Determination of ion cluster sizes and cluster-to-cluster distances in ionomers by four-pulse double electron electron resonance spectroscopy

Double electron-electron resonance (DEER) spectroscopy is introduced as a new tool for the characterization of mesoscopic structures in polymers. This method can characterize distance distributions between spin probes in a range between 1.5 and 8 nm and can be applied to the measurement of ion cluster sizes and intercluster distances in ionically end-capped polymers by using ionic spin probes that attach themselves to the surface of the ion clusters. The results on intercluster distances for systems based on homopolymers are in agreement with earlier results from small angle X-ray scattering (SAXS), while the cluster sizes can be rationalized by comparison with a force field molecular model of an ion cluster. The DEER experiment could also be applied to an ionically end-capped diblock copolymer for which a SAXS measurement of the intercluster distance failed.