Linear and non-linear viscoelastic properties of ethylene vinyl acetate/nano-crystalline cellulose composites

This paper reports on the melt rheological properties of ethylene vinyl acetate containing between 0 and 10 wt.% of nano-crystalline cellulose (NCC). A complete set of rheological tests including frequency sweeps, shear transients, and uniaxial elongations was performed. Frequency sweeps showed that at low frequencies, a pseudo solid-like behavior was obtained for NCC concentrations higher than 5%. This behavior was related to hydrogen bonding between NCC particles and the creation of particle networks as the result of particle–particle interactions. For transient shear tests, all compositions presented a stress overshoot at high shear rates before reaching a steady state. It was found that the amplitude of this overshoot depends on both NCC content and shear rate. On the other hand, the time to reach the maximum was found to be highly shear rate dependent but concentration dependence was rather weak. For uniaxial extensional flow, higher extensional viscosity was observed with increasing NCC content. On the other hand, strain hardening was found to decrease with increasing NCC content.     

Conformational Analysis of N-Alkyl-N-[2-(diphenylphosphoryl)ethyl]amides of Diphenylphosphorylacetic Acid: Dipole Moments,IR Spectroscopy,DFT Study

Experimental and theoretical conformational analysis of N-methyl-N-[2-(diphenylphosphoryl)ethyl]diphenylphosphorylacetamide, N-butyl-N-[2-(diphenylphosphoryl)ethyl]diphenylphosphorylacetamide, and N-octyl-N-[2-(diphenylphosphoryl)ethyl]diphenylphosphorylacetamide was carried out by the methods of dipole moments, IR spectroscopy, and Density Functional Theory (DFT) B3PW91/6-311++G(df,p) calculations. In solution, these N,N-dialkyl substituted bisphosphorylated acetamides exist as a conformational equilibrium of several forms divided into two groups—with Z- or E-configuration of the carbonyl group and alkyl substituent, and syn or anti arrangement of the phosphoryl-containing fragments relative to the amide plane. The substituents at the phosphorus atoms have eclipsed cis- or staggered gauche-orientation relative to the P=O groups, and cis orientation of the substituents is due to the presence of intramolecular H-contacts P=O...H−C_phenyl or p,π conjugation between the phosphoryl group and the phenyl ring. Preferred conformers of acetamides molecules are additionally stabilized by various intramolecular hydrogen contacts with the participation of oxygen atoms of the P=O or C=O groups and hydrogen atoms of the methylene and ethylene bridges, alkyl substituents, and phenyl rings. However, steric factors, such as a flat amide fragment, the bulky phenyl groups, and the configuration of alkyl bridges, make a significant contribution to the realization of preferred conformers.     

17O DOR and other solid-state NMR studies concerning the basic properties of zeolites LSX

We demonstrate complementary ¹H, ¹⁷O, ²⁷Al and ²⁹Si measurements for basic low-silica-X zeolites, which were unloaded and pyrrole and formic acid-loaded. It was found that the acid–base-system is not stabile, if the loading exceeds one pyrrole molecule or two formic acid molecules per supercage.¹⁷O DOR NMR spectra exhibit at least four lines, which are broadened by a distribution of chemical shifts in a similar extend as the ²⁹Si MAS NMR spectra are broadened by distribution of Si–O–Al angles. A strong cation influence upon ¹⁷O shifts was observed. But there was no strong influence of the acid molecules on the mean value of the ¹⁷O shift of the spectra.     

Modulation of the crystallinity of hydrogenated nitrogen-rich graphitic carbon nitrides

An hydrogenated nitrogen-rich graphitic carbon nitride, structurally related to the theoretical graphitic phase of C₃N₄, has been synthesized in a bulk well-crystallized form. This new material was prepared by thermal decomposition of thiosemicarbazide up to 600 °C at ambient pressure under nitrogen flow. Its composition was determined by elemental combustion analysis. Powder X-ray diffraction, infrared spectroscopy, thermogravimetric analysis, X-ray photoelectron spectroscopy and C¹³ MAS NMR characterizations were performed. This material can be schematically described with a two-dimensional framework and a composition close to C₃N_4.17H_1.12. In this nitrogen-rich material, C₃N₃ voids are fully occupied by water molecules which are strongly trapped into the material. A loss of crystallinity associated with a modification of the thermal behavior is observed when the amount of trapped molecules decreases in the graphitic material, order being damaged both between and in the graphitic planes.     

Advanced glycation endproducts induce photocrosslinking and oxidation of bovine lens proteins through type-I mechanism

Advanced glycation endproducts (AGEs) have been suggested as photosensitizers that are capable of mediating eye lens photo-damage during aging. In the present work, we investigate the photo-crosslinking and oxidation of bovine lens proteins sensitized by AGEs, with special regard to low oxygen conditions. A mechanistic study was conducted using different oxygen concentrations and specific additives with the aim either to scavenge or enhance Type-I or Type-II photoprocesses. Quantum yields for Trp decomposition were determined at 5%, 20% and 100% O₂, in the presence of ferricyanide and D₂O to elucidate the mechanism of action of AGEs. Type-I mechanism proved to be the most efficient pathway for AGE-sensitized Trp decomposition at low oxygen concentration. Photocrosslinking of lens proteins and crystallin fractions due to Type-I interaction was observed. The influence of the oxygen concentration and additives was also studied. The results show that both Type-I mechanism and oxygen-mediated reactions contribute to protein crosslinking. Carbonyl group formation due to protein photo-oxidation was detected with Oxyblot technique. The generation of high levels of hydrogen peroxide during the irradiations was detected and attributed mainly to Type-I reactions. The results support that AGEs act preferentially as Type-I sensitizers at the low oxygen concentration found in the lens and are capable of inducing protein crosslinking, oxidation and peroxide formation.     

The -limit sets of a flow and periodic orbits

In this paper we discuss the ω-limit sets of a flow using the Conley theory, chain recurrence and Morse decompositions. Our results generalize and improve the related result in [Schropp J. A reduction principle for ω-limit sets. Z Angew Math Meth 1996;76(6):349–56], and we also show how they can be used as a basis for some new criteria for the existence of periodic orbits.     

Scaling factors for carbon NMR chemical shifts obtained from DFT B3LYP calculations

A linear scaling of the calculated chemical shifts is used in order to improve the accuracy of the DFT predicted ¹³C NMR chemical shifts. The widely applied method of GIAO B3LYP/6-311+G(2d,p) using the B3LYP/6-31G(d) optimized geometries is chosen, which allows cost-effective calculations of the ¹³C chemical shifts in the molecular systems with 100 and more atoms. A set of 27 ¹³C NMR chemical shifts determined experimentally for 22 simple molecules with various functional groups is used in order to determine scaling factors for reproducing experimentally measured values of ¹³C chemical shifts. The results show that the use of a simple relationship (δ_scalc = 0.95 δ_calc + 0.30, where δ_calc and δ_scalc are the calculated and the linearly scaled values of the ¹³C chemical shifts, respectively) allows to achieve a three-fold improvement in mean absolute deviations for 27 chemical shifts considered. To test the universal applicability of the scaling factors derived, we have used complex organic molecules such as taxol and a steroid to demonstrate the significantly improved accuracy of the DFT predicted chemical shifts. This approach also outperforms the recently recommended usage of the Hartree-Fock optimized geometries for the GIAO B3LYP/6-311+G(2d,p) calculations of the ¹³C chemical shifts.     

Effect of fiber content and surface treatment on the mechanical properties of natural fiber composites produced by rotomolding

In this study, natural fibers (agave, coir, and pine) were surface treated with maleated polyethylene (MAPE) with two main objectives: (1) to improve the mechanical properties of natural fiber composites produced by rotational molding and (2) to increase the fiber content in the composite. The rotomolded composites were produced at 0, 10, 20, 30, and 40% wt. of fiber contents (treated or untreated) and characterized in terms of morphology and mechanical properties (hardness, impact, tension, and flexion). The results showed that MAPE surface treatment was more successful for agave and coir than for pine fibers due to their respective chemical composition. In general, surface treatment led to better fiber distribution and a more uniform composite morphology allowing the possibility to use higher fiber contents in rotational molding. At low fiber contents (10 and 20% wt.), the mechanical properties were improved using treated fiber composites (TFC) compared to the neat polymer and untreated fiber composites (UFC). Although the mechanical properties of TFC decreased at high fiber contents (30 and 40% wt.), they were substantially higher (about 160, 400, and 100% for impact, tensile, and flexural properties, respectively) than for UFC.