45694-AC7
Limits of Lubrication by Charged Polymers
In the first period of this project we have made strong progress towards completing the first two of the three main aims of our proposal (aims (a) and (b) of section 2.3 of our proposal). In addition we have extended these main aims to examine several related issues which shed further light on the proposal goals. These achievements are described in some 6 publications to date (detailed separately) with further publications in preparation.
We have for the first time grown charged polymer brushes covalently from initiator groups attached to a mica surface, utilizing atom transfer radical polymerization (ATRP) methods. In this way we were able to polymerize poly(2-Methacryloyloxyethyl phosphorylcholine) directly from a macroinitiator attached the the surface. These chains are polyzwitterionic, and each of the monomers has a phosphorylcholine-like structure, and is consequently highly hydrated, a crucial issue for their lubrication properties. We were able to characterize these polymer layers using X-ray reflectometry, XPS, multiple beam interferometry and atomic force microscopy. Furthermore, we have demonstrated the robustness of these layers with respect to friction when compared with physisorbed polyelectrolyte brushes. We found that we were able to create brushes with a remarkably high density, with a value (height/mean brush spacing) ≈ 25 in a good solvent (water), compared with maximal values of ca. 6-8 achieved with neutral brushes in good organic solvents.
The mechanism of lubrication by hydrated brushes, which is the ultimate goal of our project and which we aim to achieve in the second period, depends importantly on several aspects of the brushes themselves. These include, as noted, the behaviour of the hydration sheaths about the charges, the entropic effect of the polymer chains themselves, as well as effects of shear rate and loads. We have therefore also carried out studies to examine these effects in surface-attached polymer systems. These studies include the effect of surface morphology on the interacting hydrated chains, as for the case of hyaluronic acid interacting with enantiomorphic solid surfaces, where we have found that it is not only the polymer size or level of hydration, but also the chirality of the surface which affects the forces between polymer and interacting substrate.
We have also examined the effect of shear rates and free (mobile) polymers on the properties of the polymer brushes, and shown that polymer brushes can swell under sufficiently high shear rates, and in addition be influenced by the presence of mobile (free polymers) at concentration above overlap.
Since the lubricating properties of hydrated brushes depend strongly on the nature of the hydration sheaths about the charged monomers, we investigated also the detailed mechanisms whereby such hydration layers facilitate lubrication, for the case of hydrated quaternary ammonium groups at the interface between a surfactant layer and the adsorbing underlying substrate. We also examined for the first time how different hydrated ions in the alkali metal series affect the lubrication of charged surfaces between which they are trapped. In the same study we were able to show, surprisingly, that such ions selectively ligand to hydrated polyethylene oxide to mediate its adsorption on charged surfaces.
Finally, as we are interested in our hydrated polymer chains also in the context of biological lubrication, and as the hydration layers can also mimic those about proteins, we also made examined (briefly) the issue of protein adsorption onto nanoparticles.
As noted, these achievements are described in some 6 publications to date (detailed separately) with further publications in preparation.
In the coming period of the project we propose to address the final aim of the proposal, namely the systematic study of the lubricating properties of such hydrated brushes over a wide range of conditions including different loading pressures and sliding velocities (shear rates), with particular reference to their utility at the pressures and other conditions pertaining in biological lubrication.
