*3.4. FTIR*

The FTIR was deployed to identify the chemical groups of hybrid composites including PLA (a), PSE<sup>3</sup> (b), PSE<sup>32</sup> (c), PSE<sup>3500</sup> (d) and PSE32,500 (e) (Figure 4); through the taken analysis, the effects of both solvent and UV were assessed. The formation of oxygencontaining groups is a way by which the effect of UV on the chemical groups can be tracked [68]. The sample code PSE3, 500 signifies that the PSE<sup>3</sup> sample was placed under UV radiation for 500 h. Changes in the carbonyl groups and the absorption changes were investigated through the band at 1716 cm−<sup>1</sup> . To reduce the error caused by the thickness of samples, the band at 1895 cm−<sup>1</sup> was chosen corresponding to the CH groups as the reference; the absorption rate of each group was determined by dividing the absorption rate of that group to 1895 cm−<sup>1</sup> . The resulting films before and after the light seeing were taken at specified intervals of the FTIR spectra [47,69].

Optical-oxidative degradation of polymer samples is usually accompanied by the appearance of unsaturated bonds and various carbonyl groups by which the progression of degradation increases [70]. Two major observations—the formation of C=O (2000–1500 cm−<sup>1</sup> ) and O-H groups in (4000–3000 cm−<sup>1</sup> ) can also be found in the FTIR spectrum of polyolefin and other polymers [71].

When organic polymers are oxidized, carbonyl and hydroxyl groups are the predominant degradation products. These groups are easily characterized by an increase in bands' intensity at 1710 cm−<sup>1</sup> and 3400 cm−<sup>1</sup> . In the PLA spectrum, the band in the range of 3600–3500 cm−<sup>1</sup> belonged to the C=O and the CH<sup>3</sup> stretching vibrations were observed at 3000–2940 cm−<sup>1</sup> . The C=O stretching vibrations (1780–1761 cm−<sup>1</sup> ) and C-O

stretching vibrations (1190–1033 cm−<sup>1</sup> ) were attributed to ester bonds. The broadband in the range of 3760–3010 cm−<sup>1</sup> was attributed to the hydrogen bonds of starch hydroxyl groups. The stretching vibrations of CH<sup>2</sup> and hydro glucose starch ring appeared in the range of 1180–960 cm−<sup>1</sup> and 861–575 cm−<sup>1</sup> .

**Figure 3.** DSC curves of PSES, PSE<sup>2</sup> , PSE<sup>3</sup> , PSE<sup>22</sup> and PSE<sup>32</sup> samples.

As can be seen in the PSE<sup>3</sup> films FTIR spectrum, the absorption band related to the carbonyl group (1750–1640 cm−<sup>1</sup> ) was more intense than PLA; this phenomenon could be attributed to the presence of optimal amounts of filler. On the other hand, with an increase in the amount of filler in the polymer matrix, the OH groups with reductive properties at the end of the chains were increased and, hence, it would partially prevent the polymer's oxidation. By comparing the spectra of PSE<sup>3</sup> before and after UV radiation, it is visible that prolonging the irradiation time was synchronized with an increase in the carbonyl absorption peaks; this phenomenon is an indicator of increasing in the oxidation rate of polymers [72]. Nonetheless, the effect of solvents—acetone and ethanol—on the chemical groups of samples was investigated. It is noteworthy that using ethanol instead of acetone did not cause the appearance of new peaks in the FTIR spectra, but a shift towards lower wavenumbers (1736 to 1718 cm−<sup>1</sup> ). The possible phenomenon can be resulted from ethanol to increase the hydrogen bonds between PLA (carbonyl groups) and starch. This increment led to an increase in the bond length, followed by shifting the carbonyl peak towards lower wavenumbers [73]. Moreover, the FTIR peaks (oxygenated functional groups) of PSE32,500 experienced an increase in their intensities due to the better distribution of starch particles all over the structure.

**Figure 4.** FTIR spectra of (**a**) PLA, (**b**) PSE<sup>3</sup> and (**c**) PSE32, (**d**) PSE<sup>3</sup> ,<sup>500</sup> and (**e**) PSE32,500.
