Dipolar and Bond Vector Correlation Function of Linear Polymers Revealed by Field Cycling 1H NMR: Crossover from Rouse to Entanglement Regime

We apply field cycling NMR to study segmental reorientation dynamics in melts of linear 1,4-polybutadiene (PB) in the entanglement regime ( M ≥ M e ). Dispersion data of the spin−lattice relaxation time T 1 (ω) are transformed to the susceptibility representation χ′′(ω) = ω/ T 1 (ω), and using frequency temperature superposition master curves χ′′(ωτ s ) are constructed which reflect spectral contributions from glassy as well as polymer specific dynamics. The correlation time τ s is determined by glassy dynamics. Transforming χ′′(ωτ s ) into the time domain and studying the crossover from Rouse to entanglement regime, the full dipolar or segmental reorientational correlation function F 2 ( t/τ s ) is presented covering six decades in amplitude and 8 decades in time. Assuming F 2 ( t ) ≅ ⟨ u b ( t ) u b (0)⟩ 2 the bond vector correlation function ϕ b ( t ) = ⟨ u b ( t ) u b (0)⟩ is obtained. Reaching Z = M/M e ≤ 9, comparison with theoretical predictions by the tube-reptation model as well as renormalized Rouse theory reveals significant discrepancies whereas good agreement is found with simulations. The crossover to entanglement dynamics appears to be very protracted.