Thermoresponsive magnetic colloids

The combination of magnetic nanoparticles with thermoresponsive polymer systems leads to the formation of hybrid particle dispersions or composites with a variety of interesting properties and perspectives, including instant dispensability, thermoreversible formation of magnetic fluids, and novel magnetoresponsive properties. Special interest is gained by the magnetic heatability of magnetic particles that allows the activation of thermal effects by the application of a high-frequency electromagnetic field. This review summarizes the recent developments in this young field of research with application potential for magnetic separation, drug release systems, and for sensor and actuator purposes.     

Sorption kinetics of strontium in porous hydrous ferric oxide aggregates I. The Donnan diffusion model

Sorption of ions by hydrous ferric oxide (HFO) often shows a fast initial sorption reaction followed by a much slower sorption process. The second step is diffusion-controlled and can continue for days or months before equilibrium is reached. In this paper, we demonstrate that the diffusion rate may be explained by electrostatic interactions. The internal and external surfaces of HFO are generally positively charged and therefore repel cations. This can result in extremely low cation concentrations in pores, and therefore a significant reduction in pore diffusion rate. The theory is demonstrated here for sorption of Sr(2+) in HFO aggregates. The ion concentrations in the pore space are calculated using a Donnan model and diffusion is calculated from the Donnan concentration and potential gradients. This diffusion model is compared with nonelectrostatic pore diffusion, which does not take electrostatic interactions into account. The Donnan model predicts very low concentrations of Sr(2+) in the pores and diffusion rates that are up to 8000 times lower than predicted with a nonelectrostatic model.     

Kinetic studies of surface‐initiated atom transfer radical polymerization in the synthesis of magnetic fluids

We present results from kinetic studies on the surface‐initiated atom transfer radical polymerization in the preparation of polymer brush‐coated magnetic particles from a heterogeneous system. It is shown that a controlled reaction behavior and a reproducible surface functionalization with end‐tethered polymers are achieved, although the reaction advances gradually from a biphasic solid–liquid mixture to a stable colloidal dispersion of the nanoobjects. Although the initiator‐functional magnetite nanoparticles initially form a precipitate, the formation of a polymer layer on the particle surface in the course of the reaction contributes to a sterical stabilization in dispersion. We thoroughly investigated the development of the initial heterogeneous system with time and in various concentration regimes by simultaneously monitoring the monomer conversion, molar mass, the hydrodynamic diameter of the nanoobjects, and the magnetite content of the dispersions at different reaction times. The results indicate first‐order chain growth kinetics with respect to the monomer and narrow molar mass distributions, demonstrating good control on the particle architecture. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2009     

Prompt (n,γ) Mass Measurements for the AVOGADRO Project

The aim of the AVOGADRO project is to replace the kilogram artefact by a high-purity, perfect single crystal of natural or isotope-enriched silicon. The isotopic composition and the impurities of the silicon crystal must, therefore, be known with highest possible accuracy and precision. The only method to obtain all this information without destruction of the massive samples is prompt (n,γ)-spectrometry. The measurements are performed at a thermal neutron guide of the ILL (Institut Max von Laue Paul Langevin) in Grenoble, France. The spectrometry of γ-radiation emitted by a nucleus promptly after thermal neutron capture allows a highly precise determination of atomic mass differences, as well as the determination of isotope abundances leading to the molar mass. The uncertainties assigned to the results for the respective atomic masses determined by the mass differences amount to up to 10⁻¹⁰, while the molar mass of an isotope-enriched Si single crystal has so far been determined with an uncertainty of 1 ⋅ 10⁻⁴. A direct comparison (for example, relative value of isotope abundances determined by (n,γ)-spectrometry omitting the thermal neutron cross section) furnishes a value of 7 ⋅ 10⁻⁵. The final aim of the AVOGADRO project is to provide a well specified crystal, which allows a more accurate value of the Avogadro constant to be determined. This constant is the key input parameter for tabulated values of fundamental constants and for a new definition of the unit of mass - the kilogram itself. This revised version was published online in July 2006 with corrections to the Cover Date.     

Dipole soliton formation in a nematic liquid crystal in the nonlocal limit

The interaction of two symmetric solitary waves, termed nematicons, in a liquid crystal is considered in the limit of nonlocal response of the liquid crystal. This nonlocal limit is the applicable limit for most experimentally available liquid crystals. In this nonlocal limit, two separate cases for the initial separation of the nematicons are considered, these being large and small separation. Both spinning and nonspinning nematicons are considered. It is found that in the case of large initial separation, the nematicons can form a spinning or nonspinning bound state with a finite steady separation, this being called a nematicon dipole, when they are π out of phase. On the other hand, well separated, nonspinning, in-phase nematicons attract and merge, while well separated, spinning, in-phase nematicons can either form a bound state or merge into a single nematicon. In the limit of small initial separation, the nematicons rapidly merge when they are in phase. Modulation equations describing the nematicon interaction are derived via suitable trial functions in an averaged Lagrangian. These modulation equations are further modified to include the effect of the diffractive radiation shed as the nematicons evolve. Finally the modulation equations are approximated in order to investigate the various interaction regimes. Good to excellent agreement is found between their solutions and full numerical solutions of the nematicon equations.     

Sorption and Diffusion of Propylene and Ethylene in Heterophasic Polypropylene Copolymers

Summary: Sorption experiments of ethylene and propylene in different polypropylene powder samples, both homopolymer and heterophasic copolymers with different rubber content, have been carried out in a high-pressure magnetic suspension balance at 10 bars pressure and 70 °C. The gross solubilities measured can be well correlated with the rubber content of the polymer samples. Solubility of ethylene and propylene in the rubber phase differ from solubility in the amorphous fraction of the homopolymer, especially the concentration ratio of propylene to ethylene differs significantly between rubber phase and amorphous fraction of the homopolymer. From the slope of monomer uptake, information on kinetics of mass-transfer can be gained. No significant differences were observed in terms of mass-transfer for ethylene and propylene. With increasing rubber content, effective diffusion coefficients increased slightly. By combined sorption studies with powder samples and compressed films, information about both effective diffusion coefficients and the effective length scale of diffusion could be gained. It could be shown, that the particle radius is not the characteristic length of diffusion in the studied powder samples. Mass transfer of nearly all samples could be described by a constant diffusion length of 120 to 130 µm, independently on particle size. This indicates that the effective scale of diffusion in polymer particles is in between microparticle and macroparticle scale used in classical particle modeling.