Ethylene Polymerization-Induced Self-Assembly (PISA) of Poly(ethylene oxide)-block-polyethylene Copolymers via RAFT

Poly(ethylene oxide) (PEO) with dithiocarbamate chain ends (PEO–SC(=S)−N(CH₃)Ph and PEO–SC(=S)−NPh₂, named PEO-1 and PEO-2 , respectively) were used as macromolecular chain-transfer agents (macro-CTAs) to mediate the reversible addition–fragmentation chain transfer (RAFT) polymerization of ethylene in dimethyl carbonate (DMC) under relatively mild conditions (80 °C, 80 bar). While only a slow consumption of PEO-1 was observed, the rapid consumption of PEO-2 led to a clean chain extension and the formation of a polyethylene (PE) segment. Upon polymerization, the resulting block copolymers PEO-b-PE self-assembled into nanometric objects according to a polymerization-induced self-assembly (PISA).     

Exploring pathways to reduce the distribution of active sites in the Ziegler–Natta polymerization of propylene

Chemical treatments of classical supported Ziegler–Natta precatalysts were conducted by using additional bulky ligands to attempt to narrow and homogenize the active sites distribution in propylene polymerization. Additions of monodentate ligands such as bis(trimethylsilyl)amide, cyclopentadienyl derivates or triphenylsilanol were seen to slow down the polymerization without modifying the distribute properties of polypropylenes. In the case of multidentate ligands (porphines or biquinolines), in addition to the poisoning of active sites, an extraction of titanium from the catalyst surface is observed. A decrease of both melting point and isotacticity (II%) of polymers using these compounds suggest that the most isospecific titanium sites are first extracted from the MgCl₂‐surface. The narrowing of the molecular weight distribution confirms that the highly isospecific sites are the most active sites, producing the higher molecular weight polymers. Moreover, this study shows that the distributed properties of polymers are due to the chemical diversity of the active sites with various steric and electronic environments at the catalyst surface and not to mass transfer limitations. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3941–3948, 2007     

2T1 pumping of the R lines in ruby

We compare the effects of excitations in the ²T₁ lines and ⁴T₂ broad band in ruby. The ratios of R₂ and R₁ emissions are compared before and during bottleneck. The results lead to an approximately equal feeding ratio when the excitation takes place in the ²T₁ levels although it is strongly different for the ⁴T₂ excitation. The transition time from the ²T₁ level to the ²E level is estimated to be 2.5 ns.     

Validation of a Fast and Simple HPLC-UV Method for the Quantification of Adenosine Phosphates in Human Bronchial Epithelial Cells

A new HPLC method for the simultaneous quantitative analysis of adenosine triphosphate (ATP), adenosine diphosphate (ADP), and adenosine monophosphate (AMP) was developed and validated. ATP, ADP, and AMP were extracted from human bronchial epithelial cells with a rapid extraction procedure and separated with a C18 column (3 × 150 mm, 2.7 µm) using isocratic elution with a mobile phase consisting of 50 mM of potassium hydrogen phosphate (pH 6.80). The absorbance was monitored at 254 nm. The calibration curves were linear in 0.2 to 10 µM, selective, precise, and accurate. This method allowed us to quantify the nucleotides from two cell models: differentiated NHBE primary cells grown at the air–liquid interface (ALI) and BEAS-2B cell line. Our study highlighted the development of a sensitive, simple, and green analytical method that is faster and less expensive than other existing methods to measure ATP, ADP, and AMP and can be carried out on 2D and 3D cell models.     

Iodine-Transfer Polymerization (ITP) of Ethylene and Copolymerization with Vinyl Acetate

Controlled radical polymerization of ethylene using different commercially available, cheap, and non-toxic iodo alkyls is performed by iodine transfer polymerization (ITP) under mild conditions (≤100 °C and ≤200 bar). The formed well-defined iodo end-capped polyethylene (PE−I) species is very stable upon storage. Narrow molar-mass distributions (dispersities around 1.6) were obtained up to number average molar masses of 7300 g mol⁻¹. The ethylene copolymerization by ITP (ITcoP) with vinyl acetate allowed to form a broad range of poly(ethylene-co-vinyl acetate) (EVA) containing from 0 to 85 mol % of VAc unit. In addition, EVA-b-PE block copolymers or EVA-b-EVA gradient block copolymers with different content of VAc in the blocks were obtained for the first time using ITP. Finally, reactivity trends were explored by a theoretical mechanistic study. This highly versatile synthetic platform provides a straightforward access to a diverse range of well-defined PE based polymer materials.     

Clustering effects in optical properties of BGaAs/GaAs epilayers

We report on the further investigation of the effect of boron incorporation on GaAs grown at 580 °C temperature on GaAs (0 0 1) substrate by metal organic chemical vapor deposition (MOCVD). High-resolution X-ray diffraction (HRXRD) has been used to determine the lattice mismatch and to estimate the boron concentration. Temperature-dependent photoluminescence has been carried out to investigate B_xGa_1−xAs/GaAs epilayers with varied boron composition (x=1.64% and 3.04%). Low temperature (10 K) PL study has shown an asymmetric and broad PL band around 1.3 eV of the emission energies with a decrease of the PL intensity with increasing boron composition. The evolution of the emission energies with temperature can be described by Varshni law for the high temperature range (T?120 and 80 K) for boron composition x=1.64% and 3.04%, respectively, while a relative discrepancy has been found to occur at low temperature. Moreover, depending on the temperature range, the PL intensity quenching is found to be thermally ensured by three activation energies. These results are attributed to the localized states induced by the non-uniform insertion of boron and the clustering of the boron atom in BGaAs bulk.     

Investigation by EELS and TRIM simulation method of the interaction of Ar and N ions with the InP compound

In this study, the EELS results revealed the great sensitivity of InP compound submitted to Ar⁺ or N⁺ ions at low energy. The preliminary treatment of InP by the Ar⁺ ions was useful as part of the cleaning process of the surface. Further argon ions bombardment on cleaned InP led however to breaking of chemical bonds In–P, with desorption of phosphorus atoms and appearance of In metal distributed on InP. The damaged InP by Ar⁺ ions, constituted the diphase (In; InP) system of depth of about 30 Å, involving a superficial roughness. The In metal proportion on such a system was determined by a calculation method based on the experimental EELS spectra of pure In and InP.We submitted the heated and no heated system (In; InP) to nitrogen ions bombardment. The nitrogen reacted with the In metal to compensate the phosphorus vacancies so that InN species were formed. The heating of (In; InP) system at 450 °C, allowed the surface reconstruction with elimination of defects due to the structure and the roughness. The temperature also caused the coalescence of In metal towards the surface. Because of the physical stability of the interface of heated (In; InP) system, the nitrogen reacted with the outmost layers of In metal to form a homogeneous layer of InN of thickness estimated at 20 Å. We associated to the EELS the TRIM (Transport and Range of Ions in Matter) simulation method in order to show the mechanism of interaction Ar⁺ or N⁺ ions-InP and determine the disturbed depth as a function of the energy. The EELS alone was not able to give us with accuracy the disturbed depth of the target by these ions.     

Characterization of Ethylene methyl methacrylate and Ethylene butylacrylate Copolymers with Interactive Liquid Chromatography

Summary: Copolymers of ethylene with methyl methacrylate (EMMA) and butyl acrylate (EBA), which are of different average chemical composition and block lengths according to NMR analysis, were analyzed by size exclusion chromatography (SEC), differential scanning calorimetry (DSC), Crystallization Analysis Fractionation (CRYSTAF), and high performance liquid chromatography at high temperature (HT-HPLC). With CRYSTAF and DSC crystallizing fractions were detected only in some samples. HT-HPLC fractionated all the samples irrespective of their crystallinity. Homopolymers, PMMA and PE were also found in the copolymer samples of EMMA. EMMA and EBA were separated in HPLC according to the content of polar comonomer. A linear correlation between the MMA content and elution volume could not be established due to the presence of homopolymers as admixtures. In such a case the average chemical composition obtained by NMR does not correspond to the real chemical composition of the copolymers. Unlike EMMA the EBA samples eluted in single peaks, which was used for evaluation of their chemical composition distribution. The comparison of results obtained by fractionation via CRYSTAF and HT-HPLC clearly demonstrates the advantages of the chromatographic approach to study the chemical heterogeneity of olefin based copolymers.     

Mimicking helical antibacterial peptides with nonpeptidic folding oligomers

Unnatural oligomeric scaffolds designed to adopt defined secondary structures (e.g., helices), while retaining the chemical diversity of amino acid side chains, are of practical value to elaborate functional mimetics of bioactive alpha-polypeptides. Enantiopure N,N'-linked oligoureas as short as seven residues long have been previously shown to fold into a stable helical structure, stabilized by 12- and 14-membered H-bonded rings. We now report that eight-residue oligoureas designed to mimic globally amphiphilic alpha-helical host-defense peptides are effective against both gram-negative and gram-positive bacteria (including methicillin-resistant Staphylococcus aureus [MRSA]) and exhibit selectivity for bacterial versus mammalian cells. Circular dichroism (CD) spectroscopy studies suggest enhanced helical propensity of oligoureas in the presence of phospholipid vesicles. The utility of this new class of nonpeptidic foldamers for biological applications is highlighted by high resistance to proteolytic degradation.