Ionic interaction behavior and facilitated olefin transport in poly(N-vinyl pyrrolidone):Silver triflate electrolytes; effect of molecular weight

Interactions of cation/anion and cation/polymer in poly(N-vinyl pyrrolidone) (PVP):silver triflate (AgCF₃SO₃) electrolytes with different weight-average molecular weights (M_w's) of 1 × 10⁶ (1 M), 3.6 × 10⁵ (360 K), 4 × 10⁴ (40 K), and 1 × 10⁴ (10 K) have been studied with IR and Raman spectroscopies. According to the change of the C = O peak, coordination of silver ions by C = O in a low M_w (10 or 40 K) PVP matrix tend to be always thermodynamically favorable than high M_w (1 M or 360 K) PVP, demonstrating that the polymer matrix of low M_w dissolves silver salts more effectively. In addition, silver cations interact with both larger SO₃^- and smaller CF₃ to form ion pairs, and the former interaction is stronger than the latter in a monomer or low M_w polymer matrix (40 K, 10 K), as demonstrated by theoretical ab initio calculation or experimental spectroscopy, respectively. However, CF₃ interacts more favorably with silver cation than SO₃^- in high M_w (1 M and 360 K) PVP, which is ascribed to the steric effect of the bulky SO₃^- anion by highly entangled polymer chains. Despite the superior dissolving property of the low M_w polymer matrix, the membranes consisting of low M_w PVP and AgCF₃SO₃ exhibited poor separation performance for propylene/propane mixtures in comparison with those of high M_w, presumably because of the poor mechanical property for membrane formation in low M_w PVP.