Thickness-Dependent Properties of Polyzwitterionic Brushes

Abstract

This paper describes detailed studies on the controlled synthesis of poly[2-(methacryloyloxy)ethyl]dimethyl(3-sulfopropyl)ammonium hydroxide (polyMEDSAH) from initiator-modified gold surfaces and the properties of these brushes as a function of grafting density and thickness. Improved control over polymerization was achieved by preforming the catalytically active complex to ensure that the monomers did not coordinate to the catalyst. We observed an intriguing transition thickness (hcrit) in which the polyMEDSAH brushes switched from hydrophilic to hydrophobic due to the strong inter- and intrachain associations. We studied this transition as a function of the rate of polymerization and grafting density and found that both factors strongly influence the value of hcrit. Faster grown polyMEDSAH brushes have higher hcrit values, and brushes grown from higher grafting density have higher hcrit value. These observations suggest that the hcrit is governed by the density of the brushes, the polymer chain length, and the degree of alignment. Furthermore, the inter- or intrachain associations can be reversed by increasing the temperature, leading to a hydrophobic to hydrophilic switch, where the magnitude of the switching is governed by the grafting density.