Elucidating the H NMR Relaxation Mechanism in Polydisperse Polymers and Bitumen Using Measurements, MD Simulations, and Models.

The mechanism behind the H nuclear magnetic resonance (NMR) frequency dependence of and the viscosity dependence of for polydisperse polymers and bitumen remains elusive. We elucidate the matter through NMR relaxation measurements of polydisperse polymers over an extended range of frequencies ( = 0.01-400 MHz) and viscosities (η = 385-102 000 cP) using and in static fields, field-cycling relaxometry, and in the rotating frame. We account for the anomalous behavior of the log-mean relaxation times ∝ and ∝ (η/) with a phenomenological model of H-H dipole-dipole relaxation, which includes a distribution in molecular correlation times and internal motions of the nonrigid polymer branches. We show that the model also accounts for the anomalous and in previously reported bitumen measurements. We find that molecular dynamics (MD) simulations of the ∝ dispersion and of similar polymers simulated over a range of viscosities (η = 1-1000 cP) are in good agreement with measurements and the model. The ∝ dispersion at high viscosities agrees with previously reported MD simulations of heptane confined in a polymer matrix, which suggests a common NMR relaxation mechanism between viscous polydisperse fluids and fluids under nanoconfinement, without the need to invoke paramagnetism.