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

Abstract

The mechanism behind the ¹H nuclear magnetic resonance (NMR) frequency dependence of <i>T</i>₁ and the viscosity dependence of <i>T</i>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 (<i>f</i>₀ = 0.01-400 MHz) and viscosities (η = 385-102 000 cP) using <i>T</i>₁ and <i>T</i>₂ in static fields, <i>T</i>₁ field-cycling relaxometry, and <i>T</i>_1ρ in the rotating frame. We account for the anomalous behavior of the log-mean relaxation times <i>T</i>_1LM ∝ <i>f</i>₀ and <i>T</i>_2LM ∝ (η/<i>T</i>)^-1/2 with a phenomenological model of ¹H-¹H 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 <i>T</i>_1LM and <i>T</i>_2LM in previously reported bitumen measurements. We find that molecular dynamics (MD) simulations of the <i>T</i>₁ ∝ <i>f</i>₀ dispersion and <i>T</i>₂ of similar polymers simulated over a range of viscosities (η = 1-1000 cP) are in good agreement with measurements and the model. The <i>T</i>₁ ∝ <i>f</i>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.