Evolution of the Molecular and Supramolecular Structures of PLA during the Thermally Supported Hydrolytic Degradation of Wet Spinning Fibers

Fibers formed by the wet spinning method from commercially available polylactide (PLA) variants with different d-lactide isomer contents were exposed to thermally supported hydrolytic degradation in environments with different pH values. This paper focuses on the evolution of the supramolecular structure of PLA during the accelerated degradation progress of real fabricated materials. The disorder-to-order phase transition phenomenon was investigated using wide-angle X-ray diffraction (WAXD). Additionally, differential scanning calorimetry (DSC) was applied to determine the supramolecular changes and resulting thermal properties of the polymer. The obtained results clearly show the formation of irreversible changes in the structure of the polymer, which could be an indication of the formation of microplastics (microcrystallites), as further verified by Fourier transform infrared (FTIR) spectroscopy. Transformations at the molecular and supramolecular levels depend slightly on the pH of the environment if the process is performed at temperatures near the cold crystallization point. Rapid structural transformations lead to irreversible changes and unknown thermal properties of the polymer. Instead, these changes depend on the content of the d-lactide isomer in the PLA structure. The article shows a new extended experimental overview of the degradation of real PLA objects and creates a field for further theoretical analysis.