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RAMAN STUDY OF FREE VOLUME EFFECTS ON ION PAIRS INPOLYMER ELECTROLYTES

L.M.TORELL, S. SCHANTZ AND P. JACOBSON Chalmers University of Technology Dept. of Physics, S-412 96 Gothenburg, Sweden

ABSTRACT Ion associations are demonstrated from Raman spectra of the anion symmetric stretch in NaCF 3 SO 3 and LiCF 3 SO 3 containing salt polymer complexes based on PPO and siloxane modified PPO. The observation of increasing amount of ion pairs with increasing molecular weight and temperature can be explained by an entropically driven process. The entropy effect includes contributions from several factors related to transient cross-linking via the cations, conformational entropy of the polymer chains, and free volume dissimilarities between the solvated ions and the macromolecules. 1 INTRODUCTION We have previously observed from Raman scattering of various salt complexed polyethers that the amount of ions locked in various ion-ion configurations (pairs, triplets etc) increases with temperature[1,2]. This implies that the number of "free" ions decreases with increasing temperature in contrast to the general belief and to the weak electrolyte model suggested to hold for polymer electrolytes[3]. The "free" ions are here taken as the cations coordinated to the oxygens of the polymer chains and the unpaired anions which are believed to be more delocalized in the complex. Based on these experimental findings we have suggested another model which emphasizes the role of the difference in free volume requirements of the host polymer and the dissociated salt to explain the temperature behaviour of the complex[4,5]. It is this free volume dissimilarity which is suggested to be the driving mechanism for the phase separation and the subsequent ion-ion interaction. It is based on that the free volume of the host polymer is reduced through the chain connectivity whereas no such restriction yields for the dissociated salt. As the temperature increases the difference in the free volume (and therefore the difference in thermal expansion) of the dissociated ions and the polymer chain leads to a weakening of the cation-polymer interaction. Ultimately this may result in the salting out phenomenon observed in many polymer electrolytes[6-8]. The free volume dissimilarity model is thus directly related to the difference in the size of the involved molecules and may be cast in the oversimplified popular statement "like dissolves like". To test the relevance of the model to the molecular weight dependence of ion association we investigate in the present Raman scattering study a range of PPO based complexes of the same salt concentration and for increasing molecular weight MW. The aim is then to see whether the ion-polymer interacMat. Res. Soc. Symp. Proc. Vol. 210. @1991 Materials Research Society

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tion will weaken faster with temperature as the molecular weight increases as predicted from the model. We will also aidress entropy effects related to the degree of conformational freedom of the host polymer and also the role of end effects of the