Development of low field NMR technique for analyzing segmental mobility of crosslinked polymers

This low field NMR study established the correlation between the degree of crosslinking in rigid model systems to the proton spin lattice relaxation time (T₁) measured. For three model epoxy samples, our data have shown that as the number of crosslinks increases the T₁ minima shift toward higher temperatures. In addition, the magnitude of the T₁ minimum is also observed to shift to higher values as a function of crosslinks formed. These trends are consistent with the predictions of the Bloembergen, Purcell, and Pound analysis. For these highly crosslinked systems, it was necessary to incorporate the Fuoss Kirkwood distribution function for describing the coupled dynamics of the connected individual monomer units of each crosslink. By fitting the spin lattice relaxation data at different temperatures to the Fuoss Kirkwood modified BPP theory, the average activation energy for the molecular motion and the breadth of the relaxation spectrum were obtained. For these model systems, the increase in the activation energy to achieve mobility and the broadening of relaxation distribution have also been determined quantitatively. The results of this study provide the foundation for using T₁ to analyze the crosslinking process of polymeric systems. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 639–642