Relaxation processes and structure of ultradrawn polyethylene

Solid-state extruded polyethylene fibers have been prepared, with a wide range of draw ratios and constant processing temperature. The draw ratios vary from 4 up to 30, and the processing temperature was always 398 K. The extruded material behaves anisotropically, owing to the high degree of chain orientation in the drawing direction. The modulus and linear expansion coefficients in the fiber axis direction have been measured, over a wide temperature range, from 140 K up to 320 K. These two properties are closely related to the degree of structural continuity of the fibers. A fibrous structure model is proposed to explain the temperature effects and the values obtained for the modulus and expansion coefficients, in terms of crystallinity and volumetric fraction of extended-chains structure. At least three relaxation processes can be identified which cause the structural continuity of the fibers to change with temperature.     

Interchange free energy between COO and CH3 or CH2 groups from liquid-solid equilibrium temperatures in binary systems: Alkane + ester

The liquid-solid equilibrium temperatures in the n-tetracosane + methyl octadecanoate, methyl nonadecanoate or ethyl octadecanoate binary systems are determined in order to obtain the interchange parameters between CH₃ or CH₂ and COO groups, by means of the statistics of group interaction.     

Redox and structural properties of mixed-valence models for the active site of the [FeFe]-hydrogenase: progress and challenges

The one-electron oxidations of a series of diiron(I) dithiolato carbonyls were examined to evaluate the factors that affect the oxidation state assignments, structures, and reactivity of these low-molecular weight models for the H ox state of the [FeFe]-hydrogenases. The propanedithiolates Fe 2(S 2C 3H 6)(CO) 3(L)(dppv) (L = CO, PMe 3, P i-Pr 3) oxidize at potentials approximately 180 mV milder than the related ethanedithiolates ( Angew. Chem., Int. Ed. 2007, 46, 6152). The steric clash between the central methylene of the propanedithiolate and the phosphine favors the rotated structure, which forms upon oxidation. Electron Paramagnetic Resonance (EPR) spectra for the mixed-valence cations indicate that the unpaired electron is localized on the Fe(CO)(dppv) center in both [Fe 2(S 2C 3H 6)(CO) 4(dppv)]BF 4 and [Fe 2(S 2C 3H 6)(CO) 3(PMe 3)(dppv)]BF 4, as seen previously for the ethanedithiolate [Fe 2(S 2C 2H 4)(CO) 3(PMe 3)(dppv)]BF 4. For [Fe 2(S 2C n H 2 n )(CO) 3(P i-Pr 3)(dppv)]BF 4; however, the spin is localized on the Fe(CO) 2(P i-Pr 3) center, although the Fe(CO)(dppv) site is rotated in the crystalline state. IR and EPR spectra, as well as redox potentials and density-functional theory (DFT) calculations, suggest that the Fe(CO) 2(P i-Pr 3) site is rotated in solution, driven by steric factors. Analysis of the DFT-computed partial atomic charges for the mixed-valence species shows that the Fe atom featuring a vacant apical coordination position is an electrophilic Fe(I) center. One-electron oxidation of [Fe 2(S 2C 2H 4)(CN)(CO) 3(dppv)] (-) resulted in 2e oxidation of 0.5 equiv to give the mu-cyano derivative [Fe (I) 2(S 2C 2H 4)(CO) 3(dppv)](mu-CN)[Fe (II) 2(S 2C 2H 4)(mu-CO)(CO) 2(CN)(dppv)], which was characterized spectroscopically.     

Insights into the mechanism of electrocatalytic hydrogen evolution mediated by Fe2(S2C3H6)(CO)6: the simplest functional model of the Fe-hydrogenase active site

The di-iron complex Fe2(S2C3H6)(CO)6 (a), one of the simplest functional models of the Fe-hydrogenases active site, is able to electrocatalyze proton reduction. In the present study, the H2 evolving path catalyzed by a has been characterized using density functional theory. It is showed that, in the early stages of the catalytic cycle, a neutral mu-H adduct is formed; monoelectron reduction and subsequent protonation can give rise to a diprotonated neutral species (a-muH-SH), which is characterized by a mu-H group, a protonated sulfur atom, and a CO group bridging the two iron centers, in agreement with experimental IR data indicating the formation of a long-lived mu7-CO species. H2 release from a-muH-SH, and its less stable isomer a-H2 is kinetically unfavorable, while the corresponding monoanionic compounds (a-muH-SH- and a-H2-) are more reactive in terms of dihydrogen evolution, in agreement with experimental data. The key species involved in electrocatalysis have structural features different from the hypothetical intermediates recently proposed to be involved in the enzymatic process, an observation that is possibly correlated with the reduced catalytic efficiency of the biomimetic di-iron assembly.     

Teicoplanin-Based Enantiomeric Separations in CZE Using a Partial Filling Technique

Baseline separation of the enantiomers of a number of negatively charged amino and mandelic acid derivatives was achieved in less than 10 min by capillary electrophoresis in a polyacrylamide coated capillary, using the “partial filling method” (PFM) with submillimolar concentration of Teicoplanin (TE) as the chiral selector. The influence of the charge and concentration of TE, electrolyte solution composition and pH, on the enantioresolution was examined. Further proofs were brought to corroborate the hypothesis that the enantiorecognition takes place at the D-Ala-D-Ala binding site, whose blockade is responsible for the antibacterial activity of glycopeptide antibiotics. While the dependence of the chiral recognition capabilities of TE on electrolyte solution composition and pH could limit its applicability, improved sensitivity, reduction of TE wall adsorption, resulting in a good efficiency, and high cost reduction, due to the very small amount of chiral selector required, were shown as advantages of the PFM adopted in this study.     

Plasticization of corn gluten meal and characterization of the blends

Corn gluten meal has thermoplastic properties enhanced by it's high zein content. The addition of an efficient plasticizer is necessary to lower the glass transition temperature. Homogeneous blends of plasticizer and corn gluten meal can be obtained either by a wet mixing technique or by compounding the two substances in an heated batch mixer. The resulting raw material is suitable for “thermoplastic forming” of low cost biomaterials.