Differential water sorption studies on Kevlar 49 and As-polymerized poly(p-phenylene terephthalamide): determination of water transport properties

Water vapor sorption experiments have been conducted on Kevlar 49 at 30 degrees C over a range of water vapor pressures in 0-90% of saturation and on the as-polymerized form of the material at 30, 45, and 60 degrees C over a series of water vapor pressures of 0-60%, 0-25%, and 0-15%, respectively. For each of the differential steps in water vapor pressure, dynamic uptake curves were generated and analyzed according to a number of different mathematical models, including Fickian, Coaxial cylindrical, and intercalation models. The intercalation model was demonstrated to be the most successful model and considered two time-scales involved in the diffusion process, i.e., a penetrant-diffusive time-scale and a polymer-local-matrix-relaxation time-scale. The success of this model reinforces previously reported adsorption and desorption isotherms which suggested that water may penetrate into the surface layers of the polymer crystallite through a process known as intercalation.     

The Polymorphs of L‐Phenylalanine

The solid‐state structure of the amino acid phenylalanine (Phe) offers a potential key to understanding the behavior of a large class of important aromatic compounds. Obtaining good single crystals is, however, notoriously difficult. The structure of the common polymorph of Phe, form I, was first reported by Weissbuch et al. (as D ‐Phe) in 1990, but the correctness of the published C2 unit cell with two disordered molecules in the asymmetric unit was later questioned and other space groups suggested. The identity of form I of L ‐Phe is here established to be P2₁ with Z′=4, based on data from a well‐diffracting single crystal grown from an acetic acid solution of the amino acid. A second new polymorph, form IV, together with the two recently described forms II and III provide unprecedented information on the structural complexity of this essential amino acid. It is furthermore documented that the racemate, dl ‐Phe, does not grow proper single crystals.     

Partial Regularity for Singular Solutions to the Monge‐Ampère Equation

We prove that solutions to the Monge‐Ampère inequality in ?ⁿ are strictly convex away from a singular set of Hausdorff (n‐1)‐dimensional measure zero. Furthermore, we show this is optimal by constructing solutions to det D²u = 1 with singular set of Hausdorff dimension as close as we like to n‐1. As a consequence we obtain W^2,1 regularity for the Monge‐Ampère equation with bounded right‐hand side and unique continuation for the Monge‐Ampère equation with sufficiently regular right‐hand side. © 2015 Wiley Periodicals, Inc.     

Random Sequential Adsorption on Random Trees

When gas molecules bind to a surface they may do so in such a way that the adsorption of one molecule inhibits the arrival of others. Two models which have frequently been studied are the “dimer model” and the “blocking model”, and rather complete solutions for these are known on fixed tree structures or Bethe lattices. In this paper comparisons are made between the occupation probabilities for vertices between fixed and random trees.     

Space environmental testing of flexible coverglass alternatives based on siloxanes

With the development of thin-film, high-efficient III–V solar cells using the epitaxial lift-off technique, flexible solar panels for space applications can be designed. Besides new deployment options, this also reduces the mass and thus launch costs of a satellite. One requirement for such a flexible panel configuration is the replacement of the brittle coverglass, which shields the solar cells from the harsh space environment, by a flexible alternative. In this work we have tested several compositions of a polysiloxane candidate material for a flexible shielding layer by exposing them to high energy UV and electron radiation at elevated temperatures. It was found that irradiation by electrons with a fluence corresponding to 15 years in space produces little degradation. UV radiation, on the other hand, has a more pronounced impact on the material properties, causing a discolouration of the transparent material and for some compositions even cracking of the samples.     

A Framework for Characterizing Middle School Students' Statistical Thinking

People make use of quantitative information on a daily basis. Professional education organizations for mathematics, science, social studies, and geography recommend that students, as early as middle school, have experience collecting, organizing, representing, and interpreting data. However, research on middle school students' statistical thinking is sparse. A cohesive picture of middle school students' statistical thinking is needed to better inform curriculum developers and classroom teachers. The purpose of this study was to develop and validate a framework for characterizing middle school students' thinking across 4 processes: describing data, organizing and reducing data, representing data, and analyzing and interpreting data. The validation process involved interviewing, individually, 12 students across Grades 6 through 8. Results of the study indicate that students progress through 4 levels of thinking within each statistical process. These levels of thinking were consistent with the cognitive levels postulated in a general developmental model by Biggs and Collis (1991).