S. Cohendoz

Solid-state 13C NMR Study of Scleroglucan Polysaccharide. Effect of the Drying Process and Hydration on Scleroglucan Structure and Dynamics

Abstract High-resolution solid-state 13C CP/MAS NMR was used to study the evolution of a polysaccharide (scleroglucan) conformation from the anhydrous to the hydrated form. The influence of a thermo-mechanical treatment applied during the drying process of scleroglucan is analyzed both on the dried and rehydrated product. 13C NMR spectra, 13C relaxation times (T 1C) and 1H relaxation times in the rotating frame (T 1ρH) of scleroglucan dried by using instantaneous controlled pressure drop (Détente Instantanée Controlée™) were analyzed in order to explain the observed differences of rehydration capacity. Although the scleroglucan treated at 6 bar has the same conformational state (triple-helix) as the one treated at 1 bar, it shows two different relaxation times T 1C for the C-3 carbon involved in the interglycosidic linkage. The magnetization decay of the hydrated sample exhibits a decrease of two time constants with significant shortening of the spin-lattice relaxation times T 1C that accounts for the higher mobility of the chains. High-pressure treatment creates highly rigid and compact domains. Consequently, water molecules cannot readily access the inside of the triple-helix and relax the interchain hydrogen bonds.

Formation and Dissolution of Hydride Precipitates in Zirconium Alloys: Crystallographic Orientation Relationships and Stability after Temperature Cycling

Hydride precipitation due to the spontaneous and fast hydrogen diffusion is often pointed as causing embrittlement and rupture in zirconium alloys used in the nuclear industry. Transmission Electron Microscopy (TEM) and X-Rays Diffraction (XRD) have been used to study the precipitation of hydride phases in zirconium alloys as a function of the hydrogen content. The orientation relationships observed between the hydride phase and the substrate were similar to those previously observed in Titanium hydrides grown on Titanium. Dislocation emission from the hydride precipitates has been directly related to the relaxation of the misfit stresses appearing during the transformation. The stability of the hydride phases after several dissolution-reprecipitation cycles have been studied by DSC, TEM and XRD for different total hydrogen content in several alloys. The energy of precipitation observed is lower than that of the dissolution in each case studied. The temperature associated with these two processes slightly increase as a function of the cycle number, as a result of the homogenizing hydrogen distribution in the alloy bulk. The same hydrides phases present before cycling were also observed after 20 cycles. However, transition phases poorer in hydrogen than the dominant one may precipitate at the interface with the substrate. The evolution of these transitions phases with the temperature increase will be investigated by TEM in-situ heating in the next future.

Hydride Precipitates in Zirconium Alloys: Evolution of Dissolution and Precipitation Temperatures During Thermal Cycling Correlated to Microstructure Features

The fast and spontaneous hydrogen diffusion in zirconium alloys used in the nuclear industry leads to the hydride precipitation which is often pointed as causing embrittlement and rupture. Our studies using X-ray Diffraction (XDR), Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy coupled to Back-Scatter Diffraction (SEM-EBSD) have been demonstrating that the nature of the hydride phase precipitate depends on the hydrogen content, and can show crystallographic orientation relationships (ORs) with the substrate. Differential Scanning Calorimetry (DSC) has been used to identify the dissolution and precipitation energies at global scale. The difference of both can be associated to the misfit dislocations contribution to the precipitation. Local In-situ TEM dissolution observations confirm the dissolution temperature identified at a global scale and show the depinning of some misfit dislocations during dissolution process. The consequence of this mechanism is that dissolution and precipitation temperatures shift during thermal cyclic loading. This situation will be correlated to the nature of crystallographic hydride phases and their ORs.

Physicochemical and crystalline properties of standard maize starch hydrothermally treated by direct steaming.

The changes in physicochemical properties of standard maize starch (SMS) by three hydrothermal treatments; DV-HMT (Direct Vapor-Heat Moisture Treatment), RP-HMT (Reduced Pressurized-Heat Moisture Treatment) and DIC (instantaneous controlled pressure drop) were investigated at different processing conditions; steam pressure (SP) varied from 1 to 3bar during 20min. Starch was steamed by direct contact, whose interest was to intensify the heat transfer phenomenon but also the water transfer. The physicochemical changes of SMS depended on process conditions and their extent followed this order: DIC>RP-HMT>DV-HMT. All treatments significantly increased gelatinization temperatures and decreased the enthalpies, confirmed by loss of granules birefringence. From 2bar, the crystalline structure changed from A-type to Vh-type, revealing formation of amylose-lipid complexes during steaming. The results clearly showed that the particle size distribution depends on the melting extent of crystalline structure during treatment. At severe processing conditions the melted fraction increased and more complex aggregates of different sizes have been formed.