Multinuclear solid-state NMR of complex nitrogen-rich polymeric microcapsules: Weight fractions, spectral editing, component mixing, and persistent radicals

Abstract

The molecular structure of a crosslinked nitrogen-rich resin made from melamine, urea, and aldehydes, and of microcapsules made from the reactive resin with multiple polymeric components in aqueous dispersion, has been analyzed by ¹³C, ¹³C{¹H}, ¹H-¹³C, ¹H, ¹³C{¹⁴N}, and ¹⁵N solid-state NMR without isotopic enrichment. Quantitative ¹³C NMR spectra of the microcapsules and three precursor materials enable determination of the fractions of different components. Spectral editing of non-protonated carbons by recoupled dipolar dephasing, of CH by dipolar DEPT, and of C-N by ¹³C{¹⁴N} SPIDER resolves peak overlap and helps with peak assignment. It reveals that the N- and O-rich resin "imitates" the spectrum of polysaccharides such as chitin, cellulose, or Ambergum to an astonishing degree. ¹⁵N NMR can distinguish melamine from urea and guanazole, NC=O from COO, and primary from secondary amines. Such a comprehensive and quantitative analysis enables prediction of the elemental composition of the resin, to be compared with combustion analysis for validation. It also provides a reliable reference for iterative simulations of ¹³C NMR spectra from structural models. The conversion from quantitative NMR peak areas of structural components to the weight fractions of interest in industrial practice is derived and demonstrated. Upon microcapsule formation, ¹⁵N and ¹³C NMR consistently show loss of urea and aldehyde and an increase in primary amines while melamine is retained. NMR also made unexpected findings, such as imbedded crystallites in one of the resins, as well as persistent radicals in the microcapsules. The crystallites produce distinct sharp lines and are distinguished from liquid-like components by their strong dipolar couplings, resulting in fast dipolar dephasing. Fast ¹H spin-lattice relaxation on the 35-ms time scale and characteristically non-exponential ¹³C spin-lattice relaxation indicate persistent radicals, confirmed by EPR. Through ¹H spin diffusion, the mixing of components on the 5-nm scale was documented.

Keywords: (13)C{(14)N} NMR; Amine–aldehyde resin; Cross-linked microcapsules; NMR relaxation by persistent radicals; Quantitative (13)C NMR; Spectral editing.