Unexpected Solvent Effect Leading to Interface Segregation of Single-Chain Nanoparticles in All-Polymer Nanocomposite Films upon Solvent Evaporation

In athermal all-polymer nanocomposites (all-PNCs), single-chain nanoparticles (SCNPs) are often considered to be well miscible with polymer matrixes due to their similarity in chemical compositions. However, internal cross-linking units of SCNPs must have different chemistries from the backbone monomers and, therefore, also from matrix chains. Here, we use large-scale molecular dynamics simulations to study the influence of solvent selectivity, particularly to internal cross-linkers in SCNPs, on dispersion state of SCNPs in all-PNC films upon solvent evaporation. Surprisingly, we find distinct dispersion states of SCNPs in drying films with different solvent selectivities. When the solvent is both good for cross-linkers and backbone/matrix monomers, SCNPs can be uniformly dispersed. However, when the backbone/matrix monomers have better solvophilicity than the cross-linkers and the solvophilicity of the latter is weak enough, we find segregation of SCNPs in surface regions. Such phenomena can be attributed to the intrinsic difference in the solvent density at an interface region from that in the bulk, which eventually results in the aggregation of SCNPs at the interface region where the solvent particles are much less than in the bulk. At the interface region, cross-linkers in the SCNPs will have less contact with the solvent and, therefore, less enthalpy penalty than being located in the bulk region of the film. The results demonstrate that solvent selectivity has a non-negligible effect on the structure of the composite film, which will inevitably have impacts on macroscopic properties of the film.