Proton Conduction in Thickness-Controlled Ultrathin Polycation/Nafion Multilayers Prepared via Layer-by-Layer Assembly

Ultrathin layers of positively charged poly(allylamine hydrochloride) (PAH) and a negatively charged high-proton conductor of Nafion were alternately deposited via layer-by-layer assembly. The incremental film growth of PAH and Nafion multilayers was monitored using a quartz crystal microbalance (QCM). The results obtained using the QCM revealed that each PAH/Nafion multilayer was deposited regularly and homogeneously. The zeta-potential and thickness of the Nafion layer, which are related to the concentration of free -SO(3)(-) groups, were changed significantly by varying the pH of the PAH solution used. The thickness of one Nafion layer was controlled in the range from approximately 4 to 15 nm. The surface charge density of the PAH/Nafion multilayer was estimated using a Poisson-Boltzmann equation. A linear relationship between the proton conductivity and surface charge density of the outmost Nafion layer was observed. Furthermore, the proton mobility of the PAH/Nafion multilayer (15.2 cm(2)/(V S)) was about sevenfold higher than that of a bulk Nafion membrane (2.0 cm(2)/(V S)).