A.P. Legrand

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.

Solid state NMR to study calcium phosphate ceramics

Abstract High-resolution 31P and 1H NMR spectra at 40 and 121 MHz (31P) and 300 MHz (1H) of synthetic and biological samples of calcium phosphates have been obtained by magic angle spinning (MAS) at spinning speeds up to 6.5 kHz, and high power proton decoupling. The samples include crystalline hydroxyapatite, a deficient hydroxyapatite characterized by a Ca/P atomic ratio of 1.5, a poorly crystallized hydroxyapatite, monetite, brushite, octacalcium phosphate, β-tricalcium phosphate and rabbit femoral bone. The interactions between nuclei in unlike structures and the mobility of acid protons are discussed.

29Si and 27Al MAS NMR study of the zeolitization of kaolin by alkali leaching

Abstract The alkali leaching of two aluminosilicates, kaolinite and metakaolinite, with aqueous NaOH has been studied. Both silicates gave hydroxysodalite (HS) with or without the evanescent zeolite NaA. X-ray diffraction and high-resolution 29Si and 27Al MAS-NMR spectroscopy provide information about the reaction sequence of the clay. The conversion starts with the formation of an amorphous gel precursor at a rate which depends on the alkali concentration but not on the choice of kaolinite or metakaolinite as starting material. The rate of zeolitization of this gel is much faster when it is obtained from kaolinite, probably because it is more homogeneous.

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.

Synthesis, X-ray diffraction and solid-state 31P magic angle spinning NMR study of α-tricalcium orthophosphate

The effects of synthesis conditions on the quantitative preparation of α-tricalcium phosphate (α-TCP) have been investigated. The following parameters of the synthesis were considered: nature of the starting material-Ca-deficient hydroxyapatite, DAP, versus hydroxyapatite-anhydrous dicalcium phosphate mixtures (HAP-DCPA); Ca/P atomic ratio of the mixture, calcination temperature and time, and cooling rate. The yield and crystallinity of the final product have been estimated using X-ray diffraction (XRD) and solid state 31P magic angle spinning NMR (MAS-NMR) techniques. The results show that pure, well-crystallized α-TCP powders exhibiting nearly ideal MAS-NMR spectra, can be obtained by reactive sintering of HAP-DCPA (Ca/P=1.50...1.52) mixtures, at 1400°C for 8 h. The broadening of MAS-NMR spectra can be used as an indicator of structural order in the final product. The α-TCP yield with DAP was always less than 50%.

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.

Characterisation and conversion study into natural living bone of calcium phosphate bioceramics by solid state NMR spectroscopy

High-resolution 31P and 1H NMR spectra at 40 MHz (31P) and 300 MHz (1H) of biologically relevant synthetic samples of calcium phosphates and biopsies have been obtained by magic angle sample spinning (MASS) at spinning speeds up to 6·5 kHz and by high power proton decoupling (HPPD). The samples include crystalline hydroxyapatite, a deficient hydroxyapatite characterised by a Ca/P atomic ratio of 1·5, a poorly crystallised hydroxyapatite partially dissolved in EDTA, monetite, brushite, β-tricalcium phosphate, octacalcium phosphate, rabbit bone and human pathologic bones. The resorption of a porous β-tricalcium phosphate plate was studied; within fifty days new ossification accounts for more than half of the implant.

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.

On the silica edge, an NMR point of view

‘‘The properties of amorphous silicas of high specific surface area, from the smallest colloidal particles to macroscopic gels, depend largely on the chemistry of the surface of the solid phase’’ [1]. Silicas are useful for numerous applications. The most recent application is certainly their use in the tire manufacturing process (Energy Tires of Michelin Cie). As styrene-butadiene rubber compatibility with an organophobic silica is questionable contrarily with carbon black, it was necessary to investigate the surface composition to adapt it to such application. To do this, different methods have to be used. Infra-red spectrometry was the first physical method to demonstrate the existence of single and/or associated hydroxyls groups (Fig. 1). More recently solid state NMR was not only able to identify geminal, the existence of which was sometimes debated, but also single or associated hydroxyls.

Solid-state 13C NMR study of β-cyclodextrin/substituted aromatic ketone complexes : evidence for two kinds of complexation of the guest molecules

Abstract Solid-state 13C cross polarization magic angle spinning nuclear magnetic resonance measurements on β-cyclodextrin (β-CD)/substituted aromatic ketone inclusion complexes indicate that the interaction between the β-CD (host) and the substituted ketone (guest) depends markedly on the substituent. The carbon chemical shifts of both the host and the guest molecules depend on the substituent. In the β-CD/α-aminoacetophenone complex, each carbon of the guest molecule exhibits two 13C NMR peaks. One of these peaks is shifted with respect to that of the free molecule. This shifted peak gradually disappears when the temperature increases, whereas the unshifted peak is unchanged. The same phenomenon occurs when the inclusion complex is washed. This result is related to the existence of two kinds of complexed molecule (inside the β-CD cavity and outside but non-covalently bonded to β-CD). In β-CD/α-amino-4-bromoacetophenone, only one kind of complexed molecule exists. The disposition of α-amino-4-bromoacetophenone inside the β-CD cavity is different from that of α-aminoacetophenone.