Structure-property studies of highly conductive layer-by-layer assembled membranes for fuel cell PEM applications

Layer-by-layer (LbL) films composed of poly(diallyl dimethyl ammonium chloride) (PDAC) and sulfonated poly(2,6-dimethyl 1,4-phenylene oxide) (sPPO) (PDAC/sPPO) are studied as a result of the variation of the ionic strength of assembly solutions to determine the nature of the exceptionally high ionic conductivity of this system. Film growth is modulated from 6.91 nm/bilayer (BL) when assembled with no salt to 62.2 nm/BL when assembled with 0.5 M salt in all assembly solutions. However, at optimized assembly conditions of 1.0 M salt in only the sPPO solution, fully humidified PDAC/sPPO films have ionic conductivity values of 7.00 × 10−2 S cm−1 at 25 °C, which is the highest value reported for any LbL assembled system. Selectively screening charges by adding salt to the sPPO assembly solution decreases the ionic crosslink density of the films and increases the water uptake, yielding high ionic conductivity. Thickness measurements made at 0.5 BL increments indicate that the film composition can also be tuned by the ionic strength of the assembly baths. Additionally, PDAC/sPPO films fabricated using a recently developed LbL-Spray technique allow for the preliminary characterization of the mechanical properties of free-standing membranes.