Structures of Partially Fluorinated Bottlebrush Polymers in Thin Films

We performed multiscale molecular dynamics (MD) simulations of bottlebrush polymers with fluorinated side chains to investigate the influence of the bottlebrush architecture on the spatial distribution of fluorinated moieties. In thin films, coarse-grained MD simulations reveal that interfaces are characterized by backbone depletion with side chains at the interface oriented parallel to the surface. At the molecular level, atomistic MD simulations show that fluorine atoms in the bottlebrush are preferentially located at air-film interfaces. Both simulation results indicate enrichment of fluorinated moieties at the air-film interface. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) confirms the enhancement of fluorinated moieties in both bottlebrush and linear copolymer films. ToF-SIMS also shows that for long linear chains the difference between the concentration of fluorinated moieties at the air-film interface and the concentration at the bulk or at the middle of the film is relatively higher than those observed in shorter linear chains (macromonomers) or bottlebrushes. Although there are differences in the distributions of fluorine moieties between linear chains and bottlebrushes, simulations suggest that the effect of polymer architecture on contact angle and surface energy is only significant at low molecular weights. Measurements of the contact angle of films composed of different modestly high molecular weights of linear chains and bottlebrushes do not show a significant difference in the value of the contact angle, in a way restricting the tunability of surface energy and contact angle via polymer architecture. With these results, our study provides insights into the effective use of partially fluorinated bottlebrush polymer for directed self-assemblies at surfaces in thin films. Specifically, achieving a high degree of difference in surface energies for fluorinated thin films is limited to films with low molecular weight constituents, and the effect of molecular architecture is subtle, suggesting it is insufficient to only rely on this strategy to further lower the surface energy of these films for applications directed toward tuned wettability, adhesive interactions, and fouling resistance.