Aqueous Solvation and Functionalization of Weak-Acid Polyelectrolyte Thin Films

A combination of in situ and ex situ ellipsometric studies and infrared spectroscopy was used to evaluate the extent of solvation of a thin covalently assembled, hyperbranched poly(acrylic acid) (PAA) graft on a gold substrate. The results show that these thin films swell reversibly by up to 300% upon immersion in a pH 1.7 buffer and by more than 500% at pH 10.7. This change in thickness, which is centered at pH 4.3, results from acid-group ionization. Covalent modification of the PAA thin films can be used to control both the extent of film swelling and the pH range over which the films undergo protonation/deprotonation reactions. For example, partial fluoramidation of the PAA carboxyl groups raises the pH where ionization occurs to 6.7 and reduces the extent of swelling (compared to the dry film) to 130% and 150% at pH 1.7 and 10.7, respectively. Solvated poly(sodium acrylate) films also serve as polyvalent ion-exchange substrates for immobilization of polyvalent cations including dendrimers, poly-l-lysine, and poly(allylamine). This immobilization technique yields self-assembled nanocomposites of the polyanionic surface graft with polycationic guests. pH modulation allows the guests to be released.