Local Electrostatics within a Polyelectrolyte Multilayer with Embedded Weak Polyelectrolyte

We investigate local electrostatics within a polyelectrolyte multilayer formed from layer-by-layer self-assembly of QPVP/PSS (quaternized poly(vinylpyridine)/poly(styrenesulfonate)), through the strategy of embedding, within this multilayer, a weak polyacid, PMA (poly(methacrylic acid)). The methods mainly involved infrared measurements of these multilayer films using FTIR-ATR, Fourier transform infrared spectroscopy in attenuated total reflection, after deposition at several solution concentrations and ionic strength. A key finding is that the ionization of PMA, measured at pH = 6.0 (near its pK a), executed giant oscillations, between 30% and 80% ionization, each oscillation being in linear proportion to the quantity of strong polyelectrolyte deposited in the outermost layer of the polyelectrolyte multilayer. These oscillations persisted with a decay length (>10 layers) that far exceeded the Debye length of the aqueous solution, suggesting that this long-range electrostatic coupling stemmed from the presence of polyanions and polycations that were fixed in space within the multilayer assembly rather than mobile as supposed in a traditional analysis of screening. In addition, in the process of multilayer formation, the mass adsorbed of the outermost layer was not simply controlled by the excess charge on the preexisting multilayer. Instead, the film became responsive in the sense that its charge adjusted in the direction of maintaining neutrality within the multilayer assembly when additional charges deposited on top. The approach of embedding a weak polyelectrolyte within a multilayer of strong polyelectrolyte may find application in sensor design.