Linear 1,2-polybutadiene, glass transition temperature (T,) -18OC, is crosslinked a t -10' to -2OOC by y irradiation while strained in simple extension, with extension ratios (&) from 1.2 to 2.7. After release, the sample retracts to a state of ease (A,) a t room temperature. From equilibrium stress-strain measurements up to a stretch ratio relative to the state of ease (A) of 1.2, together with A0 and As, the concentration of network strands terminated by trapped entanglements ( U N ) is calculated. For this purpose, a three-constant Mooney-Rivlin formulation is used, in which the entanglement network is described by Mooney-Rivlin coefficients C I N and CW, whereas the crosslink network is described by the coefficient C1, only. The ratio J/N = C Z N / ( C ~N + C2N) is estimated from parallel studies of nonlinear stress relaxation of the uncrosslinked polymer, taking into account the thermal history before and during irradiation. For substantial degrees of crosslinking, i.e., for Ro' = U,/UN > 0.4 (where u, is the concentration of network strands terminated by crosslinks), and for X, < 1.8, Cm agrees rather well with the value obtained from stress relaxation of the uncrosslinked polymer in the range of time scale where it is nearly independent of time (1.87 X lo5 pascals). The corresponding value of U N is 2.3 X moles/cm3, in good agreement with that obtained from viscoelastic measurements of the uncrosslinked polymer in the plateau zone (2.5 X However,
for Ro' = 0.2, smaller values of Cw and UN are obtained, indicating that for low degrees of crosslinking the entanglements are not completely trapped. Also, for higher values of &, C2N and UN turn out to be somewhat smaller. Similar, less extensive results were obtained previously on a 1,2-polybutadiene with somewhat higher vinyl content and a higher Te Crosslinked samples of both these polymers were subjected to equilibrium stress-strain measurements in simple elongation from the state of ease a t higher strains up to A = 1.7. The results agreed closely with calculations from the three-constant Mooney-Rivlin theory.