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) wi th bis-acid chlorides, adipoyl chloride (3a), terephthaloyl chloride (3b), and 3,6,9,12-tetraoxatetradecane bis-acid chloride (3c), respectively, give s 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 la-e or NaClO4 with 1b. The polymer electrolytes containing LiClO4 a re 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 comp lete solvation of Li+ and Na+, dielectric relaxation spectroscopy shows tha t the solid polymer electrolytes Li+/1a, Li+/1b, and Li+/1c possess ionic c onductivities of sigma = 3.2 x 10(-5), 1.9 x 10(-6), and even 1 x 10(-4) S cm(-1), respectively, at 368 K. A Vogel-Tammann-Fulcher (VTF) analysis of t he ionic conductivity sigma and the relaxation time of the alpha-relaxation revealed a strong relationship between sigma and the relaxation behavior o f the chain segments. By means of a fine structure analysis of the activati on energy, the dielectric alpha-process around the glass transition was clo sely studied in the absence and presence of dissolved LiClO4 (1a-c) or NaCl O4 (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 fracti onal free volume at T-g was quantified. It increases with increasing amount of dissolved salt; this becomes in particular clear from the fine structur e analysis. Dielectric spectroscopy at T < T-g showed the presence of three secondary relaxations (gamma, beta(1), beta(2)), of which beta(1) and beta (2) strongly overlap. Two of them are assigned to local relaxations involvi ng either free (gamma) or coordinated (beta(2)) EO sequences, resulting in a decrease or increase of the relaxation strength with salt concentration, respectively. Molecular modeling supports the idea that the beta(2) process arises from a chemical relaxation by the temporary breaking up and remakin g of at least one O-Li+ coordination bond within the tetrahedral polymer-ca tion complex. The third (beta(1)) relaxation is in particular active in wea kly complexed samples exposed to ambient humidity, suggesting a local motio n involving the ester moieties.