Topological Effects of Bottlebrush Copolymer on Their Assembly at the Water/Air Interface

Amphiphilic bottlebrush copolymers offer exceptional tunability through their grafted side-chain architectures, yet the molecular principles linking topology to interfacial assembly remain incompletely understood. Here, we use coarse-grained molecular dynamics simulations to investigate the influence of four topological design parameters: block length (l), side-chain dispersity (Đ), grafting density (f), and the backbone length (N_bb) on the assembly of PEG-PS (PEG: poly(ethylene glycol), PS: polystyrene) bottlebrush copolymers at the water/air interface. The assembled structures are characterized by three methods: the interfacial area per molecule, the average radius of gyration of PEG-PS chains, and the hydration of PEG. By varying each parameter while keeping overall hydrophilic and hydrophobic content fixed, we identify the distinct physical mechanisms by which topology dictates molecular organization at the interface. Increasing block length enhances PEG-PS segregation, producing a few-percent reduction in chain dimensions and a tens-of-percent decrease in the interfacial area per molecule, making block length the strongest determinant of packing. In contrast, the dispersity and grafting density of PEG and PS induces only minor changes (∼5-10%) in the interfacial area per molecule at the studied range. The orientational distributions of side chains relative to the interface further reveal that alternating architectures exhibit strongly separated orientations of hydrophobic and hydrophilic segments, while diblock architectures show more overlapping orientations, which govern chain extension and interfacial packing. As the N_bb increases, the orientation of side chains in the diblock polymer becomes more constrained. The hydration of hydrophilic chains varies differently with the topological design parameters. The grafting density significantly impacts the hydration of PEG side chains, while the block length and the dispersity play a minor role in the local water structures. The simulations provide molecular-level insights into the experimentally observed differences in surface packing and the corresponding trends in interfacial tension of bottlebrush copolymers. This work establishes a topology-assembly relationship and provides molecular-level design principles for bottlebrush copolymers.