Periodic cation segregation in Cs0.44[Nb2.54W2.46O14] quantified by high-resolution scanning transmission electron microscopy.

The atomic structure of Cs0.44[Nb2.54W2.46O14] closely resembles the structure of the most active catalyst for the synthesis of acrylic acid, the M1 phase of Mo10V2(4+)Nb2TeO42-x. Consistently with observations made for the latter compound, the high-angle electron scattering signal recorded by scanning transmission electron microscopy shows a significant intensity variation, which repeats periodically with the projected crystallographic unit cell. The occupation factors for the individual mixed Nb/W atomic columns are extracted from the observed intensity variations. For this purpose, experimental images and simulated images are compared on an identical intensity scale, which enables a quantification of the cation distribution. According to our analysis specific sites possess low tungsten concentrations of 25%, whereas other sites have tungsten concentrations above 70%. These findings allow us to refine the existing structure model of the target compound, which has until now described a uniform distribution of the niobium and tungsten atoms in the unit cell, showing that the similarity between Cs0.44[Nb2.54W2.46O14] and the related catalytic compounds also extends to the level of the cation segregation.

Thermally induced twin polymerization of 4H-1,3,2-benzodioxasilines.

The twin monomer 2,2'-spirobi[4H-1,3,2-benzodioxasiline] (1) can be polymerized to nanostructured SiO2/phenolic-resin composite material by thermally induced twin polymerization. Thermally induced twin polymerization represents a way to produce nanocomposites simply by thermal induction of twin monomers. Besides 1, the thermal reaction of several related salicylic (2-oxybenzylic) silicon molecules has been investigated. The thermal cleavage of the molecules is studied by using several trapping reagents (e.g., vinyl compounds). A significant occurrence of quinone methide adducts indicates that the thermal mechanism proceeds not only by a ring opening at the oxymethylene position, but also with the ortho-quinone methide as a central or alternative intermediate. This is supported by product analyses of thermally initialized reactions of 1 and its substituted analogues as well as by quantum chemical calculations.

Twin polymerization at spherical hard templates: an approach to size-adjustable carbon hollow spheres with micro- or mesoporous shells.

Kitset hollow spheres: The combination of twin polymerization with hard templates makes hollow carbon spheres (HCSs) with tailored properties easily accessible. The thickness and pore texture of the HCS shells and also the diameter of the spherical cavity can be varied. The application potential of synthesized HCS is substantiated by an excellent cycling stability of lithium-sulfur batteries.

Scenarios and methods that induce protruding or released CNTs after degradation of nanocomposite materials.

ABSTRACT: Nanocomposite materials may be considered as a low-risk application of nanotechnology, if the nanofillers remain embedded throughout the life-cycle of the products in which they are embedded. We hypothesize that release of free CNTs occurs by a combination of mechanical stress and chemical degradation of the polymer matrix. We experimentally address limiting cases: Mechanically released fragments may show tubular protrusions on their surface. Here we identify these protrusions unambiguously as naked CNTs by chemically resolved microscopy and a suitable preparation protocol. By size-selective quantification of fragments we establish as a lower limit that at least 95 % of the CNTs remain embedded. Contrary to classical fiber composite approaches, we link this phenomenon to matrix materials with only a few percent elongation at break, predicting which materials should still cover their CNT nanofillers after machining. Protruding networks of CNTs remain after photochemical degradation of the matrix, and we show that it takes the worst case combinations of weathering plus high-shear wear to release free CNTs in the order of mg/m2/year. Synergy of chemical degradation and mechanical energy input is identified as the priority scenario of CNT release, but its lab simulation by combined methods is still far from real-world validation.

Elastic CNT-polyurethane nanocomposite: synthesis, performance and assessment of fragments released during use.

Intended for use in high performance applications where electrical conductivity is required, we developed a CNT-TPU composite. Such a composite can be prepared by melt processing (extrusion) on an industrial scale. Due to the known hazard upon inhalation of CNTs, we assessed three degradation scenarios that may lead to the release of CNTs from the composite: normal use, machining and outdoor weathering. Unexpectedly, we find that the relative softness of the material actually enhances the embedding of CNTs also in its degradation fragments. A release of free CNTs was not detected under any condition using several detection methods. However, since machining may induce a high acute dose of human exposure, we assessed the cytotoxicity potential of released fragments in the in vitro model of precision-cut lung slices, and found no additional toxicity due to the presence of CNTs. At very low rates over years, weathering degrades the polymer matrix as expected for polyurethanes, thus exposing a network of entangled CNTs. In a preliminary risk assessment, we conclude that this material is safe for humans in professional and consumer use.

Tin Oxide Nanoparticles and SnO2 /SiO2 Hybrid Materials by Twin Polymerization Using Tin(IV) Alkoxides.

Twin polymerization was used to prepare composite materials composed of SnO2 nanoparticles entrapped in a polymer matrix. Novel, well-defined tin-containing molecular precursors, so-called twin monomers, were synthesized by transesterification starting from Sn(OR)4 (R=tBu, tAm) to give Sn(OCH2 C4 H3 O)4 (1), [Sn(OCH2 C4 H3 S)4 ⋅HOCH2 C4 H3 S]2 (2), [Sn(OCH2 -2-OCH3 C6 H4 )4 ⋅HOCH2 -2-OCH3 C6 H4 ]2 (3), [Sn(OCH2 -2,4-(OCH3 )2 C6 H3 )4 ⋅HOCH2 -2,4-(OCH3 )2 C6 H3 ]2 (4), 2,2'-spirobi[4H-1,3,2-benzodioxastannine] (5), 2,2'-spirobi[6-methylbenzo(4H-1,3,2)-dioxastannine] (6), and 2,2'-spirobi[6-methoxybenzo(4H-1,3,2)dioxastannine] (7). 13 C and 119 Sn NMR spectroscopy in the solid state and in solution as well as IR spectroscopy and elemental analysis were used to characterize the tin alkoxides. The molecular structures of compounds 2 and 3 were determined by single-crystal X-ray diffraction analysis. The moisture sensitivity of the tin(IV) alkoxides was demonstrated by the formation of the tin oxocluster [Sn3 (µ3 -O)(µ2 -OH)(µ2 -OCH2 C4 H3 S)3 (OCH2 C4 H3 S)6 (HOCH2 C4 H3 S)]2 (2 a), a hydrolysis product of compound 2. Polymerization reactions in the melt (for 1 and 5) and in solution (for 2-4) resulted in cross-linked nanocomposites of the type polymer/SnO2 . Subsequent oxidation of the composites gave SnO2 with BET surface areas up to 178 m2 g-1 . Simultaneous twin polymerization of compounds 5-7 with the silicon derivative 2,2'-spirobi[4H-1,3,2-benzodioxasiline] resulted in the formation of polymer/SnO2 /SiO2 hybrid materials. Oxidation gave porous materials with SnO2 nanoparticles embedded in a silica network with BET surface areas up to 378 m2 g-1 . The silica acts as a crystal growth inhibitor, which prevents sintering of the SnO2 nanoparticles 20-32 nm in size.

Sulphur-doped porous carbon from a thiophene-based twin monomer.

Sulphur-doped carbon was synthesized using a thiophene-based twin monomer. While tetra(thiophene-2-ylmethoxy)-silane can be converted into sulphur containing nanocomposites, which lead to microporous sulphur-doped carbon, it is possible to produce additional mesopores by the use of templates. Thus, a variety of sulphur-doped carbon materials with tailored pore texture are available.

Carbon/carbon nanocomposites fabricated by base catalyzed twin polymerization of a Si-spiro compound on graphite sheets.

Defect-free microporous carbon layers on graphite can be produced by DABCO-catalyzed twin polymerization of 2,2'-spirobi[4H-1,3,2-benzodioxasiline] in a slurry polymerization and subsequent thermal transformation of the resulting phenolic resin into carbon.

On the lifecycle of nanocomposites: comparing released fragments and their in-vivo hazards from three release mechanisms and four nanocomposites.

Nanocomposites are the dominating class of nanomaterials to come into consumer contact, and were in general assumed to pose low risk. The first data is now emerging on the exposure from nanocomposites, but little is yet known about their hypothetical nanospecific physiological effects, giving ample room for speculation. For the first time, this comprehensive study addresses these aspects in a systematic series of thermoplastic and cementitious nanocomposite materials. Earlier reports that 'chalking', the release of pigments from weathered paints, also occurs for nanocomposites, are confirmed. In contrast, mechanical forces by normal consumer use or do-it-yourself sanding do not disrupt nanofillers (nanoparticles or nanofibers) from the matrix. Detailed evidence is provided for the nature of the degradation products: no free nanofillers are detected up to the detection threshold of 100 ppm. Sanding powders measuring 1 to 80 µm in diameter are identified with the original material, still containing the nanofillers. The potential hazard from aerosols generated by sanding nanocomposites up to the nuisance dust limit is also investigated. In-vivo instillation in rats is used to quantify physiological effects on degradation products from abraded nanocomposites, in comparison to the abraded matrix without nanofiller and to the pure nanofiller. In this pioneering and preliminary evaluation, the hazards cannot be distinguished with or without nanofiller.

Ab initio determination of the framework structure of the heavy-metal oxide Cs(x)Nb2.54W2.46O14 from 100 kV precession electron diffraction data.

The present work deals with the ab initio determination of the heavy metal framework in Cs(x)(Nb, W)(5)O(14) from precession electron diffraction intensities. The target structure was first discovered by Lundberg and Sundberg [Ultramicroscopy 52 (1993) 429-435], who succeeded in deriving a tentative structural model from high-resolution electron microsopy (HREM) images. The metal framework of the compound was solved in this investigation via direct methods from hk0 precession electron diffraction intensities recorded with a Philips EM400 at 100 kV. A subsequent (kinematical) least-squares refinement with electron intensities yielded slightly improved co-ordinates for the 11 heavy atoms in the structure. Chemical analysis of several crystallites by EDX is in agreement with the formula Cs(0.44)Nb(2.54)W(2.46)O(14). Moreover, the structure was independently determined by Rietveld refinement from X-ray powder data obtained from a multi-phasic sample. The compound crystallises in the orthorhombic space group Pbam with refined lattice parameters a=27.145(2), b=21.603(2), and c=3.9463(3)A. Comparison of the framework structure from electron diffraction with the result from Rietveld refinement shows an average agreement for the heavy atoms within 0.09 A.