Bottlebrush Block Copolymers at the Interface of Immiscible Liquids: Adsorption and Lateral Packing.

Amphiphilic bottlebrush block copolymers (BBCPs), having a hydrophilic bottlebrush polymer (BP) linked covalently to a hydrophobic BP, were found to segregate to liquid-liquid interfaces to minimize the free energy of the system. The key parameter influencing the outcome of the experiments is the ratio between the degree of polymerization of the backbone ( N BB ) and that of the side-chain brushes ( N SC ). Specifically, a spherical, star-like configuration results when N BB < N SC , while a cylindrical, bottlebrush-like shape is preferred when N BB > N SC . Dynamic interfacial tension (γ) and fluorescence recovery after photobleaching (FRAP) measurements show that the BBCP configuration influences the areal density and in-plane diffusion at the fluid interface. The characteristic relaxation times associated with BBCP adsorption ( τ A ) and reorganization ( τ R ) were determined by fitting time-dependent interfacial tension measurements to a sum of two exponential relaxation functions. Both τ A and τ R initially increased with N BB up to 92 repeat units, due to the larger hydrodynamic radius in solution and slower in-plane diffusivity, attributed to a shorter cross-sectional diameter of the side-chains near the block junction. This trend reversed at N BB = 190, with shorter τ A and τ R attributed to increased segregation strength and exposure of the bare water/toluene interface due to tilting and/or wiggling of the backbone chains, respectively. The adsorption energy barrier decreased with higher N BB , due to a reduced BBCP packing density at the fluid interface. This study provides fundamental insights into macromolecular assembly at fluid interfaces, as it pertains to unique bottlebrush block architectures.