Imaging the coil-to-globule conformational transition of a weak polyelectrolyte by tuning the polyelectrolyte charge density

Conformational changes of poly(vinylamine) (PVA) upon adsorption onto mica were determined by AFM. The polyelectrolyte charge can be tuned through the solution pH. At pH 3, where the PVA is highly charged, an extended coil conformation is observed. When the pH is increased, the polyelectrolyte charge decreases and induces a segmental collapse through pearl-necklace structures. Above pH 9, where the PVA is only weakly charged, one can observe a globular conformation. By analyzing the volume of the imaged objects, it could be clearly demonstrated that single molecules have been observed. By examining PVA adsorbed in its most globular conformation, the molecular mass distribution has been determined. The PVA molecules could be also detected after being grafted to an epoxy terminated surface, which has been obtained through a silanization reaction. The observed polyelectrolyte collapse through a sequence of pearl-necklace structures is likely related to analogous conformations in solution, which are predicted on theoretical grounds.