Application of silica nanoparticles as fillers in the preparation of nanocomposite of polymers has drawnmuch attention, due to the increased demand for new materials with improved thermal, mechanical, physical, and chemical properties. Recent developments in the synthesis of monodispersed, narrow-size distribution of nanoparticles by sol-gel method provide significant boost to development of silica-polymer nanocomposites. This paper is written by emphasizing on the synthesis of silica nanoparticles, characterization on size-dependent properties, and surface modification for the preparation of homogeneous nanocomposites, generally by sol-gel technique. The effect of nanosilica on the properties of various types of silica-polymer composites is also summarized.

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Synthesis of silica nanoparticles by sol-gel: size-dependent properties, surface modification, and applications in silica-polymer nanocomposites — a review

Polymer properties, such as their mechanical strength, barrier properties, and dielectric response, can be dramatically improved by the addition of nanoparticles. This improvement is thought to be because the surface area per unit mass of particles increases with decreasing particle size, R, as 1/R. This favorable effect has to be reconciled with the expectation that at small enough R the nanoparticles must behave akin to a solvent and cause a deterioration of properties. How does this transition in behavior from large solutes to the solvent limit occur? We conjecture that for small enough particles the layer of polymer affected by the particles (“bound” polymer layer) must be much smaller than that for large particles: the favorable effect of increasing particle surface area can thus be overcome and lead to the small solvent limit with unfavorable mechanical properties, for example. To substantiate this picture requires that we measure and compare the “bound polymer layer” formed on nanoparticles with th...

Immobilized polymer layers on spherical nanoparticles

The static and dynamic properties of poly(2-vinylpyridine)/silica nanocomposites are investigated by temperature modulated differential scanning calorimetry, broadband dielectric spectroscopy (BDS), small-angle X-ray scattering (SAXS), and transmission electron microscopy. Both BDS and SAXS detect the existence of an interfacial polymer layer on the surface of nanoparticles. The results show that whereas the calorimetric glass transition temperature varies only weakly with nanoparticle loading, the segmental mobility of the polymer interfacial layer is slower than the bulk polymer by 2 orders of magnitude. Detailed analysis of BDS and SAXS data reveal that the interfacial layer has a thickness of 4–6 nm irrespective of the nanoparticle concentration. These results demonstrate that in contrast to some recent articles on polymer nanocomposites, the interfacial polymer layer is by no means a “dead layer”. However, its existence might provide some explanation for controversies surrounding the dynamics of poly...

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Dynamics at the Polymer/Nanoparticle Interface in Poly(2-vinylpyridine)/Silica Nanocomposites

Concepts and conflicts in nanoparticles reinforcement to polymers beyond hydrodynamics

The effect of nanosized silica particles on the properties of poly(vinyl acetate) (PVAc) was investigated for a range of silica concentrations encompassing the percolation threshold. The quantity of polymer adsorbed to the particles (“bound rubber”) increased systematically with silica content and was roughly equal to the quantity shielded from shear stresses (“occluded rubber”). This bound and occluded polymer attained a level of ∼12% at a silica volume content of 28%; nevertheless, the glass transition properties of the PVAc, including the glass transition temperature, local segmental relaxation function and relaxation times, and the changes in thermal expansion coefficient and heat capacity at Tg, were unaffected by the interfacial material. That is, there is no indication that the local segmental dynamics of the chains adjacent to silica particles differ from the motions of the bulk chains. Interestingly, the volume sensitivity of the segmental dynamics, as determined from the scaling exponent γ in th...

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Effect of Silica Nanoparticles on the Local Segmental Dynamics in Poly(vinyl acetate)

The amount of immobilized polymer in PMMA SiO2 nanocomposites determined from calorimetric data

For semicrystalline polymers there is an ongoing debate at what temperature the immobilized or rigid amorphous fraction (RAF) devitrifies (relaxes). The question if the polymer crystals aremelting first and simultaneously theRAFdevitrifies or the RAF devitrifies first and later on the crystals melt cannot be answered easily on the example of semicrystalline polymers. This is because the crystals, which are the reason for the immobilization of the polymer, often disappear (melt) in the same temperature range as theRAF. For polymer nanocomposites the situation is simpler. Silica nanoparticles do notmelt or undergo other phase transitions altering the polymer–nanoparticle interaction in the temperature range where the polymer is thermally stable (does not degrade). The existence of an immobilized fraction in PMMA SiO2 nanocomposites was shown on the basis of heat capacitymeasurements at the glass transition of the polymer. The results were verified by enthalpy relaxation experiments below the glass transition. The immobilized layer is about 2 nm thick at low filler content if agglomeration is not dominant. The thickness of the layer is similar to that found in semicrystalline polymers and independent from the shape of the nanoparticles. Nanocomposites therefore offer a unique opportunity to study the devitrification of the immobilized fraction (RAF) without interference of melting of crystals as in semicrystalline polymers. It was found that the interaction between the SiO2 nanoparticles and the PMMA is so strong that no devitrification occurs before degradation of the polymer. No gradual increase of heat capacity or a broadening of the glass transitionwas found. The cooperatively rearranging regions (CRR) are either immobilized or mobile. No intermediate states are found. The results obtained for the polymer nanocomposites support the view that the reason for the restricted mobility must disappear before the RAF can devitrify. For semicrystalline polymers this means that rigid crystals must melt before the RAF can relax. 2007 Elsevier Ltd. All rights reserved.

The amount of immobilized polymer in PMMA SiO$_{2}$ nanocomposites determined from calorimetric data

The superconducting and electrotransport properties were studied in Y0.89R0.11Ba2Cu3O7−δ polycrystals, with the same concentration of rare earth metal elements (R = La, Pr) but with different valence and magnetic moments, by recording the temperature dependence ρ(T) curves of the resistivity in the temperature range (77-300) K. It was considered the influence of La and Pr on the width of the superconducting transition of the samples (ΤC), its critical temperature (determined by the onset of transition - ΤCon and the offset - Tc0), the absolute values of the resistivity of the fluctuation and normal regions ρ(300 K) and ρ(100 K)], as well as on their ratio [a = ρ(300 K)/ρ(100 K)]. It has been revealed that there is a certain correlation between the aforementioned characteristics. It is emphasized that Pr is more critical than La in terms of its effect on characteristics. It is believed that the different behavior of the correlations observed between these characteristics for La and Pr is largely due to the differences in their ionic radii, displayed valence, as well as their intrinsic magnetic moments. It is shown that these differences in the case of La and Pr are manifested in the form of the phase separation phenomenon and the "Pr- anomaly", respectively.

Superconducting and Electrotransport Properties of Y0.89R0. 11Ba2Cu3O7−δ (R=La, Pr) Polycrystals

Structural rearrangements of the lyotropic liquid–crystal system sodium salt of di–(2–ethylhexyl) ester of sulfosuccinic acid (AOT) –n–heptane – water under the influence of vitamin E were studied by X–ray diffraction. The studies were carried out in the concentration range from 50 to 80% AOT+E in a mixed solvent n–heptane/water. The studies were carried out with a gradual increase in the concentration of vitamin E in the system and with an increase in the concentration of AOT + E in a solution of n–heptane/water. It has been established that, in the model liquid crystal system under study, vitamin E penetrating into the structure of the liquid crystal phase AOT–n–heptane–water leads to structural rearrangements and compactization of the molecular packing of the “smooth” phase.

Investigation of structural changes in the liquid crystal system AOT / n–heptane–water under the influence of vitamin E by the method of X–ray analysis

Albert Sargsyan

Papers

The amount of immobilized polymer in PMMA SiO2 nanocomposites determined from calorimetric data

The amount of immobilized polymer in PMMA SiO$_{2}$ nanocomposites determined from calorimetric data

Superconducting and Electrotransport Properties of Y0.89R0. 11Ba2Cu3O7−δ (R=La, Pr) Polycrystals

Investigation of structural changes in the liquid crystal system AOT / n–heptane–water under the influence of vitamin E by the method of X–ray analysis