Novel Polymer Electrolytes Based on Amorphous Poly(ether−ester)s Containing 1,4,7-Trioxanonyl Main Chain Units. Ionic Conductivity versus Polymer Chain Mobility

Abstract

Melt condensation of 1,5-bis(9-hydroxy-1,4,7-trioxanonyl)naphthalene (2) with bis-acid chlorides, adipoyl chloride (3a), terephthaloyl chloride (3b), and 3,6,9,12-tetraoxatetradecane bis-acid chloride (3c), respectively, gives amorphous linear poly(ether−ester)s 1a−c, which contain 1,4,7-trioxanonyl (triethylene glycol) units at regular intervals in their main chain. Solid polymer electrolytes were prepared by mixing THF solutions of either LiClO4 with 1a−c or NaClO4 with 1b. The polymer electrolytes containing LiClO4 are fully amorphous, whereas in the case of NaClO4 and Na+/1b ratios larger than 0.125, crystalline NaClO4 is present. Despite the fact that the 1,4,7-trioxanonyl moieties in 1a−c are shorter than the minimum required for complete solvation of Li+ and Na+, dielectric relaxation spectroscopy shows that the solid polymer electrolytes Li+/1a, Li+/1b, and Li+/1c possess ionic conductivities of σ = 3.2 × 10-5, 1.9 × 10-6, and even 1 × 10-4 S cm-1, respectively, at 368 K. A Vogel−Tammann−Fulcher (VTF) analysis of the ionic conductivity σ and the relaxation time of the α-relaxation revealed a strong relationship between σ and the relaxation behavior of the chain segments. By means of a fine structure analysis of the activation energy, the dielectric α-process around the glass transition was closely studied in the absence and presence of dissolved LiClO4 (1a−c) or NaClO4 (1b). From the highest apparent activation energy the Tg was determined and found to agree very well with values from DSC. In addition, the fractional free volume at Tg was quantified. It increases with increasing amount of dissolved salt; this becomes in particular clear from the fine structure analysis. Dielectric spectroscopy at T < Tg showed the presence of three secondary relaxations (γ, β1, β2), of which β1 and β2 strongly overlap. Two of them are assigned to local relaxations involving either free (γ) or coordinated (β2) EO sequences, resulting in a decrease or increase of the relaxation strength with salt concentration, respectively. Molecular modeling supports the idea that the β2 process arises from a chemical relaxation by the temporary breaking up and remaking of at least one O−Li+ coordination bond within the tetrahedral polymer−cation complex. The third (β1) relaxation is in particular active in weakly complexed samples exposed to ambient humidity, suggesting a local motion involving the ester moieties.