Relative humidity (RH) dependence of the electrochemical and mechanical properties of poly(styrene sullonic acid)-grafted poly(ethylene-co-tetralluoroethylene) polymer electrolyte membranes (ETFE-PEMs) were investigated in a wide ion exchange capacity (IEC) range at 80 degrees C. ETFE-PEM proton conductivity ranges for IECs of 1.3-2.9 mmol/g at 30% and 98% RH were 0.001-0.013 and 0.16-0.52 S/cm, respectively. Unlike aromatic hydrocarbon type PEMs, ETFE-PEMs have proton conductivities that are less dependent on RH because hydrophilic grafted polymers show clearer phase separation from the hydrophobic ETFE substrate, thereby maintaining conducting channels even under dry conditions. ETFE-PEMs with IEC < 2.4 mmol/g showed higher tensile strengths compared to Nation-212 at 100% RH. The graft polymerization did not affect the crystallinity of ETFE-PEM, although sulfonation induced some damage in the crystalline domains. Further, the ETFE-PEM mechanical strength and proton conductivity were clearly related to PEM crystallinities. ETFE-PEMs with IEC > 2.7 mmol/g exhibited higher conductivity ( > 0.009 S/cm) at 30% RH and showed compatible tensile strengths of approximately 10 MPa at 100% RH and 80 degrees C. (c) 2013 Elsevier B.V. All rights reserved,