Comparison of solution-blending and melt-intercalation for the preparation of poly(ethylene-co-acrylic acid)/organoclay nanocomposites

Ethylene-acrylic acid copolymers (EAAs) and commercial montmorillonite clays organically modified with dimethyldihydrogenatedtallowammonium ions (Cloisite^® 15A and 20A) were used for the synthesis of nanocomposites by melt-compounding, static melting of polymer/clay mixtures and solution-intercalation in order to compare the effectiveness of these procedures and to shed light on the thermodynamics and the kinetics of the intercalation process. The preparation from solution was made by the use of several solvents, such as toluene, xylene, chloroform, etc., which were then removed from the hybrids by precipitation in different non-solvents or by evaporation. Particular attention was paid to the effect of the thermal treatments which are often used when processing the composites prepared from solution. X-ray diffraction (XRD) of the solution-blended composites showed that no intercalation of the EAAs inside the clay galleries can be achieved if solvent removal is made by precipitation in non-solvents or by room-temperature evaporation. On the contrary, intercalation was found to occur very rapidly (in less than 1 min) when both the hybrids prepared from solution and the mechanical blends of powdered components were melted in the absence of shear. Polymer intercalation was also found to occur, though with a lower rate, upon annealing the powder mixtures at temperatures lower than the EAA melting point. Microscopic observations made by polarized optical microscopy, scanning electron microscopy and transmission electron microscopy showed that the clay particles dispersion is appreciably lower for the composites prepared from solution, compared to those produced in the melt under shear flow conditions. The hybrids obtained by static melting of powder mixtures, on the other side, were expectedly found to comprise micron sized clay agglomerates, although intercalation was demonstrated also for these materials by XRD. The structure of the intercalated silicate layers stacks, characterized by an interlayer spacing of 4.0 nm, was shown to be independent of the preparation procedure and to correspond to thermodynamic equilibrium.     

Very mild, enantioselective synthesis of propargylamines catalyzed by copper(I)-bisimine complexes

The stereoselective addition of aryl- and alkylacetylene derivatives to imines was studied. The reaction is catalyzed by copper complexes of enantiomerically pure bisimines, readily prepared in very high yields from the commercially available binaphthyl diamine. A very simple experimental procedure allowed to obtain at room temperature optically active propargylamines in high yields and enantioselectivity. Interestingly, bisimine/copper(I) complexes were able to promote the direct, enantioselective, catalytic addition to imines of alkylacetylenes. The effects of catalyst loading and other reaction parameters on the stereochemical outcome of the transformation were investigated. The extremely convenient methodology, the mild reaction conditions, and the possibility of a modular approach for developing new and more efficient bisimine-based chiral ligands make the present methodology very attractive.     

Gadolinium(III) Complexes of dota‐Derived N‐Sulfonylacetamides (H4(dota‐NHSO2R)=10‐{2‐[(R)sulfonylamino]‐2‐oxoethyl}‐1,4,7,10‐tetraazacyclododecane‐1,4,7‐triacetic Acid): A New Class of Relaxation Agents for Magnetic Resonance Imaging Applications

Four new ligands for lanthanide ions based on the H₃do3a (=1,4,7,10‐tetraazacyclododecane‐1,4,7‐triacetic acid) structure and bearing one N‐sulfonylacetamide arm were synthesized, i.e., H₄dota‐NHSO₂R=10‐{2‐[(R)sulfonylamino]‐2‐oxoethyl}‐1,4,7,10‐tetraazacyclododecane‐1,4,7‐triacetic acids 1a – e . A ¹⁵N‐NMR study of the ¹⁵N‐labelled Eu³⁺ complex of one such ligands, 1d , showed that the coordination of the N‐sulfonylacetamide arm involves the carbonyl O‐atom rather than the N‐atom. The relaxometric properties of the corresponding Gd³⁺ complexes were investigated as a function of pH and temperature. These complexes have relaxivities in the range 4.5–5.3 mM ^?1 s^?1, at 20 MHz and 25°, and are characterized by a single H₂O molecule in their inner coordination sphere. The mean residence lifetime of this molecule is relatively long (500–700 ns) compared to other anionic complexes. The slow rate of H₂O exchange can be justified by the extensive delocalization of the negative charge on the N‐sulfonylacetamide arm. The long residence time of the coordinated H₂O allowed the observation of the effect of the prototropic exchange on the relaxivity. The study of the interaction between the complex [Gd( 1e )]‐ and HSA revealed a weak affinity constant highlighting the importance of a localized negative charge on the complex to promote a strong interaction with the protein.     

Polar substituent effects in the bicyclo[1.1.1]pentane ring system: acidities of 3-substituted bicyclo[1.1.1]pentane-1-carboxylic acids

Experimental gas-phase acidities are reported for a series of 3-substituted (X) bicyclo [1.1.1]pent-1-yl carboxylic acids (1, Y = COOH). A comparison with available calculated data (MP2/6-311++G**// B3LYP/6-311+G**) reveals good agreement. The relative substituent effects are shown to be adequately described by a much lower level of theory (B3LYP/6-31+G*). Various correlations are presented which clearly point to polar field effects as being the origin of the relative acidities.     

Lesion load quantification on fast-FLAIR, rapid acquisition relaxation-enhanced, and gradient spin echo brain MRI scans from multiple sclerosis patients.

Previous studies have addressed the issue of the usefullness of fast fluid-attenuated (fast-FLAIR), rapid acquisition relaxation-enhanced (RARE), and gradient spin echo (GRASE) sequences in small groups of patients with multiple sclerosis (MS). The aim of this study was to assess and compare the lesion volumes and the intra-rater reproducibility of such measurements using fast-FLAIR, dual echo RARE, and dual echo GRASE brain scans from a large sample of MS patients. Using a 1.5 Tesla scanner, fast-FLAIR, dual echo RARE, and dual echo GRASE scans (24 axial, 5-mm thick contiguous interleaved slices) of the brain were obtained from 50 MS patients. Total lesion loads (TLL) were assessed twice using a semi-automated local thresholding segmentation technique by the same rater from the scans obtained with the three techniques. Mean TLL were 20.3 mL for fast-FLAIR, 16.6 mL for RARE, and 17.6 mL for GRASE sequences. Mean TLL detected by the three techniques were significantly heterogeneous (p < 0.001); at post-hoc analysis, the mean lesion volume detected on fast-FLAIR images was significantly higher than that on both RARE and GRASE images (p < 0.001) and the mean TLL on GRASE scans was significantly higher than that on RARE scans (p = 0.001). The mean values of intra-observer coefficient of variation for TLL measurements were similar for the three techniques (2.69% for fast-FLAIR, 2.33% for RARE, and 2.65% for GRASE). Our results confirm that fast-FLAIR sequences detect higher lesion volumes than those detected by other magnetic resonance imaging (MRI) sequences with shorter acquisition times. However, the reproducibility of TLL measurements is comparable among fast-FLAIR, RARE, and GRASE. This suggests that when assessing MS disease burden with MRI, the choice of the pulse sequence to be used should be dictated by the clinical setting.     

On the use of elastic peak electron spectroscopy (EPES) to measure the H content of hydrogenated amorphous carbon films

Quasi‐elastic scattering of 1–2 keV electrons is considered with respect to measuring the H content in hydrogenated amorphous carbon (a‐C:H) materials. Interest in the technique lies in the fact that H cannot be typically detected by electron spectroscopic means (AES or XPS for instance). The feasibility of the approach is demonstrated and a quantification procedure is proposed. At the same time however, limitations of the technique (electron stimulated H desorption, low intensity of the H related signal and its spectral interference with the π‐plasmon peak) are discussed. Copyright © 2008 John Wiley & Sons, Ltd.     

Reaction diastereoselectivity of chiral aminoalcohols/[Co(II)NO3]+ complexes in evaporating ESI nanodroplets: new insights from a joint experimental and computational investigation

The nature of the ionic species, formed by electrospray ionization (ESI) of Co(NO₃)₂/CH₃OH solutions with a pair of aminoalcohols W and Y, has been investigated by mass spectrometric and computational methods. Collision induced dissociation (CID) of ions, formally corresponding to the [WYCoNO₃]⁺ structures, yields fragmentation patterns which reflect not only the expected [WYCoNO₃]⁺ connectivity but also that of other isomeric structures. Formation of these latter species is observed only in the presence of a tertiary aminoalcohol, like N‐methylpseudoephedrine. The CID patterns are found to be strongly dependent on the chemical form (whether the free aminoalcohol or its hydrochloride), the configuration, and the relative concentration of the W and Y aminoalcohols. This variability parallels the results of classical MD (molecular dynamics) simulations of the [WYCoNO₃]⁺ adducts which show a drastic alteration of the mechanical–dynamical features of the adducts by simply changing the charge state of W and/or Y, their absolute configuration, or by removing the solvent. The present experimental and computational study confirms the observation of fast stereoselective reactions in ESI nanodroplets before their evaporation and warns against any automatic correlation between the ESI spectrum of an analyte and its structure in solution. Copyright © 2009 John Wiley & Sons, Ltd.