In order to develop efficient proton exchange membranes used for direct methanol fuel cells, we are focusing on the design and synthesis of polyphosphazene-based copolymers. In the present study, block polystyrenes were grafted onto polyphosphazenes by atom transfer radical polymerization (ATRP) of styrene and 4-acetoxystyrene, and then alkylated with 1,4-butanesultone to yield two series of graft copolymers having branch chains of alkylsulfonic acids. They were further made into the corresponding membranes with the assist of simultaneous cross-linking reaction with 2,6-bis(hydroxymethyl)-4-methylphenol (BHMP) as the cross-linker. High proton conductivities were found for the membrane series of M-PSx-PSBOSy in the range of 0.184-0.266 S/cm, and for F-PSx-PSBOSy in the range of 0.147-0.284 S/cm under fully hydrated conditions at 80 degrees C, respectively, and were supported by the nanophase-separated structures by the transmission electron microscopy (TEM) investigations. Also, the membranes showed good methanol resistance (1.60-10.4 x 10(-7) cm(2)/s) and high oxidative stabilities. These results suggest a potential of the membranes for the DMFCs application. (C) 2015 Elsevier B.V. All rights reserved.