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Encapsulation o Silica Particles by a Thin Shell of Poly(Methyl) Methacrylate
No full textThe encapsulation of inorganic particles in a thin polymer shell generates core-shell hybrid materials. This chapter summarizes the main contributions from the literature for the preparation of a specific example of such hybrid materials, core-shell particles composed of an inner silica core and a poly(methyl methacrylate) outer shell. The spherical silica particles used in the examples have been obtained by sol-gel methods, most commonly via the Stöber synthesis and the reverse phase microemulsion method. Depending on the choice of method for their encapsulation, the surfaces of the silica particles have been modified to promote formation of poly(methyl methacrylate) at their surface. Surface modification methods may include the covalent attachment of a polymerizable organosilane which may co-polymerize with the monomer in conventional radical polymerization or the covalent attachment of an initiator for controlled living radical polymerization. The methods presented in this chapter will include conventional in situ heterophase radical polymerization methods, namely emulsion, emulsifier-free emulsion, precipitation, dispersion, and vapour phase polymerization. We also provide several examples of surface initiated atom transfer radical polymerization, for the controlled growth of poly(methyl methacrylate) brushes on silica particle
Encapsulation of submicrometer-sized silica particles by a thin shell of poly(methyl methacrylate)
No full textPolymer encapsulation of submicrometer-sized silica particles by synthesis of the polymer shell,
poly(methyl methacrylate) under static conditions in a reaction medium free of surfactants and stabilizing
agents is described. The Stöber method, a base-catalyzed hydrolysis and condensation of tetraethyl
orthosilicate is used for the synthesis of the monodisperse colloidal dispersion of silica particles.
The silica particles are subsequently modified in situ with the surface grafting of the silane coupling
agent, 3-(trimethoxysilyl)propyl methacrylate. Encapsulation is achieved using tetraethyl orthosilicate
as a reaction medium, in which a thermally initiated radical polymerization of methyl methacrylate
is promoted in the presence of the modified particles by a seeding method which leads to a thin
coating of poly(methyl methacrylate), and hence silica core–shell particles. The complete encapsulation
of individual silica spheres by poly(methyl methacrylate) is visually evidenced by TEM microscopy which
reveals the presence of a polymer shell coating up to 10 nm. Evidence for the presence of a poly(methyl
methacrylate) shell is further corroborated by DSC/TGA, DRIFT-IR and NMR measurements
Evolution of the Nonionic Inverse Microemulsion−Acid−TEOS System during the Synthesis of Nanosized Silica via the Sol−Gel Process
No full textThe cyclohexane−igepal inverse microemulsion, comprehensively established for the synthesis of silica nanoparticles in an ammonia-catalyzed sol−gel process, was alternatively studied with an acid-catalyzed sol−gel process. Tetraethyl orthosilicate (TEOS) was used as the silica precursor, while two different aqueous phases containing either HNO3 or HCl at two different concentrations, 0.1 and 0.05 M, were examined in the presence and in the absence of NaF, a catalyst of the condensation step. The evolution of the overall reacting system, specifically hydrolysis and polycondensation of reaction intermediates, was monitored in situ by time-resolved small-angle X-ray scattering. No size variation of the inverse micelles was detected throughout the sol−gel process. Conversely, the density of the micellar core increased after a certain time interval, indicating the presence of the polycondensation product. The IR spectra of the reacting mixture were in agreement with such a hypothesis. 1H and 13C NMR measurements provided information on the soluble species, the surfactant, and TEOS. The TEOS consumption was well fitted by means of an exponential decay, suggesting that a first-order kinetics for TEOS transpires in the various systems examined, with rate constants dependent not only on the acid concentration but also on its nature (anion specific effect), on the presence of NaF, and on the amount of water in the core of the inverse micelle. The self-diffusion coefficients, determined by means of PGSTE NMR, proved that a sizable amount of the byproduct ethanol was partitioned inside the inverse micelles. Characterization of the final product was carried out by means of thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and transmission electron microscopy (TEM), which concurrently confirmed that the silica isolated from the inverse nonionic microemulsion is not significantly different from the product of a bulk acid-catalyzed sol−gel synthesis. TEM micrographs illustrated particles with diameters smaller than the diameter of the inverse micelles as determined by SAXS, due to a shrinkage effect, in addition to nanostructured aggregates in the range 20−100 nm
Synthesis and optical properties of sub-micron sized rare earth-doped zirconia particles
No full textSub-micron sized crystalline particles of Eu3+ and Er3+-doped zirconia (ZrO2) were prepared via a wet chemical sol–gel route and post synthesis annealing. The doping was achieved by introduction of the respective rare earth salts into the zirconia precursor solution, with insitu generation of sodium chloride for stabilization of the particle surface during growth. A series of materials with differing europium content, nominally 0.3, 3 and 6 mol% within the ZrO2 lattice, were prepared for respective comparison and characterization of their optical properties following annealing at 700 °C. Average emission lifetimes of up to 2.3 ms were observed for the Eu3+-doped particles. Particle sizes, approximated from SEM micrographs, were observed in the range 250–400 nm. The synthesis of Er3+-doped ZrO2 particles (0.5 mol%) produced 300 nm sized particles which exhibited emission in the visible and infrared regions after annealing at 1000 °C. X-ray diffraction (XRD) with Rietveld analysis for phase quantification, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and photoluminescence (PL) spectra were used to characterize the samples
Mesoporous silica nanoparticles with tunable pore size for tailored gold nanoparticles
Full text linkThe aim of this paper was to verify a
possible correlation between the pore-size of meso-
porous silica nanoparticles (MSNs) and the sizes of
gold nanoparticles (AuNPs) obtained by an impreg-
nation of gold(III) chloride hydrate solution in the
MSNs, followed by a specific thermal treatment.
Mesoporous silica nanoparticles with tunable pore
diameter were synthesized via a surfactant-assisted
method. Tetraethoxysilane as silica precursor, cetyl-
trimethylammonium bromide (CTAB) as surfactant
and toluene as swelling agent were used. By varying
the CTAB–toluene molar ratio, the average dimension
of the pores could be tuned from 2.8 to 5.5 nm.
Successively, thiol groups were grafted on the surface
of the MSNs. Finally, the thermal evolution of the gold
salt, followed by ‘‘in situ’’ X-ray powder diffraction
(XRPD) and thermogravimetric analysis (TGA),
revealed an evident correlation among the degradation
of the thiol groups, the pore dimension of the MSNs
and the size of the AuNPs. The samples were
characterized by means of nitrogen adsorption–
desorption, transmission electron microscopy, small-
angle X-ray scattering, XRPD ‘‘in situ’’ by synchro-
tron radiation, and ‘‘ex situ’’ by conventional tech-
niques, diffuse reflectance infrared Fourier transform
spectroscopy, and TGA
In situsynthesisofEu(Tp)3 complex inside the pores of mesoporous silica nanoparticles
No full textA routeforeuropium(III)tris-pyrazolylboratecomplex[Eu(Tp)3] formationinsidetheporesof
mesoporous silicananoparticles(MSNs)hasbeenestablishedtoyieldahighlyluminescentnanostruc-
tured hybrid.Twodifferentinsituchemicalprecipitationtechniqueshavebeenexploredforthe
nanoencapsulation oftheEu3+ complex[Eu(Tp)3] insidetheporechannelsofmesoporoussilica
nanoparticles, byvaryingthesequenceorderofprecursorimpregnation.Inthe first method,theEu
salt isintroducedintotheporesbywetimpregnationandremovalofthesolvent,followedbywet
impregnation oftheligand.Inthesecondapproach,theadditionsequenceisreversed.Theimportanceof
the additionsequencewasdemonstratedbythesuccessfulformationoftheeuropium(III)tris-pyrazolyl
boratecomplexintheporenetworkbyfollowingthe first approach.Theobservedpyrazol-1-ylborate
(Tp) toEu3+ intramolecular energytransfer,i.e.,theantennaeffect,verified theformationofthecomplex.
Photoluminescencespectroscopy,X-raydiffraction,N2 adsorption, FTIRspectroscopy,andTEMwereused
to characterizethematerial
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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