K. A. Dembo

Poly(acrylamide) gels with embedded magnetite nanoparticles

Magnetic gels are a new class of soft polymer materials with their properties controlled by magnetic fields. A method was developed for the preparation of magnetite nanoparticles in a matrix of poly(acrylamide) gel. Synthesis of the magnetic spinel iron oxide Fe3O4 in a matrix of poly(acrylamide) gel was performed via coprecipitation of Fe(II) and Fe(III) in alkaline medium. The effects of the cross-link density, the concentration of polymer and salt in the swollen network during preparation on the composition, structure and properties of the magnetic gels were studied by electronography, TEM and small-angle X-ray scattering (SAXS). The crystalline lattice of the particles was determined as magnetite. The average size of the particles, d, calculated from the half-width of the diffraction peaks, is of the order of 10 nm. The obtained data demonstrate a marked difference in size distribution of magnetic particles in gels of different structure.

Determination of the size and phase composition of silver nanoparticles in a gel film of bacterial cellulose by small-angle X-ray scattering, electron diffraction, and electron microscopy

The nanoscale structural features in a composite (gel film of Acetobacter Xylinum cellulose with adsorbed silver nanoparticles, stabilized by N-polyvinylpyrrolidone) have been investigated by small-angle X-ray scattering. The size distributions of inhomogeneities in the porous structure of the cellulose matrix and the size distributions of silver nanoparticles in the composite have been determined. It is shown that the sizes of synthesized nanoparticles correlate with the sizes of inhomogeneities in the gel film. Particles of larger size (with radii up to 100 nm) have also been found. Electron microscopy of thin cross sections of a dried composite layer showed that large particles are located on the cellulose layer surface. Electron diffraction revealed a crystal structure of silver nanoparticles in the composite.

Synthesis of Ag and Cu-chitosan metal-polymer nanocomposites in supercritical carbon dioxide medium and study of their structure and antimicrobial activity

This paper is dedicated to the design of efficient antimicrobial pharmaceuticals based on chitosan. Chitosan with a deacetylation degree of 0.98 preliminarily treated with supercritical carbon dioxide (scCO2) was impregnated with organometallic complexes (silver cyclooctadiene(hexafluoroacetylacetonate) and copper hexafluoroacetylacetonate) from an scCO2 solution followed by reduction with hydrogen to form metal-chitosan composites. Their structure was studied by small-angle X-ray scattering (SAXS). A narrow virtually monodisperse size distribution with a preferable particle size of 1–3 nm was found for silver and a wider polymodal size distribution with a particle size of up to 1–40 nm was revealed for copper. An X-ray fluorescent analysis showed the content of silver and copper metal to be 6 and 4.6%, respectively. The composites prepared by fluid technology were tested for antimicrobial activity toward bacteria S. epidermidis, E. coli, and a spore form of bacteria B. cereus in comparison with Cu2+-chitosan complexes obtained by the routine solution procedure. Ag0-chitosan nanocomposites were found to show bactericidal activity toward all the noted bacteria and spore forms, whereas Cu2+-chitosan complexes exhibit bacteriostatic activity toward the above bacteria and a lack of bactericidal properties toward the spore forms. The reasons for the bactericidal activity of the prepared composites are discussed.

Acceptor-Enhanced Local Order in Conjugated Polymer Films.

Disorder in conjugated polymers is a general drawback that limits their use in organic electronics. We show that an archetypical conjugated polymer, MEH-PPV, enhances its local structural and electronic order upon addition of an electronic acceptor, trinitrofluorenone (TNF). First, acceptor addition in MEH-PPV results in a highly structured XRD pattern characteristic for semicrystalline conjugated polymers. Second, the surface roughness of the MEH-PPV films increases upon small acceptor addition, implying formation of crystalline nanodomains. Third, the low-frequency Raman features of the polymer are narrowed upon TNF addition and indicate decreased inhomogeneous broadening. Finally, the photoinduced absorption and surface photovoltage spectroscopy data show that photoexcited and dark polymer intragap electronic states assigned to deep defects disappear in the blend. We relate the enhanced order to formation of a charge-transfer complex between MEH-PPV and TNF in the electronic ground state. These findings may be of high importance to control structural properties as they demonstrate an approach to increasing the order of a conjugated polymer by using an acceptor additive.

Structure of composites prepared via polypyrrole synthesis in supercritical CO2 on microporous polyethylene

The synthesis of polypyrrole in supercritical carbon dioxide in the presence of the microporous polyethylene has been studied. Formation of polymer composites based on polypyrrole and polyethylene has been demonstrated. The structure of the test samples has been investigated by vibrational spectroscopy, SAXS, and atomic force and scanning electron microscopy. It has been discovered that the oxidized structure of polypyrrole forms during the synthesis under supercritical conditions.

Effect of polymer nature on the structure and properties of gel composites with incorporated bentonite particles

Composite gels based on polyacrylamide and poly(N-isopropyl acrylamide) with incorporated sodium bentonite particles are synthesized. It is shown that the presence of hydrophobic isopropyl groups in a polymer molecule promotes the subsequent formation of highly ordered aggregates of clay and cetylpyridinium chloride in a gel composite. An increase in temperature results in the collapse of composite gels based on poly(N-isopropyl acrylamide); however, no marked changes in the structure of lamellar aggregates of clay and surfactant are observed. It is revealed that the gel can stabilize lamellar structures formed in organoclay suspension prior to the incorporation into swollen polymer network.

Structure of mono- and bimetallic heterogeneous catalysts based on noble metals obtained by means of fluid technology and metal-vapor synthesis

Monometallic nanocomposites are obtained with the use of supercritical carbon dioxide (fluid technique) and metal-vapor synthesis (MVS), while bimetallic nanocomposites of Pt and Au noble metals and γ-Al2O3 oxide matrix are synthesized by a combination of these two methods. The structures, concentrations, and chemical states of metal atoms in composites are studied by means of small-angle X-ray scattering (SAXS), transparent electron microscopy (TEM), X-ray fluorescent analysis (XFA), and X-ray photoelectron spectroscopy (XPS). The neutral state of metal atoms in clusters is shown by XPS and their size distribution is found according to SAXS; as is shown, it is determined by the pore sizes of the oxide matrices and lies in the range of 1 to 50 nm. The obtained composites manifest themselves as effective catalysts in the oxidation of CO to CO2.

Synthesis of polyvinylpyrrolidone and its nanosilver-based polymer composites in supercritical carbon dioxide

Polyvinylpyrrolidone is synthesized in supercritical carbon dioxide at a pressure of 20 MPa and a temperature of 65°C. Polyvinylpyrrolidone films are impregnated with the organic complex silver 1,5-(cyclooctadiene)-1,1,1,5,5,5-hexafluoroacetyl acetonate, and the metal is subsequently reduced by hydrogen. The chemical structure and structure of nanometallopolymer composites are investigated by SAXS, transmission electron microscopy, FTIR and UV spectroscopy, and X-ray fluorescent analysis. It is shown that the impregnation with the complex and the reduction of the complex give rise to Ag(0) nanoparticles with a predominant size on the order of 1 nm.

Formation of silver nanoparticles in PVP matrix in supercritical CO2: Small-angle X-ray scattering and modeling

The process of silver-particle formation in a PVP matrix in supercritical carbon dioxide is investigated by means of small-angle X-ray scattering (SAXS). Application of modern methods of SAXS data interpretation, including procedure of ab initio modeling of particle structure, allowed us for the first time to reveal structural organization of both individual metal nanoparticles and of their clusters incorporated in the polymer matrix. It is shown that pores in PVP films are not simple cavities; they have complex structures which are exhibited when these cavities are contrasted (filled) with metal compounds and/or reduced metallic nanoparticles. As a result, the shape, size, and size distribution of the obtained metallic nanoparticles and their clusters depended on the structure of the polymeric matrix used as a formation medium.

Study of the gel films of Acetobacter Xylinum cellulose and its modified samples by 1H NMR cryoporometry and small-angle X-ray scattering

Gel films of Acetobacter Xylinum cellulose and its modified samples have been investigated by 1H nuclear magnetic resonance (NMR) cryoporometry and small-angle X-ray scattering. The joint use of these two methods made it possible to characterize the sizes of aqueous pores in gel films and estimate the sizes of structural inhomogeneities before and after the sorption of polyvinylpyrrolidone and Se0 nanoparticles (stabilized by polyvinylpyrrolidone) into the films. According to small-angle X-ray scattering data, the sizes of inhomogeneities in a gel film change only slightly upon the sorption of polyvinylpyrrolidone and nanoparticles. The impregnated material is sorbed into water-filled cavities that are present in the gel film. 1H NMR cryoporometry allowed us to reveal the details of changes in the sizes of small aqueous pores during modifications.