## 4 Conclusions  
The replacement of synthetic polymer materials produced from raw petroleum, which negatively influences the environment, by the ecofriendly polymer composite materials produced from the biodegradable polymers of different classes is an important modern task. The biodegradable binary compositions of the polyesters polylactide (PLA) and poly(3-hydroxybutyrate) (PHB) were synthesized from natural raw materials by chemical and microbiological methods, respectively, with polysaccharide chitosan being successfully used as an effective industrial sorbent of the heavy metal ions from aqueous media as well as for food packaging. The development of composites based on these polyesters and chitosan allows one to join the mechanical characteristics of these polyesters, which are close to characteristics of synthetic polymers, with the high sorption capacity of chitosan. Additionally, the important advantage of such compositions is their capacity to degrade into harmless substances after the end of their service life. 1. The evaluation of the sorption efficiency of both compositions as sorbents by the method of X-ray fluorescence analysis allowed to establish that the sorption of Fe\({}^{3+}\) ions by PHB-chitosan compositions from aqueous solutions is almost twice as high as that by PLA-chitosan (2.30 and 0.66 wt. %, correspondingly after sorption from 0.008 mol/L FeCl\({}_{3}\) solution during 24 h). This effect is connected with a lower crystallinity of PLA compared with PHB (43.2% and 63.5%, correspondingly), which was established under the study of the thermophysical characteristics of both compositions by DSC method and as a sequence with their various influence on the sorption activity of chitosan. 2. The DSC analysis of the PLA-chitosan composition and this composition containing sorbed iron ions showed a slight decrease in the values of T\({}_{\rm g}\) (62.8 \({}^{\circ}\)C and 61.3 \({}^{\circ}\)C, respectively), T\({}_{\rm cc}\) (113.7 \({}^{\circ}\)C and 107 \({}^{\circ}\)C, respectively), and T\({}_{\rm m}\) (Table 2) as well as an increase in the enthalpy of cold crystallization and a reduction in the degree of crystallinity \(\chi\) (24.6 and 22.9%, respectively). The DSC analysis of the PHB-chitosan and PHB-chitosan composition containing sorbed iron ions showed a minor decrease in T\({}_{\rm m}\) (Table 2) as well as a reduction in the degree of crystallinity \(\chi\) (51.5 and 50.0%, respectively). 3. High-temperature DSC studies of PHB-chitosan and PLA-chitosan compositions showed the destabilizing effect of chitosan on their thermal stability (Table 3). The reason for such a significant decrease in the thermal stability of PLA and PHB in their compositions with chitosan is connected with the presence of free amino groups in the molecule of chitosan, which accelerates the hydrolysis and depolymerization of linear polyesters. 4. The structural changes in PLA and PHB polyesters subjected to UV radiation were studied with the use of FTIR and significant changes in the spectrum were observed. Based on the analysis of the IR spectra of PHB and PLA, it was concluded that, under the action of UV radiation, the scission of ester bonds takes place, as expressed in the appearance of intense bands characterizing the formation of new structural units, leading to the decrease in the molecular weight of polyesters. It is shown that the photodegradation process of PHB mainly occurs in the amorphous phase of the polymer. 5. The one-way ANOVA statistical analysis of FTIR data, related to the most significant results, confirmed the validity of the conclusion on the formation of bands appearing in the spectra (-OH stretching vibrations of the carboxyl group) as a sequence of the predominant breaking of ester bonds during the photodegradation of PLA and PHB.