Precise Synthesis and Thin Film Self-Assembly of PLLA-b-PS Bottlebrush Block Copolymers

The ability of bottlebrush block copolymers (BBCPs) to self-assemble into ordered large periodic structures could greatly expand the scope of photonic and membrane technologies. In this paper, we describe a two-step synthesis of poly(l-lactide)-b-polystyrene (PLLA-b-PS) BBCPs and their rapid thin-film self-assembly. PLLA chains were grown from exo-5-norbornene-2-methanol via ring-opening polymerization (ROP) of l-lactide to produce norbornene-terminated PLLA. Norbonene-terminated PS was prepared using anionic polymerization followed by a termination reaction with exo-5-norbornene-2-carbonyl chloride. PLLA-b-PS BBCPs were prepared from these two norbornenyl macromonomers by a one-pot sequential ring opening metathesis polymerization (ROMP). PLLA-b-PS BBCPs thin-films exhibited cylindrical and lamellar morphologies depending on the relative block volume fractions, with domain sizes of 46–58 nm and periodicities of 70–102 nm. Additionally, nanoporous templates were produced by the selective etching of PLLA blocks from ordered structures. The findings described in this work provide further insight into the controlled synthesis of BBCPs leading to various possible morphologies for applications requiring large periodicities. Moreover, the rapid thin film patterning strategy demonstrated (>5 min) highlights the advantages of using PLLA-b-PS BBCP materials beyond their linear BCP analogues in terms of both dimensions achievable and reduced processing time.     

RESTRICTION RESPECTUEUSE ET RECONSTRUCTION DES CHAINES ET DES RELATIONS INFINITES

We show a faithful restriction theorem among infinite chains which implies a reconstructibility conjecture of Halin. This incite us to study the reconstructibility in the sense of Fraïssé and to prove it for orders of cardinality infinite or ≥ 3 and for multirelations of cardinality infinite or ≥ 7, what improves the theory obtained by G. Lopez in the finite case. For this work we had to study the infinite classes of difference which have to be a linear order of type ω, ω* or ω* + ω; this complete the theory made by G. Lopez for the finite case ([13]). We show also Ulam-reconstructibility for linear orders which have a fixed point.     

Ab initio calculation of the NH(3sigma-)-NH(3sigma-) interaction potentials in the quintet, triplet, and singlet states

We present the ab initio potential-energy surfaces of the NH-NH complex that correlate with two NH molecules in their 3sigma- electronic ground state. Three distinct potential-energy surfaces, split by exchange interactions, correspond to the coupling of the S(A) = 1 and S(B) = 1 electronic spins of the monomers to dimer states with S = 0, 1, and 2. Exploratory calculations on the quintet (S = 2), triplet (S = 1), and singlet (S = 0) states and their exchange splittings were performed with the valence bond self-consistent-field method that explicitly accounts for the nonorthogonality of the orbitals on different monomers. The potential surface of the quintet state, which can be described by a single Slater determinant reference function, was calculated at the coupled cluster level with single and double excitations and noniterative treatment of the triples. The triplet and singlet states require multiconfiguration reference wave functions and the exchange splittings between the three potential surfaces were calculated with the complete active space self-consistent-field method supplemented with perturbative configuration interaction calculations of second and third orders. Full potential-energy surfaces were computed as a function of the four intermolecular Jacobi coordinates, with an aug-cc-pVTZ basis on the N and H atoms and bond functions at the midpoint of the intermolecular vector R. An analytical representation of these potentials was given by expanding their dependence on the molecular orientations in coupled spherical harmonics, and representing the dependence of the expansion coefficients on the intermolecular distance R by the reproducing kernel Hilbert space method. The quintet surface has a van der Waals minimum of depth D(e) = 675 cm(-1) at R(e) = 6.6a0 for a linear geometry with the two NH electric dipoles aligned. The singlet and triplet surfaces show similar, slightly deeper, van der Waals wells, but when R is decreased the weakly bound NH dimer with S = 0 and S = 1 converts into the chemically bound N2H2 diimide (also called diazene) molecule with only a small energy barrier to overcome.     

Reanalysis of Chiral Discrimination of Phenoxypropionic Acid Herbicides on a Teicoplanin Phase Using a Bi-Langmuir Approach

Summary The chiral discrimination of phenoxypropionic acid herbicides by reversed-phase chromatography on a teicoplanin phase has been re-examined using the perturbation method to calculate the solute distribution isotherms. The effects of both temperature and the methanol (organic modifier) content of the mobile phase on the chiral discrimination mechanism were well described by the bi-Langmuir model. The method confirmed a change in the mechanism of enantiomer retention at a critical temperature, T^*, and showed that the mechanism was independent of (i) herbicide molecular structure, i.e. the position of the chloro group on the phenol ring, and (ii) the absolute configuration of the carbon atom. Enantioselectivity was enhanced by increasing the mobile phase methanol content. Use of this approach also revealed that secondary sites on the teicoplanin surface were involved in the processes determining both retention and selectivity. It was clearly demonstrated that these secondary sites of low affinity were not affected by the temperature change and were not involved in the chiral recognition mechanism.     

The cyano nitronyl nitroxide radical: experimental and theoretical evidence for the fourth case of the McConnell-I mechanism

The nitronyl nitroxide 2-cyano-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (1) crystallises in the tetragonal P42(1)m space group with a=7.4050(7), c=8.649(1) A. In the crystal the molecules form layers parallel to the ab plane in which they are orthogonal to each other. In the layers there are close contacts, 2.953(2) A, between the NO groups and the bridging carbon atoms of the O-N-C-N-O fragment of neighbouring radicals. The calculated spin density shows a positive population mainly and equally localised on the NO groups and small but significant negative spin densities on the bridging carbon atom and the cyano nitrogen. Absorption spectra show temperature-dependent transitions related to the magnetic behaviour. The temperature dependence of the magnetic susceptibility in the range 2-300 K reveals that couplings between the radicals are antiferromagnetic, and is interpreted by considering a two-dimensional square array of spin S=1/2 antiferromagnetically coupled (J=-10 cm(-1) and g=2.01). This is interpreted as an exchange coupling through close contact between positive and negative spin densities in orthogonal orbitals on oxygen and carbon atoms, respectively.     

The synthesis and characterisation of ferromagnetic CaMn2O4 nanowires.

The synthesis of marokite CaMn(2)O(4) nanowires using a hydrothermal method is reported. Transmission electron microscopy and electron diffraction measurements show that the nanowires are polycrystalline in nature with diameters between 10 and 20 nm and lengths ranging from approximately 100 to 500 nm. Most interestingly, in contrast with the bulk material, magnetization measurements show that these nanowires exhibit ferromagnetic ordering with a Curie temperature (T(C)) of approximately 40 K.