Effect of Salt on Phosphorylcholine-based Zwitterionic Polymer Brushes

A quantitative investigation of the responses of
surface-grown biocompatible brushes of poly(2-(methacryloyloxy)-
ethyl phosphorylcholine) (PMPC) to different types of salt has
been carried out using ellipsometry, quartz crystal microbalance
(QCM) measurements, and friction force microscopy. Both cations
and anions of varying valency over a wide range of concentrations
were examined. Ellipsometry shows that the height of the brushes is
largely independent of the ionic strength, confirming that the
degree of swelling of the polymer is independent of the ionic character of the medium. In contrast, QCM measurements reveal significant changes in mass and dissipation to the PMPC brush layer, suggesting that ions bind to phosphorylcholine (PC) groups in PMPC molecules, which results in changes in the stiffness of the brush layer, and the binding affinity varies with salt type. Nanotribological measurements made using friction force microscopy show that the coefficient of friction decreases with increasing ionic strength for a variety of salts, supporting the conclusion drawn from QCM measurements. It is proposed that the binding of ions to the PMPC molecules does not change their hydration state, and hence the height of the surface-grown polymeric brushes. However, the balance of the intra- and intermolecular interactions is strongly dependent upon the ionic character of the medium between the hydrated chains, modulating the interactions between the zwitterionic PC pendant groups and, consequently, the stiffness of the PMPC molecules in the brush layer.