Yooseong Yang

Effect of Endfunctionality of Reactive Polymers on the Reaction Kinetics at Immiscible Polymer Interfaces: A Monte Carlo Study

We are very grateful to the financial support from the Brain Korea 21 program endorsed by the Ministry of Education and the National Research Laboratory Fund by the Ministry of Science and Technology, Korea.

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.

Effect of dicarboxy terminated polystyrene on strengthening immiscible polystyrene/poly(methyl methacrylate) interface

The fracture toughness between polystyrene (PS)/poly(methyl methacrylate) (PMMA) reinforced with reactive polymers, poly(glycidyl methacrylate) (PGMA) and dicarboxy or monocarboxy terminated PS (dcPS and mcPS), was measured by the asymmetric fracture test. Molecular weight effect of mcPS, although the molecular weight distribution is rather polydisperse, on the maximum achievable fracture toughness, Gmax qualitatively agreed with the results of the monodisperse case4,5). In the case of dcPS with Mw ≅ 142 K, Gmax reached ca. 170 J/m2 which is nearly 8 times higher than that of mcPS of molecular weight of about 150K. From the mechanical point of view, dcPS with a degree of polymerization (N) greater than the ratio of chain breaking force to monomeric friction force (fb/fmono) is more effective in enhancing the interfacial adhesion than mcPS since it provides two stitches to the interface. It was also shown by Monte Carlo simulation on reactive polymer system that the di-endfunctional polymers are more effective than mono-endfunctional polymers in reinforcing the week interface between immiscible polymers.