Role of transient cross-links for transport properties in silver-polymer electrolytes

The interaction of the silver cation of AgClO4 with carbonyl oxygen in poly(2-ethyl-2-oxazoline) (POZ) was found to be similar to that of AgBF4 by infrared and Raman spectroscopic results. The two silver salts are similar in molecular size and lattice energy. However, the two silver-polymer electrolytes showed remarkably different gas transport properties. The POZ:AgClO4 membrane showed very low gas permeance compared with the POZ:AgBF4 at the 1:1 [C=O]:[Ag] mole ratio. The difference in the transport properties has been attributed mostly to the changes in chain mobility upon both the formation of transient cross-links by silver cations and the dangling of the heavy anions on the main chain. The changes in chain mobility were characterized by the glass transition temperature and the intersegmental d spacing. The extent of the transient cross-links seems to be strongly associated with the coordination number of the silver cation; the silver cation in AgClO4 has a higher coordination number than that in AgBF4. The structure difference for the two systems was also confirmed by the difference in the bond length between the cation and anion calculated from ab initio methods, despite the very similar lattice energies for two salts. Thus, the gas transport properties were successfully used to monitor the role of the transient cross-link as well as of the neighboring counteranion on the structure and physical properties of polymer electrolytes.