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

Abstract

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.

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

“…The well-established theory of paramagnetic relaxation states that 1/T 1,2 is proportional to the paramagnetic concentration (in the dilute limit). 4,5 Therefore, given the order of magnitude difference in paramagnetic concentrations between polyisobutene and Athabasca bitumen, we proposed that the similarity in T 1,2 between the two (at high viscosities and over a large range of NMR frequencies) was not due to paramagnetic surface relaxation 1,6 but rather the ubiquitous 1 H− 1 H dipole−dipole relaxation, as confirmed by MD simulations. 1,7 Figure 1a shows a higher resolution EPR spectrum of polyisobutene (B360000) using a benchtop EMXnano EPR spectrometer by Bruker, with the SpinCount module for quantitative analysis.…”

Molecular dynamics simulations of $^1$H NMR relaxation in Gd$^{3+}$--aqua

“…For instance, MD simulations revealed for the first time ever that water and alkanes do not conform to the BPP model of a mono-exponential decay in the rotational autocorrelation function, except for highly symmetric molecules such as neopentane. More complex systems such as viscous polymers [52] and heptane confined in a polymer matrix [53] have also been simulated, which again saw good agreement with measurements, and which lead to insights into the distribution in dynamic molecular modes.…”

“…The P (τ ) is binned from τ min = 10 −2 to τ max = 10 3 ps using 200 logarithmically spaced bins. τ max is chosen such that τ max = 5t max , consistent with previous studies [50,52,53]. In the present case of low-viscosity fluids (η ≃ 1 cP), the choice of τ max does not impact J(ω), and is therefore not a free parameter in the ILT analysis.…”

“…4(a), the residual between the G(t) data and the ILT fit is not dominated by Gaussian noise. As such, the regularization parameter is fixed to α = 10 −1 in accordance with previous studies [49][50][51][52][53]. As shown in Fig.…”

“…With this understanding, we note that details of the ILT procedure can be found in Refs. [50][51][52][53] and the supplementary material in Refs. [49,80].…”