A spectroscopic and conductivity comparison study of linear poly( N-methylethylenimine) with lithium triflate and sodium triflate

Abstract

Linear poly(ethylenimine) (LPEI) is an NH-containing analog of poly(ethylene oxide) (PEO). Various groups (e.g. CH 3) can be attached to the nitrogen atoms in order to decrease the crystallinity and eliminate hydrogen bonding of the host polymer. This paper focuses on linear poly( N-methylethylenimine), LPMEI, with dissolved lithium triflate (LiTf) and sodium triflate (NaTf). These electrolytes were investigated using infrared spectroscopy, differential scanning calorimetry (DSC), and impedance measurements of the conductivity. A spectroscopic comparison of LPMEI containing LiTf and NaTf over a composition range of 5:1 to 20:1 (nitrogen:cation) shows that contact ion pairs are the predominant species in both systems except for the 5:1 NaTf sample, where the “free” triflate ions are the predominant species. Furthermore, there is an increase in the degree of ionic association with increasing salt concentration in the LiTf systems. The effect of salt concentration on the glass transition temperature (Tg) of the polymer host was examined using DSC. In LPMEI, Tg occurs at −91 °C and increases to 3 °C at a 5:1 (N:Li) composition. Conductivity measurements of the LPMEI:LiTf system over a composition range of 5:1 to 20:1 are compared with the LPEI:LiTf systems for temperatures between 25 and 60 °C. In addition, X-ray powder diffraction (XRD) show a significant presence of crystalline domains in the LPMEI:NaTf samples.