Compatibilization of elastomer-based blends

The reactive compatibilization of ethylene-propylene-diene (EPDM)-based dissimilar elastomer blends has been investigated in terms of mechanical properties and swelling degree. The use of mercapto-functionalized copolymers resulted in an improvement of mechanical properties of natural rubber-EPDM blends. The mercapto-groups are able to react with the carbon-carbon double bonds of the high diene rubber, resulting in a good interaction between phases. These interactions were confirmed by the amount of insoluble material obtained in non-vulcanized blends. From dynamic mechanical properties and swelling degree, one can suggest a covulcanization process in these blends cured with sulfur-based system. Blends composed by nitrile rubber with EPDM displayed good results in terms of mechanical properties when mercapto-functionalized EVA was employed instead of functionalized EPDM, probably because of the higher polarity of the former associated to its lower viscosity. Additionally, an improvement on mechanical properties was also achieved by using EPDM functionalized with mercapto or anhydride groups in combination with nitrile rubber functionalized with epoxy or oxazoline groups.     

Conducting Elastomer Blends Based on Nitrile Rubber and Pani.DBSA

Electrically conductive elastomer blends based on polyaniline-dodecylbenzene sulfonic acid (Pani.DBSA) and nitrile rubber (NBR) were prepared by polymerization of aniline in the presence of NBR, using a direct, one-step in situ emulsion polymerization method. At the same PAni content, the conductivity of the in situ emulsion-polymerized blends is higher than that of blends produced by mechanical mixing of both components. In addition, a morphology with the presence of PAni in the form of microtubules was achieved by the in situ process. Stronger interaction between the components were also confirmed by Rheological processing analysis (RPA). The vulcanization process decreases the conductivity of the blends prepared by both methods. The in situ polymerized blends also display higher tensile strength and also higher crosslink density     

Mixing convex-optimization bounds for maximum-entropy sampling

Mathematical Programming - The maximum-entropy sampling problem is a fundamental and challenging combinatorial-optimization problem, with application in spatial statistics. It asks to find a...     

The role of pre-algebraic reasoning within a word-problem intervention for third-grade students with mathematics difficulty

ZDM – Mathematics Education - Students in the elementary grades often experience difficulty setting up and solving word problems. Using an equation to represent the structure of the problem...     

Prospect of Thiazole-based γ-Peptide Foldamers in Enamine Catalysis: Exploration of the Nitro-Michael Addition

As three-dimensional folding is prerequisite to biopolymer activity, complex functions may also be achieved through foldamer science. Because of the diversity of sizes, shapes and folding available with synthetic monomers, foldamer frameworks enable a numerous opportunities for designing new generations of catalysts. We herein demonstrate that heterocyclic γ-peptide scaffolds represent a versatile platform for enamine catalysis. One central feature was to determine how the catalytic activity and the transfer of chiral information might be under the control of the conformational behaviours of the oligomer.     

Molecular orientation,crystallinity, and flexural modulus correlations in injection molded polypropylene/talc composites

In order to promote better understanding of the structure‐mechanical properties relationships of filled thermoplastic compounds, the molecular orientation and the degree of crystallinity of injection molded talc‐filled isotactic polypropylene (PP) composites were investigated by X‐ray pole figures and wide‐angle X‐ray diffraction (WAXD). The usual orientation of the filler particles, where the plate planes of talc particles are oriented parallel to the surface of injection molding and influence the orientation of the α‐PP crystallites was observed. The PP crystallites show bimodal orientation in which the c‐ and a*‐axes are mixed oriented to the longitudinal direction (LD) and the b‐axis is oriented to the normal direction (ND). It was found that the preferential b‐axis orientation of PP crystallites increases significantly in the presence of talc particles up to 20 wt% in the composites and then levels‐off at higher filler content. WAXD measurements of the degree of crystallinity through the thickness of injection molded PP/talc composites indicated an increasing gradient of PP matrix crystallinity content from the core to the skin layers of the molded plaques. Also, the bulk PP crystallinity content of the composites, as determined by DSC measurements, increased with talc filler concentration. The bulk crystallinity content of PP matrix and the orientation behavior of the matrix PP crystallites and that of the talc particles in composites are influenced by the presence of the filler content and these three composite's microstructure modification factors influence significantly the flexural moduli and the mechanical stiffness anisotropy data (E_LD/E_TD) of the analyzed PP/talc composites. Copyright © 2009 John Wiley & Sons, Ltd.     

Organocatalyzed Cyclopropanation of α‐Substituted α,β‐Unsaturated Aldehydes: Enantioselective Synthesis of Cyclopropanes Bearing a Chiral Quaternary Center

An enantioselective cyclopropanation of α‐substituted α,β‐unsaturated aldehydes with bromomalonate has been successfully developed through a domino Michael/α‐alkylation strategy. The method allows the efficient formation of cyclopropanes bearing a quaternary carbon stereocenter at the α‐position of the aldehydes by using iminium/enamine catalysis and gives a nice extension on the organocatalytic cyclopropanation of bromomalonate and α,β‐unsaturated aldehydes previously reported by other groups. Very good yields (up to 81 %) and enantioselectivities (up to 97 % ee) have been obtained. The optically active cyclopropane derivatives are of high synthetic interest as useful targets for further elaboration into more complex structures.     

On sparse reflexive generalized inverse

We study sparse generalized inverses $H$ of a rank-$r$ real matrix $A$. We give a construction for reflexive generalized inverses having at most $r^2$ nonzeros. For $r=1$ and for $r=2$ with $A$ nonnegative, we demonstrate how to minimize the (vector) 1-norm over reflexive generalized inverses. For general $r$, we efficiently find reflexive generalized inverses with 1-norm within approximately a factor of $r^2$ of the minimum 1-norm generalized inverse.     

Distribution of chaos and periodic spikes in a three-cell population model of cancer

We study complex oscillations generated by the de Pillis-Radunskaya model of cancer growth, a model including interactions between tumor cells, healthy cells, and activated immune system cells. We report a wide-ranging systematic numerical classification of the oscillatory states and of their relative abundance. The dynamical states of the cell populations are characterized here by two independent and complementary types of stability diagrams: Lyapunov and isospike diagrams. The model is found to display stability phases organized regularly in old and new ways: Apart from the familiar spirals of stability, it displays exceptionally long zig-zag networks and intermixed cascades of two- and three-doubling flanked stability islands previously detected only in feedback systems with delay. In addition, we also characterize the interplay between continuous spike-adding and spike-doubling mechanisms responsible for the unbounded complexification of periodic wave patterns. This article is dedicated to Prof. Hans Jürgen Herrmann on the occasion of his 60th birthday.