Role of salt concentration on conductivity optimization and structural phase separation in a solid polymer electrolyte based on PMMA-LiClO₄

Abstract

Although a large number of ionic conductors based on poly(methyl-methacrylate) (PMMA) are reported in literature, an optimization of salt concentration with respect to conductivity and stability properties remains by and large neglected. We report, perhaps for the first time, such an optimization of salt (LiClO₄) concentration on structural, morphological, electrical, and ion–polymer interaction in PMMA-based solid polymer films. The active coordination site for the cation (Li⁺), out of the two possible electron donating functional groups (i.e. C=Ö and Ö–CH₃) in PMMA, has been ascertained on the basis of evidences recorded in Fourier transform infrared spectrum. The results suggested C=Ö as the only possible site in PMMA matrix for coordination with Li⁺ cation. The X-ray diffraction results have clearly indicated an optimum limit of salt dissolution in PMMA matrix corresponding to O/Li = 4 (i.e., ~21wt.%) above which “phase-separation” occurs distinctly. The effect of salt concentration on amorphous → crystalline phase changes in PMMA and its correlation to morphology have been clearly observed in terms of their impact on electrical properties. An optimum electrical conductivity of ~7.2 × 10⁻⁵S cm⁻¹ has been recorded at 100°C (~PMMA glass transition). The temperature dependence of conductivity follows typical Vogel–Tamman–Fulcher behavior.