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

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 T g was determined and found to agree very well with values from DSC. In addition, the fractional free volume at T g was quantified. It increases with increasing amount of dissolved salt; this becomes in particular clear from the fine structure analysis. Dielectric spectroscopy at T < T g showed the presence of three secondary relaxations (γ , β 1 , β 2 ), of which β 1 and β 2 strongly overlap. Two of them are assigned to local relaxations involving either free (γ) or coordinated (β 2 ) EO sequences, resulting in a decrease or increase of the relaxation strength with salt concentration, respectively. Molecular modeling supports the idea that the β 2 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 (β 1 ) relaxation is in particular active in weakly complexed samples exposed to ambient humidity, suggesting a local motion involving the ester moieties.