyrene, which is consistent with the results in the literature. We could also identify the presence of methoxy and carboxylic acid end capped polystyrenes as well as dipolystyryl ether as minor products. Among the minor products, dipolystyryl ether has not been reported yet.