AbstractThe dynamic moduli,E′ dyn, and loss tangents, tan δ, of polydimethylsiloxane and polydimethyldiphenylsiloxane polymers have been investigated by anin situtechnique during γ‐irradiation. These viscoelastic properties were calculated and plotted as a function of irradiation exposure time by measuring the free end displacements and resonance frequencies of polymeric cantilever reeds. The reeds were swept through a small frequency range from about 20 to 100 cycles/sec. The moles of effectively elastic chains per unit volume (v) of the two unfilled polysiloxahes were calculated fromin situmodulus data and compared to values obtained utilizing the swelling technique. The approximate molecular weights between entanglements,Me, of these unfilled polymers were determined by extrapolation of moduli data to zero radiation exposure. The addition of a large silica filler, SiO2, into the polymers did not alter the crosslinking rates, and the filler did not enter into polymer—filler