Dynamic fluctuations in a highly cross-linked polybutadiene rubber

Using a combined approach of different neutron scattering techniques that allows experimental separation of the self and collective dynamics, we investigated the various types of motion in a highly cross-linked 1,2-polybutadiene rubber. As a reference, the neat, fully deuterated melt was also studied. In the latter, the collective segmental relaxation around the structure factor peak S(Qmax) exhibits pronounced de Gennes narrowing, with a collective relaxation time τc(Q) that increases without plateauing toward lower momentum transfers Q. Apparently, due to the very bulky monomer structure, the correlation length ξc for the crossover to viscoelastic homogeneity is unusually large. In the highly cross-linked d1,2-PB rubber, the original structure factor peak of the pure d1,2-PB is significantly reduced. Instead, a strong low-Q peak emerges, which we attribute to correlations between the hydrogen-containing cross-linkers. Both the collective d1,2-PB relaxation rates around the former S(Qmax) and the relaxation around the new crosslink correlation peak are significantly slowed down compared to the neat melt. Finally, the cross-linking strands exhibit their own fast dynamics, which is well described by diffusion within a spherical Gaussian well, with a corresponding radius R ≅ 4 Å.