Abstract
We apply field cycling NMR to study segmental reorientation dynamics in melts of linear 1,4-polybutadiene (PB) in the entanglement regime (M ≥ Me). Dispersion data of the spin−lattice relaxation time T1(ω) are transformed to the susceptibility representation χ′′(ω) = ω/T1(ω), and using frequency temperature superposition master curves χ′′(ωτs) are constructed which reflect spectral contributions from glassy as well as polymer specific dynamics. The correlation time τs is determined by glassy dynamics. Transforming χ′′(ωτs) into the time domain and studying the crossover from Rouse to entanglement regime, the full dipolar or segmental reorientational correlation function F2(t/τs) is presented covering six decades in amplitude and 8 decades in time. Assuming F2(t) ≅ ⟨ub(t)ub(0)⟩2 the bond vector correlation function ϕb(t) = ⟨ub(t)ub(0)⟩ is obtained. Reaching Z = M/Me ≤ 9, comparison with theoretical predictions by the tube-reptation model as well as renormalized Rouse theory reveals significant discrepancie...
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