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This investigation has been part of a larger effort to explore ion motion in polymer electrolytes, via a model system consisting of single-ion conducting ionomers based on poly(ethylene oxide). Because we have learned from broadband dielectric relaxation spectroscopy that conductivity in these materials is limited not by ion mobility, but instead by a low free ion fraction, it is logical to explore where cations are located in the polymer and what local states correlate with the ability to respond to an electric field.

X-ray scattering is a standard technique for characterizing the structure of materials, including the size, shape, and electron density contrast between multiple phases. Ultra-small-angle X-ray scattering (USAXS) is sensitive to structures on a scale of nanometers to several microns. A power-law upturn in scattering intensity at small scattering angles (q values) was previously observed in sulfonated polystyrene (SPS) ionomers that form ionic clusters. While anomalous SAXS excluded voids and parasitic scattering as potential sources of this scattering, a concrete interpretation was not found, and the scattering behavior was attributed vaguely to inhomogeneities on a nanometer to micron scale [Li, Y.J., D.G. Peiffer, and B. Chu. Macromolecules 1993, 26, p. 4006-4012].

Our initial USAXS investigation of a series of PEO-based ionomers with varying ion contents revealed a similar low-q upturn whose intensity and power-law slope both increased with ion content. Further USAXS studies of these ionomers have confirmed repeatable differences in intensity and power-law slope among different ion concentrations and cation types. Another interesting observation is a feature several tens of nm in size that appears only in the highest ion concentration, is sensitive to thermal history, and moves to lower q (larger size) in combination with decreased scattering intensity. Lastly, any satisfactory explanatory theory must also address the appearance of a lower-intensity upturn in neutral versions of the ionomers with no added ions; spatial fluctuations in ion concentration that include both impurity and added ions are one possibility.

Further USAXS work was conducted on another ion-containing polymer system. Poly(vinyl methyl ether) (PVME) with varying amounts of LiClO4, NaClO4, and CsClO4 were chosen due to PVME’s amorphous structure, the known solubility of LiClO4 up to concentrations equivalent to the highest in our PEO-based ionomers, and our group’s previous characterization of ion conduction in the system [Zhang, S. and J. Runt. J Phys Chem B, 2004, 108, p. 6295-6302]. The intensity and shape of the low-q upturn change little with added LiClO4, although wide-angle X-ray diffraction (WAXD) reveals two amorphous haloes whose relative intensities change as LiClO4 is added. However, as NaClO4 is added, the upturn’s intensity increases, and a shoulder is observed at a concentration corresponding to the onset of crystallinity observed via WAXD (corresponding to insoluble quantities of NaClO4). In general, scattering intensity increases with cation atomic number at a given concentration. Overall, the data suggest that low-q scattering in PVME/perchlorate systems is related to cation-polymer interactions.

Further work will involve synthesizing this information to refine our understanding of two important issues. First, the low-q upturn is observed in many amorphous polymer systems where structure on the scale of microns is not expected; the sensitivity of this feature to added ions will aid in our understanding of the spatial fluctuations that give rise to this scattering behavior. Secondly, we aim to better understand ion structure in polymer electrolytes and its relationship to free ion fraction and conductivity, key properties when using such materials in energy storage applications.