Characterization of maleimide dimers in photo-cross-linked copolyimide films

Copolyimide membranes are established materials for the separation of gaseous and liquid mixtures. Cross-linking of the polymer strands improves the physical and chemical stability. The photo-cross-linking of a 6FDA-ODA/6FDA-DABA 4?:?1 copolyimide membrane containing maleimide side groups as linker was investigated by FTIR spectroscopy. IR absorption spectra of the copolyimide backbone, 3-hydroxypropyldimethyl maleimide and the copolyimide functionalized with 3-hydroxypropyldimethyl maleimide were measured before and after different irradiation times and compared to each other. For band assignment a normal mode analysis was performed. The backbone of the polymer and the maleimide linker can be well distinguished due to their different spectral band positions. Only the films containing a maleimide moiety perform a photoreaction, the polymer backbone does not interfere. Based on the difference spectra and the results of the DFT calculations it was shown that the trans- and the cis-cycloadduct as well as the previously suggested 2-2'-adduct without a cyclobutane ring are formed upon UV irradiation. Evidence for an oxetane-like photoproduct was not found. Different time constants for the increase of the product bands were observed. The cycloadduct accumulates with a shorter time constant (τ = 2 to 5 min) than the 2-2'-adduct (τ = 75 min). The yield of the photo-cross-linking reaction was determined by spectral deconvolution and kinetic fitting of several marker bands. For the copolyimide synthesized in this work, a maximum value of 6% was reached. The stiffness of the copolyimide backbone inhibits further photo-cross-linking.     

Syntheses of Three New Pyridyl Thienopyridine Ligands via the Hurtley Reaction

The tridentate pyridyl thienopyridines 5‐phenyl‐7‐(pyridin‐2‐yl)thieno[2,3‐c]pyridine ( L1 ), 7‐(pyridin‐2‐yl)‐5‐(thiophen‐2‐yl)‐thieno[2,3‐c]pyridine ( L2 ) and 5,7‐di(pyridin‐2‐yl)thieno[2,3‐c]pyridine ( L3 ) have been synthesized via the Hurtley reaction. L1 and L2 were synthesized by condensing 3‐bromothiophene‐2‐carboxylic acid with phenyl‐1,3‐butanedione and 1‐thienyl‐1,3‐butanedione respectively. L3 was synthesized by condensing 3‐bromothiophene‐2‐carboxylic acid with benzoylacetonitrile. Ring closure and a subsequent Negishi or Stille cross‐coupling afforded L1 , L2 , and L3 in an overall yield of 20, 3, and 6%, respectively.     

Q-switched thulium-doped photonic crystal fiber laser

We report a novel, Tm-doped photonic crystal fiber (PCF) actively Q-switched oscillator that provides ~8.9 kW peak power with 435 μJ, 49 ns pulses at 10 kHz repetition rate at 2 μm wavelength. This fiber has a mode-field area >1000 μm2, the largest of any flexible PCF providing diffraction-limited beam quality to the best of our knowledge. As an application, the oscillator is used as pump to generate >350 nm broadening in ~50 m of SMF-28 fiber.     

Stability of a spatial polling system with greedy myopic service

This paper studies a spatial queueing system on a circle, polled at random locations by a myopic server that can only observe customers in a bounded neighborhood. The server operates according to a greedy policy, always serving the nearest customer in its neighborhood, and leaving the system unchanged at polling instants where the neighborhood is empty. This system is modeled as a measure-valued random process, which is shown to be positive recurrent under a natural stability condition that does not depend on the server??s scan radius. When the interpolling times are light-tailed, the stable system is shown to be geometrically ergodic. The steady-state behavior of the system is briefly discussed using numerical simulations and a heuristic light-traffic approximation.     

Systematic study of Au6 to Au12 gold clusters on MgO(100) F centers using density-functional theory

We present an optimized genetic algorithm used in conjunction with density-functional theory in the search for stable gold clusters and O2 adsorption ensembles in F centers at MgO(100). For Au8 the method recovers known structures and identifies several more stable ones. When O2 adsorption is investigated, the genetic algorithm is used to imitate structural fluxionality, increasing the O2 bond strength by up to 1 eV. Extending the method to Au(6,10,12), strong O2 adsorption configurations are found for all sizes. However, the effect of fluxionality appears to wear off with increasing cluster size.     

Collecting all intermediates with an optimal scaling for the generalised-active-space coupled-cluster method with application to SbH

ABSTRACT

A string-based coupled-cluster method of general excitation rank, with optimal scaling and easy collection of all possible intermediates, which accounts for special relativity within the four-component framework is presented. The easy identification and collection of intermediates is achieved by extending the excitation-class formalism and introducing a contraction order for the operator indices. Initially, all possible contractions between the Hamiltonian and the intermediates with the cluster operator are found, stored and used for driving the algorithm, thereby defining an algorithm which allows for very complex state-selective, multi-reference, coupled-cluster calculations. The algorithm for solving the coupled-cluster equations can be used in both relativistic as well as non-relativistic calculations by appropriate restrictions in the excitation-class formalism. The capabilities of the new method are demonstrated in the application to the electronic ground state of the SbH molecule. In these calculations simulated multi-reference expansions with double, triple and quadruple excitations into the external space as well as the regular coupled-cluster hierarchy up to full quadruples excitations are compared.     

Enthalpy relaxation of hyperquenched glasses and its possible link to α- and β-relaxations

The enthalpy relaxation of glass fibers of different chemical compositions and thereby of different fragility steepness indices is investigated by means of the hyperquenching–annealing–DSC scanning method. Results show that the pattern of the enthalpy relaxation spectra of the glass depends on its composition, and, hence, on its fragility steepness index. The present paper suggests that the enthalpy relaxation pattern could be linked to the α- and β-relaxations that are normally used to describe relaxation spectra of supercooled liquids near the glass transition. Three observations are shown to support the suggestion.     

Ultra-high thermal expansion glass–ceramics in the system MgO/Al2O3/TiO2/ZrO2/SiO2 by volume crystallization of cristobalite

Glasses in the system MgO/Al₂O₃/TiO₂/ZrO₂/SiO₂ with and without the addition of As₂O₃ and Sb₂O₃ were thermally treated. Up to a temperature of 950 °C, this resulted in the formation of ZrTiO₄, sapphirine and high quartz solid solution. Annealing at higher temperatures led to the formation of low quartz solid solutions, ZrTiO₄ and sapphirine. This resulted in a continuous increase of density, hardness, fracture toughness and thermal expansivity. In the glass doped with As₂O₅ and Sb₂O₅ annealing temperatures >1000 °C resulted in the formation of cristobalite instead of quartz. Then the density, hardness and strength decreased again, while the fracture toughness (up to 2.8 MPa m^1/2) and the thermal expansion coefficient increased strongly. In the dilatometric curves, a steep increase in expansion is observed in the temperature range from 100 to 200 °C, which is attributed to the transformation of low cristobalite to high cristobalite. The mean linear thermal expansion coefficient (25–200 °C) is 20 × 10^?6 K^?1 and the largest up to now reported in the literature for alkali-free silicate glass–ceramics.     

A discrete interaction model/quantum mechanical method to describe the interaction of metal nanoparticles and molecular absorption

A frequency-dependent quantum mechanics/molecular mechanics method for the calculation of response properties of molecules adsorbed on metal nanoparticles is presented. This discrete interaction model/quantum mechanics (DIM/QM) method represents the nanoparticle atomistically, thus accounting for the local environment of the nanoparticle surface on the optical properties of the adsorbed molecule. Using the DIM/QM method, we investigate the coupling between the absorption of a silver nanoparticle and of a substituted naphthoquinone. This system is chosen since it shows strong coupling due to a molecular absorption peak that overlaps with the plasmon excitation in the metal nanoparticle. We show that there is a strong dependence not only on the distance of the molecule from the metal nanoparticle but also on its orientation relative to the nanoparticle. We find that when the transition dipole moment of an excitation is oriented towards the nanoparticle there is a significant increase in the molecular absorption as a result of coupling to the metal nanoparticle. In contrast, we find that the molecular absorption is decreased when the transition dipole moment is oriented parallel to the metal nanoparticle. The coupling between the molecule and the metal nanoparticle is found to be surprisingly long range and important on a length scale comparable to the size of the metal nanoparticle. A simple analytical model that describes the molecule and the metal nanoparticle as two interacting point objects is found to be in excellent agreement with the full DIM/QM calculations over the entire range studied. The results presented here are important for understanding plasmon-exciton hybridization, plasmon enhanced photochemistry, and single-molecule surface-enhanced Raman scattering.