Atomic Scale Mechanisms Controlling the Oxidation of Polyethylene: A First Principles Study

Understanding the degradation mechanisms of aliphatic polymers by thermal oxidation and radio-oxidation is very important in order to assess their lifetime in a variety of industrial applications. We focus here on polyethylene as a prototypical aliphatic polymer. Kinetic models describing the time evolution of the concentration of chain defects and radicals species in the material identify a relevant step in the formation and subsequent decomposition of transient hydroperoxides species, finally leading to carbonyl defects, in particular ketones. In this paper, we first summarize the most relevant mechanistic paths proposed in the literature for hydroperoxide formation and decomposition and, second, revisit them using first principles calculations based on Density Functional Theory (DFT). Our results partially confirm commonly accepted reaction energies, but also propose alternative, more favourable, reaction paths. We highlight the influence of the environment—crystalline or not—on the outcome of some of the studied chemical reactions. A remarkable result of our calculations is that hydroxyl radicals play an important role in the decomposition of hydroperoxides. Based on our findings, it should be possible to improve the set of equations and parameters used in current kinetic simulations of polyethylene radio-oxidation.

Lagrangian Drifter to Identify Ocean Eddy Characteristics

Deterministic–stochastic empirical mode decomposition (EMD) is used to obtain low-frequency (non-diffusive; i.e., background velocity) and high-frequency (diffusive; i.e., eddies) components from a Lagrangian drifter‘s trajectory. Eddy characteristics are determined from the time series of eddy trajectories from individual Lagrangian drifters such as eddy radius, eddy velocity, eddy Rossby number, and the eddy–current kinetic energy ratio. A long-term dataset of the Sound Fixing and Ranging (RAFOS) float time series obtained near the California coast by the Naval Postgraduate School from 1992 to 2004 at depth between 150 and 600 m is used as an example to demonstrate the capability of the deterministic–stochastic EMD.

Laser cooling of ions in a neutral plasma

Laser cooling of a neutral plasma is a challenging task because of the high temperatures typically associated with the plasma state. By using an ultracold neutral plasma created by photoionization of an ultracold atomic gas, we avoid this obstacle and demonstrate laser cooling of ions in a neutral plasma. After 135 microseconds of cooling, we observed a reduction in ion temperature by up to a factor of four, with the temperature reaching as low as 50(4) millikelvin. This pushes laboratory studies of neutral plasmas deeper into the strongly coupled regime, beyond the limits of validity of current kinetic theories for calculating transport properties. The same optical forces also retard the plasma expansion, opening avenues for neutral-plasma confinement and manipulation.

Down-Regulation of the Na+,Cl- Coupled Creatine Transporter CreaT (SLC6A8) by Glycogen Synthase Kinase GSK3ß

Background: The Na+,Cl- coupled creatine transporter CreaT (SLC6A8) is expressed in a variety of tissues including the brain. Genetic defects of CreaT lead to mental retardation with seizures. The present study explored the regulation of CreaT by the ubiquitously expressed glycogen synthase kinase GSK3ß, which contributes to the regulation of neuroexcitation. GSK3ß is phosphorylated and thus inhibited by PKB/Akt. Moreover, GSK3ß is inhibited by the antidepressant lithium. The present study thus further tested for the effects of PKB/Akt and of lithium. Methods: CreaT was expressed in Xenopus laevis oocytes with or without wild-type GSK3ß or inactive K85RGSK3ß. CreaT and GSK3ß were further expressed without and with additional expression of wild type PKB/Akt. Creatine transport in those oocytes was quantified utilizing dual electrode voltage clamp. Results: Electrogenic creatine transport was observed in CreaT expressing oocytes but not in water-injected oocytes. In CreaT expressing oocytes, co-expression of GSK3ß but not of K85RGSK3ß, resulted in a significant decrease of creatine induced current. Kinetic analysis revealed that GSK3ß significantly decreased the maximal creatine transport rate. Exposure of CreaT and GSK3ß expressing oocytes for 24 hours to Lithium was followed by a significant increase of the creatine induced current. The effect of GSK3ß on CreaT was abolished by co-expression of PKB/Akt. Conclusion: GSK3ß down-regulates the creatine transporter CreaT, an effect reversed by treatment with the antidepressant Lithium and by co-expression of PKB/Akt.

Comparison of several kinetic approaches to evaluate the pyrolysis of three Mediterranean forest fuels

The thermal degradation of three Mediterranean forest fuels was studied by using thermogravimetric analysis. The different behaviours were compared and the degradation characteristics were determined for each fuel. Then a kinetic study was performed. Simple modelling of the thermal degradation was carried out with a global scheme. These results were compared with current kinetic models used in forest fire modelling. These methods predict the mass rate evolution globally. However, they do not describe the fluctuations well owing to the different pyrolysis steps. Next, the kinetic parameters were established in function of the degree of conversion with isoconversion methods for each fuel. The results calculated by the Friedman method were compared with the experimental results, showing good agreement. These data, which take into account the degree of conversion and the fuel, can be useful to model the burning rate of forest fuels.

Bioantioxidants: From chemistry to biology

Bioantioxidants, or dietary antioxidants, are dietary substances in food that significantly decrease the adverse effects of reactive oxygen species (ROS) on normal physiological functions in humans. Inhibition of ROS-induced oxidative damages by supplementation of bioantioxidants, the so-called "antioxidant therapy", has become an attractive therapeutic strategy to reduce the risk of ROS-related diseases and has led to flourishing research in the past decade. However, many questions dealing with the correlation between chemical and biological activities, the bioavailability and "non-antioxidant" effects of bioantioxidants are still under debate. This article outlines our current kinetic and mechanistic studies on naturally occurring antioxidants, including vitamin E, green tea polyphenols (GOHs), and resveratrol, as well as their synthetic analogs in micelles, in red blood cells, in low-density lipoprotein (LDL) and in microsomes. The cytotoxicity and apoptosis-inducing activity of these antioxidants against cancer cells were also studied. It was found that there was significant correlation between the chemical and biological antioxidant activities, as well as between the antioxidant activity and the cytotoxic and apoptosis-inducing activities.

Effects of Particle Rotation on the Hydrodynamic Modeling of Segregation in Gas-Fluidized Beds

A multi-fluid Eulerian model has been improved by incorporating particle rotation using a simple kinetic theory for rapid granular flow of slightly frictional spheres. The model was implemented without changing the current kinetic theory framework by introducing an effective coefficient of restitution to account for additional energy dissipation due to frictional collisions. Simulations with and without particle rotation were performed to study the bubble dynamics in a monodispersed gas-solid fluidized bed and the segregation phenomena in a bidispersed gas-solid fluidized bed. Results were compared between simulations with and without particle rotation and with corresponding experimental results. It was found that the multi-fluid model with particle rotation better captures the bubble dynamics and time-averaged bed behavior. The model predicted lower segregation percentages and a lower segregation rate due to increased bubble intensity.