Thermodynamics controls structure, function, stability, and morphology of polymer blends. However, obtaining the precise information about their mixing thermodynamics is a challenging task, especially when dealing with complex macromolecules. This is partially because of a delicate balance between the local concentration/composition fluctuations and the monomer level (multibody) interactions. In this context, the Kirkwood-Buff (KB) theory serves as a useful tool that connects the local pairwise fluid structure to the mixing thermodynamics. Using larger scale molecular dynamics simulations, within the framework of KB theory, we investigate a set of technologically relevant poly(methyl methacrylate)-poly(lactic acid) blends with the aim to elucidate the underlying microscopic picture of their phase behavior. Consistent with the existing experiments, we emphasize the importance of properly accounting for the entropic contribution to the mixing Gibbs free-energy change $\mathrm{\Delta }{\mathcal{G}}_{\mathrm{mix}}$ that controls the phase morphology. We further show how the relative microscopic interaction details and the molecular level structures between different mixing species can control the nonlinear mechanics, ductility, and heat flow. Therefore, this study provides a guiding principle for the design of light weight functional materials with extraordinary physical properties.

• Received 8 November 2021
• Revised 9 February 2022
• Accepted 18 February 2022
• Corrected 7 December 2022

DOI:https://doi.org/10.1103/PhysRevMaterials.6.025606