SAXS studies indicate that the spatial distribution of ion-rich clusters is not uniform in radiation grafted polymer electrolytes based on a semi-crystalline polymer matrix. Due to the matrix microstructure, certain domains of the polymer electrolyte are rich in these hydrophilic clusters, while other domains are devoid of them. Therefore, the hydrophilic phase is not continuous on the nano-scale, which results in fluctuations in the volumetric number density of ion-rich clusters. The length-scale of fluctuations within the electrolyte is larger than the characteristic distance between ionic aggregates, and we argue that at low hydration level it has a negative impact on the percolation and connectivity of the aqueous phase. However, an increased number density of hydrophilic clusters within grafted domains or an increased volume fraction of grafted domains within the semi-crystalline polymer both have favorable impact on the conductivity at low-hydration levels. We anticipate that a polymer electrolyte with a less heterogenous distribution of hydrophilic clusters would have considerably improved proton conductivity at low-hydration levels, because a uniform distribution on the one hand decreases the percolation threshold of the aqueous phase and one the other hand increases the connectivity of the hydrophilic clusters. (C) 2013 Elsevier Ltd. All rights reserved.