H. Hommel

Optical absorption and electron paramagnetic resonance study of -doped phosphate glasses

We report in this paper optical absorption and electron paramagnetic resonance (EPR) results obtained on -doped phosphate glasses. Optical absorption allowed us to evaluate the crystal-field parameters , B and C. Electron paramagnetic resonance measurements indicate that ions are located in sites with low symmetry and a relatively large zero-field splitting. Both electron paramagnetic resonance and optical absorption data show that the chemical bonds of ions with the ligands are rather covalent in nature but that the ionic contribution to those bonds decreases in the order: PBa > PBaK > PBaKMg. Furthermore, it is found from this study that the crystal-field strength increases in the order: PBa < PBaK < PBaKMg.

Organization of the layers of polyethylene oxide grafted with different densities on silica

Abstract EPR spectroscopy of labeled grafted polyethylene oxide chains has been used to estimate the ratio of the population of free end segments in solution to that of adsorbed segments on a silica surface as a function of temperature. The configurations of the chains have been studied in the case of certain extreme coverages. Grafted molecules of mol wt 2000 with grafting ratios of 0.045, 0.057, 0.126, and 0.42 molecules/nm 2 were in contact with benzene. DSC thermograms are also shown. At low grafting ratios the chains lie very flat, whereas at higher grafting ratios the layer is more organized.

Solid State NMR Studies of Precipitated and Pyrogenic Silicas

Abstract A detailed study of 29 Si NMR of different silicas and the quantitative relationships that can be derived from the 29 Si spectra are discussed. Special emphasis is placed on the difference of chemical shift observed between CP/MAS and simple MAS spectra. Effects of a thermal treatment on 29 Si NMR features are also discussed.

Influence of chain length on conformations of poly(ethylene glycol) chains grafted on silica

Abstract Electron spin resonance (e.s.r.) spectroscopy of labelled grafted poly(ethylene glycol) chains has been used to estimate the ratio of the population of end segments free in solution to that of ones adsorbed on the silica surface, as a function of temperature. In particular, the influence of chain length is investigated. The grafted molecules have the following molecular weights, 44, 88, 132, 176, 400, 600, 2000, and are in contact with benzene. Taking the random walk models as reference, two different behaviours are distinguished, one for the oligomers and one for the polymers. The measured values are consistent with an overall picture of ‘brush bristles’ for oligomers and of weakly overlapping partly collapsed coils for polymers.

Dynamics of adsorption of poly(ethylene oxide) on a silica surface modified by grafting

Abstract Electron spin resonance spectroscopy of spin-labelled poly(ethylene oxide) terminally anchored on silica and in contact with solutions of poly(ethylene oxide) in different concentrations in benzene has been used to detect indirectly the adsorption process. The nitroxide free radical was 2,2,5,5-tetramethyl-3-pyrroline-1-oxyl-3-carboxylic acid. The ratios of the population of free end segments in solution to that of adsorbed segments on silica as a function of temperature have been evaluated. The conformations of the chains depend upon the concentration of polymer in solution, on their molecular weight and on the grafting ratio already achieved on the surface. The number of molecules that can be accommodated reaches a limiting value. The time involved in the reorganization of the surface layer has been evaluated to amount to several weeks.

Influence of the solvent on the conformations of poly(ethylene oxide) chains grafted on silica

Abstract Electron spin resonance (e.s.r.) of labelled grafted poly (ethylene oxide) has been used to estimate (as a function of temperature) the ratio between the populations of free-end segments in solution to those adsorbed onto the silica surface. The influence of the solvents such as C6H6, CHCl3, C4H8O2, C6F6 and CCl4 has also been investigated. Better solvents produce greater swelling of the molecules, with more of the grafted layer extending into the solution. But solvent-surface and polymer-surface interactions must be taken into account to explain the results.

Solvent and temperature effects on chain conformations in interfaces of polymers grafted on silica. Study by 1H NMR

The 1 H NMR technique is used to study the behaviour of the poly(ethylene oxide) chains grafted chemically on silica in presence or not of solvent. A noticeable influence of the different physicochemical parameters on the conformation of the grafted macromolecules is evinced. The grafting ratio, the temperature, the solvent can all in well chosen situations modify the configurations of the chains. The macromolecules can lay flat on the surface, or be swollen and spread out. Depending on the conformations the dipolar interactions between monomer detected by NMR are different and inform on the behaviour of the chains. In particular a static contribution also called pseudo-solid effect is measured, which is an indication of the local concentration of segments in the vicinity of the surface. q 2003 Published by Elsevier Ltd.

Conformations of α-ω diols and n-alcohols grafted on silica studied by solid state 13C NMR

Abstract High-resolution 13 C CP/MAS (cross-polarization/magic angle spinning) NMR of α—ω diols and n -alcohols terminally grafted on a planar pyrogenic silica, was used to determine the conformations of the molecules at the interface. For α—ω diols, the variation of the di-/mono-esterification ratio ( R ), as a function of the number of carbons in the chain was evaluated; R is highly dependent on the parity (odd or even) of this number. The results are consistent with an organization of the surface where the α—ω diols adopt a trans—trans configuration and where the n -alcohols form a compact layer which optimizes the interactions between methylene groups and the solid surface.

From isolated coils to brushes of poly(ethylene oxide) grafted on silica: a spin-labelling study

Abstract Poly(ethylene oxide) chains chemically bonded to silica with grafting ratios of 0.045, 0.057, 0.126 and 0.42 molecule nm−2 have been studied by spin-labelling. The nitroxide free radical gives electron paramagnetic resonance signals sensitive to the microscopic local motion of the segments and also information on the conformations by distinguishing between loops and tails with a fast motion and trains with a slow motion. The range of grafting ratio extends from isolated coils to strongly stretched brushes and the crossover from one regime to the other is followed by the resonance spectrum.

Electron paramagnetic resonance study of five-coordinated Cu2+ in ethylenediammonium copper(II) diphosphate monohydrate

The results of an electron paramagnetic resonance study of a new product, namely ethylenediammonium copper(II) diphosphate monohydrate of chemical formula Cu(NH3(CH2)2NH3)P2O7.H2O, are presented. The X-ray diffraction has shown the existence of two types of copper(II) coordination polyhedron for each elementary mesh in this compound. Each polyhedron consists of a copper ion surrounded by five oxygenated ligands stoning from two P2O7 groups and a water molecule. Fair agreement between these results and those obtained by X-ray diffraction is observed. In particular the organization of oxygen atoms as pyramids with a square plane around the copper(II) ions is confirmed. The existence of two magnetically nonequivalent sites of copper per mesh is evidenced.