Youngsuk Jung

Polyimide–Organosilicate Hybrids with Improved Thermal and Optical Properties

Through hydrolysis and polycondensation of amino-silane with alkyl bridged silane, a new type of polysilsesquioxane (PSSQ) was obtained. Here we use amine functionalized silane and bis(silyl)ethane to synthesize alkyl chain linked PSSQ. Compared to conventional polyhedral oligomeric silsesquioxane (POSS), this new silane compound has both enhanced thermal stability and improved compatibility with poly(amic acid). Gelation of this silane compound with poly(amic acid) provides polyimide-organosilicate composite materials. We show that films made from solutions of the composites exhibit higher optical transparency and superior dimensional stability during thermal treatment than films of pure polyimide or of polyimide composites with conventional POSS. Bridging of POSS and chemical bonding between POSS and polyimide chains significantly enhance the physical properties. These results provide useful information for designing molecular architecture for the fabrication of high-performance plastic substrates in the future display devices.

Charge mobility anisotropy of functionalized pentacenes in organic field effect transistors fabricated by solution processing

To understand and optimize the performance of thin-film electronic devices incorporating crystalline organic semiconductors, it is important to consider the impact of their structural anisotropy on the charge transport. Here we report on the charge mobility anisotropy in 6,13-bis(triisopropylsilylethynyl) (TIPS) and 6,13-bis(triethylsilylethynyl) (TES) pentacene field effect transistors, in which microstructure is controlled by solution processing conditions. Thin-film structures that range from millimetre size, crystalline domains to macroscopic, high-aspect-ratio (∼1 μm wide and >1 cm long) needles are systematically produced by controlling the substrate displacement rate during zone-cast deposition. Through precise control of the microstructure we experimentally explore the differences in charge transport anisotropy between TIPS- and TES-pentacene molecules. Aligned needles of TIPS- pentacene result in a mobility anisotropy (μ∥/μ⊥) of ∼20 (mobility of ∼0.7 cm2 V−1 s−1) whereas TES-pentacene produce an order of magnitude lower mobility (∼0.06 cm2 V−1 s−1) but much higher mobility anisotropy (>45). Such significant changes in absolute mobility and mobility anisotropy are attributed to their different packing structures, which permit 2D charge transport in TIPS-pentacene and 1D transport in TES-pentacene. Bulky TIPS- side groups (diameter ∼7.5 A) force a brick-wall type packing structure, whereas TES- side groups (diameter ∼6.6 A) pack in a 1D slipped-stack. Furthermore, through precise control of the molecular alignment, the impact of crystal orientation on charge transport is investigated. TIPS-pentacene achieves the highest mobility when the angle between the needle long-axis and charge transport directions is ∼35°, whereas in TES-pentacene it is much closer to 0°. These results are supported by theoretical simulations.

Transient color changes in oxidative-stable fluorinated polyimide film for flexible display substrates

Stable optical properties of high transmittance and low yellow index are required for a polyimide film as a flexible display substrate, but thermal processes could result in its color change by thermal imidization. To prevent the color change, anti-oxidants have been used, but as yet though, the effect of oxidation in polyimide has remained unexplored. We explore the yellow index and absorbance changes of fluorinated polyimide after thermal imidization to determine the color change kinetics. Furthermore, we investigated the effect of anti-oxidants on film color change, showing that anti-oxidants even accelerate the color change due to their decomposition. Both experimental findings and density functional theory calculations suggest that the oxidative stability for the thermal imidization could have a modest impact, and in turn yellow index is less dependent on the oxidation of fluorinated polyimide, attributed to mild interaction between fluorinated polyimide and oxygen insufficient to form strong oxidation of diamine groups.

Adhesion of a fluorinated poly(amic acid) with stainless steel surfaces

The authors elucidate an origin and probable mechanism of adhesion strength change at an interface of fluorinated poly(amic acid) and stainless steel. Fluorination provides favorable delamination with release strength weaker than 0.08 N/mm from a metal surface, once the amount of residual solvent becomes less than 35 wt. %. However, the release strength critically depends on film drying temperature. Characterization on stainless steel surfaces and thermodynamic analyses on wet films reveal a drying temperature of 80 °C fosters interaction between the metal oxides at stainless steel surface and the free electron donating groups in poly(amic acid).

Enhanced Electrochemical Stability of a Zwitterionic-Polymer-Functionalized Electrode for Capacitive Deionization

In capacitive deionization, the salt-adsorption capacity of the electrode is critical for the efficient softening of brackish water. To improve the water-deionization capacity, the carbon electrode surface is modified with ion-exchange resins. Herein, we introduce the encapsulation of zwitterionic polymers over activated carbon to provide a resistant barrier that stabilizes the structure of electrode during electrochemical performance and enhances the capacitive deionization efficiency. Compared to conventional activated carbon, the surface-modified activated carbon exhibits significantly enhanced capacitive deionization, with a salt adsorption capacity of ∼2.0 × 10-4 mg/mL and a minimum conductivity of ∼43 μS/cm in the alkali-metal ions solution. Encapsulating the activated-carbon surface increased the number of ions adsorption sites and the surface area of the electrode, which improved the charge separation and deionization efficiency. In addition, the coating layer suppresses side reactions between the electrode and electrolyte, thus providing a stable cyclability. Our experimental findings suggest that the well-distributed coating layer leads to a synergistic effect on the enhanced electrochemical performance. In addition, density functional theory calculation reveals that a favorable binding affinity exists between the alkali-metal ion and zwitterionic polymer, which supports the preferable salt ions adsorption on the coating layer. The results provide useful information for designing more efficient capacitive-deionization electrodes that require high electrochemical stability.

Correlation of stress and optical properties in highly transparent polyimides for future flexible display

High transmittance and low birefringence are desirable optical properties in polyimide films which are promising flexible substrates in next generation display devices. However, thermal processes for the fabrication of polyimide films can cause anisotropic changes in the optical properties due to its rod-like molecular structure. Here we report the changes in optical retardation in transparent fluorinated polyimide films with sub-nanometer resolution and dimensional stability induced by deformation at high temperatures. As deformation is increased, the optical retardation is changed much prominently with enhancing thermo-dimensional stability. During thermal strain, in-plane molecular orientation is preferentially improved and stress-optical coefficient that quantifies the change in out of plane optical retardation is derived to be around 6×10–6 m2/N, which is higher compared to conventional plastic optical films. The experimental findings suggest that optimized process conditions for display substrates should be determined to address both changes in the optical and thermal stabilities. We suggest that this study can be useful information for large-scale film process to be further utilized in fabrication of the transparent polyimide films.

Enhanced competitive adsorption of CO2 and H2 on graphyne: A density functional theory study

Adsorption using carbon-based materials has been established to be a feasible method for separating carbon dioxide and hydrogen to mitigate the emission of carbon dioxide into the atmosphere and for the collection of fuel for energy sources, simultaneously. We carried out density functional theory calculation with dispersion correction to investigate the physisorption characteristics of carbon allotropes such as graphene and graphyne for the competitive adsorption of CO2 and H2. It is worth noting that the graphyne represented preferable adsorption energies, short bond lengths and energy charges for both gases, compared with the characteristics observed with graphene. We found that in graphyne, both the affinitive adsorption of CO2, and the competitive adsorption of CO2 and H2, took place at the hollow site between acetylene links, which do not exist in graphene. We demonstrate that in the presence of H2, the CO2 adsorption selectivity of graphyne is higher than that of graphene, because of the improved e...

Self-standing and shape-memorable UV-curing epoxy polymers for three-dimensional (3D) continuous-filament printing

In the development of three-dimensional printable materials for high-speed and high-resolution printing, UV-curing polymers can guarantee fast and precise printing of high performance load-bearing structures, but the injected drops of the monomers tend to spread over the substrates due to their low viscosity. In this study, we imposed the self-standing and shape-memorable capability of an epoxy acrylate (EA) monomer to ensure continuous filamentary 3D printing while maintaining its low viscosity nature. Using octadecanamide (ODA) with EA, strong hydrogen-bond networks (−N−H⋯OC−, −N−CO⋯H–O–, –N–H⋯N–) were additionally achieved in the material system and the developed material distinctively exhibited rheological duality at different processing stages: a low-viscosity liquid-like behavior (viscosity of ∼50 Pa) while passing through the nozzle and a self-standing solid-like behavior (static yield stress of ∼364 Pa) right after being printed. This reversible liquid-to-solid transitional capability was quantified by viscoelastic complex moduli provided a dynamic yield stress (τy,G) of 210 Pa corresponding to the upright stacking up to ∼3.2 cm (3 wt% of ODA). The time (ty,G) required for conformational rearrangement was evaluated to be as fast as ∼10−2 s. After UV curing, the 3D printed layers exhibited no air pockets or weld lines at the stacked interfaces, which could guarantee excellent mechanical performance and structural integrity.

The Impact of the Dielectric/Semiconductor Interface on Microstructure and Charge Carrier Transport in High-Performance Polythiophene Transistors

The performance of organic field-effect transistors (OFETs) significantly depends on the properties of the interface between the semiconductor and gate dielectric. Here, we study the impact of chemically modified and morphologically controlled dielectrics on the performance of poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT) semiconductors. We find that the molecular packing, domain size, and carrier mobility of pBTTT are highly sensitive to dielectric chemistry and dielectric roughness. The large and well-oriented terraced domains that are the origin of pBTTTs high performance can develop well on certain dielectrics, but can be disrupted on others.