Department of Chemical and Biomolecular Engineering, Rice University, 6100 Main Street, Houston, Texas 77005, United States.
IFP Energies nouvelles, 1 Avenue de Bois-Préau, 92852 Rueil-Malmaison, France.
J Phys Chem B. 2020 May 21;124(20):4222-4233. doi: 10.1021/acs.jpcb.0c01941. Epub 2020 May 13.
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.
均相聚合物和沥青的 H 核磁共振（NMR）频率依赖 和粘度依赖 的背后机制仍难以捉摸。我们通过在静态场中使用 和 进行 NMR 弛豫测量，在宽频率范围（ = 0.01-400 MHz）和粘度范围（η = 385-102 000 cP）下阐明了这一问题，还使用了场循环弛豫测量和在旋转框架中的 。我们用包含分子相关时间分布和非刚性聚合物支链内运动的 H-H 偶极-偶极弛豫的唯象模型来解释 log-平均弛豫时间 和 的异常行为 ∝ 和 ∝ (η/)。我们表明，该模型还可以解释先前报道的沥青测量中的异常 和 。我们发现，相似聚合物的 ∝ 分散和 的 MD 模拟在一系列粘度（η = 1-1000 cP）下与测量和模型非常吻合。高粘度下的 ∝ 分散与先前报道的正庚烷在聚合物基质中受限的 MD 模拟一致，这表明粘性多分散流体和纳米受限流体之间存在共同的 NMR 弛豫机制，而无需引入顺磁性。
