Polymer-inorganic composite with ultradisperse gadolinium particles

The objects of investigation are polystyrene-based composites with ultradisperse particles (including nanoparticles) of metallic Gd and SiO₂. The composites prepared by milling starting materials in a barrel mill at room temperature are studied by the ferromagnetic resonance method, transmission electron microscopy (TEM), and reflection X-ray diffraction (RXD). It is found that the magnetic subsystem of the composites is formed by magnetic nanoclusters, Gd crystallites 30 ± 10 nm across, which possess volume and surface magnetic anisotropy and pass into the superparamagnetic state at 210 ± 10 K. It is also found that the Landau-Lifshitz equation with the damping term in the Landau-Lifshitz form provides the best quantitative fit to experimental data for the ferromagnetic resonance of superparamagnetic metal nanoparticles.     

Positive hole transfer between organic molecules of different classes in freon matrices

The processes of positive charge (hole) transfer between organic molecules of different classes occurring upon irradiation of their frozen freonic solutions at 77 K were studied by ESR spectroscopy. The efficiency of positive hole transfer was characterized by the “redistribution coefficient”, which was determined as ratio of concentrations of the radical cations of different compounds at equal concentrations of the parent neutral molecules. A very effective positive hole transfer to toluene was shown to occur for alkane-toluene pairs (K > 10). Meanwhile, much lower K values (～ 2 to 3) were observed for alkane-alkene pairs, in spite of a rather high difference in the ionization potentials (ca. 1 eV). This effect was explained by the conformation dispersion and distribution of environment configurations of ionized molecules for the alkane-alkene pairs. An effective positive hole transfer (K ～ 6) was observed for the pair dimethyl ether-acetone, where the conformation dispersion was absent, although the difference in ionization potentials for this pair was only 0.3 eV.     

Water-soluble interpolyelectrolyte complexes of polyisobutylene-block-poly(methacrylic acid) micelles: formation and properties

We report on interpolyelectrolyte complexes (IPECs) formed by micelles of ionic amphiphilic diblock copolymers with polyisobutylene (PIB) and poly(sodium methacrylate) (PMANa) blocks interacting with quaternized poly(4-vinylpyridine) (P4VPQ). The interpolyelectrolyte complexation was followed by turbidimetry and small angle neutron scattering (SANS). The data obtained by means of a combination of SANS, dynamic light scattering (DLS), and cryogenic transmission electron microscopy (cryo-TEM) provide evidence on the core-shell-corona structure of the complex species with the shell assembled from fragments of electrostatically bound PMANa and quaternized P4VPQ fragments, original PIBx-b-PMAAy micelles apparently playing a lyophilizing part. The complex formation is followed by potentiometric titration as well. This process is initially kinetically controlled. In the second step larger aggregates rearrange in favor of smaller complexes with core-shell-corona structure, which are thermodynamically more stable. An increase in ionic strength of the solution results in dissociation of the complex species as proven by SANS and analytical ultracentrifugation (AUC). This process begins at the certain threshold ionic strength and proceeds via a salt-induced gradual release of chains of the cationic polyectrolyte from the complex species.     

A polymer-inorganic composite material based on gadolinium, barium, and manganese compounds; elemental sulfur; and polyaniline with a giant magnetoresistance effect

The effect of a constant magnetic field up to 6000 Oe on transient current after application of a pulsed voltage of U = 5 V with a duration of 100 ??s is studied for composites based on polyaniline, manganese acetylacetonate, barium oxide hexahydrate, gadolinium chloride, and elemental sulfur. Initial powders are mixed in a ball mill. The giant magnetoresistance with a magnitude of 40?C60% is discovered. It manifests itself as a difference in transient-current curves in the absence of the magnetic field and in its presence. This effect is explained by spin-dependent transport, in which at least two charge assemblies are involved: namely, electron assemblies and proton assemblies.     

Polymer micelles with hydrophobic core and ionic amphiphilic corona. 2. Starlike distribution of charged and nonpolar blocks in corona

Mixed polymer micelles with hydrophobic polystyrene (PS) core and ionic amphiphilic poly(4-vinylpyridine)/poly(N-ethyl-4-vinylpyridinium bromide) corona (P4VP/PEVP) spontaneously self-assembled from mixtures of PS-b-PEVP and PS-b-P4VP macromolecules in dimethylformamide/methanol/water selective solvent. The fraction of PEVP units in corona was β = [PEVP]/([PEVP] + [P4VP]) = 0.05-1.0. Micelles were transferred into pure water via dialysis technique and pH was adjusted to 9, where P4VP blocks are insoluble. Structural characteristics of micelles as a function of corona composition β were investigated. Methods of dynamic and static light scattering, electrophoretic mobility measurements, sedimentation velocity, transmission electron microscopy, and UV spectrophotometry were applied. Spherical morphology with core (PS)-shell (P4VP)-corona (PEVP) organization was postulated. Micelles demonstrated a remarkable inflection in structural characteristics near β ～ 0.5-0.7. Above this region, aggregation number (m), core and corona radii of mixed micelles coincided with those of individual PS-b-PEVP micelles. When β decreased below 0.5, dramatic growth of aggregation number was observed, accompanied by growth in micelle size and stretching PEVP chains. At β below 0.2, dispersions of mixed micelles were unstable and easily precipitated upon addition of NaCl. Scaling relationships between micelle characteristics and β were obtained via minimization the micelle free energy, taking into account electrostatic, osmotic, volume, and surface contributions. Theoretical estimations predicted dramatic influence of β on aggregation number, m ～ β(-3). This result is in general agreement with experimental data and confirms the correctness of the core-shell-corona model. The inflection in micelle characteristics entails drastic changes in micelle dispersion stability in the presence of oppositely charged polymeric (sodium polymethacrylate) or amphiphilic (sodium dodecyl sulfate) complexing agents.