Electronic and Steric Effects on L-Lactide Ring-Opening Polymerization with NSSN-type Zr(IV) Complexes

In this work the mechanism of L-lactide polymerization promoted by NSSN zirconium complexes was investigated through DFT methods with the aim to understand as the electronic and steric features of the ligand affect the energy reaction. It was observed that the rate determining step of the process is the opening of the L-lactide ring and that by increasing the steric hindrance, evaluated by changing geometric parameters and topographic steric maps, or the electron-withdrawing properties of the ligand, the corresponding energy barrier increases. On the other hand, calculations foresee that a small and electron-releasing substituent on the nitrogen atom of the ligand, such as the methyl group, is desirable in order to obtain NSSN zirconium based catalysts with improved activity in the ROP of the L-lactide.     

Novel sulfur-containing rapamycin analogs prepared by precursor-directed biosynthesis

[reaction: see text] Two novel sulfur-containing analogs of the immunosuppressive natural product rapamycin (1) were obtained by feeding cultures of Streptomyces hygroscopicus with l-nipecotic acid (4) and either (S)-1,3-thiazane-4-carboxylic acid (5) or (S)-1,4-thiazane-3-carboxylic acid (6). The structures of the two new compounds, 20-thiarapamycin (2) and 15-deoxo-19-sulfoxylrapamycin (3), were determined by spectroscopic methods.     

Surface rheology of two-dimensional percolating networks: Langmuir films of polymer pancakes

We report surface rheological measurements on Langmuir films of submicron-sized polymer coils (pancakes). The dynamics of the sol phase is found to be governed by free volume as in the simplest percolation problem. The observed rheology of the percolated phase (gel-like) is compatible with predictions for compressed arrangements of highly deformed soft particles interacting through their contact interfaces.     

Particle laden fluid interfaces: Dynamics and interfacial rheology

We review the dynamics of particle laden interfaces, both particle monolayers and particle + surfactant monolayers. We also discuss the use of the Brownian motion of microparticles trapped at fluid interfaces for measuring the shear rheology of surfactant and polymer monolayers. We describe the basic concepts of interfacial rheology and the different experimental methods for measuring both dilational and shear surface complex moduli over a broad range of frequencies, with emphasis in the micro-rheology methods. In the case of particles trapped at interfaces the calculation of the diffusion coefficient from the Brownian trajectories of the particles is calculated as a function of particle surface concentration. We describe in detail the calculation in the case of subdiffusive particle dynamics. A comprehensive review of dilational and shear rheology of particle monolayers and particle + surfactant monolayers is presented. Finally the advantages and current open problems of the use of the Brownian motion of microparticles for calculating the shear complex modulus of monolayers are described in detail.     

Dynamic surface tension of aqueous solutions of ionic surfactants: role of electrostatics

The adsorption kinetics of the cationic surfactant dodecyltrimethylammonium bromide at the air-water interface has been studied by the maximum bubble pressure method at concentrations below the critical micellar concentration. At short times, the adsorption is diffusion-limited. At longer times, the surface tension shows an intermediate plateau and can no longer be accounted for by a diffusion-limited process. Instead, adsorption appears kinetically controlled and slowed down by an adsorption barrier. A Poisson-Boltzmann theory for the electrostatic repulsion from the surface does not fully account for the observed potential barrier. The possibility of a surface phase transition is expected from the fitted isotherms but has not been observed by Brewster angle microscopy.     

Diffusive liquid propagation in porous and elastic materials: the case of foams under microgravity conditions

We report the results of fluid transport experiments in aqueous foams under microgravity. Using optical and electrical methods, the capillary motion of the foam fluid and the local liquid fractions are monitored. We show that foams can be continuously wetted up to high liquid fractions ( approximately 0.3), without any bubble motion instabilities. Data are compared to drainage models: For liquid fractions above 0.2, discrepancies are found and identified. These new results on foam hydrodynamics and structure can be useful for other poroelastic materials, such as plants and biological tissues.     

Freezing transition and interaction potential in monolayers of microparticles at fluid interfaces

The structure and the interaction potential of monolayers of charged polystyrene microparticles at fluid interfaces have been studied by optical microscopy. Microparticles of different sizes have been studied over a broad range of surface particle densities. The structural characterization is based on the analysis of images obtained by digital optical microscopy. From the experimental images, radial distribution functions, hexagonal bond order correlation functions, and temporal orientational correlation functions have been calculated for different monolayer states at both the air/water and oil/water interfaces. The interaction potential has been calculated from the structure factor using integral equations within the hypernetted chain closure relationship. For particles trapped at the oil-water interface, it was found that, upon increasing the surface coverage, a freezing transition occurs, that leads to the formation of a 2D crystalline structure. We have studied the freezing densities of particle monolayers at the oil/water interface and compared them with Monte Carlo simulation results reported by H. Lo?wen. In contrast, at the air-water interface, freezing is inhibited due to the formation of particle aggregates.