Rapid method to measure diffusion of paramagnetic species: Mn²⁺ in poly(ethylene-co-methacrylic acid) ionomers

We present a facile method to measure Fickian diffusion coefficients of paramagnetic species in materials, using electron spin resonance (ESR) spectroscopy. The method requires no postannealing sample preparation, is adaptable to in situ measurement, and requires only that the spectrum of the paramagnetic species change monotonically with its local concentration. We illustrated this method with poly(ethylene-co-methacrylic acid) ionomers, using laminates prepared from the same copolymer neutralized with either paramagnetic Mn²⁺ or diamagnetic Zn²⁺. For Mn²⁺, the exchange interaction between neighboring Mn²⁺ caused the six-line hyperfine pattern to transform to a single broad signal, providing the measurement of local concentration. Diffusion coefficients of Mn²⁺ in this ionomer pair measured by ESR were compared with those for Zn²⁺ measured by conventional electron probe microanalysis and found to be in satisfactory agreement.