Elucidating the ¹H NMR Relaxation Mechanism in Polydisperse Polymers and Bitumen Using Measurements, MD Simulations, and Models

The mechanism behind the 1 H nuclear magnetic resonance (NMR) frequency dependence of T 1 and the viscosity dependence of T 2 for polydisperse polymers and bitumen remains elusive. We elucidate the matter through NMR relaxation measurements of polydisperse polymers over an extended range of frequencies ( f 0 = 0.01–400 MHz) and viscosities (η = 385–102 000 cP) using T 1 and T 2 in static fields, T 1 field-cycling relaxometry, and T 1ρ in the rotating frame. We account for the anomalous behavior of the log-mean relaxation times T 1LM ∝ f 0 and T 2LM ∝ (η/ T ) −1/2 with a phenomenological model of 1 H– 1 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 T 1LM and T 2LM in previously reported bitumen measurements. We find that molecular dynamics (MD) simulations of the T 1 ∝ f 0 dispersion and T 2 of similar polymers simulated over a range of viscosities (η = 1–1000 cP) are in good agreement with measurements and the model. The T 1 ∝ f 0 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.