Evolution of thermal behavior, mechanical properties, and microstructure in stereocomplexable poly(lactic acid) during physical ageing

Polymer glasses are metastable below their glass transition temperature (Tg) and the process of structural relaxation towards equilibrium state is known as physical ageing. However, the evolution of structure mechanics during ageing is still a long-standing question, especially the interpretation in chain level. In this work, we illustrate the critical role of intermolecular interaction on physical ageing of stereocomplexable poly(lactic acid) (PLA). By melt-blending the two enantiomers of PLA [poly(l-lactic acid) (PLLA) and poly(d-lactic acid) (PDLA)] with different ratios, we obtain a typical system with tunable intermolecular interactions and investigate the effect of such interactions on physical ageing behavior. Compared with a neat PLLA, we found that the chain mobility is lower in stereocomplexable PLLA/PDLA blend, which clearly exhibits a retarded ageing kinetics. Besides, the stereocomplexable PLLA/PDLA blend exhibits a stiffer chain network, manifested by a high elastic modulus, which improves steadily with longer ageing time. We propose that the retarded ageing kinetics and high stiffness in stereocomplexable blends are results of intermolecular interactions between PLLA and PDLA chains. This work has shed light on the critical role of intermolecular interactions on physical ageing kinetics and evolution of physical performances of glassy polymers.