Dense, Highly Hydrated Polymer Brushes via Modified Atom-Transfer-Radical-Polymerization: Structure, Surface Interactions, and Frictional Dissipation

摘要

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 NaNO3 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 mu down to mu < 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.