Determination of the nanostructure of polymer materials by electron paramagnetic resonance spectroscopy

Feature Article: Electron paramagnetic resonance (EPR) spectroscopy is one the few methods that can characterize structural features in the range between 0.5 and 5 nm in systems that lack long-range order. Approaches based on EPR spectroscopy provide good structural contrast even in complex materials, as the sites of interest can be selectively labeled or addressed by suitably functionalized spin probes using well established techniques. This article assesses the EPR experiments available for distance measurements on nanoscales in terms of the accessible distance range, precision, and sensitivity. Recommendations are derived for the proper choice of experiment for a given problem. Both simple and sophisticated methods for data analysis are described and their limitations are evaluated. It is discussed which assumptions must be made to extract a pair correlation function from EPR data. Finally, applications to the study of polymer chain conformation and the structure of ionically functionalized diblock copolymers are highlighted