Molecular Dynamics Simulations of the Structural and Thermodynamic Properties of Poly(l-lactic acid) in the Presence of Water

Abstract

In this study, we used all-atom molecular dynamics (MD) simulations to investigate the behavior of poly(l-lactic acid) (PLLA) in the presence of 0, 1, 2, 6, and 12 m-% water. We focus on understanding how the contacts between various atoms of adjacent PLLA chains evolve and how these changes influence the system’s volume, density, and mechanical and thermodynamic properties. We observe that as the solvent concentration increases, the number of contacts between the atoms of the adjacent PLLA chains decreases, leading to swelling when water penetrates between the polymer chains. However, a small amount of water is seen to facilitate contacts between the oxygen and hydrogen atoms, and the volume of the system contracts. This variation in interchain contacts correlates with changes in density. The contraction is also reflected in the thermal expansion coefficient, which decreases slightly at 1–2 m-%, but then increases at higher water contents due to swelling. In addition, the isothermal compressibility decreases and the isothermal bulk modulus increases; in other words, the resistance to compression increases as 2–6 m-% water is added. Moreover, the specific heat capacity increases after 6 m-%, by 11% at 12 m-%. Our simulation results show strong agreement with the experimental observations. The insights gained from this study improve our understanding of the molecular interactions between PLLA chains and the mechanistic degradation steps favored in the presence of water. These findings have significant implications for the design of sustainable and biodegradable materials, with applications ranging from food packaging to medical uses.