New insights into the coordination mode of silver ions dissolved in poly(2-ethyl-2-oxazoline) and its relation to facilitated olefin transport

The coordination behavior of silver ions dissolved in a polymer solvent is investigated to elucidate the facilitated transport phenomena in solid-state membranes. Silver polymer electrolyte membranes were prepared by dissolving AgCF3SO3 in poly(2-ethyl-2-oxazoline) (POZ) or poly(N-vinylpyrrolidone) (PVP). The permeance and solubility of propylene in the silver polymer electrolyte increased slowly with silver ion concentration up to the silver mole fraction of 0.25 ([C=O]:[Ag] = 3:1), after which the permeance and solubility of propylene abruptly increased. Other properties such as the Bragg d spacing, glass transition temperature (T-g), and propane permeance also showed marked changes at a silver mole fraction of 0.25. The existence of this threshold concentration is attributed to the following observations: (1) the most favorable coordination number for silver ions in polymer electrolytes under a propylene environment is 3, and (2) the coordination bond strength between silver ion and carbonyl oxygen is marginally stronger than that between silver ion and olefin molecule. As a result, it is proposed that silver ions can act as olefin carriers only when they have vacant coordination sites, i.e., only when the coordination number of silver ion for carbonyl oxygens is less than 3. Therefore, the silver ion coordination number is shown to have a strong bearing both on the activity of the silver ion as an olefin carrier and on the structure of polymer electrolytes.