Ionic conductivity in branched polyethylenimine-sodium trifluoromethanesulfonate complexes. Comparisons to analogous complexes made with linear polyethylenimine

Complexes between branched polyethylenimine and NaSO₃CF₃ were formed with a polymer repeat unit to salt ratio of 6:1, 12:1, and 20:1. All three complexes are amorphous in contrast to linear polyethylenimine which forms partially crystalline complexes at ratios above 6:1 (lower salt concentrations). The 6:1 complex of branched polyethylenimine has a higher T_g (12°C) and a lower conductivity (10^-8 S/cm at 40°C) than the corresponding complex of linear polyethylenimine (-5°C and 10^-7 S/cm). The glass transition temperature of the polymer-salt complex decreases by 43 deg upon decreasing the salt concentration from that of the 6:1 complex to that of the 20:1 complex. Correspondingly, the conductivity of the 20:1 complex at 40°C is 2 orders of magnitude higher than that of the 6:1 complex. N-H stretching frequencies of linear and branched polyethylenimines and their polymer-salt complexes were studied by infrared spectroscopy. An increase in frequency of the N-H stretching mode of both polymers is seen upon complex formation.