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The strain-optical coefficient of ethylene-methacrylic acid copolymers is measured in static and dynamic experiments as a function of temperature and degree of ionization. An alpha optical loss peak which cor­responds to a mechanical loss peak is found for the ionized salt but not for the un-ionized acid. This peak is associated with mobility of an ionized domain. Birefringence relaxation curves are found to be super­posable using horizontal and vertical shift factors. A plot of the logarithmic shift factor against (1/T) gives a broken curve exhibiting two activation energies for the salt indicating contributions by two processes.