*3.6. Biodegradation*

The biodegradation of samples was assessed during optical oxidative degradation. Due to the complexity of the biodegradation process and many factors affecting this process, various methods have been reported by researchers and some standards have been developed in this area [81–83]. However, applying them into practice often requires the use of special equipment. Biodegradation through the landfill is an easy way by which the degradation rate can be estimated [84]. The soil contains different microorganisms leading to the digestion of polymers followed by converting them into water and CO<sup>2</sup> and subsequently the sample's weight loss. The results are presented in Figure 7.

**Figure 7.** Polymer samples (**a**) rate of biodegradation and (**b**) water absorption after landfill at different times.

As can be observed in the case of control samples (PES), their weight loss was less than PLA samples. In addition, those samples, which were not in the exposure of UV, experienced less change in their weight during the period of the landfill than the samples treated with UV. The weight loss of samples was impressive, particularly after two weeks of landfill. The main mechanism governing this degradation process starts with the formation of pores in the polymers' structure after being exposed to UV and so it leaves behind more surface area leading to an increase in the oxidation and digestion of substrates. Moreover, the breakage of polymer chains due to mechanical stresses is another parameter that may lead to the creation of active radicals and the oxidation of polymer in the environment.

The hydrophobic property of polyolefin is a preventive factor that impedes the penetration of enzymes into polymers' structure and, thus, makes them useless. Moreover, many microorganisms and fungi require a suitable substrate besides a food source in order to proliferate and develop their colonies. Hence, polyolefin films are not suitable for the

growth of microorganisms [39]. To make polyolefin biodegradable, their molecular mass must be dropped massively. Although environmental factors such as light, heat and oxygen are effective for this purpose, they cannot cause massive molecular mass drop within a relatively short period of time [85].

The addition of starch to the PLA structure would improve its wettability culminating in a faster water molecules penetration. The water absorption of samples is shown in Figure 7b, where the PSEs sample showed a higher absorption capability than the PES sample. Considering that no compatibilizer was used to prepare these samples, the PLA and starch compound was found to be mixed well with a proper distribution of starch through the whole structure. It is noteworthy that the PESs polymer film should have shown at least 20% water absorption, while only about 5% was obtained. The results are in accordance with SEM studies of composites (Figure 2). As can be seen from the SEM images, the highest degradation rate after the landfill is related to PSE<sup>32</sup> due to the better PEG dispersion in the PLA/starch blend.

In the case of using ethanol as the solvent, higher water absorption in the prepared samples was observed. The reason for this phenomenon is rooted in the lack of compatibilizer, which can lead to bonding PLA and starch. Since these two are naturally hydrophobic (PLA) and hydrophilic (starch), their mixture would yield a physical bond even without using a compatibilizer leading to an increase in the compound's water absorption rate. The maximum water absorption occurred in the first week, followed by a decrease in the rate up to the second week; this can be due to the saturated hydrophilic groups and saturated pores found in the compounds.
