*3.1. Mechanical Properties Evaluation*

Tensile strength is one of the most important criteria for degradable polymers, specifically in the case of optical–oxidative degradation [50]. The rapid decrease in elongation of polymers is a meaningful sign giving valuable information about polymers' structure. If a polymer's elongation decreases, the rupture and fragmentation rate will increase under the environmental stresses [51]. The effects of PEG addition on the mechanical properties tensile strength and elongation at break—of samples are tabulated through Table 2. PSE samples showed higher tensile strength than the PES sample, indicating that polyester has higher tensile strength than PE [52].


**Table 2.** The elongation at break and tensile strength of samples without subjecting to UV.

The decrease in the tensile strength of PLA when starch was added could be attributed to the incomplete miscibility of both hydrophobic (PLA) and hydrophilic (starch) phases [53]. By incorporation of PEG as an emulsifier to PLA/starch composite, the elongation at break could be increased through the improvement of PLA and starch miscibility and stability [54]. Comparing the effect of both solvents, acetone and ethanol, it was observed that using ethanol instead of acetone as PEG solvent caused more homogeneous emulsifier distribution in the polymer's structure, which was why here the increase in elongation was observed. This phenomenon could be attributed to better emulsifier's effect on the films through the gelatinization of starch [55]. Among the PSE2, PSE3, PSE<sup>4</sup> and PSE<sup>5</sup> samples, PSE<sup>3</sup> was the one with the highest elongation percentage and the strength percentage of PSE<sup>2</sup> almost equaled PSE4. The PSE52, PSE42, PSE<sup>32</sup> and PSE<sup>22</sup> samples had the least tensile strength attributed to the solvent changing. The highest percentage of elongation belonged to the PSE52, which had the least tensile strength percentage.

As the results showed better mechanical performance for acetone-based samples, those were selected to be evaluated after UV exposure. Figure 1 shows the effect of filler on the film's tensile strength after exposure to UV light. According to Figure 1, the samples' tensile strength dwindled from the beginning. The PSEs film experienced a sharper decrease than the other samples ascribed to the particle size and their distribution in the polymer's matrix. The PSEs film sample showed a significant decrease in tensile properties after exposure to light and this reduction in tensile properties for PSE2, PSE3, PSE<sup>4</sup> and PSE<sup>5</sup> samples was less than PSEs. The reason could be attributed to the pores in the PSEs sample. These pores need the energy to get out of their normal size and stretched out. They must consume energy because of the particle's smallness, but if the particle size is large, quite the opposite is the case and the tensile strength drops. There is a direct relationship between being exposed to UV irradiation and the decrease in samples' mechanical properties [56]. The mechanical properties of the samples were declined sharply up to 500 h. The observed reduction in the mentioned properties was due to the degradation caused by UV irradiation. In the case of starch-containing samples, the reduction of tensile properties compared to the pure PLA films was due to the incompatibility of hydrophilic starch and hydrophobic PLA.

**Figure 1.** Changes of retained (**a**) tensile strength and (**b**) elongation to rupture point of polymer films after exposure to UV irradiation.

No significant change was observed in the tensile strength of PSE<sup>3</sup> comparing to PSE<sup>4</sup> and PSE5, but PSE<sup>3</sup> showed the highest elongation at break among acetone-based samples. On the other hand, PSE<sup>32</sup> exhibited the highest tensile strength among ethanolbased samples. Therefore, PSE<sup>3</sup> and PSE<sup>32</sup> are more acceptable and optimal composites for further investigations.
