Field-cycling NMR relaxometry as a tool of molecular rheology-applications on polymer melts

2^nd International Conference and Exhibition on Mesoscopic and Condensed Matter Physics

October 26-28, 2016 Chicago, USA

M Hofmann, B Kresse, F Fujara and E A Rossler

Universit�?¤t Bayreuth, Germany
Technische Universit�?¤t Darmstadt, Germany

Posters & Accepted Abstracts: J Material Sci Eng

Abstract :

NMR relaxometry provides rich information on the dynamics in complex liquids, such as polymer melts. Using the field-cycling (FC) technique, the spin-lattice relaxation rate R1, reflecting the spectral density, can be measured over a broad range of frequencies �?, appreciably reaching below the earth field, with sophisticated low-field equipment. Assuming frequency-temperature superposition, the effective window extends about ten decades in the NMR susceptibility representation �?��?��?(�?):=�?R1(�?), when master curves are constructed. In high-M polymers, the local (a-process), Rouse and entanglement dynamics is covered. The broad frequency range allows for a transformation into the time domain; a time auto-correlation function is gained. Concerning 1H, relaxation is caused by fluctuations of the dipolar interaction, comprising intra- as well as inter-molecular contributions. The isotope dilution technique allows for a separation yielding both, the re-orientational correlation function C2(t) as well as Cinter(t), providing the segmental mean square displacement (MSD). Complementing the FC data with such of field-gradient NMR reaching even longer times, the full MSD is probed in highly entangled polymers. All power-law regimes of the tube-reptation model are reproduced. Concerning C2(t), however, the predictions are only confirmed in parts. Comparing FC NMR with shear rheology, much similarity is found, regarding the local, the Rouse and the terminal regime. This renders FC NMR as a powerful tool of molecular rheology. As NMR relaxometry addresses molecular correlation functions, the slow dynamics in the entanglement regime is resolved. In particular the constrained Rouse, the reptation regime as well as the corresponding crossover times. In contrast, the shear data is governed by the rubber plateau.

Biography :

Email: marius.hofmann@uni-bayreuth.de