I. B. Meshkov

Inorganic core/organic shell hybrid nanoparticles: Synthesis and characterization

Core/shell molecular particles, promising components of polymer nanocomposites, were synthesized by condensation of tetraethoxysilane (TEOS) in acetic acid at elevated temperature with subsequent blocking of reactive SiOH groups by trimethylsiloxy groups. Particle diameters ranged from 1 to 10 nm depending on the condensation time until the blocking agent was added. The solubilities of the nanocomposites in THF, toluene, hexane, and other organic solvents made it possible to characterize them by the following physicochemical methods: gel permeation chromatography (GPC), viscometry, dynamic light scattering (DLS), and monolayer compression at the water-air interface. As a result, the particles were found to have compact spherical shapes. The physical state of the particles varied from viscous flow to crystallike depending on the sizes and hardness of their silica cores.

Silica nanoparticles with covalently attached fluorophore as selective analyte-responsive supramolecular chemoreceptors

Silica core-shell nanoparticles with SiO2 cores and an alkoxysilyl derivative of dibenzoylmethanatoboron difluoride (A-DBMBF2) fluorophore covalently attached to the core surface have been synthesized. It is shown that these nanoparticles can be used as the basis of selective sensor materials capable of detecting benzene, toluene, and xylene vapors. As benzene or its methyl derivatives are adsorbed on the surface of nanoparticles, a quenching of A-DBMBF2 fluorescence occurs while an A-DBMBF2/analyte exciplex fluorescence buildup emerges. The position of isoemissive points in the fluorescence spectra is specific for each analyte and can be used for its identification.

Synthesis and Characterization of Hybrid Core–Shell Systems Based on Molecular Silicasols

New hybrid “rigid inorganic core–soft organic shell” systems based on molecular silicasols were synthesized by applying different synthetic schemes. Inorganic core was composed of molecular silicasols, which were synthesized from hyperbranched polyethoxysiloxane and tetraethoxysilane by polymer chemistry methods. Different organic modifiers were used to form soft shell of the hybrid particles. Obtained compounds were characterized by elemental analysis, GPC, IR and NMR spectroscopy. These systems will be designated for use as model objects for investigation of nanoparticles–polymer matrix interactions in polymer nanocomposites.

Phase state and rheology of polyisobutylene mixtures with decyl surface modified silica nanoparticles

The miscibility of linear polyisobutylene and silica nanoparticles with surfaces modified by decyl groups is studied. The phase state of these systems corresponds to the amorphous equilibrium and may be described by a binodal with the UCST. As the radius of the inorganic core of nanoparticles and the molecular mass of polyisobutylene increase, the insolubility region on the phase diagram becomes wider. The addition of nanoparticles to the polymer leads to decreases in the viscoelastic characteristics of homogeneous media and provides the non-Newtonian behavior of the composition in the two-phase region. Shear deformation causes shifts of the phase equilibrium lines in the direction depended on the sizes of the nanoparticles.

Rheological and relaxation properties of MQ copolymers

The rheological properties of MQ copolymer melts are investigated under steady-state shear flow and dynamic oscillatory shear within a wide temperature range. The MQ copolymers are highly branched polycyclic compounds (densely crosslinked nanosized networks) incapable of further intermolecular interactions. The samples have identical chemical compositions, but their detailed molecular structures are different. The polymers under consideration show Newtonian behavior in a wide shear-stress range. The values of viscosity vary considerably with the molecular structure of the copolymers. The generalized frequency dependence of complex dynamic modulus components is constructed with the use of the temperature-frequency superposition method. In a first approximation, the viscoelastic behavior of the materials is satisfactorily described by the Maxwell model with a single relaxation time. In this respect, the studied materials are similar to micellar colloids. The relaxation spectra of the copolymers distinguished by narrow distributions are calculated.

Crazing of polymers in the presence of hyperbranched poly(ethoxysiloxane)

The crazing of various polymers (PET, isotactic PP, and HDPE) in the presence of branched poly(ethoxysiloxane) and its low-molecular-mass analog—tetraethoxysilane—has been studied. The hyperbranched poly(ethoxysiloxane) is shown to be an effective adsorptionally active medium for crazing of various solid polymers and development of nanoporous structures with a volume porosity of up to 60%. Depending on the nature of polymers, two mechanisms of crazing (either classical or delocalized crazing) can take place. The reactions of hydrolysis (basic and acidic) within the pores leading to formation of solid silica have been performed. Electron microscopic observations provide evidence that the transformation of a viscous adsorptionally active liquid into a solid compound directly within the volume of a polymer matrix leads to the stabilization of a highly dispersed polymer structure that arises in the course of crazing.

Preparing film composites based on crazed polymers and silica sol nanoparticles

Films of polymer/silica nanocomposites based don isotactic polypropylene and high density polyethylene are prepared via solvent-crazing in the tetrahydrofurane solution of molecular silica sol. This method allows us to disperse SiO2 particles with a diameter of 5–15 nm in a volume of polymer matrices homogeneously and at the nanometer level. The possibility of producing a fragmented riffled silica coating onto polymer surfaces is presented.

Specific features of the formation of the silicon dioxide phase in porous poly(propylene) prepared through the crazing mechanism

A new technique is described for preparing poly(propylene)-silica nanocomposites with the use of crazing of polymers in reactive liquid media that exhibit an adsorption capacity with respect to the polymer and contain functional groups able to enter into different chemical reactions, in particular, hydrolytic condensation. The advantage of this technique over conventional mixing is that the components can be mutually dispersed at the nanolevel without using additional modifying additives. The hydrolytic condensation of tetraethoxysilane and hyperbranched poly(ethoxysiloxane) in the presence of acid or base catalysts with the formation of a silica gel in a crazed polymer matrix is investigated. It is established that the morphology of the prepared composites is determined by the structure of the crazed polymer matrix, the nature of the precursor, and the hydrolytic polycondensation conditions. Composites are prepared in which the silica phase is located either inside the poly(propylene) matrix (in the form of a continuous phase or discrete particles) or on the surface of the polymer. Porous silicon plates are produced through heat treatment of the poly(propylene)-silicate nanocomposites at a temperature of 700°C.

Hydrolytic polycondensation of methylalkoxysilanes under pressure

Hydrolytic polycondensation of methytrialkoxysilanes under the pressure in water and in carbonic acid was investigated. It is shown, that both processes proceed with full conversion of the monomer to form a low molecular soluble polymethylsilsesquioxanes. However, they have a different structure: in water branched compounds were produced and in carboxylic acid polycyclic compounds were synthesized. A type of alkoxy group affects the course of the process. In the case of hydrolytic polycondensation of methyltriethoxysilane under the pressure, unlike methyltrimethoxysilane, full conversion is observed only when additional homogenization of the reaction mixture takes place by vigorous stirring or increasing temperature of the process.

Preparation of chemosensor materials based on silica nanoparticles with covalently anchored fluorophores by inkjet printing

Samples of sensor layers containing a mixture of spherical silica gel microparticles and spherical macromolecular silica sol nanoparticles have been prepared by inkjet printing. The average diameter of microparticles is 5 μm; nanoparticles about 100 nm in diameter contain covalently anchored fluorophore, dibenzoylmethane boron difluoride (DBMBF2), on the surface. The microstructure of the layers is shown to considerably affect the availability of the fluorophore indicator for gas-phase analyte molecules of the methylbenzene group. The sensitivity of the sensor layers is shown to reach 0.5 ppm with a response time of about 100 s.