NMR Observations of Entangled Polymer Dynamics: Focus on Tagged Chain Rotational Dynamics and Confirmation from a Simulation Model

Abstract

Molecular-level insights into the entangled dynamics of high-molecular-weight chains, in particular of slower chain modes in regimes II−IV of the tube model, are still rare due to the lack of methods resolving the rather long associated time scales. On the theoretical side, new computer simulation methods are just reaching the relevant time scales in sufficiently large systems. Here, we confront results from a recent multiple-quantum proton NMR method with results from a novel lattice model. We address the concern that proton NMR, relying on the dipole−dipole couplings between nearby nuclei, is intrinsically sensitive not only to intrachain rotational motions which reflect the desired details of the tube model or possibly necessary modifications, but also to the translational diffusion of chains past each other via interchain dipole−dipole couplings. In order to critically assess the influence of the latter, we here present results of isotope-dilution experiments, in which the data reflect mainly tagged-chain dynamics. We find overall weak effects of interchain dipole−dipole couplings on the shape of the extracted orientation autocorrelation function and very good agreement of the experimental and the computer simulation data. We conclude that the NMR method as well as the novel lattice model faithfully reflects the universal features of entangled chain dynamics and in particular the deviations from simple tube-model predictions on a microscopic level.

Proton spin dynamics in polymer melts: New perspectives for experimental investigations of polymer dynamics

“…In particular, the assumption of a universal behavior for all types of linear polymers will be subject to detailed investigation in the near future. Very recently, a DQ 1 H NMR study [30] has appeared which claims that the exponent α in regime does not change when isotope dilution is applied providing also agreement with simulation data [11,30,59]. This implies that ratio the of intra-and intermolecular correlation is independent of frequency or time (actually a ratio about one has been reported).…”

“…We note that the exponent has been slightly corrected with respect to the originally published one [25] applying now a derivative method to determine the smallest exponent in regime II [30]. It appears that at longest times already the terminal relaxation leads to a somewhat steeper decay.…”

“…The second line of expression (30) shows that in the limit 1 0 ωτ → , the intermolecular part of the spin-lattice relaxation rate is approaching the frequency independent plateau value: …”

“…The dynamics of the investigated systems is mainly reflected through characteristic features of nuclear spin relaxations. Important findings of recent years [7,[11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30], affecting the understanding of the proton spin dynamics in polymer melts are the subject of discussion of this paper.…”

“…The longitudinal relaxation, i.e. the spin relaxation along Z direction, is characterized by the spin-lattice relaxation time ( ) T ω possesses a non-trivial frequency dispersion covering an extremely broad frequency range which nowadays can be measured most easily by field cycling techniques in a frequency range of 100 Hz -40 MHz when earth field compensation is included [24,25,30,32]. The relaxation time ( )…”