THE EFFECT OF MOLECULAR-WEIGHT ON CATION MOBILITY IN POLYMER ELECTROLYTES

Transport of cations in a polymer electrolyte is controlled by the dynamic relaxational modes of the host polymer. For high molecular weight systems, it has been established that the most probable mechanism involves short-range segmental motion of the polymer chains together with occasional transfer of coordinated cations from one polymer strand to another. In this study, we have investigated cation mobility in poly(ethylene oxide) hosts with molecular weights ranging from 400 to 4 x 10(6), using electrochemical and pulsed field gradient NMR techniques. Above a critical limit of 3200 it is shown that the molecular weight has no significant effect on cation mobility, but that for lower values a mechanism involving molecular weight-dependent polymer chain diffusion becomes important. The mechanisms proposed are discussed in terms of the Rouse-Zimm and the de Gennes theories of polymer diffusion.