Doo-Jin Byun

Maneuvering the growth of silver nanoplates: use of halide ions to promote vertical growth

The unique shape and crystalline structure of Ag nanoplates provide an interesting model system for investigating the roles of capping agents in controlling the evolution of the nanostructure shape during growth. This article describes a simple approach to the synthesis of Ag nanoplates with well-controlled shapes in which halide ions (including Cl−, Br−, and I−) guide the well-defined kinetically controlled synthetic route. The presence of iodide ions promoted vertical growth in the nanoplate structure, resulting in small thick nanoplates. The time, during the nanoplate growth process, at which iodide ions were added could be adjusted to control the shapes of the Ag nanoplates by controlling the lateral and vertical dimensions of the nanoplates. This shape control method permitted tuning of the localized surface plasmon resonance (LSPR) peaks of the Ag nanoplates over the visible and near-IR regions of the spectrum.

Thermally stimulated current study in γ-ray irradiated low density polyethylene

Abstract Thermally stimulated current and relaxation map analysis (RMA) were used to characterize the low temperature transition of low density polyethylene (LDPE). Four current peaks were observed. The peaks at about −125°C, −25°C, and 90°C are indications of the γ-, β-, α-transition, respectively, and a new, formerly unknown peak at about 50°C will be attributed to the space charge relaxation. The RMA data showed that compensation temperature (Tc) and degree of disorder increased with increasing the γ-ray intensity and decreasing antioxidant content, whereas the compensation time (τc) decreased. It can be concluded that the cooperative molecular motion in LDPE system becomes more active as the γ-ray intensity increases and antioxidant content decreases.

Photoaging behavior of 3-D colloidal photonic crystals

Since most polymers are not inherently stable to light, their photochemical behavior remains a subject of constant interest. In this study, we investigated the photoaging behavior of polymeric colloidal photonic crystal films. As a consequence of photochemical reactions containing chain-scission, photoaging induced morphological changes in the film surface including changes in the size, surface roughness, shape, and packing structure of PS particles. These structural modifications came about the deterioration of optical properties (photonic bandgap) during photoaging. Based on our results and observations, a plausible correlation between the evolutions of the optical properties and the structural modifications during photoaging was established. Finally, the compositional modifications of the surface based on oxygen incorporation behavior caused by the photooxidation process are discussed based on the ATR-FTIR and XPS analyses.

One-Step Synthesis of Hollow Dimpled Polystyrene Microparticles by Dispersion Polymerization

The design and preparation of hollow nonspherical microparticles are of great significance for their potential applications, but the development of a facile synthetic method using only one production step remains a great challenge. In the current work, a new template-free method based on dispersion polymerization was successfully developed to produce anisotropic hollow polystyrene (PS) microparticles in a single step. In the synthesis, ammonium persulfate (APS) played a critical role in the formation and growth of highly uniform and stable hollow PS microparticles. By varying the concentration of APS and that of the stabilizer used, polyvinylpyrrolidone, we were able to control the average size of the PS particles and their degree of concavity. Based on our results and observations, a plausible mechanism for formation of these unusually shaped PS microparticles was proposed.

Deformation behavior of polymeric colloidal photonic crystals induced by UV irradiation

AbstractThe deformation behavior of polymeric colloidal photonic crystals (CPCs) during UV irradiation was investigated. A CPC grown from a polystyrene (PS) colloidal suspension was prepared and exposed to UV light. The UV irradiation resulted in photochemical reactions that induced morphological changes in the surfaces of CPC, including deformations in the sizes, shapes, and packing structures of the PS particles. The crystal morphology evolution resulting from UV irradiation differed significantly from the photoaging behavior observed during exposure to a light source that mimicked natural sunlight. The structural deformations destroyed the optical properties (by deteriorating the photonic bandgap). The UV irradiation induced photooxidation processes that altered the composition of the polymer material surface.