Christophe Depecker

Structural and mechanical behavior of nylon 6 films part I. Identification and stability of the crystalline phases

The crystalline phase of polyamide 6 (otherwise nylon 6) films produced following various physical treatments is investigated by means of thermal analysis, X-ray diffraction, and infrared spectroscopy. A recently published procedure for treating infrared spectra of multicomponent compounds without a priori knowledge of the individual component spectra allowed us to perform a semiquantitative analysis of the structural changes upon annealing, including data from drawn samples. Melt-cast films display a mesomorphic state that is thermally stable up to about 120 °C. This structure partly reorganizes into the stable monoclinic α form above 120 °C. Films in major γ form produced by iodine treatment are thermally stable up to 200 °C. Films in major α form are also prepared by superheated water treatment. No evidence is given for a Brill transition about 170 °C. This is an important fact for further understanding of the drawing behavior of PA6 at temperatures above and below this domain. The mesomorphic phase can hardly be separated from the amorphous component both from X-ray and infrared analysis. However, scanning calorimetry, which is often criticized due to possible reorganization of unstable species during the heating scan, turned out to be a very useful technique. Indeed, recrystallization from the amorphous phase or improvement of ordering from the mesomorphic state both result in exothermic effects in quite different temperature domains that allow to discriminate the two phenomena.

Low-temperature FIR and submillimetre mass absorption coefficient of interstellar silicate dust analogues

Context. Cold dust grains are responsible for the far-infrared and submillimetre (FIR/submm) emission observed by Herschel and Planck. Their thermal emission is usually expressed as a modified black body law in which the FIR/submm dust opacity, or mass absorption coefficient (MAC), is described by the MAC at a given wavelength κλ0 and the temperature- and wavelength-independent emissivity spectral index β. However, numerous data from previous space and balloon-borne missions and recently from Herschel and Planck show that the dust emission is not well understood, as revealed for example by the observed anti-correlation of β with the grain temperature. Aims. The aim of this work is to measure the optical properties of interstellar dust analogues at low temperatures to give astronomers the necessary data for interpreting FIR/submm observations such as those from the Herschel and Planck satellites. Methods. We synthesised, via sol-gel methods, analogues of interstellar amorphous silicate grains, rich in Mg and Ca, and having stoichiometry of olivine and pyroxene. The samples are characterised by various techniques to determine their composition, size, amorphisation degree. All the amorphous samples are annealed at 1100 ◦ C to study the crystallised materials for comparison. We measured the MAC of all the samples in the 2–25 μm range at room temperature and in the 100–1000/1500 μm range for grain temperatures varying from 300 to 10 K. Results. The experimental results show that, for all the amorphous samples, the grain MAC decreases when the grain temperature decreases and that the local spectral index, β, defined as the slope of the MAC curve, is anti-correlated with the grain temperature. These variations, which are not observed in the crystallised samples, are related to the amorphous nature of the samples. In addition, the spectral shape of the MAC is complex and cannot be described by a single spectral index over the 100–1500 μm range. At short wavelengths (λ ≤ 500/700 μm), β is in the range 1.6–2.1 for all grain temperature and grain composition. However, at longer wavelengths (λ ≥ 500/700 μm), β ≤ 2 for samples with a pyroxene stoichiometry and β ≥ 2 for samples with an olivine stoichiometry. Conclusions. The dust properties in the FIR/submm domain and at low temperature are more complicated than expected. The simplifying asymptotic expression based on a single temperature- and wavelength-independent spectral index used by astronomers is not appropriate to describe the dust MAC, hence the dust emission, and may induce significant errors on the derived parameters, such as the dust mass and the dust physical and chemical properties. Instead, dust emission models should use the dust MAC as a function of wavelength and temperature.

Diffuse Reflectance Infrared Spectroscopy: Experimental Study of Nonabsorbing Materials and Comparison with Theories:

This work presents a comprehensive study of the diffuse reflectance of nonabsorbing powders in the mid-infrared. The changes in the optical and physical parameters of the powder (particle size, granulometric distribution, powder density, refractive index) have been used to get a better understanding of the influence of each parameter on the diffusely reflected intensity, as well as the mechanisms of the light diffusion by the powder. A commercial diffuse reflectance attachment has been tested optically for the accuracy of measurements, the focalization of the incident beam at the sample surface being taken into account. The experimental results have been compared to results of existing diffusion theories. This comparison proves the pertinency of the observed variation of the intensity of the diffusely reflected light with the refractive index which increases from 0 for n = 1 to a maximum and then decreases with n. This behavior is related to the anisotropy of the light diffusion both at the inside and then at the outside of the particles and to the light trapping by total reflection inside the particles.

Diffuse reflectance infrared spectroscopy: an experimental measure and interpretation of the sample volume size involved in the light scattering process

This work presents the second part of a basic study of diffuse reflectance spectroscopy through a comparison of experimental observations with existing theories of diffuse reflectance of non-absorbing powders. The diffusion volume approximate shape has been assumed to be a cylinder. Its dimensions have been measured for different non-absorbing powders with a refractive index from 1.4 to 3.4, and with different distributions of particle size larger than the wavelength. The lateral extension diameter Φm of this volume is at least two times larger than that of the irradiated sample surface. The variation of the cylinder height corresponding to the penetration depth, z∞, has a behavior opposite to that of the powder diffusion coefficient, S. Its variation with the mean particle diameter is smaller than predicted by the theories. For KBr, a 1 mm thick layer can be considered as a semi-infinite layer. Comparison between existing theories of diffuse reflectance of powdered samples, shows that the statistical theories explain a good deal of the experimental observations concerning their variation with the particle parameters. This comparison shows a similarity between attenuated total reflection and diffuse reflectance spectra. Nevertheless, some discrepancies still persist; this may be due to the fact that the side-way scattering, which may be related to light transfer between neighbouring rough particles by frustrated total reflection, has only been indirectly taken into account.

Reversible strain‐induced order in the amorphous phase of a low‐density ethylene/butene copolymer

This article deals with the characterization of the structural changes induced by uniaxial extension in the amorphous phase of an ethylene/butene copolymer. Volume change measurements indicate a reversible densification relevant to a strain-induced organization in the amorphous phase. The dynamic mechanical behavior shows an improvement of the β relaxation that reveals an important immobilization of the amorphous chains. The vibrational behavior investigated by means of Raman spectroscopy suggests that the strained amorphous chains are structurally analogous to the mesomorphic interfacial component. A discussion is made about the mechanism of the transformation of the amorphous phase into a mesomorphic structure compared to the more common strain-induced crystallization phenomenon.

The deuterium/hydrogen distribution in chondritic organic matter attests to early ionizing irradiation

Primitive carbonaceous chondrites contain a large array of organic compounds dominated by insoluble organic matter (IOM). A striking feature of this IOM is the systematic enrichment in deuterium compared with the solar hydrogen reservoir. This enrichment has been taken as a sign of low-temperature ion-molecule or gas-grain reactions. However, the extent to which Solar System processes, especially ionizing radiation, can affect D/H ratios is largely unknown. Here, we report the effects of electron irradiation on the hydrogen isotopic composition of organic precursors containing different functional groups. From an initial terrestrial composition, overall D-enrichments and differential intramolecular fractionations comparable with those measured in the Orgueil meteorite were induced. Therefore, ionizing radiation can quantitatively explain the deuteration of organics in some carbonaceous chondrites. For these meteorites, the precursors of the IOM may have had the same isotopic composition as the main water reservoirs of the inner Solar System.

UV-induced reaction of H2 with germanosilicate and aluminosilicate glasses

The cores of optical fiber preforms were exposed to UV light at 244 nm after molecular hydrogen loading under high pressure. The reactions between hydrogen and the core material were investigated by Fourier transform infrared spectroscopy in the 3 mm spectral range. Various glass compositions were considered: germanosilicate glasses codoped with B, Sn, or not and aluminosilicate glasses codoped with Ce or Tb. In germanosilicate glasses (codoped or not), H2 reacts at the Ge sites only. The major defect formed is Ge-OH, some Ge-H is detected too, but at a much smaller level. In the Ce-doped aluminosilicate plates, H2 reacts with Ce and forms Ce-OH. All these materials are reduced through the UV light exposure as shown by the presence of molecular water formed by reaction of H2 with the glass network. The exceptions are with the Tb aluminosilicate preforms which show no reaction with H2 under UV exposure.

Diffuse Reflectance Infrared Spectroscopy Investigations of the Thermal Decomposition of Hexacarbonyl Molybdenum (0) Entrapped in Faujasite-type Zeolites

The temperature-programmed decomposition of Mo(CO)6 entrapped in the faujasite-type zeolites Na85X, Na56Y, and Y* has been investigated with the use of in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) as a function of the Si/Al ratio of the zeolites and the Mo(CO)6 loading level. These investigations were undertaken either under dynamic vacuum or in an atmosphere of CO or He. Evidence is presented for a reversible, sequential decomposition of Mo(CO)6 to Mo(CO)3 and then a one-step irreversible decomposition of Mo(CO)3 to Mo-metal aggregates. The Mo(CO)5 and Mo(CO)4 transient species appeared the most clearly under reduced CO pressure. The Na85X zeolite with high aluminum content (Si/Al = 1.26) promotes an easy decomposition of Mo(CO)6 to Mo(CO)3 at room temperature and stabilizes this adsorbed subcarbonyl species at temperatures above 500 K. A decrease of the aluminum content (Si/Al = 2.5) in Na56 Y increases the decomposition temperature of Mo(CO)6 and decreases the stability of the Mo(CO)3 species. The thermal behavior of Mo(CO)6 entrapped in dealuminated zeolite Y* (Si/Al = 75) was found to be analogous to that of Mo(CO)6 in the gas and solid phases.

Isochronal annealing of BG written either in H/sub 2/-loaded, UV hypersensitized or in OH-flooded standard Telecommunication fibers using ArF laser

On one hand, the stability of fiber Bragg gratings (FBG) written either in ultraviolet (UV) hypersensitized, OH flooded, H2 loaded or in pristine fibers has been investigated through isochronal annealing experiments. The stability of the modulation proved to be higher in the hypersensitized and OH-flooded fibers. In contrast the annealing-induced shift in the Bragg wavelength (lambdaB) was lower in the H2-loaded and pristine counterparts. On the other hand, germanosilicate preform plates have been sensitized and then exposed in similar conditions to those used for writing the gratings. Vacuum-ultraviolet (VUV) and infrared (IR) absorption spectroscopy has been carried out with a view of monitoring the formation (or bleaching) of defect-associated bands. These observations are discussed within the framework of a defect model

Drifts and Raman Spectroscopic Study of Intrazeolite Metal Carbonyl Chemistry

Adsorption and thermal decomposition of Mo(CO) 6 , Mn 2 (CO) 10 and Co 2 (CO) 8 in the cage system of dehydrated zeolite Na 55 Y have been studied with “in situ” DRIFTS and Raman spectroscopies. Mo(CO) 6 and Mn 2 (CO) 10 are adsorbed as intact molecules, whereas Co 2 (CO) 8 is absorbed as Co 4 (CO) 12 and cobalt subcarbonyls. The local symmetry of the absorbed species are assigned to Na + ⋅⋅⋅OC- contacts. Thermal treatment of the metal carbonyl/zeolite adducts lead to loss of CO and sometimes to reversible formation of subcarbonyls which are anchored to zeolite framework. The final products of the thermal treatment are small naked metallic clusters and probably larger crystallites sticking at the outer surface of the zeolite crystals.