Study of correlations between microstructure and conductivity in a thermoplastic polyurethane electrolyte

Micro-Raman and positron annihilation lifetime spectroscopy (PALS) have bee n used to investigate the structure of a thermoplastic polyurethane/LiClO4 solid flexible polymer electrolyte at room temperature. Correlation between the free volume and carrier concentration with ionic conductivity was obse rved. The polyurethane soft phase consisted of a poly(tetramethylene glycol -co-ethylene glycol) copolymer reinforced by condensation with hexamethyldi isocianate. The range of salt concentration between 5 and 35 wt %, which at tained the beginning of phase segregation, was also studied by differential scanning calorimetry (DSC), which showed the presence of three thermal eve nts; the soft-phase T-g, a change in heat capacity suggested as the hard-ph ase T-g, and a hard-phase ordering endotherm. The total ionic conductivity was found to be approximately 4 x 10(-6) S cm(-1) at 23 degrees C up to 27 wt % salt, whereas there were pronounced changes observed by the spectrosco pic techniques. The PALS measurements indicated a decrease of 40% of the ra tio (VfI3/(VfI3)(0)) between the free volume parameters probed by the posit ron particle. This ratio is proportional to the fractional free volume of t he system. The micro-Raman results showed an increase of ionic aggregation, although the charge carrier concentration increased significantly in the r ange of compositions studied. The opposite effects of the microstructural c hanges and the maximum conductivity value, in the observed range of concent rations, are discussed.