Proton relaxation measurements have been used to investigate the effects of crosslinking on the segmental motion in cis‐1,4‐polybutadiene samples. The temperature dependence of proton spin–lattice relaxation time T1 and spin–spin relaxation time T2 at 60 and 24.3 MHz are reported in cis‐1,4‐polybutadiene (PB) samples with different crosslink density including uncrosslinked PB and samples with 140, 40, and 14 repeat units between crosslinks. In addition, spin‐lattice relaxation times in rotating coordinate frame, T1p, have also been determined. The relaxation data are interpreted in terms of the effects of crosslinks on segmental chain motions. Because of their sensitivity to low‐frequency motion, T2 data are of major interest. At temperatures well above the T1 minimum the small T2 temperature dependence resembles solidlike behavior reflecting the nonzero averaging of dipolar interactions due to anisotropic motion of the chain segments between crosslinks. The magnitude of T2 at 60°C is found to be proportional to the average mass between crosslinks.

Polybutadiene networks were prepared by peroxide crosslinking of monodisperse 1,4‐polybutadienes both in solution and in bulk. The effect of the entangled sol fraction on the elastic modulus of high‐molecular‐weight polybutadiene was observed in stress relaxation measurements. Sol fraction was shown to make a large contribution to the Mooney–Rivlin 2C2 term. This effect was also observed on the molecular level in NMR spin‐spin relaxation measurements. For networks crosslinked in bulk the stress relaxation measurements suggest the presence of trapped entanglements. The 2C2 term is insensitive to sol extraction in these networks. NMR spin‐lattice relaxation measurements in the rotating frame at 4.68 kHz verify the presence of additional effective crosslinks in these networks.

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