Molecular modeling simulations and thermodynamic approaches to investigate compatibility/incompatibility of poly(l-lactide) and poly(vinyl alcohol) blends

This paper investigates the molecular modeling simulation approaches for understanding the blend compatibility/incompatibility of poly(l-lactide), PLL and poly(vinyl alcohol), PVA. Blends of PLL/PVA have been widely used in biotechnology as well as membranes in separation science. Realizing their importance, we thought of investigating to verify experimental observations on their compatibility/incompatibility aspects by calculating thermodynamic interactions between PLL and PVA over the entire range of blend compositions. In doing so, Flory-Huggins interaction parameter, χ, was computed for different blends using atomistic simulations to predict blend miscibility. It was found that at 1:9 blend composition of PLL/PVA, miscibility was observed, but increasing immiscibility was prevalent at higher compositions of PLL component. Computed results confirmed the literature findings on differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) and mechanical property studies, suggesting the validity of modeling strategies. Plots of Hildebrand solubility parameter, δ, and cohesive energy density, CED, supported these findings. Miscibility of PLL and PVA polymers is attributed to hydrogen-bonding effect. Literature findings have been validated to understand the nature of interactions between different groups of the polymers by computing radial distribution function, RDF, for groups that are tentatively involved in such interactions, leading to miscibility or immiscibility. RDF plot was constructed to identify the exact contribution of particular atoms of polymers to confirm miscibility/immiscibility of blends. Results of this study are correlated well with the reported data. Kinetics of phase separation was examined using density profiles calculated from the MesoDyn approach to examine miscibility/immiscibility aspects of the blends. Computed free energy from the mesoscopic simulation of blends reached equilibrium, particularly when simulation was performed at higher time step, indicating the stability of the blend at certain compositions. X-ray diffraction profiles have been constructed for individual polymers as well as for their blends, which agreed well with the reported data.