Protracted Crossover to Reptation Dynamics: A Field Cycling 1H NMR Study Including Extremely Low Frequencies

The segmental dynamics of 1,4-polybutadiene is investigated by means of electronic field cycling H-1 NMR. The frequency dependence (dispersion) of the spin-lattice relaxation time is probed over a broad range of temperature (223-408 K), molecular mass (355 <= M (g/mol) <= 441 000), and frequency (200 Hz-30 MHz). The extremely low frequencies are accessed by employing a home-built compensation for earth and stray fields extending prior reports about 2 decades to lower frequencies. Applying frequency-temperature superposition yields master curves over 10 decades in frequency (or time), and after Fourier transform the full dipolar correlation function is traced over up to 8 decades in amplitude. Several relaxation regimes can be identified, and their power law exponents are compared to the predictions of the Doi-Edwards tube-reptation model, namely the free Rouse: (I) and the constrained Rouse regime (II). Whereas the predicted value of the power law exponent of regime II is 0.25, we find that it depends on M and levels off at 0.32 for very high M = 441 000 approximate to 220M(e) (M-e: entanglement molecular mass). This is in good agreement with recent results from double quantum H-1 NMR and indicates that the actual onset of full reptation dynamics is strongly protracted