*3.5. Hydrolytic Degradation*

Figure 5 shows the hydrolytic degradation percentage of samples at pH 4, 7 and 13 and two temperatures of 23 ◦C (room temperature) and 40 ◦C in order to evaluate the effect of pH and temperature increase on degradation rate. Considering Figure 5a, degradation kinetic is faster when the temperature is increased. The highest degradation at pH = 4 occurred for the PSE32,500 according to the mechanism shown in Figure 6b. Therefore, the time duration of being exposed to the light is effective in hydrolytic degradation. Another important parameter affecting the degradation results is the solvent, which showed higher degradation in the case of ethanol. Both the effect of UV exposure and the solvent was is in accordance with mechanical and SEM results. Thus, by increasing UV exposure duration, hydrolytic degradation of the samples is increased and hydrolytic degradation is similarly increased at higher temperatures. The lowest degradation belonged to the PES, indicating that PE has a low potential for hydrolytic degradation. The difference between PES and PSE<sup>3</sup> is in the main polymeric phase in the samples due to the potential for hydrolysis [74,75].

**Figure 5.** The weight residual after hydrolytic degradation of samples at (**a**) pH = 4, (**b**) pH = 7 and (**c**) pH = 13.

**Figure 6.** Hydrolytic degradation mechanism in (**a**) alkaline environment and (**b**) acidic environment.

The percentage of hydrolytic degradation at pH = 7 was less than pH = 4. It could be attributed to the concentration of H<sup>+</sup> ions and their role in the hydrolysis of films [76]. As shown in Figure 5b, there was no significant difference between hydrolytic degradation percentages at both environments and the highest degradation was related to the PSE32,500, which experienced a higher hydrolytic degradation in 40 ◦C. The lowest hydrolytic degradation was related to PES having less degradation relative to the main sample of PSEs.

According to Figure 5c, the hydrolytic degradation percentage in alkaline environments (pH = 13) was more than the degradation percentage in neutral and acidic environments (pH = 4 and pH = 7). It can be due to the high hydrolysis power of OH− ions which can hydrolyze polymer films at a high rate. Among the samples, PSE32,500 had the highest degradation percentage and the effect of UV radiation on film's degradation was evident. Additionally, the lowest degradation percentage belonged to the PES sample. Based on the obtained results, the hydrolytic degradation of samples at pH = 13 occurred much faster than at pH = 4 and pH = 7. Making a comparison between Figure 5 revealed that the hydrolytic degradation of samples at 40 ◦C was more than that of 23 ◦C.

Figure 6 shows the mechanism of oligolactate degradation. The degradation in alkaline environments was accompanied by an intra-molecular ester exchange. The nucleophilic attack of the hydroxyl group to the second carbonyl causes the formation of a sustainable six-member interface loop and this reaction is the base of catalysis (mechanism a). According to this mechanism, oligomer lactic acid DP5 and lactic acid were produced during hydrolytic degradation. During the experiment, significant hydrolytic degradation was observed for polylactic/starch/PEG with acetone and PSEs samples which were almost fully degraded. The rupture of ester bonds in the hydroxyl groups of oligomers happens in low pH through protonation of the OH groups and through an intramolecular hydrogen bond [13]. A five-member ring may be one of the most stable intermediate structures. The formation of hydrogen bonds increases the electrophilic nature of carbonyl groups and hydrolysis through water molecules would occur through the site. According to this experiment, the degradation of various ester groups in lactic acid oligomers was independent of pH. Considering the low concentration of OH groups, the primary phase of these systems is stochastically degraded in polymer chains such that adding starch in polymer degradation increase hydrolytic degradation. Increasing starch in the matrix leads to an increase in hydrophilicity which can be attributed to the constant increase of dielectric. A

higher dielectric constant leads to faster degradation and protecting the hydroxyl groups leads to a significant decrease in the hydrolytic degradation [77–80].
