Synthesis of nanosize particles of cobalt and nickel oxides from solutions

Rounded nanosize particles of Co₃O₄ and NiO were prepared by precipitation with ammonium carbonate from cobalt and nickel nitrate solutions. Cobalt and nickel oxides and their precursors were characterized by thermal and X-ray phase analyses, electron microscopy, and IR spectroscopy.     

Structure‐Property Relationships for Model Heterocyclic Polymer Networks: Effect of Monomer Functionality

The effect of initial monomer functionality on the final properties of densely cross‐linked heterocyclic polymer networks (HPN) was evaluated from comparative studies of the viscoelastic behavior and enthalpy relaxation in the glass transition range of polyisocyanurates prepared from a bifunctional monomer D (1,6‐hexamethylene diisocyanate, HDI) and a trifunctional monomer T (the isocyanurate of HDI). Quantitative analysis of these data suggested that an HPN prepared from the monomer T was chemically more heterogeneous and possessed a lower effective network density (presumably, as a result of a lower mobility of the initial monomer T compared to monomer D).     

Polyimides reinforced with the sol‐gel derived organosilicon nanophase as low dielectric permittivity materials

Polyimide (PI) nanocomposites prepared by the in situ generation of crosslinked organosilicon nanophase (ON) through the sol‐gel process were characterized by densities, thermally stimulated depolarization currents and dielectric relaxation spectroscopy. Both a looser molecular packing of PI chain fragments adjacent to the ON and a loose inner structure of the spatial aggregates of ON were assumed to be responsible for a non‐additive decrease of the experimental values of dielectric permittivity for the nanocomposites. The pattern of composition dependence of the apparent dielectric permittivity of the ON suggested a probability of a morphological change around the composition PAAS/MTS = 100/16 (presumably, a sort of percolation transition from small‐size, individual clusters into large‐size, infinite clusters). Thus, PI reinforced with the sol‐gel derived nanophase may have a reasonably good potential as low dielectric permittivity materials. Copyright © 2004 John Wiley & Sons, Ltd.     

Stoichiometric polyelectrolyte complexes of ionic block copolymers and oppositely charged polyions

Micellization in dilute solutions of diblock copolymers with a polyelectrolyte and a hydrophilic nonionic blocks and oppositely charged polyions is studied using mean-field theory. In aqueous solutions the micelle core consists of the polyelectrolyte complex (PEC) while the corona is formed by hydrophilic blocks of the block copolymers. Describing PEC as a globule in the framework of the Lifshitz [Zh. Eksp. Teor. Fiz. 55, 2408 (1968)] globule theory we calculate the surface tension of the micellar core/solvent interface as a function of the polyion degree of ionization, solvent quality, and concentration of low-molecular-mass salt. The equilibrium aggregation number of starlike micelles formed by block copolymers and homopolymers of opposite charge at stoichiometric mixture compositions is found as a function of the system parameters. It is shown that micelles disintegrate upon addition of salt.     

Modification of carbonyl iron particles by carboxyl-containing polydimethylsiloxanes

The possibility of preparing magnetic particles with the “core–shell” structure was demonstrated. The procedure is based on the treatment of carbonyl iron particles with polydimethylsiloxanes (PDMS) containing terminal or distributed over the chain carboxyl groups attached via the alkyl or aryl spacer. The presence of carboxyl-containing PDMS grafted to the surface of carbonyl iron particles was revealed by electron microscopy. Magnetorheological fluids and magnetic elastomers containing magnetic particles with the “core–shell” structure were obtained, and their rheological properties were studied.     

Efficiency of post-Hartree-Fock force field for the interpretation of vibrational spectra ofN,N-dimethylnitramine

Harmonic force fields for the molecule ofN,N-dimethylnitramine were calculated in the RHF/6-31G^* and MP2/6-31G^** approximations. Scaling of the force fields obtained made it possible to reliably interpret the vibrational spectra of light and perdeuterated compounds reported in the literature. The assignment is confirmed by good reproducibility of experimental isotope shifts upon¹⁵N-amino- and¹⁵N-nitrosubstitution. The frequencies of intramolecular vibrations in far IR and Raman spectra as well as in neutron inelastic scattering spectra for the light and perdeuterated samples of solidN,N-dimethylnitramine were identified using the force field calculated with the inclusion of electron correlation (MP2). Although general structures of the force fields calculated in the RHF and MP2 approximations are similar, considerable differences in the force constants of the NO and NN stretching vibrations and especially in the constants of the NO_str/NO_str and NO_str/NN_str interactions remain even after scaling the force fields.     

Low-frequency rheology of magnetically controlled elastomers with isotropic structure

The method of torsion oscillations is used to measure the dynamic modulus of elasticity of magnetically controlled elastomers that comprise silicone rubber and carbonyl iron in the low-frequency (up to 100 Hz) range. The samples are synthesized in the absence of a magnetic field; therefore, they have an isotropic structure. In the measurements, a constant magnetic field (up to 24 kA/m) is superimposed along the axis of forced torsion oscillations of the sample. A simple model of the rheological behavior of magnetically controlled elastomers is proposed; the problem of torsion oscillations of a cylindrical sample is solved. From the comparison with the experiment for the materials under study, we determine the coefficients of the theoretical model and the corrections to them, which are made because of variations in the rheology of magnetically controlled elastomers under the influence of a magnetic field. The derived relations make it possible to exclude artifacts and to adequately describe dependences of the storage and loss moduli on the frequency of mechanical loading and the strength of the applied magnetic field.     

Two Types of Block Copolymer Micelles with Ion‐Containing Cores

We report a combined experimental and theoretical study of micellization of block copolymer with hydrophilic nonionic corona‐forming blocks and weak polyelectrolyte (wPE) core‐forming blocks with pH‐triggered solubility in aqueous solutions. We demonstrate that in addition to micelles with neutral cores, there exist two other types of micelles with PE‐ or ionomer‐like cores, in which monovalent counterions are released or condensed on core wPE block, respectively. The transition between the two types of micelles occurred upon changes in ionization of the PE core block and resulted in nonmonotonous changes of aggregation number as a function of pH. Such micelles with stimulus responsive cores represent promising nanocarriers for controlled delivery applications.