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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