Random Re-entry Theory of Polymer Melt Crystallization

Consideration of crystallization kinetics in high molecular weight polymers shows that adjacent re-entry is unlikely in melt crystallization and that sections of individual chains will crystallize concurrently at several sites. Surface nucleation controlled growth models can be set up which do not require adjacent re-entry but are in agreement with observations on growth rates and crystal thicknesses. The predominant process in crystallization with random re-entry is the incorporation into the crystal of a loop of chain which has both ends attached to the crystal surface. This leads to predictions of the crystallinity of quenched, spherulitic polymers. Radii of gyration of chains in the crystalline state can be calculated and are in agreement with neutron scattering results.     

Rate coefficients for the reactions of OH radicals with Methylglyoxal and Acetaldehyde

Rate coefficients have been measured for the reaction of OH radicals with methylglyoxal from 260 to 333 K using the discharge flow technique and laser-induced fluorescence detection of OH. The rate coefficient was found to be (1.32±0.30) × 10^?11 cm³ molecule^?1 s^?1 at room temperature, with a distinct negative temperature dependence (E/R of ?830 ± 300 K). These are the first measurements of the temperature dependence of this reaction. The reaction of OH with acetaldehyde was also investigated, and a rate coefficient of (1.45 ± 0.25) × 10^?11 cm³ molecule^?1 s^?1 was found at room temperature, in accord with recent studies. Experiments in which O₂ was added to the flow showed regeneration of OH following the reaction of CH₃CO radicals with O₂. However, chamber experiments at atmospheric pressure using FTIR detection showed no evidence for OH production. FTIR experiments have also been used to investigate the chemistry of the CH₃COCO radical formed by hydrogen abstraction from methylglyoxal. © 1995 John Wiley & Sons, Inc.     

The application of nuclear techniques to high temperature oxidation studies

The¹⁸O(p, α)¹⁵N reaction has been used to determine the¹⁸O distribution in the oxides of a number of alloys which have been oxidised sequentially in natural gas and¹⁸O-labelled gas. A significant feature of all the cases investigated is the penetration of¹⁸O into the oxide formed previously in the natural gas. This penetration may occur in various ways and would be difficult to study by other techniques. The evidence presented here shows that there are subtle differences in oxidation behaviour and confirms that penetration of oxygen is a feature in the oxidation of alloys in which the predominant mechanism is transport of cations. This may have implications in explanations given to account for oxidation kinetics and adhesion and buckling of scales.     

Simultaneous determination of the ground level abundances of N2O,CO2, CO and H2O

The AFCRL atmospheric line-parameter listing has been used with a non-linear, least-squares method of analysis to obtain the abundances of N₂O and CO in a sample of ground level air with a precision of about 1%. Absorption coefficients calculated for N₂O agree satisfactory with laboratory measurements but an error of 0.0267 cm^-1 in the listed position of an H₂O line at 2205.250 cm^-1 has been corrected and errors in the positions and intensities of CO₂ lines between 2230 and 2250 cm^-1 have been observed.     

Crystallization of polyethylene at high pressure: A kinetic view

A model for the crystallization kinetics of polymers is outlined and is used to interpret observations of the crystallization of polyethylene at high pressures. This model introduces a distinction between σ_e the lamellar surface energy which controls the lamellar thickness, and σ_e′, the surface nucleus surface energy which controls the growth rate. Differential scanning calorimetry and electron microscopy data for several polyethylenes crystallized at pressures of up to 8 kb are presented. From the dependence of lamellar thickness on the crystallization undercooling at 5 kb, it is found that σ_e increases markedly with pressure leading to the formation of very thick crystals at high pressures. The magnitude of the increase in σ_e is in agreement with σ_e values calculated from the dependence of melting temperatures on pressure. The nucleus surface energy σ_e′ is not expected to vary significantly with pressure, and estimates of growth rates of 5 kb which indicate that the growth rate does not vary significantly with pressure at constant under-cooling confirm this. Fractionation effects and the differences in behavior between different polyethylenes are also discussed.     

Excited ions in intense femtosecond laser pulses: laser-induced recombination

Electron-ion recombination in a laser-induced electron recollision is of fundamental importance as the underlying mechanism responsible for the generation of high-harmonic radiation and hence for the production of attosecond pulse trains in the extreme ultraviolet and soft x-ray spectral regions. By using an ion beam target, remotely prepared to be partially in long-lived excited states, the recombination process has for the first time been directly observed and studied.