Polvstyrene was grafted onto FEP by simultaneous irradiation using a (CO)-C-60 gamma-irradiator. Graft copolymers of different degree of grafting were obtained by varying the irradiation dose. Positron lifetime measurements were carried out on FEP and the grafted samples. A systematic decrease in o-Ps lifetime and intensity with grafting is seen indicating a decrease in pore size as well as total free volume fraction. Temperature dependent (95-250 K) S-parameter measurements in FEP showed saturation of free volume fraction at temperatures corresponding to the P-relaxation, the onset of which is seen at 150 K. Coincidence Doppler broadening results indicate increase in specific volume in the crystalline region of the polymer. Dynamic mechanical analysis (DMA) showed that tan 6 value for beta-relaxation of the samples decreased with increasing graft percentage. The results are discussed. (c) 2007 Elsevier Ltd. All rights reserved.