Unique starburst nanoparticles are synthesized via grafting polyacrylonitrile short chains to the cubic polyhedral oligomeric silsesquioxane (POSS) by atom transfer radical polymerization (ATRP). Introduction of these branched nanoparticles (sb-POSS) into the sulfonic perfluoro polymer (SPFP, e.g. Nafione (R)) matrix in appropriate contents gives significant improvements in the performance of SPFP membranes as direct methanol fuel cell (DMFC). This enhancement is associated with the initial clustering of sb-POSS particles in the SPFP matrix when the sb-POSS content reaches to 5 wt.% It has been found, from the differential scanning calorimetry (DSC) observation, that the SPFP molecules wage dual interactions on the sb-POSS particles, namely the hydrophobic perfluoro polymer chains of SPFP repel sb-POSS particles while the hydrophilic moieties associate with them. The content of sb-POSS strongly affects the assembly of hydrophilic channels in the membrane and, therefore, the membrane performance in a single direct methanol fuel cell (DMFC). The sb-POSS (5wt.%)-SPFP composite membrane manifests an increase of 122% in power output of DMFC at 80 degrees C. In brief, this work offers a new insight into how the unique interactions between soft nanoparticles and amphiphilic polymer chains affects performances of proton exchange membranes (PEMs) in DMFC. (C) 2008 Elsevier B.V. All rights reserved.