Switching the properties of polyelectrolyte brushes via "Hydrophobic collapse"

We have studied the changes in physical and chemical properties of cationic poly(2(methacryloyloxy)ethyltrimethylammonium chloride) brushes after collapse driven by ion-pairing interactions in the presence Of ClO4- anions. Results derived from the quartz crystal microbalance technique, atomic force microscopy, Fourier transform infrared spectroscopy, and contact angle goniometry indicate that ion-paired collapsed polyelectrolyte brushes suffer a dramatic loss of water accompanied by conformational changes leading to markedly different mechanical properties. This scenario is completely different from polyelectrolyte brushes whose collapse is simply driven by pure Coulombic screening, for example, in the presence of Cl- anions. In addition, wetting measurements indicated that ion-pairing interactions can be used to switch surface characteristics from hydrophilic to hydrophobic in a reversible manner. The immediate implications of these experimental results are related to the promising use of polyelectrolyte brushes as biolubricants and the design of "smart" surfaces exhibiting ion-sensitive reversible changes in interfacial properties.