Release characteristics of selected carbon nanotube polymer composites

Multi-walled carbon nanotubes (MWCNTs) are commonly used in polymer formulations to improve strength, conductivity, and other attributes. A developing concern is the potential for carbon nanotube polymer nanocomposites to release nanoparticles into the environment as the polymer matrix degrades or is mechanically stressed. Here, we review characteristics related to release potential of five sets of polymer systems: epoxy, polyamide, polyurethane, polyethylene, and polycarbonate. Our review includes consideration of general characteristics and use of the polymer (as related to potential MWCNT release) and its MWCNT composites; general potential for nanomaterial release (particularly MWCNTs) due to degradation and mechanical stresses during use; and potential effects of stabilizers and plasticizers on polymer degradation. We examine UV degradation, temperature extremes, acid-base catalysis, and stresses such as sanding. Based on a high-level summary of the characteristics considered, the potential for release of MWCNT with typical, intended consumer use is expected to be low. © 2013 Elsevier Ltd. All rights reserved

Differential Polarization of Spin and Charge Density in Substituted Phenoxy Radicals

We have carried out a theoretical study of a series of para-substituted phenoxy radicals in an effort to understand the factors influencing spin and charge density distribution in open-shell systems. The calculations reveal that the distribution of spin and charge are not correlated: cases were found for which spin and charge move together, whereas for other substituents the two quantities exhibit spatially distinct intramolecular polarizations. Charge density variations across the series were found to correlate well with both the Hammett (σp) and Hammett-Brown (σp+) constants for each substituent, indicating that inductive and/or resonance effects are primarily responsible for the polarization of charge within the molecule. In contrast, the distribution of unpaired spin density could not be adequately accounted for using any of the typical Hammett-type spin delocalization constants cited in the literature. We uncovered an empirical correlation between the polarization of spin density and the R-HOMO-R-LUMO gap of the substituted phenoxy radicals: this led to the development of a simple model based on a three-electron, two-orbital bonding scheme in which mixing between the HOMO of the substituent and the SOMO of the phenoxy moiety serves to define the nature and extent of unpaired spin polarization throughout the molecule. This analysis yielded a correlation coefficient of r> 0.97 for the 15 substituents examined in the study; spin polarization effects in compounds that exhibited the greatest deviation from this correlation could also be readily explained within the context of the model. The underlying reaso

A Simple Method for Measuring Ground-Level Ozone in the Atmosphere

A standard method for measuring the ozone concentration in the lower atmosphere has been adapted for high school and college chemistry students. A simple sampling apparatus is used to bubble air through a buffered potassium iodide solution. Atmospheric ozone oxidizes iodide to produce triiodide. At the end of the sampling period, starch is added to the buffered KI solution to form a violet-colored complex with triiodide. The concentration of the complex is determined with an inexpensive homemade photometer by measuring the absorption of green light. Atmospheric ozone concentration is calculated from the volume of air sampled and the triiodide concentration. Ozone levels determined with this method agree with concentrations reported from nearby monitoring stations. This laboratory experiment applies several different fundamental concepts including oxidation–reduction chemistry, the ideal gas law, and spectroscopic analysis. In addition, students are provided with unique experiences in constructing simple instrumentation and collecting environmental samples