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Supplementary Material ' PLA '' 1184 Dielectric function Absorption coefficient 1755 1091 IR reflectance spectra of PLA composites reinforced with flax fibres in the ranges 1100 – 3000 and 500 - 900 cm-1 are similar to that of pure PLA blends. Some bands show a small shift in their wavenumbers, but it does not exceeded 2 cm-1. Both measured parameters, absorption coefficient and dielectric functions vs. wavenumbers (Fig. 1 supp) exhibit similar patterns of lines in their shape and energy position for all studied samples. However, clear changes are seen when the relative intensity of absorption coefficient is compared. The intensity ratio of bands at 1755 cm-1 which correspond to the stretching (C=O) vibrations of the carbonyl group of lactic unit and 1091 cm-1 or 1184 cm-1, both assigned to two (C-O-C) vibrations of the –C(=O)-O-CH2- system for PLA, PLA+wt and PLA+M50 samples are 1.00, 0.95, 0.92, or 1.00, 0.91 and 0.95, respectively. The decrease in the intensity of the stretching (C=O) vibrations of the carbonyl group in composite might suggests hydrogen bond formation of this group with cellulose polymer from fibres. In summary the reflectance IR spectra of PLA and two studied composites (Fig. 1 supp) does not provide clear evidence for the hydrogen bond interactions between composite components but the decrease in relative band intensity of the (C=O) vibrations at 1755 cm-1 in relation to other bands of this region might suggest on it. PLA PLA+wt ' PLA+wt '' ' PLA+M50 '' PLA+M50 2000 1800 1600 1400 1200 1000 -1 Wavenumber / cm 800 600 2000 1500 1000 Wavenumber / cm 1 500 -1 Fig. 1 supp. Absorption coefficients and dielectric functions derived from the reflectance FTIR spectra of PLA, PLA+wt, PLA+M50. The suggestion is partially supported by changes of the spectral contours seen in the Raman spectra of the free PLA and its composites (Fig. 2 supp). New broad contours, not seen in the spectra of PLA, appeared at about 1850, 1600 and 750 cm-1 that could be proposed as the evidence for the hydrogen bond interaction between cellulose polymers and PLA matrix. Raman intensity PLA+M50 PLA+wt PLA 3500 3000 2500 2000 1500 1000 500 -1 Wavenumber / cm Fig. 2 supp. Raman spectra of composites PLA+wt, PLA+M50. Sheets of pure PLA were taken as a reference. The measurements were conducted as described in the Materials and Methods section. The same analysis was conducted for the IR and Raman spectra of the PCL, PCL+wt and PCL+M50 samples. The analysis of reflectance (Fig. 3 supp) IR spectra allows one to conclude that the PCL blend differs from PLA in its influence on the reinforced cellulose admixture. 2 1190 ' PCL Dielectric function 1047 Absorption coefficient 1725 '' PCL PCL+wt PCL+wt ' PCL+M50 ' '' '' PCL+M50 2000 1800 1600 1400 1200 Wavenumber / cm 1000 -1 800 600 2000 1500 1000 Wavenumber / cm Fig. 3 supp. Absorption coefficients and dielectric functions derived from the reflectance FTIR spectra of PCL, PCL+wt, PCL+M50. For the absorption coefficient the intensity ratios of bands at 1725 ((C=O) vibrations) and 1190 cm-1 ((C-O-C) vibrations of the –C(=O)-O-CH2- system) for PCL, PCL+wt and PCL+M50 samples are 1.00, 2.08 and 1.55, respectively. The respective intensity ratio of bands at 1725 and 1047 cm-1 ((C-O-C) vibrations of the –C(=O)-O-CH2- system) are 1.00, 2.33 and 1.60, respectively. Comparing the intensity of the bands at 1725 cm -1 for these samples we obtain the ratio 1 : 1.36 : 1.33, i.e. the intensity of the band corresponding to the (C=O) vibration increases for the composites in relation to the pure PCL. The result might suggest disruption of intramolecular hydrogen bonds in PCL matrix upon addition of cellulose polymers. 3 500 -1 Raman intensity PCL+M50 PCL+wt PCL 3500 3000 2500 2000 1500 1000 500 -1 Wavenumber / cm Fig. 4 supp. Raman spectra of composites PCL+wt, PCL+M50. Sheets of pure PCL were taken as a reference. The measurements were conducted as described in the Materials and Methods section. On the other hand the Raman spectra (Fig. 4 supp) of the free PCL and its composite are quite similar to those obtained for PLA and its composite suggesting hydrogen bond interaction between cellulose polymers and PCL matrix. This is however not confirmed by detailed analysis of absorption spectra recorded for PCL and its composites. Since there was no shift in bands wavenumber and their intensity in absorption spectra the lack of strong molecular interactions between the PCL and cellulose is concluded. 4