Effect of the polymer matrix on the formation of silver nanoparticles in polymer-silver salt complex membranes

When polymer-silver salt complex membranes were exposed to UV irradiation, the separation performances of both the permeance and selectivity for propylene-propane decreased, which was primarily attributed to the reduction of the silver ions in the membranes to silver nanoparticles. Here, the effect of the polymer matrix on the formation of silver nanoparticles in the polymer-silver salt complex membranes was investigated. This effect was assessed for the complexes of two kinds of silver salts (AgBF₄ and AgCF ₃SO₃) with several polymeric ligands containing three different carbonyl groups, including poly(vinyl pyrrolidone) (PVP) with an amide group, poly(vinyl methyl ketone) (PVMK) with a ketone group, and poly(methyl methacrylate) (PMMA) with an ester group. UV-vis spectra and transmission electron microscopy (TEM) images clearly indicated that the reduction rate of the silver ions has the following order in the various polymer matrices: PVP > PVMK > PMMA, whereas the size and the distribution of the nanoparticles exhibited the reverse order. The tendency to form silver nanoparticles was explained in terms of the differences between the comparative strengths of the interactions of the silver ions with the different carbonyl oxygens in the matrices, as well as that of the silver ions with counteranions, which was characterized by X-ray photoelectron spectroscopy (XPS) and FT-Raman spectroscopy. It was concluded that when the concentration of free silver ions was low due to weak polymer-silver ion and strong silver ionanion interactions, as found with PMMA, the reduction rate of silver ions to silver nanoparticles was slow. Therefore, the PMMA-silver complex membranes were less sensitive to decreases in separation performance upon UV irradiation than compared to the PVP membranes.