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- Polymer Melts Investigated by Field Cycling NMR Relaxometry: From Simple Liquid to Reptation Dynamics (2012)
- The focus of this thesis is the investigation of linear polymer melts by applying Field Cycling Nuclear Magnetic Resonance (FC NMR) relaxometry. The objective is to understand their microscopic dynamics and its dependence on the molecular mass (M) of the polymer chains. With the commercial availability of FC NMR relaxometers, the method gained attraction for studying dynamics of soft condensed matter due to its ability to detect both the structural or alpha-relaxation (identified with the segmental dynamics) and slower collective dynamics. In the case of polymer melts the latter is described most often by the Rouse model for non-entangled chains and the Doi/Edwards tube-reptation model for entangled polymers. Since 2004 a commercial relaxometer by Stelar has been operated in the Rössler group. Its capability to rapidly switch between different magnetic fields allows to measure the spin-lattice relaxation time in the proton frequency range from 10 kHz to 20 MHz. In previous works by the Rössler group the pioneering works by Kimmich and co-workers have been extended in order to combine the results of a broad temperature range: Frequency-temperature superposition is applied to construct master curves in the susceptibility representation. The key benefits are: the susceptibility is scaled by time constant of segmental dynamics and an "isofrictional" representation is achieved; the accessible frequency range is significantly increased; the time constants are provided and compared with those obtained by other techniques; the regimes of glassy and polymer dynamics can be easily distinguished; finally, the dipolar correlation function is obtained directly by Fourier transform. In this thesis by employing the above approach, the dipolar correlation function of polybutadienes (PB) melts is presented and comprises - depending on M - glassy, Rouse and entanglement dynamics. The latter two relaxation regimes can be described by different power-laws, which are compared to the predictions of the tube-reptation model. A good agreement is found for the Rouse regime (I). For the constrained Rouse regime (II) at long times, a highly protracted crossover to completely established reptation dynamics is discovered. That is, the exponent depends on M and reaches 0.32 only at M=441000, which is in accord with Double Quantum (DQ) 1H NMR results by Saalwächter and co-workers and very close to 0.25 predicted for regime II of the tube-reptation model. This is only achieved by additional relaxation experiments in cooperation with the Fujara group at TU Darmstadt, since their home-built FC NMR relaxometer is equipped with an active stray field compensation, which allows to reach extremely low frequencies down to 200 Hz. Consequently, the frequency range is extended by two decades toward lower frequencies with respect to the commercial spectrometer and the obtained correlation function stretches over 10 decades in time and 8 in amplitude for molecular masses up to 220 Me. This establishes FC 1H NMR also at long times as competitive with DQ 1H NMR. The analyses of the dipolar correlation function appear to support the applicability of the tube-reptation model. However, intramolecular and intermolecular relaxation contributions have to be discriminated and up to now the dominance of the first has been assumed implicitly. Therefore, isotopic blends of high-M protonated and deuterated PB are investigated, which allows to decompose the 1H master curves into intramolecular and intermolecular relaxation contributions. They reflect reorientational and translational dynamics, respectively. It is demonstrated that at long times or low frequencies the intermolecular contribution dominates. Consequently, the reorientational correlation function obtained from the intramolecular part exhibits a faster decay with the long-time exponent 0.49. This is ascertained by the FC 2H NMR relaxation of completely deuterated PB, which detects reorientational dynamics only. The observed exponent is significantly larger than 0.25 of regime II of the tube reptation model. Concomitantly, the segmental mean square displacement is attained from the intermolecular part following an approach by Kimmich and Fatkullin. The predicted power-laws of the tube-reptation model for the Rouse and constrained Rouse regimes are identified for the first time by FC NMR: a transition between the power-laws t^ 0.49 and t^0.19 is revealed, respectively. Thus, NMR relaxometry is designated as a method comparable to neutron scattering to study subdiffusion in polymer melts. In conclusion, the power-law predictions of the tube-reptation model are disclosed by the segmental mean square displacement, yet not by the reorientational correlation function. Thus, the simple tube-reptation model does not completely describe the microscopic dynamics of polymer melts.