Structure and Solvent Trapping in Polymer Brushes Under Poor-Solvent Conditions

Polymer brushes exhibit complex structural responses to changes in grafting density and solvent quality, particularly under poor-solvent conditions. Although brush collapse has been extensively studied, the role of an explicit solvent in determining the structure and conformational response of densely grafted polymer layers remains insufficiently understood. In this work, we investigate the structure of polymer brushes composed of flexible linear homopolymer chains grafted onto an impermeable surface in an explicit solvent. A coarse-grained lattice model was studied using Monte Carlo simulations with the Cooperative Motion Algorithm, allowing systematic variation in chain length, grafting density, and solvent quality. Chain conformations, brush height, density profiles, and solvent distribution within the polymer layer were analyzed. The simulations show that brush response to decreasing solvent quality is not universally monotonic. Depending on grafting density and chain length, the radius of gyration and its component perpendicular to the grafting surface exhibit non-monotonic changes, while the brush develops heterogeneous polymer and solvent distributions under poor-solvent conditions. In particular, solvent molecules can become locally trapped within the polymer layer, producing polymer-rich and solvent-rich regions. These structural changes arise from competition between polymer-polymer interactions promoting chain collapse and solvent mixing entropy with solvent confinement. These results demonstrate that explicit solvent redistribution can play an active role in the structural organization of polymer brushes and that solvent trapping should be considered when describing their behavior under poor-solvent conditions. The findings extend the conventional picture of brush collapse and provide a basis for understanding the coupling between chain conformation and solvent microstructure in densely grafted polymer systems.