화학공학소재연구정보센터
Journal of Physical Chemistry B, Vol.124, No.20, 4222-4233, 2020
Elucidating the H-1 NMR Relaxation Mechanism in Polydisperse Polymers and Bitumen Using Measurements, MD Simulations, and Models
The mechanism behind the H-1 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 (eta = 385-102 000 cP) using T-1 and T-2 in static fields, T-1 field-cycling relaxometry, and T-1p in the rotating frame. We account for the anomalous behavior of the log-mean relaxation times T-1LM alpha f(0) and T-2LM alpha (eta/T)(-1/2) with a phenomenological model of H-1-H-1 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 )and in previously reported bitumen measurements. We find that molecular dynamics (MD) simulations of the T-1 proportional to f(0) dispersion and T-2 of similar polymers simulated over a range of viscosities (eta = 1-1000 cP) are in good agreement with measurements and the model. The T-1 proportional to 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.