Fast and slow relaxation processes in polymer electrolytes

Abstract

We report a light scattering study of the complex dynamics of two salt-in-polymer electrolyte systems. One of these, NaCF 3SO 3 + PPG 4000, has been the subject of considerable previous study, while the second, Mg(ClO 4) 2 + PPG 4000, is little known and proves to be novel in its behavior. The measurements extend from Raman studies on the vibrational (ps) time scale, through Brillouin (10 −10s) investigations, to photon correlation studies of the 1 μs-1 s range. In the Na triflate system, Raman spectra reveal extensive ion pairing which is not seen in the Mg(ClO 4) 2 system. The Brilloin and photon correlation results show that in each system the relaxation spectrum is complex. In the Na + case, relaxation occurs by two distinct non-exponential processes. They can be related to the same processes responsible for the abnormal width of the glass transition in the Na + system and imply the presence of microscopic segregation into salt-rich and salt-free polymer regions. In the Mg(ClO 4) 2 system, which is a rubbery solid at room temperature, three distinct processes are found. The fast process is identical in character, both in time scale and non-exponentiality, to the relaxation seen in pure propylene glycol. The slower processes are related to the rubbery character of the system. The faster of the two corresponds closely, in both time and non-exponentiality, with the shear relaxation responsible for the rubber-liquid transition, while the slower is exponential in character, and must relate to the motion of larger clusters. An interpretation is again given in terms of microscopic segregation, now with Mg 2+OH chain endlinks to account for the rubbery behavior.