NMR DETERMINATION OF IONIC STRUCTURE IN PLASTICIZED POLYETHER-URETHANE POLYMER ELECTROLYTES

Solid polymer electrolytes based on amorphous polyether-urethane netwo rks combined with lithium or sodium salts and a low molecular weight c osolvent (plasticizer) have been investigated in our laboratories for several years. Conductivity enhancements of up to two orders of magnit ude can be obtained whilst still retaining solid elastomeric propertie s. In order to understand the effects of the plasticizers and their me chanism of conductivity enhancement, multinuclear NMR has been employe d to investigate ionic structure in polymer electrolyte systems contai ning NaCF3SO3, LiCF3SO3 and LiClO3 salts. With increasing dimethyl for mamide (DMF) and propylene carbonate (PC) concentration the increasing cation chemical shift with fixed salt concentration indicates a decre asing anion-cation association consistent with an increased number of charge carriers. C-13 chemical shift data for the same systems suggest s that whilst;DMF also decreases cation-polymer interactions, PC does the opposite, presumably by shielding cation-anion interactions. Tempe rature dependent Li-7 spin-lattice relaxation times indicate the expec ted increase in ionic mobility upon plasticization with a shift of the T-1 minimum to lower temperatures. The magnitude of T-1 at the minimu m increases upon addition of DMF whereas there is a slight decrease wh en PC is added. This also supports the suggestion that the DMF prefere ntially solvates the cation whereas the action of PC is limited to cou lomb screening, hence freeing the anion.