Dense, Highly Hydrated Polymer Brushes via Modified Atom- Transfer-Radical-Polymerization: Structure, Surface Interactions, and Frictional Dissipation Odeya Tairy, † Nir Kampf, † Michael J. Driver, ‡ Steven P. Armes, § and Jacob Klein* ,† † Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel ‡ Vertellus Biomaterials, Vertellus Specialties UK Ltd., Basingstoke, Hampshire RG25 2PH, U.K. § Department of Chemistry, University of Sheffield, Sheffield S3 7HF, U.K. ABSTRACT: The conditions for atom transfer radical polymerization (ATRP) of poly[2-(methacryloyloxy)ethyl phosphorylcholine] (pMPC) chains are modified to enable much more efficient growth of these poly zwitterionic chains from macroinitiator-coated mica substrates using the “grafting from” technique. In particular, we demonstrate directly that achieving a lower level of oxygen in the reaction mixtures through longer evacuation results in the creation of significantly denser and more extended pMPC brushes, with substantially improved interfacial properties both in pure water and in 0.2 M NaNO 3 salt solution. Using a surface force balance combined with atomic force microscopy and X-ray photoelectron spectroscopy, we characterize these brushes and determine the normal and especially shear interactions between them. Normal force profiles reveal that the grafting density is independent of the brush molecular weight M, and that the swollen brush thickness L scales linearly with M. Moreover, shear force measurements indicate that such pMPC brushes provide boundary lubrication that, with friction coefficients μ down to μ < 10 −4 at pressures P > 150 MPa, is superior by an order of magnitude compared to literature data for polymeric boundary layers, including pMPC brushes described earlier. We attribute this enhanced lubrication to the denser and thicker brush layers achieved in the present study, together with the hydration lubrication mechanism arising from the highly hydrated phosphorylcholine groups on the chains. 1. INTRODUCTION Polymer brushes, consisting of polymer chains densely anchored by one end on a nonadsorbing substrate in a good solvent and stretching away from its surface, are commonly used to modify surface interactions. 1−5 Most such brushes are either “grafted-to”, meaning that chains from solution attach by one end to the surface, or “grafted-from”, meaning that the chains are polymerized from initiator groups attached to the surface. Grafted-to brushes may be prepared with narrow molecular weight distributions, and are relatively easy to attach via physical bonds (e.g., electrostatic, hydrophobic, or van der Waals, or via an adsorbing block of a diblock copolymer), but display limited surface density and relatively weak attachment. 6 Grafted-from brushes are more robust as they are chemically (covalently) attached to the substrate-anchored initiator group, and may be much denser, compared with the grafted-to approach, 7−12 leading to more desirable interfacial properties. However, it is more challenging to create such brushes because of the requirement of in situ polymerization, and, in addition, the possibility of polydisperse chains comprising the surface layer. 13 Early interest in brush properties was driven by their ability to act as steric stabilizers in colloidal dispersions. 14,15 This arises because the brush chains are nonadsorbing and do not undergo bridging attractions with the opposing surface at low coverage, as would adsorbed polymer chains. Over the past 2 decades the properties of polymer brushes as boundary lubricantssurface layers that can massively reduce friction between sliding surfaceshave been discovered and widely studied. 11,16−20 Earlier work focused on friction reduction by brushes in organic solvents. 9,11,21 The striking reduction in friction between two compressed, sliding surfaces bearing neutral polymer brushes was attributed to the entropically suppressed, and thus weak, brush interpenetration on compression, which results in a relatively fluid sheared interfacial zone, because of the absence of chain entanglements within it. 22,23 Frictional dissipation, which occurs largely across this sheared but fluid zone, is thus correspondingly low. In recent years the lubrication properties of brushes in aqueous media has attracted considerable interest. 18,24,25 This is because, in addition to technological interest, the sliding of biological surfaces (as in joints or in the eye), which is always in a physiological aqueous environment, is thought to be mediated by surface-attached biomacromolecules or supramolecular assemblies. 26−29 The frictional behavior of water-soluble Received: September 19, 2014 Revised: December 14, 2014 Published: December 30, 2014 Article pubs.acs.org/Macromolecules © 2014 American Chemical Society 140 DOI: 10.1021/ma5019439 Macromolecules 2015, 48, 140−151