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

The mechanism behind the ¹H nuclear magnetic resonance (NMR) frequency dependence of T₁ and the viscosity dependence of T₂ for polydisperse polymers and bitumen remains elusive. We elucidate the matter through NMR relaxation measurements of polydisperse polymers over an extended range of frequencies (f₀ = 0.01-400 MHz) and viscosities (η = 385-102 000 cP) using T₁ and T₂ in static fields, T₁ field-cycling relaxometry, and T_1ρ in the rotating frame. We account for the anomalous behavior of the log-mean relaxation times T_1LM ∝ f₀ and T_2LM ∝ (η/T)^-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 T_1LM and T_2LM in previously reported bitumen measurements. We find that molecular dynamics (MD) simulations of the T₁ ∝ f₀ dispersion and T₂ of similar polymers simulated over a range of viscosities (η = 1-1000 cP) are in good agreement with measurements and the model. The T₁ ∝ f₀ 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.