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

© 2020, Pleiades Publishing, Ltd. Abstract: It is shown that, in the melts of linear macromolecules, the effects of dynamic heterogeneity associated with the presence of end segments are not vanishingly small in the limit N → ∞, where N is the number of Kuhn segments in the macromolecule. The effect has a frequency nature, i.e., the division of the segments into “end” ones, more mobile in comparison with the “median” ones, mainly depends on the observation time. With the increase in the observation time, symmetrical growth of the “end” regions of the polymer chain occurs from both ends of the macromolecule which covers the entire macromolecule at times on the order of the terminal relaxation time. The effect generates nontrivial experimentally observed consequences. For example, the free induction decay of deuterons in monodisperse polymer melts of macromolecules should have an extended region with the exponential decay law g(t) ∝ t−β, where β = 1 for the reptation model and β = (α − 2)−1 for isotropic renormalized Rouse models: α > 2 is the exponent of the molecular weight dependence of the terminal relaxation time of macromolecules. At α ≤ 2, the influence of the effects of dynamic heterogeneity on the shape of free induction decay is weaker, although it is observable at sufficient accuracy of the measurements

On the theory of deuteron NMR free induction decay of reptating polymer chains: Effect of end segment dynamics

© 2020 Author(s). A self-consistent approximation beyond the Redfield limit and without using the Anderson-Weiss approximation for the Free Induction Decay (FID) of deuteron spins belonging to polymer chains undergoing reptation is formulated. The dynamical heterogeneity of the polymer segments created by the end segments is taken into account. Within an accuracy of slow-changing logarithmic factors, FID can be qualitatively described by a transition from an initial pseudo-Gaussian to a stretched-exponential decay at long times. With an increase in observation time, the contribution from end effects to the FID increases. In the regime of incoherent reptation, contributions to the FID from central segments yield an exponent of 1/4 for the stretched decay and contributions from end segments yield an exponent of 3/16. In the regime of coherent reptation, the central segments generate a stretching exponent of 1/2, whereas the end segments contribute with an exponent of 1/4. These predictions are shown to be in qualitative agreement with the experimental FIDs of perdeuterated poly(ethylene oxide) with molecular masses of 132 kg/mol and 862 kg/mol