Effect of Temperature on the Electrorheological Behavior of Porous Chitosan Particles in Polydimethylsiloxane

Doklady Physical Chemistry - The rheological behavior of a 1 wt % suspension of highly porous chitosan particles in polydimethylsiloxane in a high electric field has been studied as a function of...     

Influence of the Degree of Deacetylation of Chitosan and BMP-2 Concentration on Biocompatibility and Osteogenic Properties of BMP-2/PLA Granule-Loaded Chitosan/β-Glycerophosphate Hydrogels

Compositions based on chitosan/β-glycerophosphate hydrogels with highly porous polylactide granules can be used to obtain moldable bone graft materials that have osteoinductive and osteoconductive properties. To eliminate the influence of such characteristics as chain length, degree of purification, and molecular weight on a designed material, the one-stock chitosan sample was reacetylated to degrees of deacetylation (DD%) of 19.5, 39, 49, 55, and 56. A study of the chitosan/β-glycerophosphate hydrogel with chitosan of a reduced DD% showed that a low degree of deacetylation increased the MSCs (multipotent stromal cells) viability rate in vitro and reduced the leukocyte infiltration in subcutaneous implantation to Wistar rats in vivo. The addition of 12 wt% polylactide granules resulted in optimal composite mechanical and moldable properties, and increased the modulus of elasticity of the hydrogel-based material by approximately 100 times. Excessive filling of the material with PLA (polylactide) granules (more than 20%) led to material destruction at a ~10% strain. Osteoinductive and osteoconductive properties of the chitosan hydrogel-based material with reacetylated chitosan (39 DD%) and highly porous polylactide granules impregnated with BMP-2 (bone morphogenetic protein-2) have been demonstrated in models of orthotopic and ectopic bone formation. When implanted into a critical-size calvarial defect in rats, the optimal concentration of BMP-2 was 10 μg/mL: bone tissue areas filled the entire material’s thickness. Implantation of the material with 50 μg/mL BMP-2 was accompanied with excessive growth of bone tissue and material displacement beyond the defect. Significant osteoinductive and osteoconductive properties of the material with 10 μg/mL of BMP-2 were also shown in subcutaneous implantation.     

Synthesis of monodisperse polymer suspensions with a narrow particle size distribution in the presence of water-insoluble triple block copolymers of polypropylene oxide and polyethylene oxide (pluronics)

The colloidal chemical properties of triple block copolymers of polypropylene oxide and polyethylene oxide (pluronics of various structures) were studied in comparison. All of them are shown to be surfactants but differ in interfacial tension, surface activity, surface area occupied in the adsorption layer, and adsorption layer thickness. The kinetic regularities of polymerization of styrene and methyl methacrylate were studied. The particle diameters and their size distribution were determined. Distinctions in the kinetic regularities of polymerization are shown: the shape of the conversion—time curves (for the duration of the initial and stationary stages of polymerization) and the dependences of the diameter on the surfactant concentration and monomer to water volume ratio. In the presence of the water-insoluble pluronics, the mechanism of formation of polymer—monomer particles and interfacial layer on the surface differs from that when using water-soluble surfactants, which makes it possible to distinguish these processes into an independent type of heterophase polymerization.

     

Kinetics of D,L–Lactide Polymerization Initiated with Zirconium Acetylacetonate

The kinetic of D,L-lactide polymerization in presence of biocompatible zirconium acetylacetonate initiator was studied by differential scanning calorimetry in isothermal mode at various temperatures and initiator concentrations. The enthalpy of D,L-lactide polymerization measured directly in DSC cell was found to be ΔH=−17.8±1.4 kJ mol⁻¹. Kinetic curves of D,L-lactide polymerization and propagation rate constants were determined for polymerization with zirconium acetylacetonate at concentrations of 250–1000 ppm and temperature of 160–220 °C. Using model or reversible polymerization the following kinetic and thermodynamic parameters were calculated: activation energy E_a=44.51±5.35 kJ mol⁻¹, preexponential constant lnA=15.47±1.38, entropy of polymerization ΔS=−25.14 J mol⁻¹ K⁻¹. The effect of reaction conditions on the molecular weight of poly(D,L-lactide) was shown.     

Effect of the chemical grafting of epoxy resin onto organomodified montmorillonite on the structure and heat resistance of an epoxy nanocomposite

It is shown that the noncatalyzed chemical grafting of an epoxy oligomer onto the surface of montmorillonite platelets may proceed via the reaction between the epoxy groups and the hydroxyl groups of the organic modifying agent of the layered aluminosilicate. The effect of grafting on the structure and heat resistance of the cured epoxy nanocomposite is studied. Complete exfoliation of montmorillonite particles into individual platelets is shown to be a necessary but insufficient condition for increasing the glass-transition temperature of the nanocomposite relative to that of the initial matrix. A much higher contribution to the increase in the glass-transition temperature is ensured by grafting of epoxy molecular chains onto the surface of aluminosilicate platelets; i.e., strong covalent matrix-aluminosilicate bonds form in addition to physical bonds, a process that entails development of a thicker three-dimensional network.     

Dielectric Properties of Halloysite Nanotube Suspensions in Polydimethylsiloxane

Russian Journal of Physical Chemistry A - The frequency dependences of electrical conductivity with both real and imaginary components of a dielectric permittivity of polydimethylsiloxane oil...     

Composite Material Based on Polytetrafluoroethylene and Al–Cu–Fe Quasi-Crystal Filler with Ultralow Wear: Morphology,Tribological, and Mechanical Properties

Samples of composites with polytetrafluoroethylene as the matrix and a powder of 0, 1, 2, 4, 8, 16, and 32 vol % Al–Cu–Fe quasi-crystal as the filler are prepared. Electron microscopy studies of the sample structure are carried out, the influence of the filler on the degree of crystallinity and the melting and destruction temperatures of the samples is investigated; mechanical tensile tests and tribological tests are performed. The composite samples with filler contents of 4, 8, 16, and 32 vol % show ultralow wear with the coefficient K < 5 × 10^–7 mm³/N m. The highest wear resistance exceeding that of unfilled polytetrafluoroethylene by 2200–3100 times is recorded in composites with 16 vol % filler. An increase in the wear resistance is associated with formation on the friction surface of a thin crust containing quasi-crystal particles 0.2–0.3 μm in size, revealed by scanning electron microscopy in combination with energy dispersive analysis.     

The electrical conductivity of poly-p-xylylene + CdS nanocomposites

The dependence of electrical conductivity on the concentration of CdS (c) and temperature (T) over the temperature range 10–300 K was studied for poly-p-xylylene-CdS (PPX + CdS) nanocomposites prepared by solid-state cryochemical synthesis. The results were discussed in terms of the heterogeneous conductivity model including various charge transfer mechanisms in various nanocomposite regions. Under the illumination of a film with c > 11 vol % by a daylight lamp, the conductivity increased, and the σ(T) dependence was metallic in character at low temperatures. The photoconductivity of films at larger concentrations c was caused by the appearance of photoexcited electrons in CdS nanoparticles, the separation of charges at the nanoparticle-matrix boundary, and percolation effects in films. The PPX matrix was shown to actively participate in electrical conductivity; electrons in this matrix jumped between phenyl rings. The experimental dependence of dark conductivity σ(T) at temperatures from 150 to 300 K was analyzed using the Mott hopping conductivity model with variable jump lengths. The main points of the Mott theory of hopping conductivity were discussed. Original Russian Text ? I.A. Misurkin, S.V. Titov, T.S. Zhuravleva, I.V. Klimenko, S.A. Zav’yalov, E.I. Grigor’ev, S.N. Chvalun, 2009, published in Zhurnal Fizicheskoi Khimii, 2009, Vol. 83, No. 3, pp. 534–540.     

Electrophysical properties of nanocomposites based on poly(p-xylylene) and copper nanoparticles

Poly(p-xylylene)-based nanocomposites containing various amounts of copper have been prepared by vacuum codeposition on a cooled substrate. On the basis of the relationship between the electrical conductivity of composites and the volume content of copper, the percolation threshold has been estimated as 10%. This value coincides with the corresponding parameter derived from the concentration dependence of the electrical resistance temperature coefficient. It has been shown that the conductivity of the nanocomposites increases with the moisture content and their moisture sensitivity decreases with an increase in the content of copper from a concentration of 4%. An analysis of the kinetics of the sensor response measured for various moisture contents has shown that the capacity structure of the samples rearranges during adsorption of water. According to the advanced model, the value of the sensor response is determined by a reduction in the height of the potential barrier between nanoparticles and a change in the fractal characteristics of nanocomposites that results from the adsorption of water molecules.     

Structure of a poly(2,5‐benzimidazole)/phosphoric acid complex

As‐cast films of poly(2,5‐benzimidazole) exhibit uniplanar orientation in which the planes of the aromatic rings lie parallel to the film surface. Upon doping with phosphoric acid, the original crystalline order is lost, but the doped film can be stretched to produce films with uniaxial orientation. After thermal annealing at 540 °C, nine Bragg reflections are resolved in the fiber diagram, and these are indexed by an orthorhombic unit cell with the dimensions a = 18.1 Å, b = 3.5 Å, and c = 11.4 Å, containing four monomer units of two chains. The absence of odd‐order 00l reflections points to a 2₁ chain conformation, which is probably planar so that the aromatic units can be stacked along the b axis. The water and phosphoric acid contents of the crystalline structure cannot be determined exactly because of the presence of extensive amorphous regions that probably have different solvation. The best agreement between the observed and calculated intensities is for an idealized structure containing two phosphoric acids and two water molecules per unit cell. However, the phosphoric acid is probably present mainly in the form of pyrophosphoric acid and its higher oligomers. In addition, the X‐ray data are consistent with a more disordered structure containing chains with random (up and down) polarity and a lack of c‐axis registry. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2576–2585, 2004