Solution properties and film properties of the polymer electrolyte (Nafion) were investigated by dynamic light scattering (DLS) and rheological analysis. By DLS measurement, it has been found that the Stokes diameter of Nafion molecules in a DMF solution is larger than that in a water/alcohol solution (SE5112, DuPont). This result clearly shows that the DMF is a better solvent than the solvent component of SE5112. Both SE5112 and the DMF solution behave as almost Newtonian solutions, while

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... n solution in DMF/water mixed solvent shows viscoelasticity due to the aggregation of Nafion molecules. Finally, the mechanical properties and the proton conductivities of the cast film of Nafion from SE5112 and the DMF solution were investigated. The proton conductivity is almost the same in both films, while the mechanical properties of the film made from the DMF solution are much better than those of the film from SE5112. This can be understood as a result of the weaker segregation of crystalline and amorphous regions in the former film.