pH-induced conformational changes of loosely grafted molecular brushes containing poly(acrylic acid) side chains

A series of water-soluble loosely grafted poly(acrylic acid) (PAA) brushes with four different grafting densities were synthesized by the grafting from approach using atom transfer radical polymerization (ATRP). Gel permeation chromatography (GPC) and H-1 NMR spectroscopy were used to provide evidence for formation of the well-defined backbones and the resulting brush copolymers. Atomic force microscopy Was used to study the conformation of adsorbed brushes as a function of PH. The adsorbed molecules undergo a globule-to-extended conformational transition as the solution is changed from acidic to basic. This transition was monitored on a mica surface by imaging individual molecules with atomic force microscopy (AFM). The conformational behavior was compared with 100%-grafted PAA brushes. Unlike the loose brushes, the 100%-grafted molecules remained fully extended in a broad range of pH values (pH = 2-9) due to steric repulsion between the densely grafted side chains which is strongly enhanced upon adsorption to a substrate. (C) 2008 Elsevier Ltd. All rights reserved.