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

TL;DR

This paper explores how proton spin relaxation in polymer melts is influenced by intra- and intermolecular magnetic interactions, revealing new insights into polymer dynamics and proposing experimental methods to distinguish these effects.

Contribution

It challenges the traditional view that intramolecular interactions dominate proton spin kinetics, showing the significance of intermolecular effects and their dependence on polymer dynamics models.

Findings

Intermolecular magnetic dipole-dipole interactions increase faster with time in isotropic models.

Intramolecular interactions dominate in the tube-reptation model at longer times.

Isotope dilution technique allows experimental separation of intra- and intermolecular effects.

Abstract

Significant progress was made in recent years in the understanding of the proton spin kinetics in polymer melts. Generally, the proton spin kinetics is determined by intramolecular and intermolecular magnetic dipole-dipole contributions of proton spins. During many decades it was postulated that the main contribution is a result of intramolecular magnetic dipole-dipole interactions of protons belonging to the same polymer segment. It appears that this postulate is far from reality. The relative weights of intra- and intermolecular contributions are time dependent and sensitive to details of polymer chain dynamics. It is shown that for isotropic models of polymer dynamics the influence of the intermolecular magnetic dipole-dipole interactions increases faster with increasing evolution time (i.e. decreasing frequency) than the corresponding influence of the intramolecular counterpart. On the other hand, an inverted situation is predicted by the tube-reptation model: here the influence of the intramolecular magnetic dipole-dipole interactions increases faster with time than the contribution from intermolecular interactions. The intermolecular contribution in the proton relaxation of polymer melts can experimentally be isolated using the isotope dilution technique and this opens a new perspective for experimental investigations of polymer dynamics by proton NMR.