Microstructure and Performance Evolution of Poly(l-Lactic Acid) during Physical Aging: Effect of Molecular Weight

With the growing public awareness of environmental protection, poly(l-lactic acid) (PLLA) has started to establish a presence across various industries. However, the inevitable physical aging of PLLA products leads to substantial alterations in both microstructural evolution and macroscopic properties. Therefore, it is essential to conduct an in-depth discussion of the underlying mechanism associated with physical aging. As an intrinsic parameter of PLLA, the mechanism by which molecular weight influences physical aging remains to be clarified. In this article, the regulatory mechanism of molecular weight over the physical aging timeline in PLLA is elucidated according to the cohesional entanglement theory. The ″growth process″ of cohesional entanglements within the high-molecular-weight sample system is inhibited, leading to a decrease in the effective number of such structures, which is manifested as a delay in physical aging behavior. Moreover, the effect of molecular weight on the physical aging exhibits a nonlinear increase, and a ″saturation effect″ is observed. This work provides a theoretical basis for elucidating the regulatory mechanism of molecular weight on the physical aging behavior of PLLA.