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

Abstract

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. The effect of the Molecular weight dispersity (MWD) on the interfacial width between a hPS brush and unbound dPS matrix has been investigated by NR measurement. The intermixing between hPS brush with narrow MWD and dPS matrix hardly occurred even after thermal annealing at 398 K because of dry-brush regime. In contrast, the intermixing of hPS brush with broad MWD and free dPS proceeded to increase the interfacial width like a wet-brush condition, in spite of high grafting density.