pH- and Electro-Responsive Properties of Poly(acrylic acid) and Poly(acrylic acid)-<i>block</i>-poly(acrylic acid-<i>grad</i>-styrene) Brushes Studied by Quartz Crystal Microbalance with Dissipation Monitoring

Abstract

We report on the synthesis of novel\npH- and electro-responsive\npolyelectrolyte brushes from a gold substrate by direct one-step nitroxide-mediated\npolymerization of acrylic acid (AA) or copolymerization of AA and\nstyrene (S). In the latter case, amphiphilic brushes of block–gradient\ncopolymers PAA-<i>b</i>-(PAA-<i>grad</i>-PS) comprising\none PAA block and one block with the gradient sequence of AA and S\nwere obtained. The block–gradient copolymers are initiated\nfrom the surface by the start of the PAA block. The brushes were characterized\nby XPS and ellipsometry. ¹H NMR confirmed the gradient\nsequence of the PAA-<i>grad</i>-PS copolymer block. The\npH- and electro-responsive properties of the brushes were studied\nby quartz crystal microbalance with dissipation monitoring (QCM-D)\nin combination with electrochemistry. This method provides evidence\nof swelling of the PAA brushes proportional to the contour length\nof the chains at elevated pH, whereas the response functions of the\nblock–gradient copolymers are more complex and point to intermolecular\naggregation in the brush at low pH. Monitoring of the changes in resonance\nfrequency and dissipation of the QCM-D also demonstrates that application\nof negative voltage to the substrate leads to swelling of the brush;\napplication of a positive voltage provokes only a transient collapse\nof the brush in proportion to the applied voltage.