CONDUCTIVITY BEHAVIOR BELOW AND ABOVE THE CRITICAL COMPOSITION FOR MICROPHASE SEPARATION IN POLY(PROPYLENE OXIDE) - SODIUM-SALT ELECTROLYTES

This study, performed on mixtures of NaClO4 and NaCF3SO3 with atactic poly(propylene oxide) (PPO), confirms that a microphase separation phe nomenon takes place in both low and high molecular weight PPO. Below a certain concentration, which corresponds to O/Na = 13 (O = ether oxyg en) in a M = 4 x 10(3) PPO and to O/Na = 11 in a M = 2 x 10(5) PPO, op tically clear mixtures exhibit two glass transition (T-g) features ind icating that a salt-rich microphase separates from the polymer. A comp arison with poly(ethylene oxide) (PEG) amorphous mixtures containing t he same salts shows that this phenomenon has a dramatic effect on ion conduction. It also reveals that an ion-entrapping structure, similar to that in the salt-rich microphase, is present over a range of concen trations above the critical composition. Although chain length has a m arginal effect on conductivity in PEG, it has a strong effect on condu ctivity in PPO. As evidenced by a sharper T-g splitting in the high mo lecular weight PPO, this effect is due to the formation of larger micr odomains. From this feature and the change in the microdomain composit ion, it may be argued that solvating power of PPO decreases with incre asing chain length or decreasing OH end-group density.