Untersuchung der Dynamik verhakter Polymerschmelzen mit Hilfe der Multiquanten-Kernresonanzspektroskopie
Final Report Abstract
The goal of this project was the investigation of the entangled dynamics of different polymers over a wide range of molecular weights, focusing specifically on the validity and limitations of de Gennes’ reptation and tube models of the long-time behavior. For that purpose, multiple-quantum (MQ) NMR, as pioneered by the Spiess group (MPI for Polymer Research, Mainz) and substantially improved by the applicant, had already been established as a powerful tool for the investigation of chain dynamics and local structure (crosslink and entanglement density, local packing) of polymer melts as well as dry and swollen networks. In particular, work of our group had shown that MQ NMR is superior to most other NMR techniques such as longitudinal and transverse relaxometry or other, more advanced spin-echo approaches. MQ NMR is further technically undemanding, and can be performed on inexpensive low-field instrumentation. MQ NMR as well as most of the other NMR methods are sensitive to molecular motions (rotations) on the s to ms timescale, which, unlike for instance inelastic neutron scattering, ideally matches the timescale of reptation motion. All of the mentioned methods had consequentially been applied to the investigation of entangled melt dynamics in the framework of different theoretical descriptions. With the present project, we sought to lift methodological restrictions to specific polymers and limited ranges of molecular weight and temperature, and provide a unified approach. It was suggested to apply MQ NMR to different polymers over suitable ranges of molecular weight and temperature, and developing and testing different theoretical approaches that should ultimately deliver molecular-level insight into entangled chain dynamics, making NMR experiments directly comparable to results from rheological or scattering studies. These goals were essentially achieved. In the first funding period, an encompassing NMR investigation of the entangled dynamics of different polymers over a wide range of molecular weights was carried out, achieving substantial progress in establishing a direct and meaningful NMR observable of polymer chain mobility. The dynamics was quantified in terms of a direct time-domain measure of the segmental orientation autocorrelation function over 5 decades in time. We have found universal behavior of three different polymers, namely PB, PI and PDMS, for regimes II-IV of the tube model, and observed characteristic deviations from its predictions which we attributed to constraint release (CR) effects that are active at surprisingly short times, i.e., in regime II. We could further establish a fully consistent theoretical description of the NMR signal functions on the basis of the experimental segmental correlation function. In the second funding period, we have focused in more detail on CR and contour-length fluctuation (CLF) effects by investigating pseudo D-H-D and H-D-H triblock copolymers and a large series of probe chains diluted in deuterated matrix chains of different molecular weight, respectively. We have also combined 1H and 2H MQ NMR for complementary studies of the different labeled components. We have further compared these results to computer simulations of entangled dynamics, essentially confirming the observed deviations from tube-model predictions in regime II. We have finally extended the methodology to segmental dynamics on shorter timescales (Rouse regime I) by investigating the polymers closer to their glass transition. The close interaction with the Rössler and Fujara groups at U Bayreuth and TU Darmstadt, respectively, has brought a number of surprising insights and raised relevant questions still to be addressed in the future. Initially, pioneering developments in field-cycling relaxometry in Bayreuth and Darmstadt have provided results in full support of our findings, perfectly complementing our results towards lower molecular weights and faster dynamics (regime I). Later developments, including input from the Stapf and Fatkullin groups (Ilmenau and Kazan, resp.) focusing on the separation of intra-molecular interactions (probing rotational dynamics) and inter-molecular interactions (probing translational mean-square displacements) by isotope dilution experiments have unveiled a complex picture, with the latter still supporting the approximate validity of the tube model, but the former now being in significant deviation from it. These latter findings are not supported by the MQ NMR results from this project as well as different kinds of generic simulation models, and call for a continued investigations of the origin of the discrepancy.
Publications
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NMR Observation of Entangled Polymer Dynamics: Tube Model Predictions and Constraint Release. Phys. Rev. Lett. 104, 198305 (2010)
F. Vaca Chávez, K. Saalwächter
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Time-Domain NMR Observation of Entangled Polymer Dynamics: Analytical Theory of Signal Functions Macromolecules 44, 1560 (2011)
F. Vaca Chávez, K. Saalwächter
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Time-Domain NMR Observation of Entangled Polymer Dynamics: Universal Behavior of Flexible Homopolymers and Applicability of the Tube Model. Macromolecules 44, 1549 (2011)
F. Vaca Chávez, K. Saalwächter
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NMR Observations of Entangled Polymer Dynamics: Focus on Tagged Chain Rotational Dynamics and Confirmation from a Simulation Model. Macromolecules 47, 256, (2014)
F. Furtado, J. Damron, M.-L. Trutschel, C. Franz, K. Schröter, R. C. Ball, K. Saalwächter, D. Panja
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(2017): Multiple-Quantum NMR Studies of Anisotropic Polymer Chain Dynamics. In: Graham A. Webb (Hg.): Modern magnetic resonance. 2nd edition. Cham: Springer (Springer eBook Collection), S. 1–28
K. Saalwächter
