Constrained Amorphous Interphase in Poly(l-lactic acid): Estimation of the Tensile Elastic Modulus

Understanding the mechanical behavior of semicrystalline polymers is essential for optimizing their use in various applications. While the role of crystalline regions is well-established, the contribution of the amorphous phase remains less clear. Previous studies have shown that the elastic properties of such materials depend on both the crystalline and amorphous components. However, the exact mechanical role of the rigid amorphous fraction (RAF) has not been fully resolved. This uncertainty has driven the need for a more detailed analysis of how different phases contribute to the overall modulus. The challenge lies in distinguishing the mechanical effects of the mobile and rigid amorphous fractions. Existing models often fail to account for the interfacial coupling between crystalline and amorphous domains. A clearer understanding of these interactions could improve material design. This study addresses these gaps by applying a three-phase model to quantify the contributions of each phase.

The goal of this research is to evaluate the mechanical contributions of different phases in poly(l-lactic acid) (PLLA). Specifically, the study focuses on semicrystalline PLLA samples containing either α' or α-crystals. The researchers aim to quantify the elastic moduli of the crystalline and rigid amorphous phases. They also seek to understand how the polymorphic form of the crystals affects the mechanical behavior. The motivation for this study stems from the need to better predict and control the mechanical properties of PLLA. The three-phase model is used to separate the contributions of the crystalline, mobile amorphous, and rigid amorphous fractions. This approach allows for a more accurate interpretation of the experimental data. The study is designed to provide insights that can guide the development of tailored PLLA materials for specific applications.

The researchers employed a three-phase mechanical model to analyze the elastic properties of PLLA. This model accounts for the contributions of the crystalline phase, the mobile amorphous fraction, and the rigid amorphous fraction. The model was applied to samples containing exclusively α' or α-crystals. The elastic moduli of each phase were calculated using a mathematical approach. The model also allowed for the simultaneous quantification of the moduli of the α' and α phases. The density of the rigid amorphous fractions was measured using standard techniques. The results were compared to previously reported experimental and theoretical values. The study focused on room temperature conditions to ensure consistency with prior research.

The elastic moduli of the α' and α phases were found to be 11.2 and 14.8 GPa, respectively. These values align closely with existing experimental and theoretical data. The rigid amorphous fractions associated with these phases had moduli of 5.4 and 6.1 GPa. The densities of the rigid amorphous fractions were 1.17 and 1.11 g/cm³ for the α' and α forms, respectively. The higher modulus of the α-associated rigid amorphous fraction suggests stronger chain coupling at the interface. The mobile amorphous fraction had the lowest modulus at 3.6 GPa. The overall order of moduli was E_MAF < E_RAF < E_C. These findings indicate that the rigid amorphous fraction plays a significant role in the mechanical behavior of PLLA.

The study concludes that the mechanical properties of PLLA are influenced by the contributions of all three phases. The elastic moduli of the crystalline and rigid amorphous fractions were successfully quantified. The rigid amorphous fraction associated with α-crystals showed a higher modulus and lower density than that of α' crystals. This difference is attributed to stronger chain coupling at the interface. The mobile amorphous fraction had the lowest modulus, as expected. The three-phase model provided a comprehensive interpretation of the experimental data. These results can help in the design of PLLA materials with tailored mechanical properties. The findings support the importance of considering all phases when evaluating the mechanical behavior of semicrystalline polymers.