Polymer electrolytes based on block copolymers are considered promising proton exchange membrane (PEM) materials for fuel cell applications. Anew class of block copolymers of perfluorocyclobutane (PFCB) units with pendant side chains containing sulfonic acid groups have recently been developed and studied for fuel cell applications. In order to enhance the membrane mechanical and hydrothermal stability, sulfonated PFCB copolymers are blended with fluoropolymers such as poly(vinylidene fluoride) (PVDF). Membrane fundamental properties (e.g., proton conductivity, gas permeability, water uptake) and fuel cell performance and durability were evaluated as a function of PVDF content. The proton conductivity and fuel cell performance only decrease slightly with PVDF content at PVDF blend levels of up to 30 wt.%. However, at the PVDF levels higher than 40 wt.%, the proton conductivity and fuel cell performance decrease rapidly with increasing PVDF content. The PFCB/PVDF composite membranes have lower hydrogen, oxygen and nitrogen gas permeability as well as a higher ratio of conductivity to permeability than perfluorosulfonic acid PFSA membranes (e.g., Nafion (R)). Membrane hydrothermal swelling is significantly reduced by blending sulfonated PFCB with PVDF. This reduced swelling leads to improved membrane mechanical durability as measured by increased lifetimes in RH cycling tests with increasing PVDF content. (C) 2013 Elsevier Ltd. All rights reserved.