Victor M. Litvinov

The Density of Chemical Crosslinks and Chain Entanglements in Unfilled EPDM Vulcanizates as Studied with Low Resolution, Solid State 1H NMR

Abstract The network density in unfilled EPDM vulcanizates was studied by the following methods: low resolution, proton T2 NMR relaxation, Mooney—Rivlin analysis of stress—strain curves, and equili...

Quantitative magnetic resonance imaging study of water uptake by polyamide 4,6

Abstract Magnetic resonance imaging (MRI) and NMR relaxation experiments are used to study the water absorption by polyamide 4,6 (PA46) plates. Despite the higher crystallinity, PA46 absorbs more water as compared to the polyamides 6 and 6,6 (PA6 and PA66). Relaxation measurements demonstrate that the volume averaged molecular mobility ( T 2 H ) of the absorbed water molecules in PA46 is higher than in PA6 and PA66. Quantitative relaxation results of water saturated PA46 further suggest fast exchange between free water and water molecules bound via hydrogen bonds to amide groups. MRI reveals a gradual decrease in the amount and the T 2 H relaxation behaviour of water from the surface towards the core part of PA46 plates. This can be explained by the very high crystallisation rate of PA46, which prevents the close coupling of amide groups in the amorphous phase and results in a larger mean distance between the amide groups, especially in the outer part of the PA46 plates. Density measurements (WAXS and gradient column) show an increase of the density of the amorphous phase during annealing, resulting in a lower water uptake and a lower mobility of the absorbed water molecules.

On‐Line Monitoring of Chemical Reactions by using Bench‐Top Nuclear Magnetic Resonance Spectroscopy

Real-time nuclear magnetic resonance (NMR) spectroscopy measurements carried out with a bench-top system installed next to the reactor inside the fume hood of the chemistry laboratory are presented. To test the system for on-line monitoring, a transfer hydrogenation reaction was studied by continuously pumping the reaction mixture from the reactor to the magnet and back in a closed loop. In addition to improving the time resolution provided by standard sampling methods, the use of such a flow setup eliminates the need for sample preparation. Owing to the progress in terms of field homogeneity and sensitivity now available with compact NMR spectrometers, small molecules dissolved at concentrations on the order of 1 mmol L(-1) can be characterized in single-scan measurements with 1 Hz resolution. Owing to the reduced field strength of compact low-field systems compared to that of conventional high-field magnets, the overlap in the spectrum of different NMR signals is a typical situation. The data processing required to obtain concentrations in the presence of signal overlap are discussed in detail, methods such as plain integration and line-fitting approaches are compared, and the accuracy of each method is determined. The kinetic rates measured for different catalytic concentrations show good agreement with those obtained with gas chromatography as a reference analytical method. Finally, as the measurements are performed under continuous flow conditions, the experimental setup and the flow parameters are optimized to maximize time resolution and signal-to-noise ratio.

Toughening of SAN with acrylic core-shell rubber particles: particle size effect or cross-link density?

The effect of rubber particle size on fracture toughness and tensile properties have been investigated using styrene-acrylonitrile as a matrix. Pre-formed particles with poly(butyl-acrylate) core and a poly(methylmethacrylate) shell, ranging from 0.1 to 0.6 μm in diameter, were used as a toughening agent. The morphology was checked by means of transmission electron microscopy. The test methods involved an un-notched uniaxial tensile test, notched Izod impact and notched tensile testing. The experiments were carried out with varying deformation rates and temperatures. N.m.r. experiments were used to measure network densities in the rubber core of the various particles. Uniaxial tensile tests showed that elastic modulus and yield stress of the blends were independent of particle size and network density of the rubber core. There were, however, some differences in cavitation resistance caused by the differences in network density. Easily cavitating particles produced higher toughness in notched Izod impact and notched tensile test but a clear relation with particle size could not be established for the range studied. The brittle-tough transition temperatures were much higher than for materials based on poly(butadiene) core-shell rubbers. It is suggested that the mechanical properties of the rubber particle core are the key to the toughening efficiency. It was found that high toughness could only be achieved if the particles had a low cross-link density (and a corresponding low modulus and low cavitation resistance).

Fumed Silica — Rheological Additive for Adhesives, Resins, and Paints

Fumed silica, a synthetic silicon dioxide, is a powerful rheological additive for resins and paints to introduce thixotropy or even a yield point. The rheological effectiveness of fumed silica is based on its ability to form percolating networks which immobilize large volumes of liquid. By a combination of advanced rheological experiments, spectroscopical investigations, and quantum chemical calculations it could be demonstrated that the formation and stability of the silica network is strongly influenced by particle-resin interactions. The results can be used to develop comprehensive models, which explain the rheological performance of different grades of fumed silica in different resins.

Environmentally friendly flame retardants. A detailed solid-state NMR study of melamine orthophosphate

We used solid‐state NMR spectroscopy to gain detailed information about the proton positions, proximities and the hydrogen‐bonding network in the environmentally friendly flame retardant melamine orthophosphate (MP). High‐resolution proton one‐ and two‐dimensional solid‐state NMR spectra were obtained at high external magnetic field in combination with fast magic angle spinning of the sample. Furthermore, we recorded homo‐ and heteronuclear correlation spectra of types 15N15N, 1H13C, 1H15N and 1H31P. In addition, we determined the geometry of the NH and NH2 groups in MP by 15N1H heteronuclear recoupling experiments. We were able to completely assign the different isotropic chemical shifts in MP. Furthermore, we could identify the protonation of the melamine and orthophosphate moieties. The experimental results are discussed in connection with the structural model obtained by powder X‐ray diffraction together with a combined molecular modeling‐Rietveld refinement approach (De Ridder et al. Helv. Chim. Acta 2004; 87: 1894). We show that the geometry of the NH2 groups can only be successfully estimated by solid‐state NMR. Copyright