Thickness dependences of proton conductivity for ultrathin Nafion multilayers prepared via layer-by-layer assembly

Nafion ultrathin layers were deposited on a glass substrate by using layer-by-layer assembly technique, and proton conductivity of the multilayers was investigated. Two different types of polycations, poly(diallyldimethylammonium chloride) (PDDA) and poly(allylamine hydrochloride) (PAH) were used for the deposition of anionic Nafion layers. The incremental film growth of multilayers was monitored using a quartz crystal microbalance (QCM). QCM results revealed that the polycation and Nafion layers were deposited regularly and homogeneously. The thicknesses of one Nafion layer were estimated to be 1.5 and 10 nm for (PDDA/Nafion)n and (PAH/Nafion)n multilayers, respectively (n means the number of layer). In the case of PDDA/Nafion multilayer, proton conductivity decreased significantly with increasing the number of layer n. On the other hand, no significant decreases in proton conductivity were observed for the PAH/Nafion multilayer. The weights of absorbed water for ultrathin multilayers were also measured and the results revealed that the formation of ionic pair of -SO3−⋯N+- affects both the water uptake and proton conductivity.