Constraint release effects on the dynamics of polymer melts

The dynamical behavior of polymers with molecular weight distribution is analyzed from the standpoint of reptation and tube renewal. In a binary blend where the entanglements between longer chains are prominent, the shorter chain relaxes only by reptation, whereas the longer chain shows more complex behavior, i.e., reptation in the original tube, short-range tube renewal causing the tube enlargement, and thereafter reptation again in the expanded tube. Dynamic moduli data from literature are used for determining the compositional dependence of the relaxation times. Also on the basis of the relaxation mechanism considered here, the critical composition due to the onset of entanglements between different longer chains is proposed as a function of their component molecular weighl ratio.     

Control of the photorefractive effect using a low Tg sol-gel glass

Summary We prepared the photorefractive sol-gel glass based on organic-inorganic hybrid materials containing a charge transporting molecule, second-order nonlinear optical chromophore, photosensitizer, and plasticizer. Carbazole and 2-{ 4-[(2-hydroxyethyl) methylamino]benzylidene}malononitrile were reacted with 3-isocyanatopropyl triethoxysilane and the functionalized silanes were employed to fabricate the efficient photorefractive media including 2,4,7-trinitrofluorenone (TNF) to form a charge transfer complex. The simplest way to vary the composition in the matrix was to mix the desired amount of the functionalized alkoxysilane. The prepared sol-gel glass samples contained a large amount of nonlinear optical (NLO) chromophore compared to that of the charge transporting molecules. They showed a large net gain coefficient and high diffraction efficiency at certain conditions. Received:27 May 2002/Revised version:23 August 2002/Accepted:7 September 2002 Correspondence to Dong Hoon Choi E-mail: dhchoi@khu.ac.kr     

Rotational diffusion coefficient of rod-like polymer with a slight flexibility in semidilute and concentrated solutions

Summary A confined stiff chain model is suggested for the prediction of the rotational diffusion coefficient of a rigid rodlike polymer with a slight flexibility above the region of dilute solution (c1/L³). It shows a fairly good agreement with the experimental data of various polymers. Among them, PBLG and PBT with more rigidity are more consistent with the model when the log-jamming effect is considered. The predicted rotational diffusivity shows approximately the inverse seventh-power of length, which is less than 9 of Doi-Edwards tube model, but larger than the experimental value 5.7 of M-virus, while it shows the inverse power of concentration is a little larger than the value 2 of tube model except for the rodlike virus M-13.     

Enhanced surface morphologies of screen-printed carbon nanotube films by heat treatment and their field-emission properties

A heating process for obtaining free-standing carbon nanotube emitters is presented with the aim of improving field-emission properties from the screen-printed multiwalled carbon nanotube (MWCNT) films. Using an atmosphere with an optimum combination of nitrogen and air for heat treatment of CNT films, the CNT emitters can be made to protrude from the surface. This allows for a high emission current and the formation of very uniform emission sites without special surface treatment. The morphological change of the CNT film by this technique has eliminated additional processing steps, such as surface treatment which may result in secondary contamination and damage to the film. Despite its simplicity the process provides a high reproducibility in emission current density which makes the films suitable for practical applications.     

Preparation and characterization of polypropylene/layered silicate nanocomposites using an antioxidant

Polypropylene (PP)/layered silicate nanocomposites were prepared via simple melt mixing of three components, PP, layered silicates modified with octadecylamine (C18-MMT) and antioxidant, to investigate the role of antioxidant. TEM and X-ray scattering results confirmed the intercalated state of silicates in PP/layered silicate nanocomposites with antioxidant. In rheological and mechanical study, the nanocomposites with antioxidant showed higher properties than those of the unfilled PP. The nanocomposite with 5 wt% C18-MMT and 0.5 phr antioxidant exhibited about 1.4 times higher tensile modulus and 1.3 times higher storage modulus than the unfilled PP. However, PP/C18-MMT without antioxidant showed lower rheological values owing to the thermal decomposition of PP and the poor compatibility between PP and C18-MMT. It could be concluded that antioxidants played an important role in enhancing the compatibility between PP and C18-MMT. According to the real time X-ray diffraction, the nanocomposite showed the weak ordering of PP crystals than the unfilled PP in the load-extension plateau region of elongation.     

Phase Relationships in the ZrO2-Sc2O3 and ZrO2-In2O3 Systems

Zirconia-rich subsolidus phase relationships in the ZrO₂–Sc₂O₃ and ZrO₂–In₂O₃ systems were investigated. Phase inconsistencies in the ZrO₂–Sc₂O₃ system resulted from a diffusionless cubic-to-tetragonal ( t' ) phase transformation not being recognized in the past. Through three different measuring techniques, along with microstructural observations, the solubility limits of the tetragonal and cubic phases were determined.     

A kinetic model for polystyrene (PS) pyrolysis reaction

A mathematical model for the pyrolysis reaction of polystyrene (PS) in a semi-batch reactor has been presented. The thermal degradation of PS was flexibly modeled by a combination of random and specific chain-end scissions. Numerical simulation was used to investigate the effect of operating conditions on the PS products spectrum, the results of which were validated by the experimental data. It was found that as the reaction temperature increased (decreased), the monomer fraction in the products became lower (higher) while the trimer higher (lower). No significant variation in the product composition was, however, observed while constant temperature was maintained. These results indicate the reaction temperature is an effective manipulated variable for the control of products composition of PS pyrolysis. The calculation of the optimum temperature trajectories through the optimization study can thus be of interest for achieving productivity enhancement in plastics pyrolysis processes. This paper is dedicated to Professor Dong Sup Doh on the occasion of his retirement from Korea University.     

Intricate CSTR dynamics

Various dynamical characters of continuous stirred tank reaclors (CSTR) are introduced with respect to the effects of reaction types, extra thermal capacitance, periodic forcing, and coupling of CSTRs. The subject includes the classical dynamics of two-dimensional model and the variety of complex dynamics in three or higher dimensional systems such as periodic bifurcations to torus or chaos, aperiodic oscillations on invariant torus, and universal dynamics of alternating periodic-chaotic sequences with k.2^v-cycles for every natural number k. Particularly this review intends to bring about the problems that the engineers must be prepared to encounter in solving various physical systems including chemically reacting systems.     

The European,Japanese and US protective helmet,gloves and boots for firefighters: thermoregulatory and psychological evaluations

The purpose of this study was to investigate the physiological and subjective responses of the European, Japanese (JPN) and US firefighters' helmet, gloves and boots for international standardisation. Three experimental conditions were evaluated (clothing mass: 9.4, 8.2 and 10.1 kg for the three conditions, respectively) at the air temperature of 32°C and 60% relative humidity. The results showed that there was no significant difference among the three conditions in oxygen consumption, heart rate, total sweat rate, rectal temperature and mean skin temperature, whereas peripheral temperatures and subjective perceptions were lower in the JPN condition than in the other conditions (P < 0.05). These results indicate that a 0.5-kg reduction in helmet mass and a 1.1-kg reduction in boot mass during exercise resulted in a significant decrease in head and leg temperatures and subjective perceptions, while a 1.9-kg reduction in total clothing mass had insignificant influences on the metabolic burden and overall body temperature.     

Highly Efficient Overall Water Splitting Through Optimization of Preparation and Operation Conditions of Layered Perovskite Photocatalysts

The layered perovskite materials were found to give the high photocatalytic activity in water splitting reaction under UV irradiation, where the electronic structure of perovskite slab constructing the layered structure (the total cation valency) was the most crucial factor to the high photocatalytic activity. Both the excessive cation valency and the layered structure were required for active photocatalysts, while the slab thickness of layered perovskites had an insignificant effect on water-splitting activity. In order to identify key variables that affected photocatalytic activity and to optimize the performance of (110) layered perovskite, La₂Ti₂O₇ was modified by various methods. The optimum amount of loaded nickel had a great effect and the amount depended on the surface area of the perovskite phase. When an alkaline-earth element such as Ba, Sr, and Ca was doped on La₂Ti₂O₇, the photocatalytic activity was enhanced markedly. Introduction of an alkaline hydroxide into the reaction system as an external additive enhanced the activity further showing extremely high quantum yields close to 50%.