Nam-Goo Kang

Structural and Electrical Characterization of a Block Copolymer‐Based Unipolar Nonvolatile Memory Device

Electronic devices based on a series of synthesized block copolymers are demonstrated. In particular, a block copolymer system with a lamellar structure exhibits unipolar switching behavior. This study provides a simple strategy based on the adjustment of the block ratio in block copolymers to control the polymer morphology and thus the electrical and switching properties of polymer-based memory devices.

Polymer electrophosphorescent device: comparison of phosphorescent dye doped and coordinated systems

Abstract We have synthesized a new polymer, (poly(Ir(ppy)2(2-(4-vinylphenyl)pyridine))-co-vinylcarbazole)) [Ir complex copolymer], containing both carbazole and iridium(III) complex as side groups and fabricated electrophosphorescent polymer light emitting devices. The 1MLCT transition of Ir complex copolymer is almost same position and absorption intensity of 8% Ir(ppy)3 doped PVK. At a lower molar concentration of the Ir complex copolymer, the polymer exhibited emission only from the carbazole units. However, with gradual increase in the concentration of the Ir complex copolymer, emission from the Ir complex became dominant. Present results indicated the energy transfer takes place by the intermolecular energy transfer not the intramolecular energy transfer. From the PL and EL of film state, the emission maximum was observed at 512 nm, due to the radiative decay from the 3MLCT state to the ground state, confirming almost a complete energy transfer from carbazole to Ir complex. The device showed 4.4% maximum external quantum efficiency and 5.0 lm/W power efficiency and peak luminance of 12,900 cd/m2.

Reversible and pH-Sensitive Vesicles from Amphiphilic Homopolymer Poly(2-(4-vinylphenyl)pyridine)†

The self-assembly of amphiphiles in varying sizes is of great importance to material and biomedical sciences due to their spontaneous generation of well-defined structures under thermodynamic equilibrium conditions. Polymeric amphiphiles form stable structures with a broader range of properties than small-molecule surfactants. In addition to long-term stability and enhanced loading capacity for guest molecules, self-assembling systems have demonstrated the capability to promptly and appropriately respond to external stimuli, such as pH, solvent polarity, and temperature. In this regard, amphiphilic block copolymerswithmicrometerto nanometersize aggregated structures warrant further investigation to prove their effectiveness in the encapsulation of a variety of guest molecules and their ability to spontaneously respond to external stimuli. Reversibility of the vesicles of diblock and triblock copolymers with pH has also been recently reported in a selective solvent. In the chemical architecture of block copolymer colloidal aggregates, the polarity of the block polymer segments and solvent plays a major role in defining their physicochemical characteristics. Recently, a new class of graft copolymer of polystyrene or polyacrylamide with miktoarm hydrophilic and hydrophobic substitution has been utilized to form various colloidal aggregates in selective solvents. Thayumanavan et al. demonstrated the effects of miktoarm side-chain length on the formationofnanometer-sizeaggregates fromagraftpolymerof polyacrylamide. The polyacrylamide derivative, in which the nitrogen of the acrylamide functionality and the nonglycine carboxylate functionality are separated by five or seven methylene units, formed reverse micelles. On the other hand,

Unexpected molecular weight effect in polymer nanocomposites

The properties of the interfacial layer between the polymer matrix and nanoparticles largely determine the macroscopic properties of polymer nanocomposites (PNCs). Although the static thickness of the interfacial layer was found to increase with the molecular weight (MW), the influence of MW on segmental relaxation and the glass transition in this layer remains to be explored. In this Letter, we show an unexpected MW dependence of the interfacial properties in PNC with attractive polymer-nanoparticle interactions: the thickness of the interfacial layer with hindered segmental relaxation decreases as MW increases, in sharp contrast to theoretical predictions. Further analyses reveal a reduction in mass density of the interfacial layer with increasing MW, which can elucidate these unexpected dynamic effects. Our observations call for a significant revision of the current understandings of PNCs and suggest interesting ways to tailor their properties.

Thermally cross-linkable hole transporting polymer synthesized by living anionic polymerization for effective electron blocking and reduction of exciton quenching in multilayer polymer light emitting diodes

A new cross-linkable polymer (d-PBAB), which has triphenylamine as the hole transporting moiety and ethynyl group as the thermal cross-linker, was synthesized by a living anionic polymerization and deprotection process. The thermally cross-linked d-PBAB layer had a smooth surface and excellent solvent resistance. The PLED fabricated with cross-linked d-PBAB as hole transporting layer (HTL) showed approximately one and half times higher luminance and four times higher luminous efficiency than those obtained from PLED with PEDOT:PSS. This is ascribed to better abilities of cross-linked d-PBAB to block electrons and prevent exciton quenching than those of PEDOT:PSS at the emitting layer (EML) interface. These results strongly suggested that cross-linked d-PBAB can be a promising material to replace conventional PEDOT:PSS.

Effect of solvent composition on transformation of micelles to vesicles of rod-coil poly(n-hexyl isocyanate-block-2-vinylpyridine) diblock copolymers.

The self-aggregation behavior of an amphiphilic rod-coil block copolymer of poly(n-hexyl isocyanate-block-2-vinylpyridine) (PHIC(189)-b-P2VP(228)) (f(P2VP) = 0.78, M(n) = 24.5K) in a tetrahydrofuran (THF)/water system was examined using dynamic light scattering (DLS), transmission electron microscopy (TEM), and field emission scanning electron microscopy (FE-SEM). The presence of a certain amount of water in the THF-based polymer solution induced a morphological transition from spherical solid micelles to open mouth platelike vesicles. The size of the aggregates increased with an increase in water content in the mixed solvent of THF/water. In the range of 30-40% water, the polymer formed vesicles with an interdigitated architecture of poly(n-hexyl isocyanate) (PHIC) at the center of the membrane and with the poly(2-vinylpyridine) (P2VP) block forming the outer layers and pointing toward the solvent. However, at higher water contents, the thickness of the bilayer increased due to the rearrangement of the vesicle membrane from a flip-flop to a lamellar architecture. After the degradation of the PHIC from the vesicles at basic pH, hollow spherical aggregates remained stable. After removing the THF from the mixed solvent using dialysis, large-sized compound vesicles were formed.

Block Copolymers: Synthesis, Self-Assembly, and Applications

Research on block copolymers (BCPs) has played a critical role in the development of polymer chemistry, with numerous pivotal contributions that have advanced our ability to prepare, characterize, theoretically model, and technologically exploit this class of materials in a myriad of ways in the fields of chemistry, physics, material sciences, and biological and medical sciences. The breathtaking progress has been driven by the advancement in experimental techniques enabling the synthesis and characterization of a wide range of block copolymers with tailored composition, architectures, and properties. In this review, we briefly discussed the recent progress in BCP synthesis, followed by a discussion of the fundamentals of self-assembly of BCPs along with their applications.

Uni-molecular hollow micelles from amphiphilic homopolymer poly(2-(4-vinylphenyl)pyridine).

Amphiphilic homopolymer poly(2-(4-vinylphenyl)pyridine) (PVPPy) forms hollow micelles with uni-molecular thickness in a tetrahydrofuran/water (95/5 v/v) azeotropic solvent. Depending on the pH of the media, the micelles may be transformed to vesicles.

Molecular Level Ordering in Poly(2-vinylpyridine)

The reaction between atactic poly(2-vinylpyridine) and 1,4-dibromobutane leads to formation of long-range 3D molecular ordering in polymer chains mainly because the side group (pyridine) of the polymer chain changes to a syndotactic configuration. This may enable the production of functional molecular devices that operate on a 3D atomic scale.

The fabrication of nanopatterns with Au nanoparticles-embedded micelles via nanoimprint lithography

We fabricated nanopatterns with Au nanoparticles-embedded micelles (Au-micelles) by self-assembly of block copolymers via nanoimprint lithography. The micelle structure prepared by self-assembled block copolymers was used as a template for the synthesis of Au nanoparticles (Au NPs). Au NPs were synthesized in situ inside the micelles of polystyrene-block-poly(2-vinylpyridine) (PS- b-P2VP). Au-micelles were arranged on the trenches of the polymer template, which was imprinted by nanoimprint lithography. The fabrication of line-type and dot-type nanopatterns was carried out by the combined method. In addition, multilayer nanopatterns of the Au-micelles were also proposed.