Nucleation and growth rates of poly(L-lactic acid) microparticles during precipitation with a compressed-fluid antisolvent

We measured nucleation and growth rates of poly(L-lactic acid) (PLLA) microparticles produced during precipitation with a compressed-fluid antisolvent (PCA). The injector/precipitator used in this study satisfied the constraints and assumptions incorporated in the development of the mixed-suspension, mixed-product-removal population balance theory. A semicontinuous operation mode with batch product filtering was developed, and results from product particle size distributions allowed nucleation and growth rates to be determined through the use of population balances. Kinetic data, obtained by operating the precipitator under various degrees of supersaturation and suspension density, were used to generate a nucleation rate model for PLLA. Model results indicate a relative kinetic order of 1 and a linear dependence of the nucleation rate on the suspension density. First-order dependence of the nucleation rate on suspension density suggests secondary nucleation mechanism(s) are operative within this PCA flow system and may explain the relative insensitivity of particle size distributions to changes in PCA operating conditions.     

Fundamental noise limitations to supercontinuum generation in microstructure fiber

Broadband noise on supercontinuum spectra generated in microstructure fiber is shown to lead to amplitude fluctuations as large as 50% for certain input laser pulse parameters. We study this noise using both experimental measurements and numerical simulations with a generalized stochastic nonlinear Schr?dinger equation, finding good quantitative agreement over a range of input-pulse energies and chirp values. This noise is shown to arise from nonlinear amplification of two quantum noise inputs: the input-pulse shot noise and the spontaneous Raman scattering down the fiber.     

Donor-substituted peralkynylated "radiaannulenes": novel all-carbon macrocycles with an intense intramolecular charge-transfer

A novel class of planar, highly conjugated all-carbon macrocycles, which we christened "radiaannulenes", have been prepared based on acetylenic scaffolding using tetraethynylethene (TEE) building blocks; these structures are powerful electron acceptors and, upon peripheral substitution with electron-donating N,N-dialkylanilino groups, display intense intramolecular charge-transfer.     

Comparison of three sequential extraction procedures used to study trace metal distribution in an acidic sandy soil

On an acid sandy soil contaminated with trace metals (Fe, Mn, Cu, Pb and Zn), three sequential extraction procedures were compared to determine the efficiency of the reagents used and the effects of the step order on the fractionation of metal species. In all cases, a magnesium nitrate solution (MgNIT) was previously used to extract exchangeable forms. In the first procedure (I), the next extraction step was performed with sodium acetate buffer (NaOAc), as used on calcareous soils, to dissolve active calcium carbonate. Then trace metals bound to different forms of oxi-hydroxides (NH(2)OH, TAMOx and TAMAs fractions) were extracted before organic matter/sulfide oxidation with hydrogen peroxide at pH 2.0 in nitric acid medium (OMHyd). Finally, residual bound metals (RESID) in each procedure were extracted with a nitric-hydrofluoric-perchloric acid mixture. The second procedure (II) was the same as I, but without the NaOAc step, because of the absence of carbonate in the study soil. In procedure III, the NaOAc step was omitted and the oxidizable organic/sulfide fraction was extracted with sodium hypochlorite at pH 8.5 (OMOCl) before the reducible fractions. This study first showed that NaOAc may remove considerable amounts of metals (especially Mn and Zn) in other forms than exchangeable ones. Procedures II and III give similar results for Fe, Mn and Zn forms, which were mainly found in fractions of inorganic soil components, but not for Cu and Pb. Copper distribution was affected by the position of the oxidation step in the sequence. In procedure II, where the oxidation step (OMHyd) ended the sequence, Cu was mainly recovered in the TAMOx fraction. However, in procedure III, where the oxidation step (OMOCl) preceded the NH(2)OH, TAMOx and TAMAs steps, Cu was found in both OMOCl and TAMOx fractions. Lead distribution varied with oxidation reagent: it was partly removed in the OMHyd fraction of procedures I and II, and to a much lower extent in the OMOCl fraction of procedure III, probably due to the alkaline pH of the reagent in the latter case.     

Continuous-wave ultrahigh-repetition-rate pulse-train generation through modulational instability in a passive fiber cavity

Thanks to a passive cavity configuration, modulational instability in fibers is successfully observed, for the first time to our knowledge, in the continuous-wave regime. Our technique provides a new means of generating all-optically ultrahigh-repetition-rate pulse trains and opens up new possibilities for the fundamental study of modulational instability and related phenomena.     

Synthesis of self‐healing supramolecular rubbers from fatty acid derivatives,diethylene triamine,and urea

We describe the synthesis of supramolecular self‐healing elastomers from vegetable oil fatty acid derivatives, diethylene triamine, and urea. Our strategy to obtain materials that are self‐healing but do not flow relies on the use of a wide molecular distribution of randomly branched oligomers equipped with self‐complementary and complementary hydrogen bonding groups. We prepared such oligomers with a two steps procedure. In the first step, diethylene triamine was condensed with dimer acids. In the second step, the oligomers obtained were allowed to react with urea. The molecules were characterized by NMR and IR spectroscopies and Monte‐Carlo simulations were used to analyze the molecular size distribution. The sensitivity to small variations of the experimental conditions has been examined and the robustness of the synthetic procedure optimized. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7925–7936, 2008     

Synthesis of new [2]rotaxane including a macrocyclic receptor and a photochromic unit

The novel photochromic [2]rotaxane based on chromene molecule introduced into a crown-containing macrocyclic receptor was synthesized. The photochemical properties of rotaxane could be modified by the complexation of the crown ether moiety.     

Metal-acetylide addition to tetracyanoethylene

Two push-pull chromophores that have shown utility in the field of molecular electronics and non-linear optics are DDMEBT (1, 2-(4-dimethylamino)phenyl)-3-((4-(dimethylamino)phenyl)ethynyl)buta-1,3-diene-1,1,4,4-tetracarbonitrile) and TDMEE (2, 4-(4-dimethylamino)phenyl)but-1-en-3-yne-1,1,2-tricarbonitrile). Unfortunately, the methods reported for their synthesis give variable yields, use toxic solvents, and only provide small amounts of material. We report improved synthetic protocols, providing access to larger quantities of material. By investigating multiple metal-acetylides of 4-ethynyl-N,N-dimethylaniline and their subsequent addition to TCNE, we obtained various products depending on the identity of the metal ion. This led to the simple synthesis of push-pull chromophoric compounds.