VIBRATIONAL-SPECTRA AS EXPERIMENTAL PROBES FOR MOLECULAR-MODELS OF ION-CONDUCTING POLYETHER SYSTEMS

Atomistic molecular modelling has been used to construct polymer elect rolyte materials consisting of poly(ethylene oxide) (PEO), poly(ethyle ne oxide) with sulfonic acid end groups (PEO sulfonic acid) and water in an amorphous cell with periodic boundary conditions. A conformation al analysis of PEO sulfonic acid was made using the PCFF forcefield, a nd for one linkage also the CVFF forcefield was used in molecular mech anics calculations. In addition, the energy contour map for the bond c onnecting the sulfonic acid group to the polyether chain and the next carbon-carbon bonds was constructed using RIS theory. The results obta ined with the different forcefields were fairly similar. For all linka ges several minima with low energy barriers were found. The relative e nergies of the minima vary very little. The maxima are due to steric e ffects. The conclusion is drawn that all rotations in PEO sulfonic aci d are restricted at room temperature. Two polymer electrolyte systems were constructed, one water-free, and one containing water. The releva nce of the model, and the most suitable forcefield, were tested by com paring the calculated vibrational spectra with experimental i.r. and R aman spectra. Very good agreement between measured and calculated vibr ational frequencies was found with both forcefields for the modes attr ibuted to the PEO chain. There was a significant difference between va lues for the S=O stretch calculated in the CVFF forcefield and in the PCFF forcefield, respectively. Since the S=O stretch calculated with t he PCFF forcefield corresponds very well with literature values, the c onclusion was drawn that the PCFF forcefield is better suited to model PEO sulfonic acid than is the CVFF forcefield. In the modelling studi es, hydrogen bonds between the sulfonic acid group and water molecules were formed. In the experimental system an absorption was found which is attributed to a hydronium sulfonate ion pair. The correlation of t his finding with experimental measurements of water transport in proto n-conducting membranes is discussed. The proton coordination study sho ws that the model can be developed by adding sulfonic acid anions to t he amorphous cell. The model will be further developed for studies of proton conducting mechanisms. (C) 1997 Elsevier Science Ltd.