Heterogeneous Polymer Dynamics Explored Using Static ¹H NMR Spectra

NMR spectroscopy continues to provide important molecular level details of dynamics in different polymer materials, ranging from rubbers to highly crosslinked composites. It has been argued that thermoset polymers containing dynamic and chemical heterogeneities can be fully cured at temperatures well below the final glass transition temperature (T_g). In this paper, we described the use of static solid-state ¹H NMR spectroscopy to measure the activation of different chain dynamics as a function of temperature. Near T_g, increasing polymer segmental chain fluctuations lead to dynamic averaging of the local homonuclear proton-proton (¹H-¹H) dipolar couplings, as reflected in the reduction of the NMR line shape second moment (M₂) when motions are faster than the magnitude of the dipolar coupling. In general, for polymer systems, distributions in the dynamic correlation times are commonly expected. To help identify the limitations and pitfalls of M₂ analyses, the impact of activation energy or, equivalently, correlation time distributions, on the analysis of ¹H NMR M₂ temperature variations is explored. It is shown by using normalized reference curves that the distributions in dynamic activation energies can be measured from the M₂ temperature behavior. An example of the M₂ analysis for a series of thermosetting polymers with systematically varied dynamic heterogeneity is presented and discussed.