Mass absorption coefficient of the samples X35 (upper left panel), X40 (upper right panel), X50A (lower left panel) and X50B (lower right panel). For each sample we show the spectra measured for a grain temperature of 300 K (red), 200 K (orange), 100 K (green) and at 10 K (blue). The shaded area represents the uncertainty on the experimental data (see Sect. 2.3). The sharp band at 7.2 μm is not associated with the samples and is due to the PE matrix.