Ionic conductivity of polymer electrolyte membranes based on polyphosphazene with oligo(propylene oxide) side chains

A polyphosphazene [NP(NHR)2]n with oligo[propylene oxide] side chains − R = –[CH(CH3)–CH2O]m–CH3 (m = 6 – 10) was synthesized by living cationic polymerisation and polymer-analogue substitution of chlorine from the intermediate precursor [NPCl2]n using the corresponding primary amine RNH2. The polymer had an average molecular weight of 3.3 × 105 D. Polymer electrolytes with different concentrations of dissolved lithium triflate (LiCF3SO3) were prepared. Mechanically stable polymer electrolyte membranes were formed using UV radiation induced crosslinking of the polymer salt mixture in the presence of benzophenone as photoinitiator. The glass transition temperature of the parent polymer was found to be − 75 °C before cross linking. It increases after crosslinking and with increasing amounts of salt to a maximum of − 55 °C for 20 wt.% LiCF3SO3. The ionic conductivity was determined by impedance spectroscopy in the temperature range 0–80 °C. The highest conductivity was found for a salt concentration of 20 wt.% LiCF3SO3: 6.5 × 10− 6 S·cm− 1 at 20 °C and 2.8 × 10− 4 S cm− 1 at 80 °C. The temperature dependence of the conductivities was well described by the MIGRATION concept.