Controlling the permeability of multilayered polyelectrolyte films through derivatization, cross-linking, and hydrolysis

Partial Fischer esterification of poly(acrylic acid) allows tailoring of the hydrophobicity and charge density of multilayered films containing poly(allylamine hydrochloride) (PAH) and derivatized poly(acrylic acid) (d-PAA). As hydrophobicity and charge density strongly affect film permeability, control over these properties is vital for possible applications of PAH/d-PAA films as ion-separation membranes and sensors. The hydrophobicity of these films depends on both the extent of esterification and the nature of the derivatizing alcohol. Even though PAH/d-PAA films are composed of polyelectrolytes, the presence of hydrophobic ester groups results in advancing water contact angles as high as 101 degrees. The hydrophobicity of these coatings allows them to effectively passivate underlying electrodes as shown by minimal peak currents in cyclic voltammograms (CVs) of RU(NH3)(6)(3+) and Fe(CN)(6)(3-). Cross-linking of hydrophobic PAH/ d-PAA films via heat-induced amidation stabilizes coatings over a wide pH range but does not significantly decrease the already low film permeability to Ru(NH3)(6)(3+) and Fe(CN)(6)(3-). Stabilization due to cross-linking does, however, allow base-promoted hydrolysis of the ester groups of PAH/d-PAA coatings. After hydrolysis, films are extremely hydrophilic and selectively permeable to Ru(NH3)(6)(3+) over Fe(CN)(6)(3-) due to,the high density of newly formed -COO- groups. In the case of some hydrolyzed films, the presence of small concentrations of Ca2+ results in dramatic current decreases in CVs of RU(NH3)(6)(3+), suggesting possible use of these films in sensing applications.