Jaime Retuert

Hydrogels based on modified chitosan, 1. Synthesis and swelling behavior of poly(acrylic acid) grafted chitosan

A large number of carboxylic groups were introduced onto chitosan by grafting with poly(acrylic acid) as an efficient way of modification. The reactions were carried out in a homogeneous aqueous phase by using potassium persulfate and ferrous ammonium sulfate as the combined redox initiator system. The efficiency of grafting was found to depend on monomer, initiator, and ferrous ion concentrations as well as reaction time and -temperature. It was observed that the level of grafting could be controlled to some degree by varying the amount of ferrous ion used as a co-catalyst in the reaction. Evidence of grafting was obtained by comparison of FTIR spectra of chitosan and the graft copolymer as well as solubility characteristics of the products. The swelling behavior of chitosan samples, grafted with 115% and 524% poly(acrylic acid), was studied as function of pH, in distilled water and in aqueous NaCl solution. The sample that has 115% grafting swells considerably more both in distilled water (pH 5.8) as well as higher pH values than the sample with 524% grafting. In contrast, an inverse swelling behavior at pH 2 was observed. An unusual swelling behavior at pH 2 was found for the sample with 524% grafting, which swells to a greater extent than in distilled water. This was attributed to the amphiphilic nature of the modified chitosan from which complex inter- and/or intramolecular interactions could originate.

Homogeneous grafting reaction of vinyl pyrrolidone onto chitosan

Modification of chitosan by grafting of vinyl pyrrolidone (VP) was carried out in homogeneous phase using potassium persulfate as redox initiator. The effect of the reaction variables on the extent of grafting was studied systematically. Values for grafting percentages up to 290% were reached. It was observed that the solubility of chitosan was markedly reduced after grafting with vinyl pyrrolidone. The grafted product is insoluble in common organic solvents as well in dilute organic and inorganic acids. However, the solubility of the grafted chitosan after adsorption of copper ions changed substantially, becoming completely soluble in dilute hydrochloric acid. This was attributed to the effect of complex formation produced by coordination of amino groups of chitosan with copper ions.

Nanoparticles prepared by the sol–gel method and their use in the formation of nanocomposites with polypropylene

Hybrid layered aluminosilicate nanoparticles (HLNP) containing octadecylamine (ODA) as the organic part, and silica nanoparticles with spherical morphology containing ODA (HSNP) or without ODA (SNP) were prepared by the sol–gel method and used for the formation of nanocomposites with polypropylene. The polypropylene matrices, of different molecular weight and polydispersity, were prepared using polymers obtained via Ziegler–Natta or metallocene catalysts. A strong influence of the morphology and the presence of ODA on the surface of the nanoparticles was found on the formation and characteristics of the nanocomposites. The mechanical properties and thermal stability of these materials were determined and compared with those of nanocomposites prepared with 2:1 phylosilicate clays such as montmorillonite and hectorite in similar polymer matrices. X-ray diffraction, transmission electron microscopy, and the study of mechanical properties showed that the use of HLNP allows nanocomposites with considerably improved mechanical properties to be obtained, compared with nanocomposites prepared with exfoliated clays. In the case of nanocomposites prepared with spherical particles functionalized with ODA (HSNP), materials with high specific strength combined with high elongation before rupture were obtained. The thermal stabilization of polypropylene matrices containing the synthesized nanoparticles (HLNP, HSNP or SNP) occurs about 50 °C higher than that attained with clays.

Cocatalyst effect in potassium persulfate initiated grafting onto chitosan

Methyl methacrylate and methyl acrylate were grafted onto chitosan by using potassium persulfate alone as redox initiator and in combination with MnCl2 and CuCl2, as inorganic, and both ammonium tartrate and oxalate as organic cocatalysts. The extent of grafting was found to depend mainly on the nature of the cocatalysts used.

Porous silica derived from chitosan-containing hybrid composites

In this paper, we report the preparation by the sol-gel technique of organic-inorganic hybrid composites containing the biopolymer chitosan incorporated in a siloxane-based inorganic network. The hybrid xerogels were transformed into porous silica particles by elimination of the organic phase. Surface characteristics of the silica samples can be easily tailored. In this way Brunauer-Emmett-Teller areas, pore volume, and pore diameter of the prepared silica can be predetermined within a wide range. Morphology of the particles at longer length scales can be designed to obtain either irregularly shaped particles with layered morphology or spherical particles. The results are explained on the basis of the cationic polyelectrolytic properties of chitosan, which allows easy association with siloxane oligomers, the precursors of silica in forming hybrid nanocomposites.

The effect of reaction parameters on catalytic activity in the polymerization of ethylene using supported and unsupported metallocene catalysts

Abstract The catalytic activities of two metallocene compounds, EtInd2ZrCl2 and Ind2ZrCl2, in the polymerization of ethylene depend heavily on certain parameters, such as Zr concentration in the reaction, type of the reactor, and stirring speed for homogeneous catalysts. One way or another, all these factors influence the catalytic activity of these systems, but catalyst concentration and stirring speed are the most important ones. In the case of heterogeneous catalysts of the same metallocenes, the most pronounced influence is related to the type of support and its porous nature. The influence of the support on activity and morphology is shown.

Study of metallocene supported on porous and nonporous silica for the polymerization of ethylene

Porous and nonporous silicas are good supports for the synthesis of coordinative heterogeneous catalysts for ethylene polymerization. The catalytic activity of nonporous Aerosil depends critically on the percentage of incorporated zirconium. The calcination of the support also affects the activity, showing the best results with nonporous silica at 1000°C, and with porous silica at 800°C.

Transparent semiconductor-polymer hybrid films with tunable optical properties{

Size confined CdS nanoparticles with controlled size and narrow particle size distribution were synthesized in transparent adhesive polyelectrolyte films, which are composed of chitosan (CHI), poly(monomethyl itaconate) (PMMI) and silica (SiO2) in variable amounts. This room temperature synthesis in an aqueous environment avoids the toxic and high temperature conditions usually associated with the synthesis of high quality semiconductors. The semiconductor particle size could be controlled via the Cd2+ concentration and the amine groups of chitosan were identified to be of importance for the coordination of the semiconductor. The films were characterized by UV-Vis and fluorescence spectroscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectra and the adsorption behavior of Cd2+ to the films was characterized by Langmuir and Freundlich isotherms.

EFFECT OF CROSSLINKED CHITOSAN AS A CONSTRAINED VOLUME ON THE IN VITRO CALCIUM CARBONATE CRYSTALLIZATION

The present work deals with the effect of the constrained volume given by crosslinked chitosan (CHI) as a sphere on the in vitro CaC03 crystallization. Crosslinked CHI was obtained with formaldehyde (FA), glutaraldehyde (GA), epichlorhydrine (EPCH) and poly (propylene glycol) diglycidyl ether (PPDGE) as crosslinking agents. Determination of swelling percentage (%) of the crosslinked CHI spheres was carried out in TRIS buffer at pH 9. Spheres of high molecular weight were prepared using drops of CHI solution on NaOH. In vitro CaC03 crystallization using gas-diffusion method was done. In addition, a synthetically sulphonated containing polymethylsiloxane (S-PMS) was used as modifying additive on the CaC03 crystals growth in a confined space of CHI. The degree of the crosslinking altered the diffusion of C02 gas through the CHI spheres during the CaC03 crystallization resulting in different and specific crystals morphologies

PREPARATION AND PROPERTIES OF CHITOSAN HYBRID FILMS FROM MICROWAVE IRRADIATED SOLUTIONS

The effect of microwave irradiation (MW, 1.55 kW at 2450 MHz on the dissolution chitosan(CHI) and on the properties of the solutions was studied and the results compared with those obtained by using mechanical stirring. MW effects were tested by analysing the viscosity of the solutions as well as the properties of the films of the nanocomposite the chitosan with poly(3-(aminopropyl)triethoxysilane)(pAPS) they produce. Microwave irradiation induces rapid dissolution, leading to solutions with relatively low viscosity, which asymptotically increases with further irradiation. Dissolution-precipitation-redissolution experiments with samples prepared using both methods indicate that MW effects are more related with the aggregation state of the macromolecule in the solution than with the chitosan degradation. CHI/pAPS hybrid materials appear to be different when the solutions of chitosan used as precursor are obtained by MW irradiation. Thus, MW appears to affect the chitosan self-association so changing the equilibria between the micro domains involved in the macromolecular complex formation.