Intermolecular magnetic dipole-dipole-interaction in 1H NMR of polymer melts and intermolecular dynamic correlations in polymer chains

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The dynamics of polymers in the melt has been explained for several decades by two complementary models, the Rouse ansatz and the reptation model. They describe the interactions in the limiting cases of short and long chains, respectively, where the transition occurs at the onset of entanglements. The mean squared displacement (MSD) of individual segments is frequently employed as a suitable parameter for these dynamics, with the segments themselves being assumed as abstract units – internal rotations and vibrations occur on much shorter timescales and are not covered by the mentioned models.The MSD is directly accessible by, for instance, NMR field gradient methods or neutron scattering, and the theoretical predictions, i.e. the characteristic time dependences, were confirmed repeatedly, so that Rouse and reptation models are considered as satisfactory descriptions. In addition, indirect methods exist which determine related quantities, first and foremost one needs to mention NMR relaxation which has successfully been employed in polymer melt studies for about 30 years. Only during recent years methods have been developed that are able to separate between intra- and intermolecular dipolar interactions of the chain molecules by making use of isotopic dilution in deuterated polymers. While this approach initially served only the purpose to isolate the MSD from relaxation data, it does contain valuable information about long-range dynamic correlations in the melt which is insufficiently described by the classical models. In the context of these studies, systematic deviations from the predictions of the reptation model have been found, first in NMR relaxometry, later also by alternative methods, in particular by analysis of echo decay functions within the first period of this research project. Specifically, we have found, for three chemically different polymers of high molecular weight, a contribution of the intermolecular interaction that is clearly at variance with reptation theory and suggests the validity of isotropic dynamic models rather than reptation which assumes the existence of a – on a certain timescale – rigid tube. In the planned continuation of this project, we would like to cover a broad range of combinations of molecular weights and temperatures as well as boundary conditions (confinement) for poly (ethylene oxide) as a suitable model polymer; with this approach we want to verify our earlier findings and deepen our understanding of polymer dynamics. To this end, we also employ deuteron and possibly carbon relaxation studies. The project will be accompanied by Prof. Nail Fatkullin who will further refine the theoretical description – for instance by specifically computing the dynamics of chain ends, or by addressing the temporal evolution during encoding periods of NMR experiments – and who will support the choice of optimized experimental parameters for allowing discrimination between concurring model approaches.