From simple liquid to polymer melt. glassy and polymer dynamics studied by fast field cycling NMR relaxometry: Rouse regime

We apply fast field cycling NMR to study the dispersion of the ¹H spin-lattice relaxation time T₁(ω) of linear 1,4-polybutadienes with molecular weight M (g/mol) ranging from M = 355 to 817 000. By this, the crossover from glassy dynamics through Rouse to reptation becomes accessible. Analyzing the data in the susceptibility form ω/T ₁(ω) and applying frequency-temperature superposition, spectra extending over up to 8 decades in ω are obtained. Characteristic polymer spectra are revealed when the underlying glassy dynamics are accounted for. Instead of describing the unentangled melt by the full Rouse mode spectrum, the emergence of a limited number of modes is identified which saturates when entanglement sets in. A quantitative analysis yields the molecular weight of a Rouse unit M_R ≅ as 500, and the entanglement weight M _e ≅ 2000, at which first entanglement effects are observed. Moreover, the dynamic order parameter S(M) and the behavior of the terminal time τ_max(M) are obtained. Both quantities allow to identify three dynamic regimes, namely simple liquid, Rouse, and reptation dynamics. The temperature dependence of the segmental relaxation time τ_s(T) coincides with the corresponding dielectric relaxation times which were measured additionally, and the M dependence of the glass transition temperature T _g shows distinctive kinks at M_R and M_e, indicating that glassy dynamics are modified by polymer dynamics.