Silica/poly(methyl methacrylate) nanocomposite latex particles have been synthesized by emulsion polymerization of methyl
methacrylate using a nonionic surfactant: nonylphenol poly(oxyethylene) and three different initiators, namely: 2,2′-azobis(2-amidinopropane)
dihydrochloride (AIBA), potassium persulfate (KPS) and azobis(isobutyronitrile) (AIBN), being cationic, anionic and nonionic,
respectively. A silica sol with an average diameter of 68 nm was used as the seed. The polymerization reaction was conducted
under alkaline conditions in order to evaluate the role of the surface charge of the hydrophilic silica on the coating reaction.
AIBA was found to be adsorbed on the silica surface owing to electrostatic interactions of the amidine function of the cationic
initiator with the silanolate groups of the oxide surface, while the anionic and the nonionic initiators did not adsorb on
silica under the same conditions. Nonetheless, whatever the nature of the initiator, polymerization took place on the silica
particles as evidenced by transmission electron microscopy. The extent of interaction between the inorganic surface and the
polymer particles was quantified by means of ultracentrifugation and a material balance. As much as 65% by weight of the total
polymer formed was found to be present at the silica surface using AIBA, while only 40% for KPS and 25% for AIBN was found
to cover the silica particles under alkaline conditions. We demonstrate that by using a cationic initiator and by controlling
the pH of the suspension it is possible to significantly decrease the amount of free polymer. Coating of the silica particles
took place through a kind of in situ heterocoagulation mechanism.
Received: 8 December 2000 Accepted: 22 February 2001 相似文献
Chitosans having three different degrees of acetylation (DA) were studied in acid solution using the uranyl staining technique and electron microscopy. Strings of approximately spherical aggregates were seen. The aggregates were interpreted as micelle-like agglomerates formed by almost fully acetylated polysaccharide, interconnected by blocks of almost fully deacetylated polysaccharide stretched by electrostatic repulsion. These agglomerates include NH3+ groups which produce electrostatic swelling of the agglomerates, giving a radius proportional to the degree of deacetylation. The length of the strings are also proportional to the deacetylation degree. These strings are extended because of the electrostatic repulsion between charged ammonium groups. 相似文献
Fumed silica Cab-O-Sil HS-5 grafted with 3-aminopropyldimethylsilyl (APDMS), butyldimethylsilyl (BDMS), octadecyldimethylsilyl (ODDMS), and trimethylsilyl (TMS) groups of different concentrations were studied using photon correlation spectroscopy, electrophoresis, potentiometric titration, and nitrogen adsorption methods. Calculations of both electrophoretic mobility and zeta potential were performed taking into consideration the topological structure of "porous" aggregates of primary particles of fumed silica using two different approaches. Changes in surface charge density and the textural, aggregative, and electrophoretic characteristics of modified silicas depend on the type of grafted OSC, its chain length, polarity, and surface concentration. Particle swarms of initial and modified silicas in aqueous suspension are typically characterized by bimodal size distributions between 20 and 300 nm (aggregates) and 1-3 microm (agglomerates of aggregates). The difference between parameter values computed according to the Smoluchowski theory and improved approaches increases with increasing pH and is concordant for both mobility and zeta potential. 相似文献
Structures of the micelles which are formed by the chiral oligopeptide surfactant N-dodecanoyl-L-proline tripeptide anions have been examined using small-angle neutron scattering spectral analysis. Results show that the
chiral N-dodecanoyl-L-proline trimeric anions may form a spherical micelle with an aggregation number of 36 and that the oligopeptide portions
with a poly-L-proline I-type helical structure are saturated with water.
Received: 21 March 2001 Accepted: 5 April 2001 相似文献
The full configuration interaction method in the space of fractionally occupied unrestricted natural orbitals (UNO-CAS method)
is extended to excited states as well as to strongly correlated and reactive systems with large active spaces. This is accomplished
by␣using restricted active space (RAS) wave functions introduced by Olsen et al. [(1988) J Chem Phys 89: 2185] and using the
UNOs without the expensive orbital optimization step. In RAS, the space of active orbitals is subdivided into three groups:
a group with essentially doubly occupied orbitals (RAS1), the usual CAS space (RAS2), and a space with weakly occupied active
orbitals (RAS3). We select these spaces on the basis of the occupation numbers of the UNOs. All possible electron distributions
are allowed in the usual CAS space, but the number of vacancies is limited in RAS1 and the number of electrons is limited
in RAS3. We discuss an efficient algorithm for generating a RAS wave function. This is based on the Handy-Knowles determinantal
expansion with an addressing scheme adopted for the restricted expansion. Results for both ground and excited states of azulene
and free base porphyrin are presented.
Received: 16 July 1998 / Accepted: 7 August 1998 / Published online: 19 October 1998 相似文献
Mesoporous 1,4‐phenylene‐silica nanorings were prepared using cetyltrimethylammonium bromide (CTAB) and (S)‐2‐methyl‐1‐butanol as a chiral dopant in concentrated aqueous NH3 solutions. Transmission electron microscopy images of the samples indicated that the nanorings were formed by bending nanorods 360°. With increasing the stirring speed or the (S)‐2‐methyl‐1‐butanol/CTAB molar ratio, the morphologies of mesoporous 1,4‐phenylene‐silicas changed from helical nanofibers to nanorings, and then to nano‐saddles. Circular dichroism spectra of these hybrid silicas indicated that they were chiral. 相似文献
A size-consistent ab initio formalism to calculate correlation corrections to ionization potentials as well as electron affinities
of periodic systems is presented. Our approach is based on a Hartree-Fock scheme which directly yields local orbitals without
any a posteriori localization step. The use of local orbitals implies non-zero off-diagonal matrix elements of the Fock operator,
which are treated as an additional perturbation and give rise to localization diagrams. Based on the obtained local orbitals,
an effective Bloch Hamiltonian is constructed to second order of perturbation theory with all third-order localization diagrams
included. In addition, the summation of certain classes of diagrams up to infinite order in the off-diagonal Fock elements
as well as the Epstein-Nesbet partitioning of the full Hamiltonian are discussed. The problem of intruder states, frequently
encountered in many-body perturbation theory, is dealt with by employing the theory of intermediate Hamiltonians. As model
systems we have chosen cyclic periodic structures up to an oligoethylene ring in double-zeta basis; however, the theory presented
here straightforwardly carries over to infinite periodic systems.
Received: 30 April 1998 / Accepted: 27 July 1998 / Published online: 7 October 1998 相似文献
Indentation tests were carried out on a carbon black filled rubber sample on different length scales. The experiments covered
the range from aggregation of particles on the submicron scale up to structures which represent the bulk properties of the
sample on the millimeter scale. The local stiffness was used to visualize the areas investigated; therefore, mechanical images
were obtained for all length scales. So-called “mechanical units” were defined for every scale. The size distribution curves
for the mechanical units were analyzed and they were found to be non-Gaussian-shaped for every scale. Moreover, the distribution
curves of the mechanical units are similar to the distribution curves of particles and aggregates obtained by electron microscopy
reported in the literature. Evaluation by means of fractal analysis led to fractal dimensions for the mechanical units. It
could be shown in the present case that the fractal dimension D≈1.24 of the mechanical units in the range of submicrons up to several hundred microns is in good agreement with that of the
filler aggregates proposed in the literature. Furthermore, D is constant over a wide range of about 6 decades in area scale starting from aggregates up to the size of agglomerates. This
leads to the conclusion that the local arrangement of the filler ensembles seems to be self-similar from the smallest scale
of aggregation of particles up to the largest formation observed by indentation testing on the millimeter scale.
Received: 15 May 2001 Accepted: 18 August 2001 相似文献
We studied the adsorption of two neutral polymers [poly(vinyl pyrrolidone) and poly(vinyl alcohol) (PVA)] on negatively charged
liposomes composed of 25:2:3 (molar ratio) soy lecithin/dicetyl phosphate/cholesterol.The liposomes were prepared in buffered
solution at pH 7.4 and were mixed with the solution of the polymers in the desired polymer/lipid ratios. Adsorption was measured
by determination of the equilibrium bulk concentration of the polymer. Protamine hydrochloride was used to aggregate the liposomes
with polymers adsorbed on their surface and to facilitate their separation from the equilibrium bulk solution. In the case
of PVA, quantitative adsorption measurements with a specific reagent were possible. Adsorption isotherms were recorded at
25 ± 0.2 °C. It was concluded that adsorbed and nonadsorbed PVA molecules are in equilibrium even at low polymer/lipide ratios.
The results were confirmed by dynamic laser light scattering, X-ray diffraction and thermal activity monitoring experiments.
Received: 13 October 2000 Accepted: 8 March 2001 相似文献
The electrochemical redox reactions: Fe(CN)64−−e−↔ Fe(CN)63−, Ru(NH3)63++e−↔ Ru (NH3)62+ and Fc(CH2OH)2−e−↔ Fc(CH2OH)2+ (Fc–ferrocene) were investigated in tetrabutylammonium halide hydrates at temperatures below and above the electrolyte melting
point. They were studied by cyclic voltammetry, potential step chronoamperometry and impedance spectroscopy. Freezing of
the electrolyte affects both the shape and position of the cyclic voltammogram on the potential scale. Also the shapes of
the current-time relationship and the impedance spectra change at temperatures below the melting point. It has been proposed
that this behaviour is caused by slow transport of the reactant and the heterogeneous nature of the electrolyte. The activation
energies of reactant transport are about four times larger in frozen electrolytes than those in liquid. It has been concluded
that reactant transport is restricted to the intergrain space of the electrolyte.
Received: 16 December 1997 / Accepted: 10 February 1998 相似文献
Polystyrene/styrene–divinylbenzene copolymer composite particles with different cross-linking densities were produced by
seeded copolymerization for (styrene/divinylbenzene)-swollen polystyrene particles prepared by utilizing the dynamic swelling
method (DSM) which was proposed by the authors in 1991. Using the cross-linked, composite particles as seeds, styrene-adsorbed
(swollen) composite particles having snowman shapes were prepared by the DSM. With a decrease in the cross-linking density
in the composite particles, the volume of the composite particle which was embedded in a spherical styrene phase in the snowman-shaped,
styrene-adsorbed particle increased and the contact angle of the styrene phase on the composite particle decreased. In the
DSM process, the absorption stage of styrene in the composite particles and the adsorption stage thereon were clearly observed.
This suggests that the cross-linking density of the composite particles greatly affects the morphology of the snowman-shaped
particles.
Received: 13 December 2000 Accepted: 14 March 2001 相似文献
Two synthetic routes to surface-aminated polypyrrole–silica nanocomposite particles have been investigated. Route 1 involved
the initial synthesis of homopolypyrrole – silica particles as described previously, followed by surface amination using 3-aminopropyltriethoxy-silane.
In Route 2 aminated polypyrrole–silica particles were synthesized directly by copolymerising an N-substituted aminopyrrole comonomer with pyrrole in the presence of an ultrafine silica sol. Both types of aminated particles
were characterized in terms of their particle size and morphology, long-term colloid stability and degree of amination using
transmission electron microscopy, disc centrifuge photosedimentometry and zeta potential measurements, respectively.
Received: 19 May 1998 Accepted: 15 June 1998 相似文献
Several series of fumed silicas and mixed fumed oxides produced and treated under different conditions were studied in gaseous and liquid media using nitrogen and water adsorption-desorption, mass spectrometry, FTIR, NMR, thermally stimulated depolarization current (TSDC), photon correlation spectroscopy (PCS), zeta potential, potentiometric titration, and Auger electron spectroscopy methods. Aggregation of primary particles and adsorption capacity (Vp) decrease and hysteresis loops of nitrogen adsorption-desorption isotherms becomes shorter with decreasing specific surface area (S(BET)). However, the shape of nitrogen adsorption-desorption isotherms can be assigned to the same type independent of S(BET) value. The main maximum of pore size distribution (gaps between primary nonporous particles in aggregates and agglomerates) shifts toward larger pore size and its intensity decreases with decreasing S(BET) value. The water adsorption increases with increasing S(BET) value; however, the opposite effect is observed for the content of surface hydroxyls (in mmol/m2). Associative desorption of water (2(SiOH)-->SiOSi+H2O) depends on both the morphology and synthesis conditions of fumed silica. The silica dissolution rate increases with increasing S(BET) and pH values. However, surface charge density and the modulus of zeta-potential increase with decreasing S(BET) value. The PCS, 1H NMR, and TSDC spectra demonstrate rearrangement of the fumed silica dispersion depending on the S(BET) value and the silica concentration (C(SiO2)) in the aqueous suspensions. A specific state of the dispersion is observed at the C(SiO2) values corresponding to the bulk density of the initial silica powder. 相似文献
Alkyl (S)-2-ammonium-2-isobutylacetate p-toluenesulfonate formed organogel in nonpolar solvents. The gels exhibited thermally reversible sol–gel phase transitions.
UV spectroscopic study suggested that dodecyl (S)-2-ammonium-2-isobutylacetate p-toluenesulfonate forms reversed micelle-like aggregate at low concentration in a nonpolar solvent. Circular dichroism spec-troscopy
indicated that component molecules of the reversed micelle-like aggregate are cooperatively organized and result in chiral
aggregate. The huge fibrous aggregate responsible for gelation was observed with transmission electron microscopy. The accumulation
and rearrangement of reversed micelle-like aggregate resulted in the formation of huge fibrous aggregates. A gathering of
numerous fibrous aggregates formed the three-dimensional network to immobilize the isotropic liquid.
Received: 24 June 1997 Accepted: 3 October 1997 相似文献
Assembly of mesostructured silica using Pluronic P123 triblock copolymer (EO(20)-PO(70)-EO(20)) and n-butanol mixture is a facile synthesis route to the MCM-48-like ordered large mesoporous silicas with the cubic Iad mesostructure. The cubic phase domain is remarkably extended by controlling the amounts of butanol and silica source correspondingly. The extended phase domain allows synthesis of the mesoporous silicas with various structural characteristics. Characterization by powder X-ray diffraction, nitrogen physisorption, scanning electron microscopy, and transmission electron microscopy reveals that the cubic Iad materials possess high specific surface areas, high pore volumes, and readily tunable pore diameters in narrow distribution of sizes ranging from 4 to 12 nm. Moreover, generation of complementary pores between the two chiral channels in the gyroid Iad structure can be controlled systematically depending on synthesis conditions. Carbon replicas, using sucrose as the carbon precursor, are obtained with either the same Iad structure or I4(1)/a (or lower symmetry), depending on the controlled synthesis conditions for silica. Thus, the present discovery of the extended phase domain leads to facile synthesis of the cubic Iad silica with precise structure control, offering vast prospects for future applications of large-pore silica materials with three-dimensional pore interconnectivity. 相似文献
Silica bodies of orchard grass (Dactylis glomerata L.) were analysed by transmission electron microscopy and X-ray microanalysis. Silica bodies are inserted into the epidermis
of the leaf and consist of a rather compact amorphous silica and low carbon. In particular regions of the interface between
silica body and cell wall, silica is crossed by electron opaque strands, which are similar to filaments of the cell wall and
most probably represent cellulose fibrils. The presence of a higher carbon content in this region than in the compact silica
supports this notion. Cells with silicified walls or lumen were found apart from typical silica bodies. They most likely represent
early or intermediate stages of silica body formation. Some epidermal cells showed silica deposits between protoplast and
cell wall with small but regularly detected nitrogen and phosphorus peaks indicating a particular extracellular chemical composition
probably involved in silica accumulation. 相似文献
Silver nanoparticles (AgNPs) were obtained by a redox reaction, using a glucose-containing cyclosiloxane as a reduction agent and stabilizer. Then the AgNPs aqueous solution was used as the reaction medium for the sol-gel process, starting from tetraethylorthosilicate (TEOS) as silica precursor. The nanocomposite material resulted (SilAg) after solvent removal, aging and calcination and was investigated by infrared spectroscopy (FT-IR), atomic force microscopy (AFM), scanning electron microscopy coupled with energy dispersive X-ray system (SEM/EDX), transmission electron microscopy (TEM), energy-dispersive X-ray fluorescence spectroscopy (EDXRF), X-ray diffraction (XRD) and dynamic vapor sorption (DVS). The results were compared to model silicas obtained without silver. A higher condensation degree in SilAg was obtained due to the basic medium used in the first step and was confirmed by a sorption capacity lower than for the model silicas. The solid surface area calculated with GAB analysis using DVS data for the water vapors is 210 m2 g?1. The nanocomposite showed good catalytic activity for hydrogen peroxide decomposition. 相似文献
Summary: Three dimensional (3D) nanostructures of particulate silicas in natural rubber (NR) were observed for the first time by use of 3D transmission electron microscopy (3D‐TEM) combined with electron tomography. The method enabled us to visualize and evaluate structural characteristics in 3D space, such as the size and the volume of in situ silica generated in the NR matrix by the sol‐gel reaction of tetraethoxysilane, at nanometer scale resolution.
The reconstructed mass density view of the silica in an in situ silica‐filled natural rubber vulcanizate, as determined by 3D‐TEM. 相似文献
In this study, the formation of silica–chitosan hybrid materials via sol–gel process under pH values of 2–6 were investigated
using N2 sorption analysis, scanning electron microscopy, transmission electron microscopy, thermal analysis and zeta potential analyzer.
The hierarchical structure consisting of meso- and macropore was formed when pH value was higher than 2. Mesopores were formed
as the interparticle channels of silica nanoparticles aggregates, whereas macropores were the void between the aggregates
(clusters). The clusters size was decreased with increasing the pH value, resulting in the increase of the macroporosity.
The thermal conductivity of the products was controlled in the range of 0.06 and 0.13 W m−1 K−1 by varying the product porosity between 88 and 69% (pH 6 and pH 2, respectively). 相似文献