NMR and Entangled Chain Dynamics in Molten Polybutadiene. Molecular Weight Dependent Segmental Friction

The chain dynamics in high molecular weight polymer melts observed from proton magnetic relaxation generates a reference frequency, Δ ≈ 10 3 rad·s -1, closely related to the chain entanglement interaction. Observing polybutadiene, we report on the slight molecular weight dependence of the segmental correlation time associated with the time scale 1/Δ ≈ 10 -3 s, (41 × 10 3 ≤ M ≤ 375 × 10 3 ); is shown to exhibit an apparent power law M 0.31 . Correspondingly, the reptation time is expected to go as M 3+(0.31±0.03) within the intermediate range 41 × 10 3 ≤ M ≤ 375 × 10 3 . This semilocal property and the chain self-diffusion M -β (β = 2.3 ± 0.1 instead of 2) dependence, recently reported for many polymers and solutions, suggest a chain dynamics governed by a semilocal friction effect varying as an apparent M 0.3 power law instead of M 0 . A thorough analysis of the pattern of proton relaxation curves is proposed; end-submolecules (≈60 monomeric units) are distinguished from inner ones from their magnetization dynamics; these are described as nonexponential time functions in accordance with the observation of pseudo-solid echoes (PSOSES). The molecular weight dependent pattern of four species of curves (two relaxation types and two PSOSE types) corresponding to 40 recorded curves is quantitatively analyzed using only four independent adjustable parameters.