Thermal Behavior of Biodegradable Compositions of Polylactide and Poly(3-hydroxybutyrate) with Chitosan and the Effect of UV Radiation on Their Structure

Abstract

The negative influence of water pollution by heavy metals ions on human health represents a serious ecological problem which requires effective methods in the search for its solution. The creation of eco-friendly biodegradable polymer materials capable of performing the sorption of the water media of heavy metals followed by decomposition into harmless substances after the end of their service life presents an actual task. To this aim, binary compositions synthesized from natural raw polyesters polylactide (PLA) and poly(3-hydroxybutyrate) (PHB) with polysaccharide chitosan, corresponding to these requirements, were obtained in the liquid phase. The polyesters have mechanical characteristics close to the characteristics of synthetic polymers, while the chitosan containing the amino groups is capable of performing the sorption of heavy metals. The use of compositions on their base allows one to create the new inexpensive biodegradable sorbents stable in aqueous media as well as apply them as packing materials. The sorption capacity of PLA–chitosan and PHB–chitosan compositions in relation to iron ions from aqueous solutions was explored by a method of X-ray fluorescence analysis and it was established that the sorption of Fe3+ ions by PHB–chitosan composition is more than twice as high as that by the PLA–chitosan composition (2.30 and 0.66 wt. %, correspondingly, after sorption from 0.008 mol/L FeCl3 solution during 24 h). A comparative study of thermophysical parameters and the degree of crystallinity of PLA and PHB, as well as in their initial compositions and compositions, containing sorbed iron ions, was carried out by DSC method. The DSC analysis of the PLA–chitosan and PHB–chitosan compositions, containing sorbed iron ions, showed a slight decrease in the values of Tg, Tcc, and Tm as well as an increase in the enthalpy of cold crystallization and a reduction in the degree of crystallinity of these polyesters. At the same time, an increasing of the thermal stability of polyester compositions in the presence of iron ions was established. The influence of UV irradiation on the structure of PLA and PHB for 2, 5, 24, and 144 h was analyzed by FTIR spectroscopy 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 destruction of ester bonds takes place, which is expressed in the appearance of intense bands characterizing the formation of new structural units, resulting in the decrease in the molecular weight of polyesters.