Influence of DGEBA crosslinking on Li+ ion conduction in poly(ethyleneimine) gels

We characterize the DC conductivity (σ0) of solution electrolytes prepared by adding LiCF3SO3 (LiTf) salt to a (50/50 w/w) solution of branched poly(ethyleneimine) (PEI) in N,N-dimethylformamide (DMF). The value of σ0 increases with decreasing LiTf concentration over the range of compositions studied due to the formation of contact ion pairs at higher LiTf concentrations, with the highest value of σ0 exceeding 10−3 S cm−1 at 20 °C. Rubber-like gel electrolytes are prepared by epoxide-amine crosslinking of selected solutions by addition of diglycidyl ether of bisphenol A (DGEBA). Holding the [N]:[Li] mole ratio fixed, increasing the crosslink density dramatically decreases σ0 at all temperatures studied. The decrease in σ0 cannot be attributed to an increase in the glass transition temperature, as little variation in Tg is noted amongst the samples due to their high solvent content. Rather, we propose that the decrease in conductivity is due to loss of fast segmental motions associated with chain ends, which become tethered to the network upon crosslinking.