Boknam Chae

Sequence of Rubbing-Induced Molecular Segmental Reorientations in the Nanoscale Film Surface of a Brush Polymer Rod

Poly(p-phenylene-3,6-bis(4-(n-butoxy)phenyloxy)pyromellitimide) (C4-PMDA-PDA PI), a well-defined model brush polymer composed of a rodlike polymer backbone with two bristles per repeat unit, was the first reported polyimide to align liquid crystals perpendicular to the rubbing direction at the rubbed film surface. In the present study, we used polarized infrared (IR) spectroscopy and 2D correlation analyses of the resulting IR spectra to study nanoscale films of C4-PMDA-PDA PI rubbed at various rubbing densities. The results of these studies allowed us to establish the nature and sequence of the rubbing-induced segmental reorientations that occur in the polymer molecules at the film surface. The rubbing process was found to reorient the fully rodlike polymer backbones and the n-butyl bristle end groups such that they lay parallel to the rubbing direction. In contrast, rubbing caused the phenyloxy bristle units to reorient to a direction perpendicular to the rubbing direction. These reorientations of the polymers main chain and bristles became more pronounced with increasing rubbing density, and the rubbing process had a greater effect on the polymers main chains than on the bristles. The rubbing-induced reorientations of the polymer segments were found to follow the sequence PDA (phenyl ring), imide ring, phenyloxy unit, imide C-N bond, and n-butyl group. It was additionally evident that the rubbing process reorients the imide rings biaxially, that is, both along the rubbing direction and out of the plane. This biaxial reorientation was found to be accompanied by a biaxial reorientation of the bristles chemically bonded to the PMDA unit that includes the imide rings. In particular, increasing the rubbing density enhanced the out-of-plane reorientation of the imide rings. In contrast, no rubbing-induced inclination of the reoriented imide rings (i.e., the polymers main chains) was detected.

Rubbed Thin Films of Well-defined Brush Polyimides for Flat-Panel Liquid Crystal Displays: Surface Morphology, Molecular Orientation, and Liquid Crystal Alignability

A series of well-defined brush polyimide (PI) composed of two 4-n-alkyloxyphenyloxy bristles per repeat unit on a semi-rigid poly(4,4′-methylenyldiphenylene pyromellitimide), Cm-PMDA-MDA PIs, were synthesized and their nanoscale thin films prepared by conventional spin-coating of their soluble poly(amic acid) precursor solutions and subsequent drying and thermal imidization in a nitrogen atmosphere. All the PIs were determined to be a positively birefringent polymer. The surface morphology and molecular orientation of each PI in films before and after rubbing were investigated in detail by atomic force microscopy, optical retardation analysis, and linearly polarized infrared spectroscopy. The sequence of the rubbing-induced polymer segmental orientations was further investigated in detail. In addition, the liquid crystal alignment and pretilt ability of the rubbed PI films were examined, and their thermal stability investigated. The present study provides important information on the sequence of the polymer segmental orientations induced by rubbing and additionally the mechanisms of the alignment and pretilt of liquid crystal molecules in contact with the rubbed PI film surface.

Molecular orientation of new photosensitive polyesters for liquid crystal alignment

Abstract New polyesters with the main-chain cinnamoyl group, but with the different length of alkyl side-chain have been investigated their photoreactivity and photoalignment characteristics using UV–VIS absorption and FTIR spectra. In the case of Cn-polyesters, chromophores in cinnamoyl group are found to involve in the (2+2) cycloaddition reaction. The remained chromophores demonstrate a preferred orientation in the direction perpendicular to the exposure direction of the linearly polarized UV light, whereas photodimers do not display any anisotropy.

A study of threshold switching of NbO2 using atom probe tomography and transmission electron microscopy.

Threshold switching is a phenomenon where the resistivity of an insulating material changes and the insulator exhibits metallic behavior. This could be explained by phase transformation in oxide materials; however, this behavior is also seen in amorphous insulators. In this study, through an ex-situ experiment using transmission electron microscopy (TEM), we proved that threshold switching of amorphous NbO2 accompanies local crystallization. The change in I-V characteristics after electroforming was examined by evaluating the concentration profile. Atom probe tomography (APT) combined with in-situ TEM probing technique was performed to understand the threshold switching in amorphous NbO2. The local crystallization in amorphous NbO2 was validated by the observed difference in time-of-flight (ToF) between amorphous and crystalline NbO2. We concluded that the slower ToF of amorphous NbO2 (a-NbO2) compared with crystalline NbO2 (c-NbO2) is due to the resistivity difference and trap-assisted recombination.

Two-dimensional correlation analysis study of the photo-induced molecular reorientations of photosensitive polyester film containing 1,4-phenylenediacryloyl units in the backbone.

The photochemical reaction and molecular reorientation of a novel photosensitive polyester, poly[oxy(4-n-butyl-3,5-benzoate)oxy-1,4-phenylenediacryloyl] (PPDA-C4BZ), which contains n-butyl side groups and 1,4-phenylenediacryloyl units (PDA chromophores) in the main chain, are reported in detail. We applied two-dimensional (2D) correlation analysis for the infrared (IR) and ultraviolet (UV) absorption spectra of nanoscaled films of PPDA-C4BZ to establish the sequence of the photo-induced segmental reorientations that result from UV irradiation. The photochemical reaction was found to have a greater effect on the polymers main chains than on its side groups and to induce the reorientation of the polymer molecules. In particular, a cycloaddition process occurs first in the PDA chromophore units and then the local reorientation of the polymer molecules is induced. Namely, such photodimerization of the PDA chromophores induces the molecular reorientations of the PDA chromophores and the benzoate units in the main chain. The photo-induced molecular reorientations occur in the following sequence: photodimerization → benzoate units → PDA chromophores → n-butyl side groups. In addition, a two-dimensional map of the first derivatives of the UV absorption spectra with respect to the exposure energy provided evidence of the formation of head-to-head aggregates (i.e., H-aggregates) of PPDA-C4BZ molecules.

Understanding the structural differences between spherical and rod-shaped human insulin nanoparticles produced by supercritical fluids precipitation.

In this study, the thermal denaturation mechanism and secondary structures of two types of human insulin nanoparticles produced by a process of solution-enhanced dispersion by supercritical fluids using dimethyl sulfoxide (DMSO) and ethanol (EtOH) solutions of insulin are investigated using spectroscopic approaches and molecular dynamics calculations. First, the temperature-dependent IR spectra of spherical and rod-shaped insulin nanoparticles prepared from DMSO and EtOH solution, respectively, are analyzed using principal component analysis (PCA) and 2D correlation spectroscopy to obtain a deeper understanding of the molecular structures and thermal behavior of the two insulin particle shapes. All-atom molecular dynamics (AAMD) calculations are performed to investigate the influence of the solvent molecules on the production of the insulin nanoparticles and to elucidate the geometric differences between the two types of nanoparticles. The results of the PCA, the 2D correlation spectroscopic analysis, and the AAMD calculations clearly reveal that the thermal denaturation mechanisms and the degrees of hydrogen bonding in the spherical and rod-shaped insulin nanoparticles are different. The polarity of the solvent might not alter the structure or function of the insulin produced, but the solvent polarity does influence the synthesis of different shapes of insulin nanoparticles.

Two strategies for the self-assembly of gold nanoparticles: Photoreaction and radical reaction

Gold nanoparticles modified with a newly synthesized cinnamate moiety (CI-AuNPs) were prepared using the phase method. The cinnamate moiety, which is well known for its photoreactive properties, could be reacted by means of radical attack and UV irradiation. Crosslinking of this moiety by the two reactions causes aggregation of the CI-AuNPs, which demonstrates the feasibility of fabricating self-assembled spherical structures by means of chemical crosslinking of gold nanoparticles through two distinct mechanisms, namely, photocycloaddition and radical reaction upon addition of an initiator. Both methods yielded monodispersed spherical CI-AuNPs assemblies, and especially the radical-induced system showed the ability to construct linear-chain structures. Here, we suggest the preparation of gold nanoparticles modified with a simply synthesized photoreactive group and their self-assembly via two selective strategies (photoreaction and radical reaction). We expect that this study will contribute to the strategic fabrication of self-assembled nanostructures.

Preparation and Characterization of Transparent Polyimide/Silica Composite Films by a Sol-Gel Reaction

A series of transparent, 100 μm thick, polyimide (PI) composite films were prepared by a sol-gel reaction using a thermal imidization process between poly(amic acid) (PAA) and various amounts of tetraethyl orthosilicate (TEOS) in N,N-dimethylacetamide. The PAA were prepared using 4,4’-(hexafluoroisopropylidene) diphthalic anhydride (6FDA), 2,2’-bis(trifluoromethyl)benzidine (TFMB) and 3,5-diaminobenzoic acid (DABA). The chemical structure and thermal stability of the PI/silica composite films were examined in detail by Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). The PI/silica composite films showed high thermal stability, and the degradation behavior was found to be dependent on the amount used. In addition, the optical and thermo-mechanical properties of the films were investigated, including the haze values, yellow indices and in-plane coefficients of thermal expansion (CTE). The CTE values of the PI/silica composite films were 52.0 - 43.5 ppm/°C, and were dependent on the amount of TEOS used. The haze values and yellow indices increased with increasing TEOS concentration. Optically transparent PI/silica composite films were obtained with a TEOS content of up to 20 wt%.

Liquid Crystal Alignment Properties on Polyamide Films Bearing a Phenylenediacryloyl Moiety in the Main Chain

Soluble photosensitive polyamide, which has a photoreactive 1,4-phenylenediacryloyl (PDA) moiety in the main chain with biphenyl side groups, was synthesized with high molecular weights. The polymer produced high quality films through conventional spin-casting and dry processing with relatively good thermal stability. The photochemical reactions of the polyamide in the films were investigated by UV-Vis spectroscopy; the polymer in the film demonstrated excellent photoreactivity to UV light. Linearly polarized ultra-violet light (LPUVL) irradiation induced anisotropic reorientations of the polymer chains in the film as a result of direction-selective photoreaction of the PDA moieties. The polyamide thin films were found to have excellent unidirectional orientation ability as a result of photo-exposure with LPUVL; the direction-selective photoreaction of the PDA moiety in the main chains induced alignment of nematic liquid-crystals (LCs) on the surface. The pretilt behaviors of the LC molecules on the LPUVL irradiated film surfaces were controlled by both the exposure dose of LPUVL and the annealing temperature.

Comparison of Liquid Crystal Alignments on Rubbed and Linearly Polarized UV-Irradiated Polyimide Surfaces

Liquid crystals’ surface morphologies, molecular reorientation, alignment, and nematic pretilt angles on polyimide surfaces were studied. The polyimide molecules contained cinnamoyl side groups; they were treated by irradiation with linearly polarized ultra-violet light or rubbing. Root-mean-square surface roughnesses over 1.0 μm × 1.0 μm areas were 0.282, 0.260, and 2.073 nm for untreated, irradiated (1.0 J/cm2), and rubbed (strength, 65 cm) films, respectively. The optical axis of the irradiated film was perpendicular to the lights direction of polarization; in the rubbed film it was parallel to the rubbing direction. Rubbing resulted in higher retardation than irradiation. The orientations of liquid crystal molecules on the films coincided with their polymer chain orientations. Fabricating LC cells with rubbed film resulted in higher pretilt angles than using irradiated film.