Poly(lactic acid) - PLA - is one of the most promising biobased and biodegradable polymers able to replace several fossil-based plastics for packaging and other applications. However, PLA is subjected to hydrolytic degradation, affecting its service performance, end-of-life, and environmental sustainability. This dissertation investigated the hydrolytic degradation of PLA to elucidate how the three-phase semicrystalline structure model of a polymer is responsible for the kinetic and mechanistic insights into the hydrolysis of PLA.Modifying a polymer crystallinity is the simpler and more practical approach for enhancing the properties of materials. The crystalline structure exhibits high stability and impermeability to water, consequently reducing the hydrolysis process throughout the entire materials. According to the three-phase structure model, a rigid amorphous fraction (RAF), the interphase between the crystal fraction (CF) and the mobile amorphous fraction (MAF), has more geometrical constraints, causing unique characteristics. So, the amount of RAF may be responsible for some of the discrepancies between the theoretical prediction and the experimental results in PLA hydrolysis. Molecular weight and three-phase fraction analyses were performed during hydrolysis at an elevated temperature of 85 °C to evaluate the morphological change and estimate the kinetic rates using phenomenological models. The different L-lactide content and crystallization methods significantly affected the three-phase fractions, consequently affecting the hydrolysis behavior. The amorphous PLA film with higher L-lactide content had more potential to crystallize, leading to a higher degree of crystallinity. Moreover, the specific structure of the nano-confined crystals from the melt-stretching method provided a high CF and less RAF, demonstrating the lowest hydrolysis rates among the samples. For cold-crystallized samples, a higher initial amount of RAF resulted in accelerated hydrolysis, effectively counterbalancing the crystallinity effect. The influence of varying hydrolysis temperatures on the three-phase structure of PLA was studied, providing insights into the temperature-dependent three-phase behavior. PLA films were crystallized by melt-crystallization at 120 °C with variations in crystallization time to achieve a desirable quantity level of the three-phase structure. Results revealed that temperature significantly impacted PLA degradation. Above the glass transition temperatures (75 and 85 °C), the hydrolysis rates of PLA were comparable among samples with different crystallinity. In contrast, the higher crystallinity sample exhibited a faster hydrolysis rate below and near the glass transition temperatures (45 and 65 °C). These findings highlight the impact of RAF under various temperature conditions. The amount of RAF shows less significant impact at higher temperatures and exhibits similar characteristics to MAF. While at lower temperatures, the chains in RAF were immobilized and retained their defects. This study provides insights into the complex interplay among temperature, crystallinity, and hydrolysis kinetics, establishing a predictive framework to comprehend PLA degradation behavior fully. Finally, a population balance was introduced to describe the evolution of the entire molecular weight distribution (MWD) during the hydrolysis. Our predictions show a promising agreement in the weight location and distribution shape with the experimental MWDs. The evolution of the MWDs by hydrolysis can be explained by considering it as a random process with noncatalytic and autocatalytic reactions, along with specific chain scission of a particular length. The simulation of the evolution of MWD was constructed for real-life hydrolytic degradation to provide comprehensive guideline information for degradation under various applications.
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