This nanorheological study addresses the water transport hindrance in perfluorinated ionomer membranes (Nafion (R) H+ form) in conjunction with structural relaxation properties of the polymer matrix. A low-temperature relaxation of the '' dry '' Nafion (R) matrix at 79 degrees C, about 35 degrees C below the glass transition temperature, was found to be responsible for a steady improvement of the water transport above 70 degrees C. The low-temperature structural transition at 79 degrees C was observed for the dehydrated membrane, and thus, is specific to the polymer matrix and not a sole consequence of the kinetics of the solvent-containing aggregates within the apolar perfluorocarbon matrix. Water was found to be trapped within a hydrated perfluorinated ionomer membrane below 70 degrees C. The direct observation of water transport is possible with a novel scanning force microscopy (SFM) method, flux-lateral force microscopy (F-LFM). With this technique, local fluid permeation fluxes are directly obtained at the membrane downstream surface. The structural transition properties were acquired by another highly sensitive nanoscopic technique, shear modulation force microscopy (SM-FM). (c) 2006 Elsevier B.V. All rights reserved.