Peter Gerstel

Selective Dispersion of Single-Walled Carbon Nanotubes with Specific Chiral Indices by Poly(N-decyl-2,7-carbazole)

Physico-chemical methods to sort single-walled carbon nanotubes (SWNTs) by chiral index are presently lacking but are required for in-depth experimental analysis and also for potential future applications of specific species. Here we report the unexpected selectivity of poly(N-decyl-2,7-carbazole) to almost exclusively disperse semiconducting SWNTs with differences of their chiral indices (n - m) ≥ 2 in toluene. The observed selectivity complements perfectly the dispersing features of the fluorene analogue poly(9,9-dialkyl-2,7-fluorene), which disperses semiconducting SWNTs with (n - m) ≤ 2 in toluene. The dispersed samples are further purified by density gradient centrifugation and analyzed by photoluminescence excitation spectroscopy. All-atom molecular modeling with decamer model compounds of the polymers and (10,2) and (7,6) SWNTs suggests differences in the π-π stacking interaction as origin of the selectivity. We observe energetically favored complexes between the (10,2) SWNT and the carbazole decamer and between the (7,6) SWNT and the fluorene decamer, respectively. These findings demonstrate that subtle structural changes of polymers lead to selective solvation of different families of carbon nanotubes. Furthermore, chemical screening of closely related polymers may pave the way toward simple, low-cost, and index-specific isolation of SWNTs.

Boron-modified polysilazane as a novel single-source precursor for SiBCN ceramic fibers: synthesis, melt-spinning, curing and ceramic conversion

A novel boron-modified polysilazane functionalised with Si- and N-bonded methyl groups has been synthesised and characterised by means of FT-IR and NMR spectroscopies as well as elemental analysis. Both Si- and N-bonded methyl groups inhibited extensive cross-linking to yield a tractable polymer which was successfully processed into polymer green fibers by a melt-spinning process. After the shaping processing, the N-bonded methyl groups offered the capability of facile polymer cross-linking in an ammonia atmosphere at 200 °C to increase the ceramic yield of the polymer and avoid inter-fiber fusion. The as-obtained cured fibers were subsequently pyrolysed at 1400 °C in a nitrogen atmosphere to provide amorphous and stable Si3.0B1.0C5.0N2.4 ceramic fibers with ca. 1.3 GPa in tensile strength, ca. 170 GPa in Youngs modulus and ca. 23 μm in diameter.

Correlation of boron content and high temperature stability in Si-B-C-N ceramics II

Abstract This study treats the preparation of polyborosilazanes which are obtained by hydroboration of oligovinylsilazane [–(H 2 CCH)SiHNH–] n with different amounts of H 3 B . SMe 2 . Pyrolysis of these precursors yields amorphous ceramic materials, which only differ in their relative boron content whereas their Si/C/N ratio is very similar. High temperature thermogravimetric analysis reveals that with increasing boron content the materials thermal degradation is shifted toward higher temperatures. Furthermore, the observed mass loss due to nitrogen evaporation during the materials decomposition is noticeably smaller in boron-rich materials than thermodynamically calculated. X-ray diffraction (XRD) experiments support the presence of crystalline silicon nitride in heat treated samples with a minimum boron content of 5 at.%. The decomposition temperature of this phase strongly depends on the amount of boron and can exceed 2000°C. However, high temperature stable materials are not only characterized by a definite B/N ratio, but also by the ability to develop appropriate microstructures.

Single chain self-assembly: preparation of α,ω-donor–acceptor chains via living radical polymerization and orthogonal conjugation

Alpha,omega-hydrogen donor/acceptor functional polymer strands are prepared via a combination of living radical polymerization and orthogonal conjugation and subsequently self-assembled as single chains to emulate--on a simple level--the self-folding behaviour of natural biomacromolecules.

Dynamic Covalent Chemistry on Surfaces Employing Highly Reactive Cyclopentadienyl Moieties

Silicon substrates coated with a bromide-terminated silane are transformed into highly reactive, cyclopentadiene covered analogues. These surfaces undergo rapid cycloaddition reactions with various dienophile-capped polymers. Mild heating of the substrates causes the retro-Diels-Alder reaction to occur, thus reforming the reactive cyclopentadiene surface, generating an efficiently switchable surface.

DNA-templated synthesis of ZnO thin layers and nanowires

In this paper, we report a novel synthetic approach towards electrically conductive ZnO nanowires close to ambient conditions using lambda-DNA as a template. Initially, the suitability of DNA to assemble ZnO nanocrystals into thin coatings was investigated. The ZnO nanowires formed on stretched and aligned lambda-DNA molecules were prepared via chemical bath deposition (CBD) of zinc acetate in methanol solution in the presence of polyvinylpyrrolidone (PVP). After 10 deposition cycles, the nanowires exceed 10 microm in length and the height can be varied from 12 to around 40 nm. The nanocrystalline structure of the ZnO wires was confirmed by high-resolution transmission electron microscopy (HRTEM). The electrical conductivity was found to be of the order of several Omega cm at room temperature in two terminal measurements.

Polymer-derived Si-C-N ceramics reinforced by single wall carbon nanotubes

Abstract Nanocomposites made of polymer-derived Si–C–N ceramic reinforced by single-wall carbon nanotubes (SWCNTs) were prepared for the first time. The synthesis procedure involved ultrasonic dispersion of the nanotubes into a liquid polysilazane precursor polymer, followed by cross-linking and thermolysis. With the aid of nanoindentation testing, dependence of the mechanical properties of the composites on the concentration and agglomeration state of SWCNTs, was studied. The nanotube-filled composites showed improved mechanical performance, as reflected by an increase in Youngs modulus which was found to be correlated with the microstructure of the composites, in particular the degree of dispersion of the nanotubes inside the matrix, whereas the hardness is hardly affected.

RAFT Mediated Polymerization of Methyl Methacrylate Initiated by Bergman Cyclization: Access to High Molecular Weight Narrow Polydispersity Polymers

The first RAFT mediated polymerization of methyl methacrylate initiated by diradicals derived from Bergman cyclization was performed employing 3,4-benzocyclodec-3-ene-1,5-diyne (BCDY) as diradical source and cyanoisopropyldithiobenzoate (CPDB) as RAFT agent. The polymerization was conducted in bulk at 80 °C for 3 h. The concentration of the enediyne was kept constant at 3.0 x 10⁻² mol · L⁻¹ and the RAFT agent concentration was varied between 0.0 mol · L⁻¹ and 2.4 x 10⁻¹ mol · L⁻¹. A detailed ESI-MS analysis reveals the absence of intramolecular termination reactions (ring formation) in the RAFT mediated system, which usually makes diradicalic initiation unfavorable. The presence of polymeric chains propagating at both ends could be confirmed. The conversion of the RAFT mediated polymerization was up to more than two times higher than the RAFT free polymerization at identical conditions. Thus, polymers with narrow polydispersities (1.1 ≤ PDI ≤ 1.5) even at very high molecular weights (near 400,000 Da) were obtained within modest reaction times.

Aqueous solution deposition of indium hydroxide and indium oxide columnar type thin films

Abstract Crystalline In(OH)3 and UV-emitting In2O3 thin films with a unique columnar morphology have been successfully prepared for the first time via a simple aqueous solution deposition method. In the present study, sulfonate-terminated self-assembled monolayers on single-crystal Si wafers were used as substrates for the film deposition. Uniform In(OH)3 thin films formed on self-assembled monolayers in aqueous solutions of InCl3.4H2O at 60°C. These films, containing columnar In(OH)3 crystals, showed preferential growth orientation along the [100] direction. The current understanding of the film formation mechanism was discussed. After annealing in air at 400°C, these films were converted to nanocrystalline In2O3 films without modification of the morphology. Photoluminescence emission from In2O3 films was reported at approximately 3.4eV, which was ascribed to the near-band-edge emission.

Nanostructured SiC/BN/C ceramics derived from mixtures of B3N3H6 and [HSi(Me)C≡C]n

Abstract Three borazine-modified polycarbosilanes were synthesized by reaction of poly[1,2-ethynediyl-(methylsilylene)], [HSi(Me)C≡C]n, and borazine, B3N3H6 using different stoichiometries. The polymeric precursors were transformed into inorganic Si–B–C–N materials by solid state thermolysis at 1400°C each in 88% yield. High temperature thermogravimetric analysis in an argon atmosphere showed no substantial decomposition reactions below 1800°C. Crystallization behavior up to 2000°C in a nitrogen atmosphere was studied by post-thermolysis heat treatment of as-obtained ceramics at various temperatures and subsequent examination of samples at room temperature using powder X-ray diffraction. Ceramics annealed at 1800°C were additionally characterized by transmission electron microscopy.