What is it about?
In polymer melts the molecular dynamic of an individual chain depends on the overall chain length distribution of the melt. In a melt consisting of short chains, the dynamic of an individual chain is little influenced by the other chains. With increasing chain length the chains become more and more entangled, like a ball of earthworms, which affects the chain dynamic severely. There are two well-known models that describe either the first (Rouse model) or the second situation (Tube-Reptation model). In previous works, these models were further developed to take into account the dynamical heterogeneity of a polymer chain, i.e. the dependence of the dynamic properties of the segments of a polymer chain on their distance to the end segments. In this study, we investigate melts of four deuterated polyethylene oxides, which differ in molecular weight, e.g. in chain length, to study four different dynamic situations. With the used nuclear magnetic resonance (NMR) relaxation technique we measure the return of the nuclear spin magnetization from a perturbed state back to the equilibrium state. The time constants as well as the shape of the resulting signal decay provide information about the molecular dynamics of the sample, since the relaxation originates from fluctuations in the local magnetic field around the spins, which are caused by molecular motions. The measured signal decays are then compared with the signal decays predicted by the aforementioned models including the dynamical heterogeneity. We observe a continuous transition from the situation of short chains with little entanglements to the situation of long chains and many entanglements. While the two extremes are well (short chains) to sufficiently (long chains) described by the used models, the poorly matching in the transition region indicates remaining flaws of the theory.
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Why is it important?
In this study, it becomes clear that, especially in the transition region between the two models, the molecular dynamics of polymer melts can only be described inadequately. It is therefore still necessary to further develop the existing theories by refining the assumptions on the basis of real conditions. In this context, systematic experimental investigations are indispensable to enable a constant comparison between theory and experiment.
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This page is a summary of: The deuteron NMR Hahn echo decay in polyethylene oxide melts, AIP Advances, July 2022, American Institute of Physics, DOI: 10.1063/5.0099293.
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The Effect of the End Segments on the Dynamics of a Polymer Melt: The Frequency Nature of the Effect and Possibility of Experimental Observation in the Free Induction Decay of Deuterons
This paper shows that the dynamical heterogeneity, due to the presence of end segments, is not negligible in polymer chains, especially not at larger observation times. It is shown how the dynamical heterogeneity can be taken into account for shorter polymer chains to describe the NMR deuteron free induction decay (FID) in deuterated polymer melts accordingly.
On the theory of deuteron NMR free induction decay of reptating polymer chains: Effect of end segment dynamics
In this paper, the consideration of the dynamical heterogeneity in the description of the NMR deuteron free induction decay in deuterated polymer melts is expanded to long polymer chains and first experimental results are shown.
NMR Hahn echo decays in deuterated polyethylene oxide melts
Corresponding data set, containing the raw and processed experimental results, as well as MATLAB scripts for computing the signal decays according to the used models.
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