Interpretation of 1H and 2H spin–lattice relaxation dispersions: Insights from molecular dynamics simulations of polymer melts

Abstract

We demonstrate that molecular dynamics simulations are a versatile tool to ascertain the interpretation of spin–lattice relaxation data. For 1H, our simulation approach allows us to separate and to compare intra- and inter-molecular contributions to spin–lattice relaxation dispersions. Dealing with the important example of polymer melts, we show that the intramolecular parts of 1H spectral densities and correlation functions are governed by rotational motion, while their inter-molecular counterparts provide access to translational motion, in particular, to mean-squared displacements and self-diffusion coefficients. Exploiting that the full microscopic information is available from molecular dynamics simulations, we determine the range of validity of experimental approaches, which often assume Gaussian dynamics, and we provide guidelines for the determination of free parameters required in experimental analyses. For 2H, we examine the traditional methodology to extract correlation times of complex dynamics from relaxation data. Furthermore, based on knowledge from our computational study, it is shown that measurement of 2H spin–lattice relaxation dispersions allows one to disentangle the intra- and inter-molecular contributions to the corresponding 1H data in experimental work. Altogether, our simulation results yield a solid basis for future 1H and 2H spin–lattice relaxation analysis.