A series of FOSS containing star-shaped block copolymers were synthesized by atom transfer radical polymerization (ATRP), with POSS-(Cl)(8) as initiator, polymethyl methacrylate as the first building block, and polystyrene as the second building block. Sulfonation of the polystyrene block yielded star-shaped ionic polymers POSS-(PMMA-b-SPS)(8) that were evaluated as proton exchange membranes (PEMs) subsequently. Under low relative humidities (RHs), the PEM with longer SPS block exhibited higher proton conductivity than the PEM with shorter SPS block when compared at same hydration number (lambda) conditions, which was attributed to the better connected hydrophilic domains of the former as evidenced by electron microscopes. However, this conductivity trend for the two PEMs was reversed at 100% RI-is. Low field nuclear magnetic resonance analysis revealed that the PEM with shorter SPS block had more loosely bonded water than the PEM with longer SPS block at 100% RH, giving an explanation of why the conductivity trend was reversed. This study suggested that both ionic domain structure and water-polymer interaction are important parameters for achieving high proton conductivities. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.