Catch and Release of Oil Droplets by Block-Copolymer-Grafted Surfaces: Coarse-Grained Molecular Dynamics Simulations

Stimuli-responsive polymer brushes enable the selective capture and release of oil droplets from an aqueous phase. We study pH-switchable poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) grafts on weakly and strongly oleophilic surfaces. Under basic conditions, PDMAEMA is neutral and collapses, absorbing oil; under acidic conditions, it becomes polycationic, extends, and expels the droplet. On a weakly oleophilic substrate, this mechanism releases 86% of the oil, but on a strongly oleophilic substrate, only 0.5% is expelled due to the substrate's strong attraction. To overcome this, we design a block copolymer brush with a PDMAEMA top layer above a pH-insensitive, permanently cationic, water-swollen poly([2-(methacryloyloxy)ethyl]trimethylammonium chloride) (PMETAC) cushion that blocks oil access to the substrate. Upon acidification, this architecture releases 99% of the oil, even from strongly oleophilic surfaces. Furthermore, although low grafting density compromises switchability for the homopolymer, the block brush retains efficient catch-and-release performance.