Understanding Microstructure Formation in Drying Nanoparticle–Polymer Films through Evolving Interparticle Interactions

The microstructure of nanoparticles in dried nanoparticle–polymer composite films influences their final properties and performance. However, a well-dispersed suspension does not always yield a uniform film, challenging the assumption that the initial dispersion state determines the microstructure of the dried film. We attribute this discrepancy to the evolution of interparticle interactions during drying. Brownian dynamics simulations were performed using a fixed interaction model (FIM) and an evolving interaction model (EIM). While the FIM showed limited agreement with the experimental trends, the EIM more closely reproduced the reported uniform structure at pH 3 and a heterogeneous structure at pH 10, despite the contrasting initial dispersion states. This results from interaction evolution governed by particle–polymer affinity, becoming more repulsive under good affinity (pH 3) and more attractive under poor affinity (pH 10). The drying rate, characterized by the Peclet number (Pe), further governs the influence of evolved interactions, with low Pe enabling rearrangement and high Pe increasingly preserving the initial structure. These results demonstrate that understanding microstructure formation in nanoparticle–polymer films requires consideration of the evolution of interparticle interactions beyond conventional drying kinetics.