Fabrication and Interfacial Properties of Polymer Brush Gradients by Surface-Initiated Cu(0)-Mediated Controlled Radical Polymerization

Abstract

Surface-initiated Cu(0)-mediated controlled radical polymerization (Si-CuCRP) can be successfully applied to fabricate poly[(oligoethylene glycol)methyl ether methacrylate] (POEGMA) brushes in one pot, presenting a grafting-density gradient across the surface. This is achieved by continuously varying the distance (d) between a copper plate, used as a source of Cu species, and the initiator-functionalized substrate. X-ray photoelectron spectroscopy (XPS) analysis of monolayers of CuI-selective ligands demonstrates that a higher concentration of activator species diffuses to the initiating substrate in areas closer to the copper plate, a progressive decrease in activator concentration being observed upon increasing the distance between the two surfaces. As confirmed by the SI-CuCRP kinetics measured at different positions along the gradient, radical-termination reactions between propagating chains limit the grafting density of POEGMA grafts where the diffusion of activators is favored (i.e., at d → 0). This effect decreases with increasing d, ultimately yielding a gradual variation of POEGMA grafting density across the substrate. We have investigated the influence of grafting-density variation across the gradient on the swelling of POEGMA brushes as well as on their nanomechanical and nanotribological properties, measured by a combination of variable angle spectroscopic ellipsometry (VASE), colloidal-probe atomic force microscopy (CP-AFM), and lateral force microscopy (LFM). The results of these tests highlight how loosely grafted POEGMA chains incorporating a substantial amount of water can be significantly deformed by a shearing AFM probe, exhibit relatively high friction, and generate friction-vs-load (Ff–L) profiles that follow a sublinear trend described by a Johnson–Kendall–Roberts (JKR) model—typical of deformable films of high surface energy. In contrast, more densely packed POEGMA brushes incorporate less solvent and display very low friction, with Ff–L data following a linear progression according to Amontons’ law.