Lock/unlock mechanism of solvent-responsive binary polymer brushes: density functional theory approach.

A density functional theory (DFT) approach based on a weighted density approximation has been employed to study the perpendicular microphase separation of symmetric binary polymer brushes with weak incompatibility in explicit solvents with different selectivities. Characterized by the relation between the grand potential and vertical structures (including nonlayered and layered structures), a dry binary brush can be categorized as W-type or U-type according to whether the characteristic relation contains a structure that undergoes spontaneous symmetry breaking. A W-type brush can memorize the selectivity of the induced solvent in one of its two layered structures after the removal of solvent, which can be seen as a kind of lock state with the nonselective solvent used as its key to unlock. A U-type brush is lockless but can adapt to the environment without the nonselective solvent's triggering. Also, the boundary described in chain-length-incompatibility space is investigated by the DFT approach, which also verifies that the spontaneous symmetry breaking of the W-type brush originates from the molecular contributions to asymmetry, such as the enthalpic contribution of incompatibility and the entropic contribution of chain connectivity.