Chain-mixing behavior at interface between polystyrene brushes and polystyrene matrices

The effect of the molecular weight dispersity (MWD) on the interfacial width between an end-tethered polymer brush and a free polymer matrix has been experimentally investigated by neutron reflectivity (NR) measurements. We prepared high-density hydrogenous polystyrene (hPS) brushes with different MWDs by surface-initiated controlled radical polymerization. Unbound (free) deuterated polystyrene (dPS) with a similar molecular weight to the hPS was mounted on the hPS brush to form bilayer films and annealed at 398 K for 2-120 min. The interfacial width between the narrow-MWD hPS and dPS matrix was relatively unchanged after thermal annealing because of the high graft density and similar molecular architectures (dry-brush regime). However, the broad-MWD hPS brush showed a large interfacial width (wet-brush regime) due to the extension of the longer chains into the dPS matrix and intermixing. The interface width increased rapidly from 2 to 20 nm after annealing for 5 min and increased slowly to 30 nm after 120 min. The volume fraction profile of the broad-MWD hPS brush showed a characteristic gradient distribution as the distance from the substrate surface increased, consistent with the theoretical simulations by Laub and Koberstein. 7 Polydisperse brushes are expected to improve the adhesion and interfacial toughness at solid-polymer interfaces.