Accessing molecular motion activation parameters with ¹H low-field time-domain NMR: Examples from glass transition in...

A practical and accessible approach for analyzing complex molecular dynamics in heterogeneous systems from the temperature dependence of low-field ¹H time-domain NMR signals is presented. Based on Dipolar Filtered Magic Sandwich Echo (DFMSE) experiments, the method can be adapted to any measurement where the fraction of mobile or rigid segments can be monitored as a function of temperature. A theoretical framework based on the Anderson-Weiss model, using analytical expressions, is described, enabling the extraction of activation parameters and their distributions from low-field ¹H time-domain NMR data. A theoretical expression for the DFMSE signal as a function of temperature and filter time used in the experiment is derived, and an approximation is proposed to simplify the model while retaining the essential features of the DFMSE-temperature curves. Assuming Arrhenius or Vogel-Fulcher-Tammann (VFT) temperature dependence of correlation times, a relation is obtained linking the curve's inflection temperature to the filter time, with activation parameters as fitting variables. This allows an initial estimation of activation parameters, later refined by fitting full DFMSE-temperature curves, including a distribution of values. The method is validated by determining activation parameters for motions associated with the glass transition dynamics of atactic polypropylene (aPP), yielding results consistent with previous NMR studies. It is further applied to a styrene-butadiene rubber (SBR) sample, whose parameters were independently obtained by field-cycling NMR, showing good agreement.