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 κ λ 0 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.

The plastic behavior of polyamide‐6 (or nylon‐6) films under uniaxial and biaxial tensile drawing is studied in relation to structural features. Quenched films in the mesomorphic β form are more ductile than films in the predominant stable α form. Films in the major γ crystalline form are intermediate between films in the β and α forms. Under uniaxial drawing, a great part of the β phase undergoes strain‐induced phase change into the α form, involving strain hardening of the material. The β→α phase change is more pronounced above 120 °C because of additional thermal reorganization. The γ form that is thermally stable up to 200 °C also undergoes a strain‐induced phase change above 120 °C, but the reorganization is much less important than for the β phase. Biaxial drawing can only be achieved below 120 °C with quenched films. This is likely due to the high mechanical anisotropy of the H‐bonded sheetlike structure of both the α and γ phases that is suspected to develop catastrophic splitting under a normal stress component, notably above 120 °C because of the collapse of the paraffin‐like van der Waals interactions between the sheets. The disordered distribution of the H bonds in the mesomorphic β structure of the quenched films is more appropriate for biaxial drawing. The strain‐induced β→α phase reorganization, which might have been a serious prejudice, proved to be much reduced during biaxial drawing below 120 °C. The mechanism of the strain‐induced phase change is discussed. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 1224–1236, 2001

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Context. The submillimeter spectral domain has been extensively explored by the Herschel and Planck satellites and is now reachable from the ground with ALMA. A wealth of data, revealing cold dust thermal emission, is available for astronomical environments ranging from interstellar clouds, cold clumps, circumstellar envelops, and protoplanetary disks. The interpretation of these observations relies on the understanding and modeling of cold dust emission and on the knowledge of the dust optical properties. Aims. The aim of this work is to provide astronomers with a set of spectroscopic data of realistic interstellar dust analogues that can be used to interpret the observations. It pursues the experimental effort aimed at characterizing the spectroscopic properties of interstellar dust analogues at low temperature in the mid infrared (MIR) to millimeter spectral domain. Compared to previous studies, it extends the range of studied dust analogues in terms of composition and of structure of the material. Methods. Glassy silicates of mean composition (1-x)MgO -xSiO 2 with x = 0.35 (close to forsterite, Mg 2 SiO 4 ), 0.50 (close to enstatite, MgSiO 3 ) and 0.40 (close to Mg 1.5 SiO 3.5 or MgSiO 3 :Mg 2 SiO 4 = 50:50) were synthesized. The mass absorption coefficient (MAC) of the samples was measured in the spectral domain 30 -1000 µm for grain temperature in the range 300 K -10 K and at room temperature in the 5 -40 µm domain. Results. We find that the MAC of all samples varies with the grains temperature and that its spectral shape cannot be approximated by a single power law in λ −β . In the FIR/submm, and above 30K, the MAC value at a given wavelength increases with the temperature as thermally activated absorption processes appear. The studied materials exhibit different and complex behaviors at long wavelengths (λ ≥ 200 to 700 µm depending on the samples).These behaviors are attributed to the amorphous nature of dust and to the amount and nature of the defects within this amorphous structure. We do not observe MAC variations in the 10-30 K range. Above 20 µm, the measured MAC are much higher than the MAC calculated from interstellar silicate dust models indicating that the analogues measured in this study are more emissive than the silicates in cosmic dust models. Conclusions. The underestimated value of the MAC deduced from cosmic dust models in the FIR/submm has important astrophysical implications because masses are overestimated by the models. Moreover, constraints on elemental abundance of heavy elements in cosmic dust models are relaxed.