1,720,962 research outputs found
Defined Chitosan-based networks by C-6-Azide-alkyne “click” reaction
No full textChitosan was converted to its -NH(2)-protected derivative by reacting with phthalic anhydride and then selectively functionalized with an azido group in C-6 position. Two different procedures were employed. The first was a "one-pot" procedure and the second was a two steps reaction through the tosylated intermediate. Both methods resulted in an effective functionalization of chitosan with azido groups. For the two before mentioned procedures, the functionalization degree was estimated by elemental analysis as number of azido groups per pyranoside ring and was 0.28 mol/mol and 0.26 mol/mol, respectively. The azido-functionalized derivatives underwent further modification by Cu(I) catalyzed dipolar cycloaddition with mono- or di-alkynes. The reaction with phenylacetylene produced soluble derivatives that were fully characterized at molecular level by FT-IR and NMR spectroscopies, elemental analysis and size exclusion chromatography (SEC). Crosslinked derivatives were obtained by reactions with 1,7-octadiyne or 1,4-diethynylbenzene and subsequently deprotected to restore the free amino groups. These last systems showed selective swellability in acid medium
Poly(ethyl acrylate) surface-initiated ATRP grafting from wood pulp cellulose fibers
No full textWood pulp cellulose fibers were functionalized by partial esterification of the superficial -OH groups with alpha-bromoisobutyryl bromide (BIBB) under mild (room temperature and THF as solvent) and harsh (85 degrees C and pyridine as solvent) conditions. Samples were characterized by ATR spectroscopy, EDXS, elemental analysis and SEM. Functionalization degrees was varied up to 2% mol Br/OH (by elemental analysis) by varying reaction time and method, without affecting the fiber morphology, except for the highest modification degree, where external cell-wall was partially degraded.
The BIBB-functionalized wood pulp cellulose was used as initiator for surface-initiated ATRP of ethyl acrylate in the presence or not of free-"sacrificial" initiator. Poly(ethyl acrylate) (PEA) modified fibers were characterized by ATR spectroscopy, SEM, thermogravimetric (TGA) and calorimetric (DSC) analyses. while ungrafted polymer was characterized by SEC. A direct correlation was found between the function-alization degree by BIBB and the amount of grafted polymer; products from 3% to 80% wt of grafted poly (ethyl acrylate) were obtained.
Thermal destabilization of wood pulp fibers occurred because of the BIBB functionalization, which was recovered by the successive PEA grafting. Moreover, an increase of the glass transition temperature of the grafted polymer with respect to ungrafted one was observed
Synthesis and photochromic response of a new precisely functionalized chitosan with "clicked" spiropyran RID E-9799-2011
No full textA new light responsive polysaccharide based on a precisely defined N-phthaloyl chitosan with a covalent bound photochromic spiropyran moiety in C-6 was synthesized through Cu(I) catalyzed azide–alkyne [3+2] dipolar cycloaddition between 6-azido-6-deoxy, N-phthaloyl chitosan and a new spiropyran derivative containing an alkynyl group (SPCC). The reaction product was characterized by NMR and FT-IR spectroscopies for confirming the molecular structure. The UV–vis spectroscopy analyses of films prepared by solution casting on glass from DMF solutions of the functionalized polysaccharide, before and after exposure to an UV lamp (in the 280–350nm spectral range) or sunlight, showed very slow
thermal decay of the photo-induced colored form which was substantially maintained after few months at room temperature. On the contrary, fast color fading was observed for SPCC in ethanol or diethyl ether solution where the color disappeared completely in few minutes at the same temperature. Intermediate color durability was found in diethyl ether suspensions, which indicated the solid polysaccharide phase as responsible of the observed color durability. Considering that the absorption spectra of the polymers are similar under the different conditions examined, the observed effect in the solid state is discussed with reference to the chromophore environment providing molecular and supramolecular hindrance by steric effect and weak binding
Oxidation of Glycogen by Periodate to Functional “Molecular Nanoparticles”
No full textGlycogen is a natural polymer having a very high molecular weight (Mw ~7 106 g/mol). It is made of macromolecules having mostly spherical shape with 21.3 nm diameter (from DLS). It is thus a sort of natural nanoparticle that can be used to prepared nanostructured functional materials. A possible strategy to provide glycogen with functional properties for optical or biological application relies on the insertion of specific functional groups on the surface of this nanoparticle by chemical modification.
The oxidation of glycogen with periodate groups seems a promising strategy since this reaction occurs under mild conditions with the formation of aldehyde groups. These lasts are very reactive groups and can be converted easily in a variety of chemical and biological functionality by reactions suitable amines derivatives.
Glycogen was oxidized with IO4- amount per glycosidic unit in the range 0.5-20 % mol/mol. Water, methanol, propanol and N,N-dimethylformamide (DMF) were used as reaction solvents. The reaction products were characterised by NMR spectroscopy and base titration after treatment with hydroxylamine chlorohydrate to obtaining the oxidation degree. The selective cleavage of the oxidized units of glycogen followed by SEC analysis provided information on the distance of the aldehyde groups from the glycogen chain-ends, that roughly corresponds to the distance from the surface of the nanoparticle.
The experimental results showed that a good control over the aldehyde group amount can be obtained by controlling the periodate amount in the reaction feed while the distribution of the groups depends on the glycogen accessibility in the reaction solvent. In particular fictionalization only at the surface was obtained in DMF
Oxidation of Glycogen “Molecular Nanoparticles” by Periodate
No full textGlycogen, a natural hyperbranched polymer of glucose characterized by a substantially spherical macromolecular structure, was oxidized by periodate to obtain reactive intermediates for the preparation of nanostructured materials. The reaction was carried out at room temperature in water, methanol, isopropanol or N,N-dimethylformamide as solvents. The oxidized glycogen was analyzed by H-1 NMR and the oxidation degree was determined by titration. The repartition of the aldehydes between the surface and the volume of the macromolecular nanoparticles was studied by selective cleavage of the oxidized monomeric units under Smith degradation conditions and SEC analysis. Tuning of the oxidation degree in the range of 1-40% (mole fraction of aldehyde per glucoside unit) was achieved by controlling the reagent feed ratio (periodate/glycogen), while the topology of the aldehyde group distribution was controlled by exploiting the kinetics of mass transport. When the reaction was carried out in water, at low oxidation degree values (0.5-3 mol% of oxidized monomeric units), the aldehyde groups were found to be mostly near the nanoparticle surface. A gradient of distribution of the aldehydes from the surface to the particle core was obtained at higher oxidation degrees (>5%). When the reaction was carried out in organic solvents the functionalization occurred mostly at the nanoparticle surface and also at a high oxidation degree. In particular, in N,N-dimethylformamide at 5% oxidation degree, the aldehyde groups were found to be near the macromolecular chain-ends, mostly located on the particle external shell
New Functional Materials from Polysaccharides Modified by Copper Catalyzed Azide-Alkyne Cycloaddition
No full textAmphiphilic amylose-g-poly(meth)acrylate copolymers through "click" onto grafting method
No full textPeriodate oxidation and subsequent reductive amination with propargylamine was adopted for the controlled functionalization of amylose with alkyne groups, whereas ATRP polymerization was exploited to obtain end-(alpha)-or end-(omega)-azide functionalized poly(meth)acrylates to be used as "click" reagents in Cu(I) catalyzed azide-alkyne [3 + 2] dipolar cycloaddition. Amy lose was effectively grafted with poly(n-butyl acrylate), poly(n-butyl methacrylate), poly(n-hexyl methacrylate), and poly(dimethylaminoethyl methacrylate) with this strategy. Their structure and composition were confirmed by FT-IR, NMR spectroscopies, and thermogravimetric analysis (TGA). Dynamic and static light scattering analyses, as well as TEM microscopy showed that the most amphiphilic among these hybrid graft copolymers self-assembled in water, yielding nanoparticles with ca. 30 nm diameter
Amphiphilic copolymers from polysaccharides and (meth)acrylates by “click” promoted coupling
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Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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