Molecular architecture of block-polymer brushes for underwater oil droplet catch-and-release: A constant-pH hybrid Monte Carlo molecular-dynamics study

We quantify how block-brush architecture controls the underwater catch-and-release of oil droplets by poly([2-(methacryloyloxy)ethyl]trimethylammonium chloride) (PMETAC)-poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) diblock polymer brushes, mounted on a generally oleophilic substrate surface. Using a Martini 3 coarse-grained model combined with a constant-pH hybrid Monte Carlo Molecular-Dynamics (MC/MD) scheme, we vary grafting distance d ∈ {2.4, 3.0, 4.0} and the length of the pH-responsive PDMAEMA block LD ∈ {25, 50, 75}, while keeping the permanently charged PMETAC underlayer fixed (LM = 50). In its neutral, deprotonated state, PDMAEMA collapses into laterally heterogeneous domains; shorter LD and lower grafting density yield more numerous, smaller islands. When oil droplets are captured by the PDMAEMA block, their wrapping by the polymer and their deformation strengthen with increasing LD and with decreasing d, consistent with droplet-brush contact statistics. Upon acidification (PDMAEMA is positively charged), the release of oil droplets is generally robust and proceeds in near synchrony with PDMAEMA protonation: the MC/MD cycle at which polymer-droplet contacts vanish closely follows the cycle at which the degree of protonation reaches unity. Release slows with increasing LD and accelerates with decreasing grafting density, while strongly wrapped architectures retain more residual oil within the brush after detachment of the majority. A distinct failure case emerges at low grafting density and long PDMAEMA (d = 4.0 nm, LD = 75), where the droplet penetrates the brush and establishes persistent contact with the oleophilic substrate, preventing pH-triggered detachment. These results provide architectural guidelines for designing responsive brushes that balance robust capture with reliable release.