Polyelectrolyte Multilayers of Weak Polyacid and Cationic Copolymer: Competition of Hydrogen-Bonding and Electrostatic Interactions
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We report on the competitive role of hydrogen bonding and electrostatic interactions in growth and stability of polyelectrolyte multilayers (PEMs) in a wide range of pH. The system was a cationic copolymer of acrylamide and dimethyldiallylammonium chloride (PAAm-DMDAAC) containing 50% cationic units and poly(methacrylic acid) (PMAA) as a polyacid. The amounts of polymers adsorbed and ionization of carboxylic groups within a film were quantified using in situ FTIR-ATR (Fourier transform infrared spectroscopy in attenuated total reflection). The results were contrasted with the stability of films formed from polyacrylamide (PAAm) and PMAA. In the PAAm/PMAA system, interlayer adhesion occurred through hydrogen-bonding interactions, and the multilayer decomposed at pH > 5.5. However, the PAAm-DMDAAC/PMAA multilayers produced at law pH could be stabilized at a pH as high as 8 with transition to electrostatic interactions. With films deposited at pH = 2 from low ionic strength solutions, asymmetric release of PMAA was observed with no mass loss for the polycation in the range of pH 2-7. With thicker PAAm-DMDAAC/PMAA films, deposited at pH = 2 in the presence of 0.15 M NaCl, multilayers showed significant mass loss at pH > 5.5 However, stable polymer multilayers could be produced in this pH range by growing PAAm-DMDAAC/PMAA multilayers at pH = 2 and exposing them to higher pH values in the presence of 0.4 mg/mL PAAm-DMDAAc solutions.
