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Kookheon Char

Publications and source records attributed to Kookheon Char.

14 recordsLinked to original sources
PubMed2006

Transient solidlike behavior near the cylinder/disorder transition in block copolymer solutions.

A nearly symmetric polystyrene-block-polyisoprene diblock copolymer dissolved at a concentration of 40% in styrene-selective solvents exhibited a cylinder-to-disorder transition upon heating. The solvents used were diethyl phthalate (DEP) and 75:25 and 50:50 mixtures of DEP with di-n-butyl phthalate (DBP). In DEP, the most styrene-selective of the three solvents, rheological measurements indicated a distinct plateau in the temperature-dependent elastic modulus across the 8 degrees C interval above the order-disorder transition temperature, T(ODT) = 116 degrees C. Previous small-angle neutron scattering measurements in this regime indicated the equilibrium phase to be a liquidlike solution of approximately spherical micelles. An isothermal frequency sweep in this regime indicated a very long relaxation time. Annealing eventually led to the recovery of liquidlike rheological response, over a time scale of hours. Qualitatively similar phenomena were also observed in 75:25 DEP/DBP and 50:50 DEP/DBP solutions, except the fact that the temperature window of the transient response is narrow and the time scale for the recovery diminishes significantly. Neither small-angle X-ray scattering nor static birefringence gave any clear signature of the transient structure. The structure that leads to the transient rheological response is attributed to micellar congestion due to the slow relaxation of anisotropic micelles into an equilibrium distribution of micelles. Possible origins of the remarkable solvent selectivity dependence are also discussed.

Butadienes↗
PubMed2006

Nanoporous block copolymer micelle/micelle multilayer films with dual optical properties.

We introduce a novel and versatile approach for preparing self-assembled nanoporous multilayered films with tunable optical properties. Protonated polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) and anionic polystyrene-block-poly(acrylic acid) (PS-b-PAA) block copolymer micelles (BCM) were used as building blocks for the layer-by-layer assembly of BCM multilayer films. BCM film growth is governed by electrostatic and hydrogen-bonding interactions between the opposite BCMs. Both film porosity and film thickness are dependent upon the charge density of the micelles, with the porosity of the film controlled by the solution pH and the molecular weight (M(w)) of the constituents. PS(7K)-b-P4VP(28K)/PS(2K)-b-PAA(8K) films prepared at pH 4 (for PS(7K)-b-P4VP(28K)) and pH 6 (for PS(2K)-b-PAA(8K)) are highly nanoporous and antireflective. In contrast, PS(7K)-b-P4VP(28K)/PS(2K)-b-PAA(8K) films assembled at pH 4/4 show a relatively dense surface morphology due to the decreased charge density of PS(2K)-b-PAA(8K). Films formed from BCMs with increased PS block and decreased hydrophilic block (P4VP or PAA) size (e.g., PS(36K)-b-P4VP(12K)/PS(16K)-b-PAA(4K) at pH 4/4) were also nanoporous. This is attributed to a decrease in interdigitation between the adjacent corona shells of the low M(w) BCMs, thus creating more void space between the micelles. Multilayer films with antireflective and photochromic properties were obtained by incorporating a water-insoluble photochromic dye (spiropyran) into the hydrophobic PS core of the BCMs assembled in the films. The optical properties of these films can be modulated by UV irradiation to selectively and reversibly control the transmission of light. Light transmission of higher than 99% was observed with accompanying photochromism in the (PS(7K)-b-P4VP(28K)/PS(2K)-b-PAA(8K)) multilayer films assembled at pH 4/6. Our approach highlights the potential to incorporate a range of materials, ranging from conventional hydrophilic materials with specific interactions to hydrophobic compounds, into the assembled BCMs to yield multifunctional nanoporous films.

Journal Article↗
PubMed2006

Substituent effect on the luminescent properties of a series of deep blue emitting mixed ligand Ir(III) complexes.

The syntheses of the bright deep blue emitting mixed ligand Ir(III) complexes comprising two cyclometalating, one phosphine and one cyano, ligands are reported. In this study, a firm connection between the nature of the excited states and the physicochemical behavior of the complexes with different ligand systems is elucidated by correlating the observed crystal structures, spectroscopic properties, and electrochemical properties with the theoretical results obtained by the density functional theory (DFT) methods. The cyclometalating ligands used here are the anions of 2-(4',6'-difluorophenyl)-pyridine (F2ppy), 2-(4',6'-difluorophenyl)-4-methyl pyridine (F2ppyM), and 4-amino-2-(4',6'-difluorophenyl)-pyridine (DMAF2ppy). The phosphine ligands are PhP(O-(CH2CH2O)3-CH3)2 and Ph2P(O-(CH2CH2O)n-CH3), where Ph = phenyl and n = 1 (P1), 3 (P3), or 8 (P350). The thermal stabilities of the complexes were enhanced upon increasing the "n" value. The crystal structures of the complexes, [(DMAF2ppy)2Ir(P1)CN], (P1)DMA, and [(F2ppyM)2Ir(P3)CN], (P3)F2M, show the cyano and phosphine groups being in a cis configuration to each other and in a trans configuration to the coordinating Cring atoms. The long Ir-Cring bond lengths are ascribed to the trans effect of the strong phosphine and cyano ligands. DFT calculations indicate that the highest occupied molecular orbital (HOMO) is mainly contributed from the d-orbitals of the iridium atom and the pi-orbitals of cyclometalating and cyano ligands, whereas the lowest unoccupied molecular orbital (LUMO) spreads over only one of the cyclometalating ligands, with no contribution from phosphine ligands to both frontier orbitals. Dimethylamino substitution increases the energy of the emitting state that has more metal-to-ligand-charge-transfer (MLCT) character evidenced by the smaller vibronic progressions, smaller difference in the 1MLCT and 3MLCT absorption wavelengths, and higher extinction coefficients (epsilon) than the F2ppy and F2ppyM complexes. However, the increase in the basicity of the dimethylamino group in the DMAF2ppy complexes in the excited states leads to distortions and consequent nonradiative depopulation of the excited states, decreasing their lower photoluminescence (PL) efficiency. The effect of the substituents in the phosphine ligand is more pronounced in the electroluminescence (EL) than in the PL properties. Multilayer organic light emitting devices (OLEDs) are fabricated by doping the Ir(III) complexes in a blend of mCP (m-bis(N-carbazolyl benzene)) and polystyrene, and their device characteristics are studied. The (P3)F2M complex shows a maximum external quantum efficiency (etaex) of 2%, a maximum luminance efficiency (etaL) of 4.13 cd/A at 0.04 mA/cm2, and a maximum brightness of 7200 cd/m2 with a shift of the Commission Internationale de L'Eclairage (CIE) coordinates from (0.14, 0.15) in film PL to (0.19, 0.34) in EL.

Journal Article↗
PubMed2006

Effect of the casting solvent on the morphology of poly(styrene-b-isoprene) diblock copolymer/magnetic nanoparticle mixtures.

We investigate the effect of the casting solvent on the phase behavior of a cylinder-forming poly(styrene-b-isoprene) (PS-b-PI) diblock copolymer mixed with marginally selective gamma-Fe2O3 nanoparticles. Notably, for the mixtures of PS-b-PI/gamma-Fe2O3 nanoparticles cast with toluene, which is a neutral solvent for both PS and PI blocks, the latticelike aggregates of gamma-Fe2O3 nanoparticles, originating from magnetic dipole interactions, induce an intriguing transition from hexagonal cylinders to body-centered cubic spheres via undulated cylinders whereas the neat block copolymer does not show such an order-order transition. In contrast, when PI-selective hexane is used, the nanoparticles are selectively incorporated into the PI domains, and the microphase-separated hexagonal cylinder morphology gradually loses its long-range order upon increasing the nanoparticle concentration. The structural information obtained from small-angle X-ray scattering is also in good agreement with transmission electron microscopy images and differential scanning calorimetry results.

Journal Article↗
PubMed2006

Modulating the pattern quality of micropatterned multilayer films prepared by layer-by-layer self-assembly.

Patterned multilayer films composed of poly(allylamine hydrochloride) (PAH) and poly(sodium 4-styrenesulfonate) (PSS) were prepared using dip and spin self-assembly (SA) methods. A silicon substrate was patterned with a photoresist thin film using conventional photolithography, and PAH/PSS multilayers were then deposited onto the substrate surface using dip or spin SA. For spin SA, the photoresist on the substrate was retained, despite the high centrifugal forces involved in depositing the polyelectrolytes (PEs). The patterned multilayer films were formed by immersing the PE-coated substrates in acetone for 10 min. The effect of ionic strength on the pattern quality in dip and spin multilayer patterns (line-edge definition and surface roughness of the patterned region) was investigated by increasing the salt concentration in the PE solution (range 0-1 M). In dip multilayer patterns, the presence of salt increased the film surface roughness and pattern thickness without any deformation of pattern shape. The spin multilayer patterns formed without salt induced a height profile of about 130 nm at the pattern edge, whereas the patterns formed with high salt content (1 M) were extensively washed off the substrates. Well-defined pattern shapes of spin SA multilayers were obtained at an ionic strength of 0.4 M NaCl. Multilayer patterns prepared using spin SA and lift-off methods at the same ionic strength had a surface roughness of about 2 nm, and those prepared using the dip SA and lift-off method had a surface roughness of about 5 nm. The same process was used to prepare well-defined patterns of organic/metallic multilayer films consisting of PE and gold nanoparticles. The spin SA process yielded patterned multilayer films with various lengths and shapes.

Journal Article↗
PubMed2005

Intercalation mechanism and interlayer structure of hexadecylamines in the confined space of layered alpha-zirconium phosphates.

Well-defined hexadecylamine (HDA) intercalated structures, either interdigitated layers, bilayers, or hybrid layers of both, in a confined space of highly functionalized layered alpha-zirconium phosphates (alpha-ZrPs) have been prepared based on the two-step intercalation mechanism and these distinct intercalated structures can serve as a model system to investigate the interactions of two monolayers whose amphiphilic tails are adjacent to each other. At the first intercalation step, the electrostatic interaction between HDAs and alpha-ZrP is dominant and results in an interdigitated layer structure (d(001) = 3.0 nm) and the interdigitated layer is saturated at around phi = 50%, where phi is the weight fraction of intercalated HDAs in the intergallery of alpha-ZrP. For phi higher than 50%, the bilayer structure (d(001) = 4.3 nm) emerges due to further hydrophobic interaction between HDAs initially grafted to alpha-ZrP and unanchored HDAs and the relative fraction of the bilayer structure over the interdigitated layer increases with the increase in the intercalated amount of HDAs. The intriguing morphology of alpha-ZrP tactoids intercalated with HDAs in coexisting bilayers and interdigitated layers is observed by using microtomed TEM and the two-step intercalation has also been verified with TGA and FT-IR. Also, a structural transition from the bilayers to the interdigitated layers is monitored by using in situ synchrotron WAXS showing that the hydrophobically intercalated HDAs are selectively deintercalated at a relatively low decomposition temperature around 220 degrees C.

Journal Article↗
PubMed2005

Conformational behavior of dodecyldiamine inside the confined space of montmorillonites.

We have investigated the change of the intercalated amount and conformational behavior of dodecyldiamine (C(12)H(28)N(2), di-C(12)amine) inside the confined space of the montmorillonite (MMT) intergallery at different pHs of the intercalation solution (H(2)O/ethanol/Na(+)-MMT/di-C(12)amine), and these results were compared with those of dodecylmonoamine (C(12)H(27)N, mono-C(12)amine). The mono-C(12)amine with one end-functional amine (-NH(2)) has a constant intercalated amount independent of the pH of the intercalation solution, confirming the tail conformation in the MMT intergallery. On the other hand, the intercalated amount of di-C(12)amines remains quite low at low pH due to the long-range Coulombic repulsion among protonated amines. The di-C(12)amines dominantly take the bridge conformation inside the MMT intergallery up to pH 9.5 and adopt a fraction of the tail conformation at pH 11.5, which is verified by Fourier transform infrared spectra with an abrupt frequency shift of the CH(2) stretching band and the emergence of a new NH(2) scissoring band at 1598 cm(-)(1) related to unanchored free amines. The d-spacing and the square of the half-width at half-maximum of the MMT based on X-ray diffraction measurements are also constant up to pH 9.5, while these two variables are suddenly increased at pH 11.5. The bridge conformation of di-C(12)amines in the confined space of the MMT intergallery prevents poly(ethylene oxide) chains from intercalation into the MMT intergallery, called the gluing effect.

Alkylation↗
PubMed2005

Interplay between cubic and hexagonal phases in block copolymer solutions.

The phase behavior of a symmetric styrene-isoprene (SI) diblock copolymer in a styrene-selective solvent, diethylphthalate, was investigated by in situ small-angle X-ray scattering on isotropic and shear-oriented solutions and by rheology and birefringence. A remarkable new feature in this phase diagram is the coexistence of both body-centered cubic (bcc) and hexagonally close-packed (hcp) sphere phases, in a region between close-packed spheres (cps) and hexagonally packed cylinders (hex) over the concentration range phi approximately 0.33-0.45. By focusing on the transitions among these various ordered phases during heating and cooling cycles, we observed a strong hysteresis: supercooled cylinders persisted upon cooling. The stability of these supercooled cylinders is quite dependent on concentration, and for phi > or = 0.40, the supercooled cylinders do not revert to spheres even after quiescent annealing for 1 month. The spontaneous formation of spheres due to the dissociation of cylinders is kinetically hindered in this case, and the system is apparently not amenable to any pretransitional fluctuations of cylinders prior to the cylinder-to-sphere transition. This contrasts with the case of cylinders transforming to spheres upon heating in the melt. The application of large amplitude shear to the supercooled cylinders is effective in restoring the equilibrium sphere phases.

Journal Article↗
PubMed2004

Temperature-dependent micellar structures in poly(styrene-b-isoprene) diblock copolymer solutions near the critical micelle temperature.

The temperature dependence of the micelle structures formed by poly(styrene-b-isoprene) (SI) diblock copolymers in the selective solvents diethyl phthalate (DEP) and tetradecane (C14), which are selective for the PS and PI blocks, respectively, have been investigated by small angle neutron scattering (SANS). Two nearly symmetric SI diblock copolymers, one with a perdeuterated PS block and the other with a perdeuterated PI block, were examined in both DEP and C14. The SANS scattering length density of the solvent was matched closely to either the core or the corona block. The resulting core and corona contrast data were fitted with a detailed model developed by Pedersen and co-workers. The fits provide quantitative information on micellar characteristics such as aggregation number, core size, overall size, solvent fraction in the core, and corona thickness. As temperature increases, the solvent selectivity decreases, leading to substantial solvent swelling of the core and a decrease in the aggregation number and core size. Both core and corona chains are able to relax their conformations near the critical micelle temperature due to a decrease in the interfacial tension, even though the corona chains are always under good solvent conditions.

Journal Article↗
PubMed2004

Effect of layer integrity of spin self-assembled multilayer films on surface wettability.

We investigated the correlation between surface wettability and internal structure of polyelectrolyte (PE)/PE and PE/inorganic multilayer films prepared by the spin self-assembly (SA) method. Spin self-assembled poly(allylamine hydrochloride) (PAH)/poly(sodium 4-styrenesulfonate) (PSS) multilayer films deposited from PE solutions of 10 mM show the distinct oscillation in contact angles with variation of the outermost PE layer, representing the saturated values in contact angles of individual PAH and PSS layers. These contact angles are also well consistent with the angles measured from respective PE layers (i.e., PAH and PSS) of the spin SA (PAH/CdS-COO-) and (CdS-NH3+/PSS) films carrying the flat interface between PE and inorganic CdS nanoparticle layers as confirmed by X-ray reflectivity. Furthermore, based on the contact angle of CdS-NH3+ layer in the ordered (CdS-NH3+/PSS) films, the change in surface wettability of CdS-NH3+ layers of two different spin SA (CdS-NH3+/poly(methacrylic acid) (PMAA)) multilayer films with ordered and disordered internal structure is also investigated. The films with ordered and disordered internal structure were fabricated by the pH adjustment of PMAA. The CdS-NH3+ layer in both CdS-NH3+/PSS and CdS-NH3+/PMAA multilayer films with the ordered internal structure has the contact angle of about 25 +/- 2 degrees irrespective of the PSS or PMAA sublayer. As a result, the same surface wettability of PE or inorganic layers, despite different sublayers, strongly indicates that the spin SA method in optimum condition allows the top surface to be completely covered with a low level ofinterdigitation with a sublayer at each deposition step, and this leads to the conclusion that physical and chemical characteristics of the sublayers have no significant influence on those of the outermost layer.

Journal Article↗
PubMed2004

Origin of the thermoreversible fcc-bcc transition in block copolymer solutions.

The thermoreversible fcc-bcc transition in concentrated block copolymer micellar solutions is shown to be driven by decreases in the aggregation number as the solvent penetrates the core, leading to a softer intermicelle potential. Small-angle neutron scattering measurements in a dilute solution are used to quantify the temperature-dependent micellar characteristics. The observed phase boundary is in excellent agreement with recent simulations of highly branched star polymers.

Journal Article↗
PubMed2004

Gelation of PEO-PLGA-PEO triblock copolymers induced by macroscopic phase separation.

The gelation behavior of aqueous solutions of poly(ethylene oxide-b-(DL-lactic acid-co-glycolic acid)-b-ethylene oxide) (PEO-PLGA-PEO) triblock copolymer containing short hydrophilic PEO end blocks is investigated using dynamic light scattering, rheology, small-angle neutron scattering (SANS), and differential scanning calorimetry (DSC). For polymer concentrations between 5 and 35 wt %, four distinct regions of the turbidity change depending on temperature were observed. Interestingly, in the turbid solution region, gel phase is formed for polymer concentrations above 14 wt % and an extremely slow relaxation was detected. In fact, a power law, which takes into account the dynamics of percolation clusters, dominates the correlation function. In rheological measurements, the local maximum in G' is observed at around the temperature of maximum turbidity. We further found that G" > G' and G' is highly dependent on frequency at the gel state implying viscoelastic characteristics, which is quite different from general concepts of gels, typically formed by the micellar packing. SANS profiles showing multiple peaks in the sol state rather than in the gel state as well as a DSC exotherm at the temperature of gels can also serve as the evidence of different gel states. Based upon the experimental data obtained in the present study, a new gelation mechanism induced by the macroscopic phase separation of triblock copolymers containing short hydrophilic PEO end blocks such as PEO-PLGA-PEO is proposed. The effect of the type ofhydrophobic middle blocks on the gelation is also discussed.

Journal Article↗
PubMed2004

Highly stable mesoporous metal oxides using nano-propping hybrid gemini surfactants.

Noble Gemini surfactants containing a siloxane moiety have been designed and successfully synthesized in the present study and are utilized as structure-directing agents for mesoporous metal oxides such as zirconia, titania, and vanadia. The siloxane moiety is believed to play an important nano-propping role during the surfactant removal by direct calcination, yielding thermally stable mesoporous metal oxides. It is also believed that the synthesis strategy described here can be applied to the synthesis of robust nanostructured materials such as nanoparticles and nanorods in addition to mesoporous materials.

Journal Article↗