Effects of hydrothermal/thermal treatments on the water-uptake of Nafion membranes and relations with changes of conformation, counter-elastic force and tensile modulus of the matrix

A kinetic investigation of the water-uptake of Nation 117 membranes after different hydrothermal and thermal treatments was performed. Long equilibration times (150-225 h) and a decrease of kinetic rate with the increasing of temperature were found. This behaviour suggests that the kinetics of the water-uptake is the result of two distinct processes: one very fast that can be attributed to the time of osmotic equilibration necessary for the water diffusion within the thin membrane and the other, very slow, that has been associated with a slow modification of the Nation conformation with the temperature.The irreversibility of the hydration process with the temperature was related to the irreversibility of the conformational changes. The memory of the thermal treatment is due to the fact that ionomers are essentially constituted as an amorphous matrix in which some microcrystalline phases are embedded. All the amorphous ionomers can give metastable phases that can appear kinetically stable although, in fact, they are thermodynamically unstable.Previous osmotic models for ionomers were re-visited in order to relate the water-uptake isotherms of the various treated samples to the counter-elastic force of their matrix. It was found that an index proportional to the counter-elastic force of the matrix can be simply derived by determining the water-uptake of the samples in liquid water at 20 degrees C. Furthermore, some useful relations between this index and tensile modulus and the shape of water-vapour sorption isotherm of the samples are reported and discussed.We believe that the researches on the modifications of PFSA membranes by thermal and hydrothermal treatments are very important: (1) in order to have a better understanding of the fundamental properties of this important class of membranes; (2) for obtaining reliable comparisons between the properties of different PFSA membranes and (3) for improving the stabilization of the Nafion membranes at temperatures higher than 90 degrees C. (C) 2007 Elsevier B.V. All rights reserved

On the decay of Nafion proton conductivity at high temperature and relative humidity

The irreversible conductivity decay exhibited by Nafion 117 membranes above certain values of temperature and relative humidity (RH) has been investigated by two-probe impedance measurements carried out at 120 degrees C with the electric field normal to the membrane surface, under controlled applied pressure on the electrodes. The analysis of the evolution of both frequency response and normal conductivity during the decay has suggested that the decay arises from changes in the bulk transport properties of the Nafion membrane. This has been confirmed by determining, under stability conditions, the conductivity of membranes pre-treated under decay conditions. The results of these measurements have shown that the decay occurs only if the membrane undergoes an anisotropic deformation along the direction parallel to the electrode surface. Four-probe impedance measurements with the electric field parallel to the membrane surface have also been carried out to determine the membrane tangential conductivity before and after the decay. Comparison of normal and tangential conductivity has indicated that the decay is associated, to a certain extent, with an increase in the conduction anisotropy. (c) 2006 Elsevier B.V. All rights reserved

Multicomponent diversity oriented synthesis of multivalent glycomimetics containing hexafluorovaline

The chemoselective introduction of fluorinated moieties into biologically relevant scaffolds is now an established strategy to modulate and to study the properties of molecular leads. In this article we propose, for the first time, the diversity oriented synthesis of multivalent glycomimetics incorporating hexafluorovaline through a straightforward multicomponent sequential process, which occurs with high yield and in mild conditions. The same process has been successfully applied to the chemistry of aminoglycosides producing a neomycin-hexafluorovaline-galactose conjugate and providing a general, efficient strategy to functionalized aminoglycosides with sugar-hexafluorovaline tags

Diversity oriented combinatorial synthesis of multivalent glycomimetics through a multicomponent domino process

Both multicomponent reactions and diversity oriented synthesis are indispensable tools for the modern medicinal chemist. However, their employment for the synthesis of multivalent glycomimetics has not been exploited so far although the importance that such compounds play in exploring multivalency on glycoside inhibition. Herein, we report the combinatorial synthesis of diversity oriented hetero di- and trivalent glycomimetics through a multicomponent domino process. The process is high yielding and very general, working efficiently with easily accessible sugar starting materials such as glycosylamines, glycosylazides, and glycosylisothiocyanates, having the reactive functional groups tethered either directly to the anomeric carbon, through a suitable linker, or to the primary 6 position of hexoses (or 5 position of pentoses), leading, in the latter case, to glycomimetics with artificial enzymatically stable backbone. The process has been also exploited for the multicomponent synthesis of aminoglycoside (neomycin) conjugates

Multicomponent diversity oriented synthesis of multivalent glycomimetics containing hexafluorovaline

The chemoselective introduction of fluorinated moieties into biologically relevant scaffolds is now an established strategy to modulate and to study the properties of molecular leads. In this article we propose, for the first time, the diversity oriented synthesis of multivalent glycomimetics incorporating hexafluorovaline through a straightforward multicomponent sequential process, which occurs with high yield and in mild conditions. The same process has been successfully applied to the chemistry of aminoglycosides producing a neomycin-hexafluorovaline-galactose conjugate and providing a general, efficient strategy to functionalized aminoglycosides with sugar-hexafluorovaline tags

Diversity Oriented Combinatorial Synthesis of Multivalent Glycomimetics Through a Multicomponent Domino Process

Both multicomponent reactions and diversity oriented synthesis are indispensable tools for the modern medicinal chemist. However, their employment for the synthesis of multivalent glycomimetics has not been exploited so far although the importance that such compounds play in exploring multivalency on glycoside inhibition. Herein, we report the combinatorial synthesis of diversity oriented hetero di- and trivalent glycomimetics through a multicomponent domino process. The process is high yielding and very general, working efficiently with easily accessible sugar starting materials such as glycosylamines, glycosylazides, and glycosylisothiocyanates, having the reactive functional groups tethered either directly to the anomeric carbon, through a suitable linker, or to the primary 6 position of hexoses (or 5 position of pentoses), leading, in the latter case, to glycomimetics with artificial enzymatically stable backbone. The process has been also exploited for the multicomponent synthesis of aminoglycoside (neomycin) conjugates

Multicomponent Domino Synthesis and Antibacterial Activity of Neomycin-Sugar Conjugates

An efficient multicomponent domino process that works under mild conditions was used for the synthesis of systematically modified neomycin-sugar conjugates. The final aminoglycoside derivatives were tested against methicillin-resistant Staphylococcus aureus, Klebsiella, and E. coli strains, and show activity comparable to the parent antibiotics. Such a strategy can impact multicomponent combinatorial syntheses of diverse biologically active conjugates

Naked‐Eye Heterogeneous Sensing of Fluoride Ions by Co‐Polymeric Nanosponge Systems Comprising Aromatic‐Imide‐Functionalized Nanocellulose and Branched Polyethyleneimine

Heterogeneous colorimetric sensors for fluoride ions were obtained by cross‐linking TEMPO‐oxidized cellulose nanofibers (TOCNF) with chemically modified branched polyethyleneimine 25 kDa (bPEI). Functionalization of bPEI primary amino groups with aromatic anhydrides led to the formation of the corresponding mono‐ and bis‐imides on the grafted polymers (f‐bPEI). A microwave‐assisted procedure allowed the optimization of the synthetic protocol by reducing reaction time from 17 h to 30 minutes. Hydrogels obtained by mixing different ratios of TOCNF, bPEI and f‐bPEI were lyophilized and thermally treated at about 100 °C to promote the formation of amide bonds between the amino groups of poly‐cationic polymers and the carboxylic groups of cellulose nanofibers. This approach generated a series of cellulose nanosponges S1‐S3 which were characterized by FT‐IR and by solid state 13C CPMAS NMR. These sponge materials can act as colorimetric sensors for the selective naked‐eye recognition of fluoride ions over chloride, phosphate and acetate ions at concentrations of up to 0.05 M in DMSO. Moreover, when the sponges were functionalized with perylene tetracarboxylic diimide, successful naked‐eye detection was achieved with only 0.02 % w/w of chromophore units per gram of material