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 (gamma) 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 (tau(A)) and reorganization (tau(R)) were determined by fitting time-dependent interfacial tension measurements to a sum of two exponential relaxation functions. Both tau(A) and tau(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 tau(A) and tau(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.