*3.4. Chemical Structure of Composite Films*

Functional group analysis of the PP and PP composites can be performed using FTIR spectroscopy. Figure 6 depicts the typical FT-IR spectra of pristine PP, SiPs, CNFs, and PP composite. The appearance of characteristic absorption bands of SiPs at 1054 and 1048 cm−<sup>1</sup> were assigned to the siloxane Si–O–Si bonds for SiP SS and SP, respectively. Furthermore, transmittance peaks at 794 and 797 cm−<sup>1</sup> indicated the presence of hydroxyl groups on the surface [23]. The C-H stretch vibration at 2966, 2898 and 1392 cm−<sup>1</sup> were the typical characteristics of polyalkyl siloxane on the SiP SP as hydrophobic surface treatments [24].

**Figure 6.** FTIR spectra of SiP Sylosphere 200, Sylophobic 200, CNF, and PP composites.

For the CNFs, the broad peak at 3400–3300 cm−<sup>1</sup> was attributed to the stretching vibration of O–H bonding from absorbed water molecules of the cellulose chains. This peak also included inter- and intra-molecular hydrogen bond vibrations in hydroxyl groups in cellulose I [25,26]. The peak at 2895 cm−<sup>1</sup> was assigned to the C–H stretching vibration of all hydrocarbon constituent in polysaccharides and the peak at 894 cm−<sup>1</sup> attributed to the β-glycosidic linkages of the cellulose chain [27].

Furthermore, the peaks at 2950–2835 cm−<sup>1</sup> were contributed by C–H stretching vibrations while peaks at 1450 and 1376 cm−<sup>1</sup> were assigned to CH2 and CH3 bending vibration in neat PP [28]. Further, the transmittance peak located at 840 cm−<sup>1</sup> was assigned to C–CH3 stretching vibration [29]. This peak is a typical characteristic of PP polymers.

New transmittance peaks of the PP nanocomposites at 1087 and 1076 cm−<sup>1</sup> of the asymmetric vibration of O–Si–O bonds indicated that PP with SS/CNF and SP/CNF contained silica fillers. In addition, increasing peak intensity at 806 cm−<sup>1</sup> indicated stretching vibrations of Si–O. From the FT-IR spectra, it is confirmed that the hybrid fillers did not damage the composite materials via denaturing through the extrusion process. However, the characteristic bonds of the C–O group in CNFs were undetected due to the overlap with the O–Si–O band [30].
