Biocompatible polymer brushes grown from model quartz fibres: synthesis, characterisation and in situ determination of frictional coefficient

Abstract

Poly(2-(methacryloyloxy)ethyl phosphorylcholine) (PMPC) chains were grown from near-monodisperse 13 µm diameter quartz fibres using surface-initiated atom transfer radical polymerisation (ATRP) at ambient temperature in 1 : 1 methanol–water at 20 °C. A covalently-bound siloxane-based ATRP initiator provided a sufficiently high grafting density to obtain polymer brushes from this model substrate. X-Ray photoelectron spectroscopy indicates that the PMPC brush coverage on this model fibre substrate is both thick and uniform, since the Si signals due to the underlying quartz are obscured. Thermogravimetric analysis (TGA) on selected PMPC-coated quartz fibres indicates a monotonic increase in PMPC brush thickness and reasonably good agreement was obtained with ellipsometry data, which was obtained indirectly for the corresponding PMPC brushes grown in situ from planar silicon wafers placed in the same reaction vessel as the quartz fibres. The dimensions of the thickest PMPC brush grown from the fibres could also be estimated by scanning electron microscopy. FT-IR spectroscopy confirmed a linear increase in the ester carbonyl band intensity for increasing target brush thickness, as expected. Rutherford back-scattering spectrometry (RBS) analyses also reported brush thicknesses that correlated linearly with this ester carbonyl band, but this technique appears to underestimate brush dimensions relative to TGA and ellipsometry. This is presumably due to in situ beam damage during RBS analysis. Friction force microscopy studies were conducted for both PMPC-coated and bare fibres immersed in water. Compared to the bare fibres, the presence of the PMPC chains led to a reduction in the relative frictional coefficient by more than an order of magnitude. A smaller, but still significant, reduction in the friction coefficient was also obtained using ethanol instead of water. This suggests possible applications for PMPC brushes in the context of aqueous lubrication.