**3.1 Infrared spectroscopic study**

The transmittance of samples was examined by Fourier transform infrared (FTIR) spectroscopy (FTIR spectrometer, VERTEX 70/70v from Bruker™ Optics) in the wavelength range of 1–25 μm. **Figure 2** shows the transmittance spectra of the SiO2/LDPE films in different ratios.

Different absorption peaks could be identified in the MIR range. The first one at ~3 μm caused by OH group and other peaks at ~9 μm, ~12 μm, and ~21 μm, due to Si—O—Si resonance mode of vibrations [18]. Some of these peaks also involve the LDPE substrate in the IR absorption spectra. The peak at 9 μm gives the SiO2 its importance and allows it to be used in this application. We observe a decrease in transmittance when the mixing ratio of SiO2 increases. The changes in the average transmittance for wavelengths ranging from 7 to 10.5 μm are shown in **Figure 3**. We notice a sharp decline in transmittance when the ratio of SiO2 is increased.
