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 LiClO 4 with 1a − c or NaClO 4 with 1b . The polymer electrolytes containing LiClO 4 are fully amorphous, whereas in the case of NaClO 4 and Na + / 1b ratios larger than 0.125, crystalline NaClO 4 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 LiClO 4 ( 1a − c ) or NaClO 4 ( 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.