Study of the multilayer assembly and complex formation of poly(diallyldimethylammonium chloride) (PDADMAC) and poly(acrylic acid) (PAA) as a function of pH

Abstract

The assembly of polyelectrolyte multilayers and the formation of complexes in a solution of poly(diallyldimethylammonium chloride) (PDADMAC) and poly(acrylic acid) (PAA) have been studied as a function of pH. Quartz Crystal Microbalance with Dissipation (QCM-D) in combination with Ellipsometry shows that PAA–PDADMAC multilayers display a supralinear growth pattern at pH 3 with a corresponding change in the frequency response of more than 650 Hz for 17 polyelectrolyte layers. Atomic Force Microscopy (AFM) confirmed the formation of a 200 nm thick film. For pH values higher than 6, assembly is drastically reduced resulting in values below 150 Hz in the QCM-D for the same number of layers. Isothermal Titration Calorimetry (ITC) and Dynamic Light Scattering (DLS) were used to study complex formation. At pH 3, an exothermic heat of complex formation of −2.11 kcal mol⁻¹ was measured. With increasing pH, the heat of complex formation has been seen to decrease. At pH 10, the heat of complex formation tends to be more endothermic and is approximately 0 at pH 13. The addition of urea to PAA at pH 13 resulted in an exothermic heat of complex formation of 1.41 kcal mol⁻¹ during titration with PDADMAC, highlighting the role of hydrogen bonding with water of PAA in governing the interaction between PAA and PDADMAC.