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 Cu Iselective 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 (F f −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 F f −L data following a linear progression according to Amontons' law.

Facile Fabrication of Bio‐ and Dual‐Functional Poly(2‐oxazoline) Bottle‐Brush Brush Surfaces

“…In the case of SI‐Cu 0 CRP, PIPOx brush gradients can be prepared straightforward using a tilted Cu‐plate (Figure a). Because the gradual variation of the distance (D) between the Cu source and initiating‐substrate allows gradually changed polymerization rate and thus leads to polymer brush gradients . Even after side chain grafting, the gradient conformation was retained, but their thickness were systematically increased.…”

“…More importantly, the SI‐Cu 0 CRP is oxygen tolerant and requires very limited amount of monomers (μL), and therefore can be used to prepare polymer brushes with low cost . In addition, by simply variation of the distance ( D ) between the copper plate and the initiating surface, various structured polymer brushes can be prepared …”

“On Water” Surface‐initiated Polymerization of Hydrophobic Monomers

“…Recently, we optimized the CuCRP for polymer brush synthesis via surface‐initiated polymerization CuCRP (SI‐CuCRP) by simply using a copper plate as the catalyst, a ligand and the monomer of choice but no additional Cu I/II sources . The SI‐CuCRP was found to be the fastest SI‐CRP for many monomers, and resulted in very dense brushes and homogeneous brushes even on the wafer scale –. Very recently, this method was also successfully adopted for SI‐ATRP .…”

Effect of Architecture of Thermoresponsive Copolymer Brushes on Switching of Their Adsorption Properties

“…They are also able to demonstrate switchable properties due to the stimuli‐responsive behavior of the polymer chains that were grafted onto the surface . The responsiveness of these brushes was utilized to design surfaces with tunable wetting properties with regard to adhesion, adsorption, friction, and other properties . In one approach for the design of stimuli‐responsive brushes, the switching is provided by changing the properties (charge and hydrophobicity) of the polymer themselves .…”